Optical switches - Materials and design

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Optical switches

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Related titles: Handbook of advanced dielectric, piezoelectric and ferroelectric materials (ISBN 978-1-84569-186-8) This comprehensive book covers the latest developments in advanced dielectric, piezoelectric and ferroelectric materials. It presents current research from leading innovators in the field. Sections cover topics under the general headings: high-strain highperformance piezo- and ferroelectric single crystals; electric-field-induced effects and domain engineering; morphotropic phase boundary related materials and phenomena; high-power piezoelectric and microwave dielectric materials; nanoscale piezo- and ferroelectrics; piezo- and ferroelectric films; novel processing, new materials and properties. Science and technology of advanced piezoelectric materials (ISBN 978-1-84569-534-7) Science and technology of advanced piezoelectric materials provides an up-to-date comprehensive review of piezoelectrics. Written by a distinguished team of international contributors, this book focuses on the most promising piezoelectric materials and their applications. In the first part of the book, the properties of key piezoelectrics are discussed; materials include PZT, relaxor ferroelectrics, Pb-free ceramics, quartz, lithium niobate and tantalate, single crystals, polymers and composites. Part II reviews the preparation techniques of piezoelectrics during manufacturing and in the laboratory. Applications oriented materials development is discussed in Part III, focusing on actuators, high-power applications and performance under stress. Details of these and other Woodhead Publishing materials books can be obtained by: • visiting our web site at www.woodheadpublishing.com • contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 130; address: Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, UK) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: francis. [email protected]). Please confirm which subject areas you are interested in.

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Optical switches Materials and design

Edited by Baojun Li and Soo Jin Chua

iii © Woodhead Publishing Limited, 2010

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Published by Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, UK www.woodheadpublishing.com Woodhead Publishing, 525 South 4th Street #241, Philadelphia, PA19147, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2010, Woodhead Publishing Limited © Woodhead Publishing Limited, 2010 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publisher cannot assume responsibility for the validity of all materials. Neither the authors nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-84569-579-8 (print) ISBN 978-0-85709-041-6 (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used Have met acceptable environmental accreditation standards. Typeset by RefineCatch Limited, Bungay, Suffolk, UK Printed by TJI Digital, Padstow, Cornwall, UK

iv © Woodhead Publishing Limited, 2010

Contents

Contributor contact details 1

Introduction to optical switches

ix 1

S.J. Chua, National University of Singapore, Singapore, and B.J. Li, Sun Yat-Sen University, China

2

Electro-optical switches



B.J. Li, Sun Yat-Sen University, China

5

2.1 2.2 2.3 2.4 2.5 2.6

Introduction Theory and principles of electro-optical switches Materials and fabrication of electro-optical switches Device structures of electro-optical switches Performance and challenges References

5 6 10 12 58 59

3

Thermo-optical switches

61

L. Sirleto, G. Coppola, M. Iodice, M. Casalino, M. Gioffrè and I. Rendina, National Research Council – Institute for Microelectronics and Microsystems, Naples, Italy

3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8

Introduction Theory and principles of thermo-optic effect Materials for thermo-optical switches Device structures of thermo-optical switches Conclusions List of abbreviations List of symbols References

61 62 69 75 86 89 90 91 97

4

Magneto-optical switches



J. Tioh, R.J. Weber and M. Mina, Iowa State University, USA

4.1 4.2

Introduction History of optical communication

97 97 v

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Contents

4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10

All-optical switches Magneto-optical switches Theory and principles of magneto-optical switches Material Characterization of Faraday rotation Summary Appendices References

102 104 105 116 118 129 129 132

5

MEMS-based optical switches

136

L.L.P. Wong and J.T.W. Yeow, University of Waterloo, Canada, and A.A. Goldenberg, University of Toronto, Canada

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

Introduction Optical systems Optical switch architectures Actuating principles of MEMS-based optical switches Materials and fabrication of MEMS-based optical switches Challenges surrounding MEMS-based optical switches Conclusions List of abbreviations References

136 136 138 144 149 153 155 155 155

6

SOA-based optical switches

158

A. Assadihaghi, H. Teimoori and T.J. Hall, University of Ottawa, Canada

6.1 6.2 6.3 6.4 6.5 6.6

Introduction SOA-based switching strategy SOA structure SOA design criteria Summary References

158 158 165 171 178 178 181

7

Switching based on optical nonlinear effects



M.P. Fok and P.R. Prucnal, Princeton University, USA

7.1 7.2 7.3 7.4 7.5 7.6

Introduction Nonlinear effects for optical switches Nonlinear devices for optical switches Structure of nonlinear-effect-based optical switches The ‘ideal’ nonlinear-effect-based optical switch? References

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181 182 185 189 202 203



8

Contents

Liquid crystal optical switches

vii

206

C. Vázquez García, I. Pérez Garcilópez and P. Contreras Lallana, Universidad Carlos III, Spain, and B. Vinouze and B. Fracasso, Telecom Bretagne, France

8.1 8.2 8.3 8.4 8.5 8.6

Introduction Liquid crystal theory and principles Liquid crystal switches and applications Future trends Acknowledgments References

206 208 215 230 235 236

9

Photonic crystal all-optical switches

241

K. Asakawa, Y. Sugimoto, N. Ikeda and Y. Watanabe, National Institute for Materials Science, Japan, N. Ozaki, Wakayama University, Japan, Y. Takata, Kyocera Corporation, Japan, Y. Kitagawa, Stanley Electric Co. Ltd, Japan, S. Ohkouchi and S. Nakamura, NEC Corporation, Japan, A. Watanabe, Meijo University, Japan, and X. Wang, National Institute of Advanced Science and Technology, Japan

9.1 Introduction 9.2 Theory and principles of photonic crystal all-optical switches 9.3 Design and fabrication of advanced 2DPC waveguide for PC-SMZ 9.4 Growth and characterization of optical QDs for PC-FF 9.5 Device structures and performances of photonic crystal all-optical switches 9.6 Conclusion 9.7 Acknowledgments 9.8 References 10 Fiber, holographic, quantum optical and other types of optical switches

241 243 251 257 267 271 272 273 276



Y. Zhang and B.J. Li, Sun Yat-Sen University, China

10.1 10.2 10.3 10.4 10.5 10.6

Introduction Fiber switches Holographic switches Quantum optical switches Other switches References

276 277 294 296 305 309

11

Summary: key trends in optical switches

313

B.J. Li, Sun Yat-Sen University, China, and S.J. Chua, National University of Singapore, Singapore

   Index

316

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viii

Contributor contact details

(* = main contact)

Chapter 1

Chapter 3

S.J. Chua Department of Electrical and Computer Engineering National University of Singapore E4-05-48, 4 Engineering Drive 3 Singapore 117567

L. Sirleto*, G. Coppola, M. Iodice, M. Casalino, M. Gioffrè and I. Rendina National Research Council – Institute for Microelectronics and Microsystems via P. Castellino 111 I-80131 Naples Italy

E-mail: [email protected]

E-mail: [email protected]

Chapter 2 B.J. Li State Key Laboratory of Optoelectronic Materials and Technologies School of Physics and Engineering Sun Yat-Sen University Guangzhou 510275 China E-mail: [email protected]

Chapter 4 J. Tioh, R.J. Weber* and M. Mina High-speed Systems Engineering Department of Electrical and Computer Engineering Iowa State University Ames, IA 50011 USA E-mail: [email protected]; weber@ iastate.edu; mmina@engineering. iastate.edu

ix © Woodhead Publishing Limited, 2010

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Contributor contact details

Chapter 5

Chapter 7

L.L.P. Wong and J.T.W. Yeow Department of Systems Design Engineering University of Waterloo 200 University Avenue West Waterloo, Ontario Canada N2L 3G1

M.P. Fok* and P.R. Prucnal Department of Electrical Engineering Princeton University Engineering Quadrangle, Olden Street Princeton, NJ 08544 USA

E-mail: [email protected]

E-mail: [email protected]; prucnal@ princeton.edu

A.A. Goldenberg* Department of Mechanical and Industrial Engineering University of Toronto 5 King’s College Road Toronto, Ontario Canada M5S 3G8 E-mail: [email protected]

Chapter 6 A. Assadihaghi, H. Teimoori and T.J. Hall* Centre for Research in Photonics at the University of Ottawa School of Information Technology and Engineering (SITE) 800 King Edward Avenue University of Ottawa Ottawa, Ontario Canada K1N 6N5 E-mail: [email protected]

Chapter 8 C. Vázquez García*, I. Pérez Garcilópez and Pedro Contreras Lallana Grupo de Displays y Aplicaciones Fotónicas Dpto. Tecnologia Electrónica Escuela Politécnica Superior Universidad Carlos III de Madrid Av. Universidad 30 28911 Leganés Madrid Spain E-mail: [email protected]

Bruno Vinouze and Bruno Fracasso Optics Department Telecom Bretagne Brest France E-mail: [email protected]

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Contributor contact details

Chapter 9

Y. Kitagawa Stanley Electric Co. Ltd. 400, Soya Hatano, 257–8555 Japan

K. Asakawa* National Institute for Materials Science 3–13, Sakura Tsukuba 305–0003 Japan E-mail: [email protected]

Y. Sugimoto and N. Ikeda National Institute for Materials Science 1–2–1, Sengen Tsukuba 305–0047 Japan E-mail: [email protected]. jp; [email protected]

Y. Watanabe National Institute for Materials Science 3–13, Sakura Tsukuba 305–0003 Japan E-mail: [email protected]

N. Ozaki Wakayama University 930, Sakaedani Wakayama 640–8510 Japan E-mail: [email protected]

xi

E-mail: yoshinori_kitagawa@stanley. co.jp

S. Ohkouchi and S. Nakamura NEC Corporation 34, Miyukigaoka Tsukuba 305–8501 Japan E-mail: [email protected]; [email protected]

A. Watanabe Meijo University, Tenshiroku Nagoya 468–8502 Japan E-mail: [email protected]

X. Wang National Institute of Advanced Industrial Science and Technology 1–1–1, Umezono Tsukuba 305–8568 Japan E-mail: [email protected]

Y. Takata KYOCERA Corporation 660–10, Shimonocho Ise 516–8510 Japan E-mail: [email protected]

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Contributor contact details

Chapter 10

Chapter 11

Y. Zhang and B.J. Li* State Key Laboratory of Optoelectronic Materials and Technologies School of Physics and Engineering Sun Yat-Sen University Guangzhou 510275 China

B.J. Li State Key Laboratory of Optoelectronic Materials and Technologies School of Physics and Engineering Sun Yat-Sen University Guangzhou 510275 China

E-mail: [email protected]

E-mail: [email protected]

© Woodhead Publishing Limited, 2010

1 Introduction to optical switches S.J. CHUA, National University of Singapore, Singapore, and B.J. LI, Sun Yat-Sen University, China Abstract: A number of technologies are used for implementation of optical switches. It ranges from simple mechanical movements to deflect the light beam to using external stimuli to change the optical properties of materials. This chapter summarizes the mechanisms used for implementation of optical switches. Key words: optical switches, thermo-optical switch, magneto-optical switch, micro-electro-mechanical (MEMS), electro-optical switch, liquid crystal optical switch.

Optical communication using semiconductor lasers as sources and optical fiber as the transmission medium is the only solution to handle the massive growth of both telecom and datacom traffic. A single strand of fiber offers a bandwidth of 25 000 GHz, and a cable containing about 1000 optical fibers can carry six billion simultaneous full-screen videophone conversations – one for every person on earth. With the introduction of new services such as high-definition television (HDTV) and grid computing, bandwidth demand is expected to rise. Grid computing provides on-demand access to both local and remote computational resources for storage and visualization and encourages the effective and productive use of expensive resources to simulate scientific, engineering and commercial applications. Three-dimensional (3D) movies, which are now introduced in cinemas, will soon see their emergence in the home entertainment arena. To fully realize the potential bandwidth available on these optical fibers, other components of the optical network system have to be developed, ranging from detectors to multiplexers, buffers and switches to match the transmission rate and bandwidth. This book addresses the different technologies which could be applied to switching optical signals, applications of which depend on the topology of the optical network, the switching fabric and the switching speed required. In general, a switch is concerned with the routing of message information in response to supervisory control signals. The message information could be large blocks of multiplexed traffic in the optical core network or a large number of lower bit channels to be delivered to the users in the optical access network. However, the application of an optical switch may not just be limited to the communication networks but will also be incorporated in the communication cores of a large multi-processor computer where the data rates may exceed 100 Gbit/s. With new schemes being experimented for secure communication and for computing using quantum phenomena, new architecture will be required for switches that do not interrupt the phase information of the quantum packets. 1 © Woodhead Publishing Limited, 2010

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Optical switches

An optical switch functions by selectively switching an optical signal delivered through an optical fiber or an integrated optical circuit to another. Several methods are available and each relies on a different physical mechanism for its operation. The various physical mechanisms used are briefly summarized below, following the order of presentation in each of the chapters in this book.   1 Decrease in refractive index due to the presence of charge carriers in the semiconductor device forming the switch. By injecting charge carriers at a material interface, the refractive index at one side of the interface can be reduced, which can cause total internal reflection (TIR) to take place when a beam travels from a high to a low refractive index media at the interface. Thus, the beam is reflected rather than transmitted across the interface, enabling the beam to be switched. Changes in refractive index of one beam path relative to another cause a phase difference between the beams which can lead to constructive or destructive interference when they arrive at the outputs of the two different arms forming the beam paths. Electro-optical switches make use of this effect in an interferometric device.   2 Change in refractive index with temperature. Refractive index of materials generally decreases with increase in temperature. Thus by incorporating this property change into an interferometric device, for example, switching can be realized. This effect is made use of in thermo-optical switches.   3 Change in polarization of light as it travels through the medium interacting with the magnetic field. The rotation of the plane of polarization, known as the Faraday effect, is proportional to the intensity of the applied magnetic field in the direction of propagation of the light beam. With a polarizer at the output, the beam can be cut off when the rotation causes the plane of polarization to be perpendicular to the transmission axis of the polarizer. This effect is made use of in magneto-optical switches.   4 When a free-space beam is deflected by a micro-mirror, the deflected beam can be made incident at a number of optical fibers by precisely controlling the deflection of the micro-mirror. Such micro-mirrors can be implemented on micro-electro-mechanical (MEMS) systems often implemented by etching the silicon surface into arrays of flat beams and membranes. The movements of micro-mirrors, for example, form the basis of MEMS-based optical switches.   5 As light propagates through an optical gain medium, its wavelength, polarization, phase and amplitude can be changed and the gating function can be performed by putting an element sensitive to the property altered by the amplifier. Such elements can be a grating polarization beam splitter while a Mach–Zehnder interferometer can act upon variations in wavelength, polarization and phase. The integration of the semiconductor optical amplifier with the gating elements forms the basis of semiconductor optical amplifier (SOA) switches.   6 With optical nonlinearities such as the Kerr effect, changes in the refractive index of a material take place in response to an applied electric field. In the

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Introduction to optical switches

3

case where the electric field is due to the light itself, it is known as the optical Kerr effect or AC Kerr effect. This causes a variation in index of refraction which is proportional to the local irradiance of the light. This refractive index variation is responsible for the nonlinear optical effects of self-focusing and self-phase modulation. As the beam propagates, it experiences a phase shift due to the change in refractive index that is related to the intensity of the beam itself. Thus by applying a gating element at the output of the medium, a switching action can be implemented.   7 In liquid crystals, the orientation of the rod-like molecules causes the polarization state of a linearly polarized light transmitted through the medium to vary. Thus, if the orientation of the rod-like molecules is continuously varied from the top to the bottom of a layer by 90°, through the application of a voltage across the layer, the state of the linearly polarized light transmitted through the layer will undergo a 90° rotation with respect to that at the input. With the use of a polarizer at the output, the beam is blocked if the output polarization is perpendicular to the polarizer axis, whereas it would be transmitted when the voltage is not applied. Thus, a switching function can be performed when the input signal is distributed over several outputs, for example, and with a polarizer at each end.   8 Photonic crystals are periodic optical nanostructures, typically a hexagonal patterned array of holes in an optical slab waveguide, designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. By appropriate choice of hole diameter and period, specific wavelengths of light cannot propagate through the guide. Thus by eliminating a row of these holes, the light can be guided through the regions where there are no holes. By changing the refractive index of the semiconductor where the light is guided, say by a control pulse, phase shift can occur. Such a phase change can be made the basis of a switching action.   9 By physically moving sideways two fibers aligned end-on using a piezoelectric element, switching action can also be performed. When they are perfectly aligned, transmission takes place and the signal can be switched between fibers. 10 Quantum confined stark effect is the change in the quantized energy in a quantum well when an electrical field is applied across the quantum well. This results in a reduction of the transition energy between the lowest quantized energy levels of the hole and electron. The optical absorption of the quantum wells is increased for a designed wavelength with the application of the external voltage. This effect is being made used of in quantum optical switches. Each of the chapters deals with a different principle for the operation of the switch. However, they are considered in greater detail with discussions on the choice of materials, fabrication technique and treatment of the complexity of the switch design in affecting the performance and to satisfy the network topology and the switching speed. The control signal for the switches can be electrical in

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Optical switches

origin, viz. current or voltage, or it can be an external optical signal pulse which changes the physical properties and conditions of the switch material. In the alloptical network, the signal does not undergo any optical-to-electrical-to-optical (OEO) conversion in the transmission path from source to destination. In such a case, optical control can be achieved by the intensity of the signal beam in affecting the nonlinear optical properties, and this class of switches is important for implementation in an all-optical network. For switches, several performance criteria are specified such as switching speed, insertion loss, crosstalk, on/off ratio, power consumption and reliability. Switching speed is defined as the time it takes for the connection to be made for the signal to be transferred from the input to the output ports. It is the time it takes for the output port to see the signal after the control signal has been activated. It is a function of the delay encountered within the switch and, depending on the physical mechanism employed for the switch, can vary from nanoseconds to microseconds. Insertion loss is the amount of power loss in the signal in coupling to the output port. Crosstalk is defined as the ratio of light power in the unwanted output port to the power in the desired output port. The unwanted signal that is leaked out contributes as noise on the unintended output ports. On/off ratio is the ratio of the power in the output port when the switch is on to the power when it is switched off. In the ideal case, when the switch is off, no signal should be transmitted. As many switches are operating millions of times a second in the system, their power consumption is by no means negligible. Thus, it is important to minimize the power required to perform a switching function. Finally, for the switch to be accepted, it has to be reliable and should meet the performance parameters under a wide variety of environmental conditions. Thus, while essentially any physical mechanism can be used for making a switch, it is finally the practicality and features such as physical size, cost and stringent requirements on performance that see the switch being commercially adopted.

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2 Electro-optical switches B.J. LI, Sun Yat-Sen University, China Abstract: This chapter introduces the developments of electro-optical switches and mainly focuses on Si-based semiconductor materials because of the very mature fabrication technology. The chapter first discusses theory and principles of single-mode, multi-mode interference, and plasma dispersion effect, followed by materials and fabrication of electro-optical switches. The chapter then discusses eight kinds of electro-optical switches. Finally, a brief discussion on the performance and challenges of electro-optical switches is given. Key words: electro-optical switch, plasma dispersion effect, carrier injection, Si-Ge.

2.1

Introduction

Fiber-optic communication networks are experiencing a continuing increase in demand for telephone, cable TV, digital video, data and internet services. The continuing development of fiber-optic communication networks to accommodate future demands will depend on the availability of cheap, reliable and robust components for routing, switching and detection. Among them, optical switches are essential components for 1310–1550-nm fiber-optic communications and optical networks. They can reduce the cost of the network and increase fiber transmission capacity and at the same time, distribute optical signals to different subscribers. The basic technologies for the design and production of optical switches are now in place, but there is not yet a clear winner in the area of materials. GaAs- or InP-based quaternary compound semiconductor materials are widely employed for optical switches, due mainly to their potential for integration with active devices such as lasers and photo-detectors operating at the fiber-optic windows of 1310–1550 nm. However, its fabrication technique is not compatible with the very mature Si technology and remains complex and expensive. Silica and/or glass on silicon are widely used in integrated optics. They can offer the advantage of larger cross-section waveguides and low losses, but monolithic integration with lasers and photo-detectors is difficult. Polymer-based materials are used in optical devices, but their stability needs to be improved. In this chapter, we will introduce the developments of electro-optical switches and mainly focus on Si-based semiconductor materials because of the very mature fabrication technology.

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Optical switches

2.2

Theory and principles of electro-optical switches

2.2.1 Single-mode principle Ridge waveguide is a fundamental structure for constructing optical switches. To connect a switch with a single-mode optical fiber with a core diameter of about 9 to 10 µm, the first step of the design is to get a large-scale single-mode ridge waveguide as shown in Figure 2.1. The design criteria for the single-mode ridge waveguide are as follows: (1) The numerical aperture of ridge waveguide must be matched with that of single-mode fiber (0.2–0.3); (2) The ridge waveguide must have a large cross-section, and the ridge height and ridge width should be equal to the core diameter of the singlemode fiber (9–10 µm); (3) The ridge waveguide must support single-mode. As a design example, we use Si-based materials. Figure 2.2 shows the crosssection of a ridge waveguide. It was formed by a Si-Ge layer with a refractive index of n1 grown on (100) Si substrate. The ridge width is w = 2aλ, the inner ridge height is h = 2bλ, and the etched depth of the ridge is h´ = 2b(12r)λ. In these expressions, λ is the free-space optical wavelength, and r is the fractional height of the side regions compared to the ridge center (the out-inner ratio). The three dielectric materials have refractive indices of n0, n1, and n2, respectively, at the wavelength of interest. To propagate the single-mode light in the input and the output waveguides, the lateral dimensions and the transverse dimensions must satisfy the single-mode ridge waveguide condition. Based on the single-mode ridge waveguide condition, the ratio a/b should be1

n0 n1 n2 n0

n1 n2

2.1  Cross-section view of a ridge waveguide.

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Electro-optical switches

7

w = 2aλ h ' = 2b(1–r)λ

n0

h = 2bλ

n1 Si-substrate

n2

2.2  Cross-section view of SiGe/Si ridge waveguide.

[2.1] [2.2] where γ0,2 = 1 for HE mode, γ0,2 = (n0,2/n1)2 for EH mode, a, b, r, and λ are the ridge width factor, ridge height factor, etching depth factor, and wavelength in free space, respectively, and n0, n1, and n2 are the refractive index of cladding layer, guiding wave layer, and substrate, respectively. It is well established that the pseudomorphic, dislocation-free SiGe alloy layers can be grown on a Si substrate, provided that their thickness is less than a critical thickness hc. This thickness is defined as the thickness above which the misfit dislocations are generated. It also depends remarkably on the Ge fraction x. For SiGe layers grown on Si (100) substrates, the function hc(x) can be expressed by2



[2.3]

where a(x) ≈ 0.554 nm is the mean bulk lattice constant of SiGe, b is the Burger’s vector modulus, ν is the Poisson’s ratio, and fm(x) is the substrate-alloy misfit parameter. For SiGe alloys, the lattice constant can approximately be related to the lattice constant of Si and Ge according to Vegard’s rule for a (100) Si substrate with fm(x) = 0.04117x. To reduce the misfit dislocation of SiGe-Si interface, the thickness of strained SiGe alloy layer, which is grown on the Si (100) substrate, must be less than hc. For the SiGe optical waveguide, the Ge content must be less than 15%. According to the calculation, the optimum Ge content is x = 0.03 to 0.05 (Figure 2.3). Here we choose x = 0.04. According to Eq. (2.3), for x = 0.04, the hc = 6.5 µm. Therefore, the thickness of the ridge waveguide was chosen to be 2.5 µm. Figure 2.4 shows the

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8

0.5

0.4

0.3

NA

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Optical switches

0.2

0.1

0.0 0.00

0.02

0.04

0.06

0.08

0.10

Ge content, x 2.3  Numerical aperture of a SiGe/Si waveguide with Ge mole fraction, x.

critical value of the ratio a/b as a function of b for λ = 1.3 and 1.55 µm, respectively, using the factors r = 0.5 and 0.8 as parameters3 using Eq. (2.1). In Figure 2.4, we observe that the HE 00 and EH 00 modes are essentially identical. To simplify the design, only the HE 00 mode is discussed here. For a thickness of 2.5 µm and r = 0.5, an etched depth of 1.2 µm and a width of 3 µm were chosen, respectively.

2.2.2 Multi-mode interference principle From the viewpoint of integration, a small size is desirable. The main advantages of optical devices based on the multi-mode interference (MMI) effect are low loss, compact size, and large fabrication tolerances. They are quite easy to design and fabricate. Because of the excellent properties of MMI devices, optical switches were demonstrated based on MMI effect. The operation of the optical MMI switch is based on the self-imaging principle. Self-imaging is a property of a multi-mode waveguide by which an input field is reproduced in single or multiple images at periodic intervals along the propagation direction of the waveguide. In the MMI switch, a multi-mode waveguide is designed to support a large number of modes. According to the self-imaging theory4, when an input light beam is coupled into the multi-mode waveguide from a single-mode input waveguide, the input optical field will be reproduced in single or multiple images at periodic intervals along the propagation direction. In general, taking the Goos-Hähnchen shifts into account, an effective width We of the multi-mode waveguide can be expressed as:

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Electro-optical switches

9 [2.4]

where σ = 0 for the TE mode and σ = 1 for the TM mode, λ0 is a free-space wavelength; nr and nc are effective refractive indices of ridge waveguide and cladding layer, respectively; WM is the width of the multi-mode waveguide. (a)

8

Rib width/Rib height, a/b

7

HE 00

l = 1.3 mm

6

EH00

Ge0.04 Si0.96 /Si

5 4

2

r = 0.5

1 0

Multi-mode region

r = 0.8

3

single-mode region 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Rib height factor, b (b)

8

Rib width/Rib height, a/b

7

HE 00

l = 1.55 mm

6

EH00

Ge0.04 Si0.96 /Si

5 4 r = 0.8

3 2

r = 0.5

1 0

Multi-mode region

single-mode region 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Rib height factor, b 2.4  Critical a/b versus b for SiGe ridge waveguide shown in Fig. 2: (a) λ = 1.3 µm and (b) λ = 1.55 µm.

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4.0

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Optical switches

By defining Lπ as the beat length of the two lowest-order modes:



[2.5]

where β0 and β1 are the propagation constants of the fundamental and the first-order lateral modes, respectively. When the width WM and length L of the multi-mode waveguide satisfy the condition: L = p (3Lπ)    p = 0, 1, 2, …

[2.6]

the input light field will be repeated and single image can be obtained. For the multi-mode waveguide with a length of z = 3Lπ, according to the partial index-modulation principle for the multi-mode waveguide5, when a π phase shift is introduced at around the position z = 3L π /2, a transformation between the even and odd modes will take place. Thus after a further propagation of z = 3L π /2, the input optical signal will be outputted from another corresponding output waveguide.

2.2.3 Plasma dispersion effect The principle of the plasma dispersion effect is that the refractive index of materials is related to its carrier concentration6. As an example, for SiGe/Si materials, when the Ge composition is low (x < 20%), the free-carrier plasma dispersion effect in Si12xGex will lead to a variation of refractive index, which can be described by the relation as follows: ∆n = – (q2λ2/8π2c2nε0)·[(∆Ne/mce*) + (∆Nh/mch*)]

[2.7]

where q is the electron charge, ε0 is the permittivity of free space, n is the refractive index of Si12xGex, λ is the wavelength, c is the light velocity, ∆Ne and ∆Nh are the concentration changes of electrons and holes, respectively, mce* and mch* are the conductivity effective masses of electrons and holes of Si1-xGex and could be given by

2.3

mce* = mce*(Si) (12x) + mce*(Ge) x

[2.8]

m* ch = m* ch(Si) (12x) + m* ch(Ge) x

[2.9]

Materials and fabrication of electro-optical switches

Materials that can be used to fabricate optical waveguide switches are LiNbO3, III-V compound conductors, polymers, and Si-based materials. To achieve good reliability and highly monolithic integration with Si-based chips, SiGe material was used to fabricate multi-functional photonic devices because of low propagation loss (< 0.5 dB/cm) in the wavelength region of λ = 1.3–1.55 µm. SiGe epitaxy has

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Electro-optical switches

11

the advantage that its fabrication techniques are compatible with large-scale Si integration. Due to greater electron mobility and smaller bandgap in germanium, remarkable improvements in the performance of devices can be achieved, with virtually no changes in existing all-silicon designs. SiGe is also relatively easy to fabricate using existing silicon facilities. For Si-based materials, they were usually grown by molecular beam epitaxy (MBE) or UHV-chemical vapor deposition (CVD). By using an MBE, two waveguide switches are described as follows. One contains a lateral p+-n junction (Fig. 2.5) and the other contains a vertical p+-n junction (Fig. 2.6). p-SiGe

p-SiGe

p-Si substrate

p-Si substrate

(a)

(b)

SiO2

SiO2 p-SiGe

n+

p-Si substrate

p-Si substrate

(c)

(d)

SiO2

SiO2

Al electrode

+ p

n+

p-Si substrate

+ p

n+

p-Si substrate

(e)

(f)

2.5  Fabrication procedure: (a) SiGe material growth; (b) dry etching to form ridge waveguide; (c) deposition of SiO2; (d) phosphorus ion implantation to from n+ carrier injection regions; (e) boron ion implantation to form p+ collector; and (f) sputtering deposition of aluminum films for ohmic contacts. SiO2

32.2 μm

5 nm, n+-Si cap, 1 × 1018 cm–3

1.0 μm 2.5 μm

p-SiGe, 2 × 1016 cm–3

5 nm, p-Si cladding, 2 × 1016 cm–3 p+-Si sub, 2 × 1018 cm–3

2.6  A schematic diagram of a waveguide cross-section view of an optical switch.

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Optical switches

2.3.1 Fabrication of a lateral p+-n junction switch The sample was made in the stages as shown in Figure 2.5. First, SiGe materials were grown by MBE. A 50-nm p-type Si buffer and a lightly doped p-type SiGe (Ge content at ~4%) layer with the thickness of 2.6 µm was grown by molecular beam epitaxy on a p-Si(100) substrate. The boron doped-concentrations in the buffer layer and waveguide layer are approx. 5 × 1016 cm23. The substrate temperature during the growth was kept at 600°C. Second stage is the fabrication of the device. The device was fabricated in a 3-µm manufacturing process line. The ridge waveguides were formed by reactive ion etching (RIE) technique with etching rate of 250–300 Å min21. A 550-nm thickness SiO2 film was deposited at 400°C by plasma-enhanced chemical vapor deposition (PECVD) on the top of the sample surface to serve as the ion implantation mask and the surface passivation layer. The n+ carrier injection regions of the switch were realized using phosphorus ion implantation with energy of 60 keV and a dose of 5 × 1015 cm22 (Fig. 2.5d). The p+ collector was formed by boron ion implantation with energy of 80 keV and the same dose. The n+ and p+ ohmic contacts were formed by sputter deposition of aluminum films with thickness of 2.0 µm and followed by alloying at 440°C for 30 min in N2 ambient. The last stage is end polishing. The wafer was cut and the input and output facets of the waveguides were polished by a mechanical method in order to couple the incident light from a single-mode optical fiber.

2.3.2 Fabrication of a vertical p+-n junction switch A 5-nm p-type Si lower cladding layer with a concentration of about 2 × 1016 cm23 was grown by UHV-CVD (900°C) on a p+-type Si(100) substrate. This is followed by the growth of a 2.5-µm p-type strained Si0.96Ge0.04 core waveguide layer with a concentration of 2 × 1016 cm23 (Figure 2.6). On the top of the sample, an abrupt n+-p junction was formed by growing a 5-nm n+-type Si cap layer with a concentration of 1 × 1018 cm23. The whole device was fabricated using siliconoptical bench technology. Waveguides were developed based on lithography and plasma etching of silicon. The waveguide with an abrupt n+-p junction was formed by removing other n+-Si cap layer. The device surface was next passivated by a SiO2 layer. The n+ and p+ ohmic contact electrodes were deposited by evaporating 1.0-µm thick aluminum layer followed by alloying at 450°C. The chips were diced and the input/output facets of the switch were mechanically polished in order to have easy coupling of the incident light from a single-mode fiber.

2.4

Device structures of electro-optical switches

2.4.1 1 × 1 switch 1 × 1 optical switch is usually a 1 × 1 optical modulator. It can be fabricated in III–Vs materials, Si(Ge) materials, LiNbO3, or polymers. Figure 2.7 shows a 1 × 1

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Electro-optical switches

13

p+

n+

n+

n-Si Si-sub.

SiO2

2.7  Schematic diagram of 1 × 1 optical switch/modulator in SOI.

optical switch fabricated in silicon-on-insulator (SOI)7. A single-mode rib waveguide with a width of W is composed of an n-type silicon guided-wave layer on a SiO2 layer. An abrupt p+-n junction is formed below the top surface of the ridge waveguide to inject the carriers into the waveguide. If the p+-n junction of the waveguide is forward-biased when guided-mode optical signals are end-coupled into the rib waveguide, a large number of carriers will be injected into the guided-wave layer of the waveguide, and the refractive index in the waveguide will decrease because of the plasma dispersion effect, which can make the guided-mode convert into the radiation mode of the substrate and the cover. This causes a lot of guided-mode energy to be lost and absorbed in the rib waveguide, which will cause the rib waveguide to cut off, resulting in the so-called waveguide ‘vanishing.’ Consequently, there is no output light in the waveguide, and thus switching is achieved. For the fabrication, SOI material is used. This is produced by cleaning followed by oxidizing the substrate wafer (the SiO2 is 400–500 nm thick), bonding at a high temperature (in O2, at 1200°C, for 2 h), and thinning (grinding precisely rear face down to 20 µm), and then, polishing to a thickness of 6 µm with r.m.s. roughness <±0.5 µm. The second stage is the preparation of the SOI ridge waveguide, which consists of lithography, oxidizing, photo-etching, and potassium hydroxide liquidphase anisotropic etching at 80°C. Silicon (3 µm) is etched away to form the ridge waveguide, i.e. a ridge height of 3 µm and a width of 6 µm. The third stage is the fabrication of the optical waveguide intensity modulator, and the processes involved are the making of diffusion masks, photo-etching, diffusing phosphorus

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Optical switches

to form two n+ collectors either side of the ridge waveguide, oxidizing, photoetching, and diffusing boron to form a p+-n junction on the ridge waveguide. The gap between each n+ region and the ridge waveguide is 12 µm and the area of the p+-n junction is 6 × 200 µm2. The metal contacts are obtained by sputtering a Ti/Al electrode. To improve the response speed of the switch, electron irradiation is performed at an electron energy of 14 MeV. After that, the wafer is diced to an overall length of 6 mm, but the real operation length of the switch is <0.5 mm. The chip is end-polished and wire-bonded for characterization. The switching current of the switch is 45 mA. The modulation depth is 96% at an injection current of 45 mA. The insertion loss is 3.65 dB at a wavelength of 1.3 µm. The switching time is about 160 ns.

2.4.2 1 × 2 switch Optical switches can be implemented using Y-shaped optical waveguides. The Y-shaped optical waveguide switch consists of one input waveguide and two output waveguides. The input waveguide is connected to the output waveguides at a branching point with a small branching angle. The 1 × 2 Y-branching digital photonic splitting (DPS) switch has an advantage in that the output optical power of the switch is insensitive to voltage variations or drift in the applied controlling voltage. When an appropriate controlling voltage is applied to one of the two output waveguides, propagation of the light is allowed in one of the output optical waveguides and blocked in the other output optical waveguide. The optical output power in either of the output waveguides can also be controlled and modulated by an appropriate voltage. Desirable properties of the 1 × 2 DPS are low insertion loss, low crosstalk, low switching voltage, high switching speed, good reliability, and capability for highly integrated implementation. With the use of modal analysis tools, the Y-shaped DPS exhibits low wavelength sensitivity, polarization insensitivity, and highly desirable digital response. Figure 2.8 shows a Y-branch digital optical switch with varied local branch angle8. The waveguide was fabricated in z-cut LiNbO3 substrate by diffusing Ti to a depth of 60 nm and width of 7 µm at 1050°C for eight hours. The SiO2 buffer layer has a thickness of 250 nm. The length of the electrode is 10 mm. Figure 2.9 shows a voltage or current-driven digital optical switch based on carrier effects in InP/GaInAsP for both TE and TM polarizations of light9. The driving voltage of the switch is 210 V and the low driving current is 6 mA. The crosstalk is better than 12.1 and 152 dB for polarization independent operation with a voltage as low as 210 V and a current as low as 6 mA. Figure 2.10 shows the configuration of the silicon 1 × 2 optical switch10. The Y-junction is formed by a silicon single-mode ridge waveguide. In the ridge waveguide an impurity-induced index step at the n/n+ interface is used for vertical confinement of the optical wave. Lateral confinement is provided by the Si/Si3N4 ridge wall and the effective index induced by its geometry. Below the top surface of each branching

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Electro-optical switches

15

Waveguide

Electrode

h

0

L

z

2.8  Diagram of Y-branch digital optical switch with varied local branch angle.

waveguide, an abrupt p+-n junction is made to inject the carriers into the waveguide when it is forward-biased. The switch was characterized at 1.3 µm. The switch current is 290 mA, the extinction ratio and insertion loss are 22.3 and 8.2 dB, respectively. Figure 2.11 shows a Y-branch digital switch with a double-etch waveguide structure to greatly enhance the loss and crosstalk performance11. The double-etch design increases the coupling between the two tapered Y-branch waveguides, which allows for efficient switching while decreasing the radiation loss that arises from the branching angle. This combined advantage also allows us to reduce the switch length, which in turn results in an efficient high-speed design. The

0.2 μm p*GaInAsP 1.1 μm p InP

Electrodes

0.4 μm nid InP 0.5 μm n GaIn AsP 1 μm n InP n+ InP substrate

<110>

2.9  Schematic view of digital optical switch with epilayers.

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Optical switches Output waveguide 1

2

p+ n junction n epitaxial layer n+ doped silicon

Input waveguide

2.10  1 × 2 optical waveguide switch.

double-etch and dual-taper Y-branch switch/modulator was designed with InGaAsP multiple-quantum wells (MQWs). The waveguide layers consist of a 1-µm-thick p-doped InP cladding layer, a 0.6-µm-thick intrinsic layer which has 30 periods of InGaAsP–InGaAsP MQWs (6 nm well, 7 nm barrier), and an n-type InP substrate or lower cladding layer. At the input is a 3-µm-wide and 1.6-µm-deep single-mode waveguide which adiabatically widens to a width of 5 µm where it splits into two symmetric active waveguide branches with an angle of 0.4°. Biases can be applied to these active branches in order to produce index changes. These branches are trapezoidal in shape and maintain a constant gap of 0.75 µm between them throughout the active length which is 900 µm long. In this active section of the switch, the gap is etched only to the bottom of the p-InP cladding layer so that electrical isolation between the two active branches is achieved while allowing for a strong optical coupling to occur between them. The outer sides of the active

Passive input section

Passive fanout sections

Port 1

900 mm

Input light

250 mm

Active section Port 2 3 mm

2.11  Schematic diagram of a double-etch and dual-taper digital optical switch.

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Electro-optical switches

17

waveguides as well as all the passive waveguides are etched to the bottom of the i-MQW layer. At the end of the active waveguides, the two waveguide branches continue as passive tapers down to the original width of 3 µm with a smaller angle (~0.05°). Then, these waveguides fan out to 250 µm center–center separation between them via deep-etch S-bends of approximately 350-µm radius. The measured crosstalk and loss are 219 and 3 dB, respectively, for a 3-mm-long switch. Figure 2.12 shows a compact 1 × 2 waveguide digital optical switch (DOS) with electrically reconfigurable output waveguide arms12. The DOS p-i-n waveguide heterostructure was grown by chemical beam epitaxy in InGaAsP–InP. The ridge waveguides were etched, and silicon nitride was used as the insulating layer and for planarization. Ti/Au electrical contacts were placed on each branch, as shown in Figure 2.12a. The region between the two electrical contacts was implanted to a depth of 400 nm with O+ ions to electrically isolate the contacts at the Y-junction vertex. The switch exhibits a 20-dB switching contrast ratio. The device length is 1400 µm.

(a)

Electrode 1

Output waveguide 2

Intput waveguide

Electrode 2

Output waveguide 1 Output waveguide 2

(b) Electrode 1 (biased) Intput waveguide

Electrode 2 (unbiased) Output waveguide 1

2.12  (a) Configuration of the digital optical switch Y-junction switch, and (b) effective waveguide configuration when the index under the biased electrode matches the refractive index of the surrounding slab region. The shaded areas indicate high effective index, while the clear areas indicate low effective index.

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Optical switches

Figure 2.13a shows a plan view of a symmetrical Y-branch DPS with two widened carrier-injection regions and Figure 2.13b shows its cross-section perpendicular to A–A13. It was designed for free carrier injection for operation at λ = 1.55 µm. In the ridge waveguide, an impurity-induced index step at the p/p+ interface is used for vertical confinement of the light. Lateral confinement is provided by the SiGe/SiO2 ridge wall, and an effective index is induced by its geometry. Below the top surface of each branching waveguide, an abrupt n+-p junction is made to inject the carriers into the waveguide when it is forwardbiased, as shown in Figure 2.13b. The principle of the DPS is based on the plasma dispersion effect, in which the variation of the refractive index of SiGe is related to its carrier concentration. During operation, the incident light with a wavelength of λ = 1.55 µm will be split into the branches 1 and 2 of the Y-branch. When the n+-p junction of the Y-branch is forward-biased, a large number of electrons will be injected to the p-region of the waveguide causing an increase of carrier concentration. This will lead to a decrease of the refractive index of the SiGe layer at λ = 1.55 µm which will be cut-off at branch 1 and/or branch 2. The thickness of the ridge waveguide was chosen to be 2.5 µm for the singlemode ridge waveguide operation. The widths of input ridge waveguide at branch 1 and branch 2 were chosen to be 8 µm. To enhance the switch performance and therefore to reduce the switching power consumption, the widths of the two carrier-injection regions were chosen to be 10 µm, wide enough to support single-

(a)

A

450mm

Light

w1

150mm

w2

Electrode

Branch 1

16mm

1.27°

Electrode 300mm (b)

Material

Index

SiO2

1.45 1.2 mm

SiGe Si-wafer

2.5 mm

3.492

Branch 2

A

Type n+ 10 mm

3.48

p p+

2.13  Schematic diagram of a symmetrical waveguide Y-branch digital photonic splitting switch (a) top view and (b) A–A cross-section view. (not in scales)

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Electro-optical switches

19

mode operation. The separation of the two output waveguide at the ends is only 16 µm, permitting the use of only a single-output fiber at one time. The etching depth of all ridge waveguides was 1.2 µm for Ge content of 4%. The effective index method was employed to analyze the behavior of the mode propagation and determine the branch angle of the switch. In our optimum design, the Y-branching angle of the switch is optimized to be 1.273°. The total length of the switch is 900 µm and the length of the active region is Lactive = 150 µm. The distance of the electrode to the facet of the output waveguide is 50 µm. The device performance is simulated by the beam propagation method (BPM) and the results are shown in Plate I. From Plate Ia, we can see that for a Y-branch, the output optical powers at the two arms are 47.6% when a switch is ON for both the branches. This means the insertion loss, which is defined as 210log10(Ptotal-output/Ptotal-input), is 0.2 dB. For a DPS response as shown in Plate Ib, the output optical powers in the two arms are 58.7% and 3%, whenever a switch is ON for branch 2 and OFF for branch 1. Exactly similar results are obtained whenever the branch 2 is OFF and branch 1 is ON, as shown in Plate Ic. The calculated insertion loss is 2.1 dB and the crosstalk is 212.9 dB in both the cases. For this kind of switch, without applying modulation voltage on both n+-p junctions, two bright output patterns of λ = 1.55 µm were observed from branch 1 and 2, respectively. With the branch 1 at forward-bias (active state) and the branch 2 at zero-bias (passive state), the output optical power of branch 1 decreases and the output optical power of branch 2 increases with the applied voltage at branch 1. Figure 2.14 shows the output optical power versus the applied forward-bias. It illustrates that the switch operates as an on/off device at a threshold voltage of 1.0 V and a corresponding threshold current of 85 mA on one of the output waveguide arms. In this situation, the calculated insertion loss is 3.2 dB. From Figure 2.14, we can further see that when the forward-bias reaches 2 V, the active branch was totally cut-off. When the branch 1 is at zero bias while the branch 2 is at forward-bias, the output optical power of branch 2 decreases and the output optical power of branch 1 increases with the applied voltage on branch 2. The totally cut-off voltage of the light at branch 2 is also 2 V. Under this condition, the injection current density is 6.3 kA/ cm2 and the power consumption is 190 mW. The measured insertion loss is 5.2 dB, and the crosstalk is 29.6 dB. Figure 2.15 shows the near-field output intensity patterns observed by a vidicon at the end facets of the optical waveguide switch.

2.4.3 2 × 2 switch The 2 × 2 optical switch is a kind of fundamental device in integrated optics and has already received much attention in recent years, especially for Si-based optical waveguide switches. These switches are generally based on the principle of total internal reflection (TIR), and require the precise reflection at interfaces in the switch structure. The reflection interface of the optical switch is controlled by refractive index, which can be altered significantly by means of carrier injection.

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Injection current (mA) 85

1.0

Output optical power (arb. units)

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Optical switches

0.8

95

105

115

125

135

145

Threshold current Passive branch

0.6

0.4

0.2

0.0

Active branch

0

1

2

3

4

5

6

7

8

Forward bias (V)

2.14  Output optical power versus the applied forward bias.

Figure 2.16 shows a 2 × 2 carrier injection TIR optical switch for λ = 1.55 µm operation14. It is a 2 × 2 SiGe waveguide structure with an input Y branch and an output Y branch connected to an intersection region. Below the top surface of the intersection, an abrupt p-n+ junction is made to inject the carriers under the

Output spot

Output field

2.15  Near-field output intensity patterns observed by a vidicon at the end facets of the switch.

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Electro-optical switches

21

forward-bias into a region, which is called the carrier injection region. The injected carriers in this region diffuse and drift toward a collector which is located on the surface of the guiding layer and apart from the p-n+ junction for a distance of d. The decrease of the refractive index in the carrier injection region due to the plasma dispersion effect in SiGe leads to the formation of a reflection region at the intersection. This switch is really a transverse injection switch which improves the carrier distribution at the interface of the reflection region. In order to enhance the carrier utilization ratio, the drift distance of the carriers (d) has been extended and the reflection region is not placed at the center of the intersection. This is the so-called asymmetric switch. The n+ region is placed off-center because the electric field lines crowd to one side of the n+ region and the off-center plasma fills the half waveguide better, too. Without the formation of reflection region by carrier injection, the input light beam from port 1 will propagate basically to port 4. When the p-n+ junction in the structure is forward-biased and the reflection region is formed, the input light from port 1 suffers the reflection. Then TIR occurs and the incident laser beam from port 1 is reflected to port 3, and the switching operation is realized. The thickness and width of the ridge waveguide are 3 and 8.5 µm, respectively, and the etching depth is 1.2 µm for a Ge content of 0.04. For λ = 1.55 µm optical switch, the optimized branch angle is 5°–6° and the intersection length is determined to be 162.5–195 µm.

(a)

1

3

n+ q d

2

4 p+

(b)

Al-electrode

SiO2 p-SiGe

n+

p+

p-Si substrate

2.16  Schematic diagram of (a) the SiGe asymmetric optical waveguide switch and (b) cross-section view of the intersection region.

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Optical switches

The sample for the TIR switch was made by the following steps: A lightly doped p-type Si0.96Ge0.04 layer with the thickness of 3 µm was grown by molecular beam epitaxy on a p-Si(100) substrate. The doped concentration in the waveguide layer is about 5 × 1016 cm23. The substrate temperature during the growth was kept at about 600°C in order to improve the epitaxial quality of the layers. The ridge waveguide was formed by plasma ion etching. The p-n+ junction for carrier injection and the p+ collector were formed by ion implantation. A SiO2 film was deposited on the top of the sample surface to serve as the ion implantation mask and the surface passivation layer. The n+ region of the switch was realized using phosphorus ion implantation with a dose of 5 × 1015 cm2 at the energy of E = 80 keV, and the p+ contact region was formed by boron ion implantation with the same dose at the energy of E = 120 keV. The p-n+ junction area is 4 × 174 µm2. For maximum carrier utilization, the collector is placed at d = 20 µm from the p-n+ junction. The n+ and p+ ohmic contacts were formed by sputtering deposition of Al films followed by alloying at 450°C. The input and output facets of the waveguide were polished in order to couple the incident light from a single-mode optical fiber. The total length of the device is 5 mm. For this switch, the experiment shows that under the zero bias of the p-n+ junction, only the output port 4 has optical power outputted. The switch is in a cross state and the measured power ratio P3/P4 is 0.01, the crosstalk is about 220 dB, and the insertion loss is about 2.84 dB at zero bias. As the forward-bias of the p-n+ junction increases, the injection current increases accordingly, and the output power at port 4 decreases accompanied by an increase of the output optical power at port 3. Figure 2.17 shows the measured output power at port 4. Figure 2.18 is the measured modulation depth against the modulation voltage of the switch. From Figure 2.18 it can be seen that the modulation depth reaches 90% at a bias voltage of 1.3 V. The modulation current and injection current density are 110 mA and 15.8 kA/cm2, respectively. At the bias voltage of 1.4 V, the device reaches a maximum optical switching state with a maximum switching current of 120 mA, and a corresponding current density of 17.2 kA/cm2, which is slightly higher than the theoretical value of 15.0 kA/cm2. This small deviation between the measured and calculated results might be caused by the carrier-injection-induced optical absorption from the waveguide and the active region that was ignored in the theoretical calculation. At 1.3 V bias voltage, the losses of the switch for λ = 1.55 µm operation are as follows: the insertion loss is 2.86 dB, the Fresnel losses at both waveguide ends are 3.2 dB, the absorption losses of the imaging lens are 5.16 dB, and the coupling loss is 0.8 dB. So, the overall loss is 12 dB. The crosstalk of the switch, which is defined as 10 log10(P4/P3), is less than 218.5 dB at the switching current of 110 mA, and the extinction ratio is larger than 34 dB. The modulation depth is 90% at an injection current of 110 mA and the switching time is about 0.2 µs. The device reaches a maximum optical switching at the injection current of 120 mA. Most 2 × 2 optical switches utilizing carrier-induced refractive index change are generally based on the principle of total internal reflection, and require the

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2.17  Normalized output optical power at port 4 for input power in port 1 versus modulator voltage of the optical switch.

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2.18  Measured modulation depth versus modulator voltage of the optical switch.

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Optical switches

precise reflection interfaces in the switch structure. These switches consist of two single-mode waveguides intersecting at an angle ‘θ’ with a straight electrode. It is shown that the switching characteristics with a straight electrode would be satisfactory only if large refractive index variations are achievable and the intersection angle is limited to about the critical angle. To widen the reflection angle of this type of switches, an optical intersecting-waveguide switch with a widened angle of deflection has been reported by Nayyer et al.15 Figure 2.19 shows a schematic diagram of the intersectional ridge optical waveguide switch with a bow-tie electrode designed in SiGe/Si material16,17. The two ridge waveguides with a width of w and composed of a p-type SiGe guiding layer grown on a p-Si substrate intersect at an angle of θ. A bow-tie electrode with an angle of θw is located at the intersection region. On the top surface of the intersection, a bow-tie abrupt p-n+ junction is made to inject the carriers under the forward-bias into a region, which is called the carrier injection region. We know that the decrease of the refractive index in the carrier injection region due to the plasma dispersion effect in SiGe leads to the formation of a reflection interface at the intersection region. Under the zero bias of the p-n+ junction, the input light beam from port 1 will propagate basically to port 4. When the bow-tie p-n+ junction in the structure is forward-biased and the reflection region is formed, the input light from port 1 suffers the reflection. Then total internal reflection occurs and the incident light beam from port 1 is reflected to port 3, and the switching operation is achieved. The branch angle of the switch is θ = 2° and the bow-tie angle is θw = 1.5°. The thickness and width of the ridge waveguide are 2.6 and 9 µm, respectively, and the etching depth was 1.0 µm. The bow-tie electrode length is L = 600 µm and the total length of the switch is 5 mm. The sample for the switch was made by the following steps: A 50 nm p-type Si buffer and a lightly doped p-type SiGe (Ge content at around 4%) layer with the thickness of 2.6 µm were grown by molecular beam epitaxy on a p-Si(100) 1 W

3

L

n+ θW

θ

p+ 2

4

2.19  Schematic diagram of a 2 × 2 intersectional ridge optical waveguide switch with a bow-tie electrode.

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Electro-optical switches

25

substrate. The boron-doped concentration in the buffer layer and the waveguide layer are about 5 × 1016 cm23. The substrate temperature during the growth was kept at 600°C. The ridge waveguide of the device was formed by reactive ion etching technique. A 550-nm thickness SiO2 film was deposited at 400°C by plasma-enhanced chemical vapor deposition on the top of the sample surface to serve as the ion implantation mask and the surface passivation layer. The n+ carrier injection region of the switch was realized using phosphorus ion implantation with an energy of 60 keV and a dose of 5 × 1015 cm22. The p+ collector was formed by boron ion implantation with the energy of 80 keV and the same dose. The n+ and p+ ohmic contacts were formed by sputter deposition of Al films with a thickness of 2.0 µm and followed by alloying at 440°C for 30 min. After that, the wafer was diced, the input and output facets of the waveguides were polished in order to couple the incident light from a single-mode optical fiber. For 1.3 µm operaton, it was discovered that under the zero bias of the p-n+ junction, only the output port 4 has an output optical profile and the switch is in a cross-state. The near-field output intensity spot from the output waveguides port 4 is shown in Figure 2.20a. As the forward-bias of the p-n+ junction increases, the injection current increases simultaneously, and the output power at port 4 decreases. The decrease of output optical power at port 4 is accompanied with the increase of the output optical power at port 3. At an injection current of 85 mA, the input light was totally internally reflected into port 3 and the near-field output intensity spot is shown in Figure 2.20b. Figure 2.21 shows the measured output power at port 3 and the modulation depth at various forward-biases. From Figure 2.21 we can see that at a bias voltage of 1.0 V, the output optical power is very much higher and the modulation depth reaches 90%. At this state, the injection current is 85 mA. At the bias voltage of 1.2 V, the device reaches a maximum optical switching state and the maximum switching current is 95 mA. At the bias voltage of 1.0 V, the measured power ratio P4/P3 is 0.011, the crosstalk of the switch, which is defined as 10 log10(P4/P3), is 219.6 dB. If no switch was there, the mean power ratio Pout/Pin was 0.695, so that the absorption loss of the a

b

Port 3

Port 4

Port 3

Port 4

2.20  Near-field output bright spots observed by a vidicon at the end facets of the optical waveguide switch at 1.3 µm. (a) at zero injection current, a bright spot can be seen at port 4, and (b) at 85 mA injection current, a bright spot is seen at port 3.

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2.21  Normalized output optical power at port 3 for input at port 1 and modulation depth versus forward bias voltage for optical waveguide switch with a bow-tie electrode at the wavelength of 1.3 µm.

lens is thus equal to 1.58 dB. If a switch was there, the ratio of output and input powers was 0.187. The insertion loss, which is defined as 210 log10(Pout/Pin), is 7.28 dB. This loss includes the absorption loss of the lens. So the net insertion loss is 5.70 dB. The insertion loss of the device includes the Fresnel-losses at the two waveguide ends and the mode mismatch loss at the fiber-waveguide coupling. Subtracting the above loss by the Fresnel-loss of 1.6 dB at both waveguide ends and an estimated mode mismatch loss of 0.8 dB, the waveguide loss of 1.70 dB is estimated. The extinction ratio, which is defined as 10 log10(P4/P3) under the zero bias of the p-n+ junction, is determined to be 38.5 dB. For 1.55 µm operation, at zero bias of the p-n+ junction, the near-field output intensity spot from the output waveguides port 4 is shown in Figure 2.22a. The ratios of output and input powers were 0.69 and 0.189 without and with the inserted device, respectively. The results show that the absorption loss of the lens is 1.61 dB and the waveguide loss is 1.63 dB, which does not include the Fresnellosses and the mode mismatch loss. The extinction ratio is determined to be about 39 dB and the crosstalk is 221.8 dB. As the forward-bias of the p-n+ junction increased, the decrease of output optical power at port 4 is accompanied with the increase of the output optical power at port 3. At an injection current of 78 mA, the input light was totally internally reflected into port 3 and the near-field output intensity spot is shown in Figure 2.22b. Figure 2.23 shows the measured output power at port 3 and the modulation depth at various forward-biases. From Figure 2.23 we can see that the modulation depth reaches 90% at a bias voltage of

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Electro-optical switches a

27

b

Port 3

Port 4

Port 3

Port 4

2.22  Near-field output bright spots observed by a vidicon at the end facets of the optical waveguide switch at 1. 55 µm. (a) at zero injection current, a bright spot can be seen at port 4, and (b) at 78 mA injection current, a bright spot is seen at port 3.

0.9 V, and at this time, the injection current is 78 mA. Further measurement finds that at the bias voltage of 1.1 V, the injection current is 88 mA, the modulation depth reaches 100%, and the switch reaches a maximum optical switching state. The optical response of the switch was measured by an oscilloscope. A current pulse signal was applied to the bow-tie p-n+ junction of the switch. The output light beam of the switch was recorded by a p–i–n Ge detector and observed by an oscilloscope synchronously with the applied pulse signal. Figure 2.24 shows the optical response of the switch. The response time obtained from 90% to 10% of Injection current (mA) 53

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2.23  Normalized optical power and modulation depth versus forward bias for optical waveguide switch with a bow-tie electrode at the wavelength of 1.55 µm.

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Optical switches T = 3.33 μS

2.24  Optical response of the SiGe optical waveguide switch: the upper curve is the input modulation signal, and the lower curve is the output response signal.

the maximum response at the falling edge is estimated to be about 180 ns. It should be noted that during the experiment, the thermo-optical effects are negligible because the device is mounted on a metal heat sink. It should be emphasized that at a required switching characteristic, the present structure enables a much wider intersection angle to be achieved. So the size of the switch can be reduced. The performances of the switch with a bow-tie electrode are better than those with a straight electrode and very suitable for Si-based hybrid and monolithic optoelectronic integration. The 2 × 2 optical switch can also be fulfilled utilizing MMI principle and freecarrier plasma dispersion effect. Figure 2.25 shows an MMI switch using an ultra-compact directional coupler in a strongly confining ridge structure18. The device was implemented in the InP/InGaAsP quantum well pin layer structure. The width of the single-mode waveguide is 2 µm. The etching depth of the ridge waveguide is 3 µm. The device yields almost 10 dB of contrast between outputs at 3 V bias, with 4.3 dB of excess coupler loss. The total cleaved device

S

W

θ

WSM

L

2.25  Schematic of compact multi-mode coupler switch.

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Electro-optical switches

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length is only 1.5 mm, and high bandwidth (15 GHz) operability is anticipated. Later, a zero-gap directional coupler switch integrated into a silicon-on insulator for 1.3-µm operation has been fabricated (Figure 2.26)19. Four single-mode ridge waveguides with width W are composed of an n-type silicon guided-wave layer on a SiO2 layer, and they intersect at an angle of a. The width of the intersection region of the switch is twice that of the waveguides. The dimensions of the zero-gap directional coupler switch are Lc = 1103 µm, W = 6 µm. The SOI is fabricated by potassium hydroxide anisotropic etching. Its insertion loss and cross talk are less than 4.81 dB and 218.6 dB, respectively, at a wavelength of 1.3 µm and a switching voltage of 0.91 V. Response time is about 210 ns. The above-mentioned optical waveguide switches have a single carrier injection region. Figure 2.27 shows a two-mode interference (TMI) photonic waveguide switch with double carrier injection that has been designed and fabricated for application in fiber-optic communications20,21. It consists of an input Y-branch with single-mode ridge waveguides, a TMI waveguide coupling section, and an output Y-branch with single-mode ridge waveguides. In Figure 2.27a, two singlemode optical waveguides, 1 and 2, cross at an angle θ forming a Y-branch and function as the inputs of the switch, while single-mode optical waveguides, 3 and 4, cross at an angle θ forming another Y-branch and function as the outputs of the switch. Waveguide 5 is a TMI section and serves as a refractive index modulation region. Here 6 and 7 serve as the carrier injection region while 8 serves as the carrier collection region. Figure 2.27b shows the sectional structure view of the optical waveguide, which is taken along line II-II of Figure 2.27a. In the structure, label 9 is the p-Si(100) substrate and 10 is the p-SiGe core waveguide layer. Label 11 is the SiO2 insulator film. 12 and 13 are the two n+ abrupt carrier injection regions which are, respectively, located on the top of the TMI region and on the left side of the TMI region. 14 is the abrupt carrier collection junction which is located on the right side of the TMI region. Lc

X

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– p+

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2W

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W

n+

2.26  SOI zero-gap directional coupler switch.

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2.27  Schematic diagram of (a) two-mode interference photonic waveguide switch and (b) cross section of intersection region.

The width of the intersection region of the switch is two times that of the singlemode waveguide and supports two modes. The two abrupt p-n+ junctions are made to inject carriers into the region when they are forward-biased. The injection current of both the p-n+ junctions, 12 and 13, increases simultaneously as the switch is forward-biased. This is the so-called double carrier injection, and thus, the TMI region may be achieved by a higher refractive index change than that of a single-mode waveguide and single-injection p-n+ junction optical switches. If an input light A is coupled into the single-mode ridge waveguide 1, as indicated by arrow P1 in Figure 2.28, at the input port of the TMI region, it is excited as a fundamental mode B and a first-order mode C. The modes B and C propagate with different propagation phase constants β00 and β10, respectively.

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Electro-optical switches

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These two modes interfere along the propagation direction. After the propagation length L, the light power coupled into each output arm is given by P3/P1 = sin2(∆φ/2),

[2.10]

P4/P1 = cos2(∆φ/2),

[2.11]

and

where P3 and P4 are output optical powers of the waveguides 3 and 4, respectively. ∆φ is the phase difference of mode B and mode C and satisfies ∆φ = ∆bL,

[2.12]

where ∆β = (β00 – β10). If no forward-bias is applied at both the p-n+ junctions, (β00 – β10)L = π, and thus the input light will be output from the waveguide 3, and there is no output light in the waveguide 4. The switch is in OFF-state. If both the carrier injection junctions are forward-biased simultaneously, as shown in Figure 2.27b, a large number of carriers (I1 + I2) will be injected into the optically modulated region, i.e. optical filed profile region 15 from carrier injection regions 12 and 13. In this case, the refractive index in the region 15 will decrease. This will cause the propagation constants β00 and β10 to be changed. If β00, β10 and the changes of ∆β00, ∆β10 satisfy (b00 – b10 + ∆b00 + ∆b10)L = 0,

[2.13]

the light power will be output from the waveguide 4, as (B2+C2) as indicated by arrow P4 and as shown in Figure 2.28, and the waveguide 3 will be cut off (B1+C1 = 0). So the switching is achieved and the switch is in ON-state. In Eq. (2.13), β00 and –

P1

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C1 3

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C 4

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00

β + 10

∇

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2.28  Schematic plan view showing the two-mode interference photonic switch with double carrier injections output from a waveguide at forward bias.

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Optical switches

β10 are the respective propagation constants of modes B and C under zero forwardbias; and ∆β00 and ∆β10 are the respective changes of β00 and β10 under forward-bias. The thickness and width of the ridge waveguide were chosen to be 2.6 µm and 8 µm, respectively, and the ridge height of the waveguide is 1.0 µm. From Eqs (2.10)–(2.13), the length of the two-mode section is decided to be L = 1438 µm and the branching angle of the Y-junction is θ = 2–3°. The behavior of the mode propagation has been simulated using the beam propagation method (BPM). Plates IIa, b show the BPM field intensity distribution in SiGe two-mode interference photonic switch based on general resonance at zero injection current. The sample was made in the following stages. First, SiGe materials were grown. A 50-nm p-type Si buffer and a lightly doped p-type SiGe (Ge content at around 4%) layer with the thickness of 2.6 µm were grown by molecular beam epitaxy on a p-Si(100) substrate. The boron-doped concentrations in the buffer layer and waveguide layer are about 5 × 1016 cm23. The substrate temperature during the growth was kept at 600°C. The second stage is the fabrication of the device. The device has been fabricated in a 3-µm manufacture process lines. The ridge waveguides were formed by reactive ion etching (RIE) technique with etching rate 250–300 Å min21. A 550-nm thickness SiO2 film was deposited at 400°C by plasma-enhanced chemical vapor deposition (PECVD) on the top of the sample surface to serve as the ion implantation mask and the surface passivation layer. The n+ carrier injection regions of the switch were realized using phosphorus ion implantation with an energy of 60 keV and a dose of 5 × 1015 cm22. The p+ collector was formed by boron ion implantation with an energy of 80 keV and the same dose. The n+ and p+ ohmic contacts were formed by sputtering deposition of aluminum films with thickness of 2.0 µm and followed by alloying at 440°C for 30 min in N2 ambient. The last stage is end polishing. The wafer was diced, the input and output facets of the waveguides were polished by a mechanical method in order to couple the incident light from a single-mode optical fiber. The switch is characterized by using a 1310-nm InGaAsP/InP heterostructure laser diode. When both the p-n+ junctions are zero-biased, only the waveguide 3 has optical profile in the video monitor as shown in Figure 2.29a. The output optical power of the waveguide 3 is roughly the same as the input optical power of the waveguide 1. As the forward biases of the p-n+ junction increase, the injection current of both the p-n+ junctions increases simultaneously, and the output optical power of the waveguide 3 decreases and the output power of the waveguide 4 increases. The device reaches the maximum optical switching state and the output optical power is only in the output waveguide 4 when the total switching current is about 110 mA. The near-field output intensity pattern at the end port 4 of the switch is shown in Figure 2.29b. In this case the current I1 of the p-n+ junction which is placed on the top of the TMI region is 80 mA and the current I2 of another p-n+ junction is 30 mA. The current density J1 of the p-n+ junction placed on the top of the TMI region is 347.8 A/cm2 because the area of the junction is 2w × L, and the current density J2 of another p-n+ junction with an area of w × L is 260 A/cm2. To

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Electro-optical switches

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b

a

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Port 3

Port 4

2.29  Near-field output intensity patterns observed by a vidicon at the end facets of the optical waveguide switch at 1.3 µm. (a) at zero injection current, a bright spot can be seen at port 3, and (b) at 110 mA injection current, a bright spot is seen at port 4.

our knowledge, this is extremely low current density reported for switches fabricated in SiGe/Si structure material. Further measurement, if I1 is zero and I2 is 110 mA, J1 is zero and J2 is 956 A/cm2, respectively. If I1 is 110 mA and I2 is zero, J1 is 478 A/cm2 and J2 is zero, respectively. In these two cases, the device does not reach the maximum optical switching state. Further measuring the losses of device at 1.3 µm, the insertion loss is less than 2.74 dB, other losses are as follows: the absorption loss of the imaging lens is 1.58 dB, an estimated mode mismatch loss is 0.8 dB, and the Fresnel-losses at both waveguide ends are 3.2 dB. So, the overall loss is 8.32 dB. The insertion loss of 2.74 dB is due primarily to the absorption produced by the n+ contact on the 2w waveguide. It includes the optical wave transmission loss and the absorption loss of the injection electrons. The crosstalk of the switch, which is defined as 10·log(P3/P4), is less than 215.5 dB at the total switching current of 110 mA. The optical response of the device was measured by an oscilloscope. A pulse signal was applied to the p-n+ junction of the device. The output light beam of the device was recorded by a germanium detector with a bandwith of >1 GHz and observed by an oscilloscope synchronously with the applied pulse signal. At a low frequency of modulation square-wave pulse signal, the response pulse was also a square-wave signal. Figure 2.30 shows the optical response of the device at input modulation signal of 300 kHz. The response time obtained from 90% to 10% of the maximum response at the falling edge was estimated to be about 180 ns. When the frequency of the signal applied to the p-n+ junction was increased, the trace of the output response signal became worse compared to that of lower frequency. Figure 2.31 shows the optical response of the switch at input modulation signal of 10 MHz. The response time of switch was estimated to be about 30 ns. The switching time could be improved by electron irradiation. It should be noted that during the experiment, the thermo-optical effects are negligible because the device is mounted on a metal heat sink.

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Figure 2.32 shows a 2 × 2 optical switch which consists of two paired-input MMIs serving as 3-dB couplers (one splitter and one combiner), two modulation arms, and four access waveguides. To conveniently couple with optical fiber arrays, at input/output ports, the spaces between the two neighboring access waveguides are enlarged to 127 µm by using S-shaped waveguide bends22. The width of singlemode waveguide is 7.5 µm. The access waveguides are widened to 12 µm wide. The bending radius of the S-shaped waveguides is 20 mm. The two MMIs are both 40 µm wide and 2655 µm long. The total length of the 2 × 2 switch is about © Woodhead Publishing Limited, 2010



Electro-optical switches

P0

35 Pbar

MMI combiner

MMI splitter Rib waveguide

Modulation arms

Px

2.32  Schematic view of the 2 × 2 optical switch.

20 mm. The whole device was fabricated using a CMOS technology platform. The insertion loss of the switch is 3.44 dB and the switching speed is 300 ns. The crosstalk and extinction ratio are 215.5 and 14.9 dB, respectively.

2.4.4 2 × 3 switch Optical switch with input/output ports beyond 2 × 2 would be desired for multichannel applications. To get multi-ports optical switch, a 2 × 3 index-modulation switch is designed. Figure 2.33 shows a schematic structure of the proposed 2 × 3 switch23. Figure 2.33a is the top view whereas Figure 2.33b is the cross-section 3Lp/2

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2.33  2 × 3 index-modulation switch: (a) top view and (b) cross-section view of the index-modulation regions.

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view of the index-modulation region. Figure 2.33a illustrates that the switch consists of three sections: an input section, a central section, and an output section. The input section consists of two input waveguides A and B, while the output section consists of three output waveguides 1, 2, and 3. All the input and output waveguides are single-mode waveguides. The central section consists of an MMI waveguide with two electronically controlled index-modulation regions I and II. Each index-modulation region is designed as a p-n junction, as shown in Figure 2.33b. When a forward-bias voltage is applied to any one of the two p-n junctions, a decrease in a refractive index in the index-modulation regions I and II will be caused due to the plasma dispersion effect. This decrease in refractive index leads to a change in propagation constant of optical waveguide and hence, the input optical signals will be switched to any one of the three output ports. The thickness of the ridge waveguides is 2.6 µm p-SiGe with 4% Ge content. The ridge height is chosen to be 1.0 µm for all waveguides. For ease of butt coupling of optical signals between the ridge waveguides and single-mode fibers, all the input and output single-mode waveguides are set to be 6 µm in width. The MMI section shown in Figure 2.33a is designed to support a large number of modes according to the self-imaging theory. As the width of the input and output waveguides is set to 6 µm in this design, the width of the MMI is chosen to be 30 µm with 6 µm spacing between output port waveguides. Switching characteristics are demonstrated theoretically by a BPM for 1.55 µm operation. When a 1.55 µm light is coupled into the input waveguide A without applying a forward-bias voltage to both the p-n junctions of I and II, the refractive index of the index-modulation regions I and II remains unchanged. In this case, the input optical signal will be output mainly from the output port 3, as shown in Plate IIIa. The calculated insertion loss is 0.17 dB while the calculated crosstalks are 232.8 dB and 229.8 dB for the ports 1 and 2, respectively. When a proper forward-bias voltage is applied to the p-n junction of region I, a 0.3% decrease of refractive index will be achieved in the region. As a result, the output light will be mainly switched to the output port 1, as shown in Plate IIIb. The calculated insertion loss is 0.24 dB while the calculated crosstalks are 229 dB and 229.8 dB for the ports 2 and 3, respectively. Similarly, when a proper forward-bias voltage is applied to both the p-n junctions of regions I and II simultaneously, the output light will be switched to the output port 2, as shown in Plate IIIc. The calculated insertion loss is 1.12 dB while the calculated crosstalks are 218 dB and 218.8 dB for the ports 1 and 3, respectively. When the 1.55-µm optical signal is coupled into the input waveguide B without applying a forward-bias voltage to both the p-n junctions of regions I and II, the refractive indices of both the regions remain unchanged. In this case, the input optical signal will be output from the output port 1, as shown in Plate IVa. The calculated insertion loss is 0.17 dB while the calculated crosstalks are 229.8 dB and 232.8 dB for the ports 1 and 3, respectively. Second, when a proper forward-bias voltage is applied to the p-n junction of region I, the output light will be switched to

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Electro-optical switches

37

the output port 3, as shown in Plate IVb. The calculated insertion loss is 0.19 dB while the calculated crosstalks are 230.8 dB and 229 dB for the ports 1 and 3, respectively. Third, when a proper forward-bias voltage is applied to both the p-n junctions of regions I and II, simultaneously, the output light will be switched to the output port 2, as shown in Plate IVc. The calculated insertion loss is 1.08 dB while the calculated crosstalks are 218 dB and 219.8 dB for the ports 1 and 3, respectively. When the two 1.55-µm optical signals with same original phase, amplitude, and polarization are coupled into the input ports A and B, respectively and without applying a forward-bias voltage to both the p-n junctions of regions I and II, the input signal coupled from the input port A will be switched to the output port 3 and the input light coupled from the input port B will be switched to the output port 1. In this case, the switch is at cross-state and functions as an optical crossconnection, as shown in Plate Va. The calculated insertion loss and crosstalk are 0.18 dB and 229.8 dB, respectively. Second, when the two 1.55-µm lights with the same original phase, amplitude, and polarization are coupled into the input ports A and B, respectively and with a properly applied forward-bias voltage to the p-n junction of region I, the input light coupled from the input port A will be switched to the output port 1 and the input light coupled from the input port B will be switched to the output port 3. In this case, the switch is at bar-state and also functions as an optical cross-connection, as shown in Plate Vb. The calculated insertion loss and crosstalk are 0.35 dB and 226 dB, respectively. Similarly, when the two 1.55-µm lights are coupled into the input ports A and B, respectively and with a properly applied forward-bias voltage to both the p-n junctions of regions I and II simultaneously, the input lights coupled from the input ports A and B are combined and will be output from the output port 2. In this case, the switch functions as an optical power combiner, as shown in Plate Vc. The normalized output power in the output ports 1, 2, and 3 is 0.9%, 70%, and 1.0%, respectively. The calculated insertion loss and crosstalk are 1.43 dB and 219 dB, respectively.

2.4.5 3 × 2 switch Single-mode-based 3 × 2 switch The 2 × 3 and 3 × 2 switches can be used as an optical cross-connect and/or an optical combiner for multi-wavelength signals in the optical networks. To scale to a large number of input/output ports, a single-mode-based 3 × 2 optical switch was also designed beyond 2 × 2 switches. Figure 2.34a shows a schematic diagram of the proposed 3 × 2 switch with a multi-wavelength cross-connect structure24. It consists of an input section, a central section, and an output section. The input section consists of one straight waveguide 1 and two S-shaped waveguides 2 and 3 while output section consists of two S-shaped waveguides 4 and 5. The central section is an intersecting part, which is the key part of this photonic switch. Figure 2.34b shows an enlarged

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Optical switches (a) 30 mm

2

4 R = 30 mm 2q = 3°

1 25 mm

5

3

3 mm

(b) 2

Reflected interfaces (mirrors) II’

4

I’

θ 1

2θ θ

II 3

5

I

Carrier injection regions

10 mm

(c)

1.1 mm

2.5 mm

SiO2

p-SiGe, 2 × 1016 cm–3 5 nm, p-Si cladding, 2 × 1016 cm–3 p+-Si sub, 2 × 1018 cm–3

2.34  A schematic diagram of a photonic switch: (a) top view, (b) enlarged central section view, and (c) waveguide cross-section view. © Woodhead Publishing Limited, 2010



Electro-optical switches

39

central section with two electronically controlled carrier injection regions, which is basically an n+p junction. As the figure shows, the two carrier injection regions, I and II, are arranged in such a way that they can be controlled individually to switch optical signals from the three input ports to the two output ports. The two reflecting interfaces, viz. mirror I’ and mirror II’, are actuated by applying forward-bias voltage. Each of the mirrors has two states, i.e., mirror-ON and mirror-OFF. If a forward-bias voltage is applied to the carrier injection region, the carrier concentration will increase. As a result, the refractive index of the material will decrease in the region and a reflecting interface will be formed. This state is called a mirror-ON state. On the other hand, when no forward-bias voltage is applied, the refractive index of the material remains unchanged and there is no reflecting interface formed. Such a state is called a mirror-OFF state. The switches are designed for Si-based SiGe material with raised ridge waveguide structures for the single-mode operation. For design of all ridge waveguides, a thickness of 2.5 µm and width of 10 µm are chosen. The etching depth of all the ridge waveguides is designed to be 1.1±0.1 µm for a Ge content of 4%. The effective index method is used to analyze the behavior of the mode propagation and to determine the branch angle of the switch. In the optimum design, two input intersecting angles are θ = 1.5° and one output branching angle is 2θ =3°. Figure 2.34c shows the cross-section view of the ridge waveguide layer structure. In the ridge waveguide, an impurity-induced index step at the p-SiGe/p-Si interface is used for vertical confinement of input light. Lateral confinement is provided by a SiGe/SiO2 ridge wall and the effective index induced by its geometry. The operating principle of the switch is based on the total internal reflection caused by the plasma dispersion effect of the SiGe material. It is well known that the refractive index of SiGe is related to its carrier concentration. When the n+p junction of the switch is forward-biased, a large number of electrons will be injected to the p region of the SiGe waveguide causing an increase of carrier concentration. As a result, this will lead to a decrease in the refractive index of the SiGe layer and, hence, a reflecting mirror is formed to reflect the light beams to branch 4 or branch 5. Theoretical results for the typical C-band wavelengths of λ1 = 1540 nm, λ2 = 1550 nm, and λ3 = 1560 nm are as follows: 1 If only λ1 is coupled to the input waveguide 1 without applying a bias voltage to both the n+p junctions I and II, λ1 is split into two parts and is divided between waveguides 4 and 5, as shown in Plate VIIa. If a bias voltage is applied to the n+p junction I, mirror I’ is ON and the input light is totally reflected to the waveguide 5, as shown in Plate VIIb. Similarly, if a bias is applied to the n+p junction II, the light is reflected to the waveguide 4 by mirror II’. In this case, the device functions as an optical power splitter and one-wavelength 1 × 2 digital switch.

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2 If input λ2 is are coupled to the waveguide 2 or λ3 to the wavegude 3 and at the same time, applied a forward-bias to the junctions I or II individually, the device will function as a one-wavelength 2 × 2 switch. The two operating examples are as shown in Plates VIIc and VIId. 3 If input λ2 and λ3 are coupled to the input waveguides 2 and 3, respectively, the two wavelengths will be output from the cross-waveguides 5 and 4 at zero bias. The device functions as a passive OXC switch, as shown in Plate VIIe. 4 If input λ1 and λ3 (or λ1 and λ2) are coupled to the input waveguides 1 and 3 (or the waveguides 1 and 2), respectively, the two input wavelengths will be combined and reflected to the output waveguide 4 or 5 by the mirror II’ or I’, respectively, at the forward applied bias. The device functions as an optical add multiplexer and an asymmetrical two-wavelength 2 × 2 switch, as shown in Plates VIIf and VIIg. 5 If the three inputs λ1, λ2, and λ3 are coupled to the device simultaneously and no bias is applied, the device will function as an optical combiner and optical add/drop multiplexer, as shown in Plate VIIh. With bias voltage at the junctions I or II, for example, at junction I, the device will function as a wavelength division multiplexer and/or a three-wavelength switch, as shown in Plate VIIi. It should be noted that if both the n+p junctions I and II are at forward-biased, the two mirrors I’ and II’ are ON simultaneously. In this case, the device will be totally cutoff. The samples of the switch were grown in an UHV-chemical vapor deposition (CVD) system. On a p+-type Si(100) substrate, a 5-nm p-type Si lower cladding layer with a concentration of about 2 × 1016 cm23 was grown followed by a 2.5µm p-type strained Si0.96Ge0.04 core waveguide layer with a concentration of 2 × 1016 cm23. On the top of the sample, an abrupt n+p junction was formed by growing a 5-nm n+-type Si cap layer with a concentration of 1 × 1018 cm23. The whole device was fabricated using silicon-optical bench technology. Waveguides were developed based on lithography and plasma etching of silicon. Two abrupt n+p junctions, I and II, were formed by removing the other n+-Si cap layer of the waveguides. Then, the device surface was passivated by a SiO2 layer. The n+ and p+ ohmic contact electrodes were deposited by evaporating 1.0-µm-thick aluminum layer followed by alloying at 450°C. Figure 2.35 shows the fabricated device array. The chips were diced and the input/output facets of the switch were mechanically polished in order to allow easy coupling of the incident light from a single-mode fiber (Figure 2.36). In measurement, a tunable laser source with a tuning range 1525–1575 nm was used as an input light source. Three wavelengths of λ1 = 1540 nm, λ2 = 1550 nm, and λ3 = 1560 nm were coupled to the input waveguides 1, 2, and 3, respectively. By measuring output optical power from the waveguides 4 and 5 for different wavelengths at different applied bias voltages, insertion loss, crosstalk, and

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2.35  Fabricated optical switch array on a 4-inch Si wafer.

extinction ratio of the device were calculated. The insertion loss of the device is defined as 210 log10(Pout/Pin), where Pin is the input power, Pout is the output power. The measured insertion loss is 1.98 dB, which includes total on-chip propagation and structure loss. The crosstalk is defined as 10 log10(P4/P5) and/or 10 log10(P5/P4), where P4 and P5 are the output powers from the waveguides 4 and 5, respectively. The measured crosstalk is between 220 and 225 dB. Further, the ON–OFF ratio is calculated by measuring the total output power (P4 + P5) at the mirror-ON and OFF, respectively. For the designed device, the optimum bias voltage for the mirror ON state is 1.3 to 1.4 V. At this bias, the ON– OFF ratio is greater than 30 dB. The injection current with the mirror ON is measured as 120 mA. The switching speed is measured to be 100–200 ns. The device has advantages of single-mode operation, polarization independence, and wavelength insensitivity because of utilizing carrier-induced refractive index change. It is an ultra-compact, low-cost, and highly reliable device as it uses a wellintegrated structure of SiGe/Si and is based on very mature Si fabrication technology. It is very suitable for monolithic integration with other Si-based optoelectronic devices. The switch can serve network functions other than a traditional 2 × 2 switch. It can be used in fiber optic communications systems, photonic-integrated circuits and wavelength division multiplexed networks as an optical power splitter, optical cross-connect, optical add-drop multiplexer, and wavelength division multiplexer simultaneously or individually. The function of the device is to combine multi-wavelengths from different input channels and to switch them to different output channels.

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Optical switches (a)

(b)

2.36  (a) Three input waveguides microscope image of the fabricated 3 × 2 optical switch. The inset (top left) is the whole image of the central part of the switch. (b) Waveguide facets of the input and output waveguides of the switch.

Multi-mode-based 3 × 2 switch A multi-mode-based 3 × 2 optical switch was also designed beyond 2 × 2 switches. Figure 2.37 shows a schematic diagram of the 3 × 2 switch25, which consists of three input waveguides and two output waveguides. All input and output

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Electro-optical switches

43

(a)

Bird’s eye view

V

500 μm

(b)

784 μm

2

A 1209 μm

507 μm 500 μm 4

25 μm 32.2 μm

1 25 μm 3

r = 3 cm Spacing = 6 μm

r = 3 cm

30 μm 5

B

2.37  Schematic diagram of an intelligent integration of optical power splitter with optically switchable cross-connect which consists of three input single-ridge waveguides, one MMI central section, and two output single-ridge waveguides; (a) bird's eye view, (b) top view.

waveguides are single-mode ridge waveguides and are designed for operation in the 1545–1555 nm wavelength range. The whole device has a 3 × 2 configuration with a MMI coupler in its central section. It can be simply regarded as a stack of a conventional 1 × 2 MMI optical power splitter and a conventional 2 × 2 MMI coupler switch. Figure 2.37a shows a bird’s eye view and Figure 2.37b shows a top view. It consists of a p-SiGe core waveguide layer, a bottom p-Si cladding layer, and a top n+-cap layer. The top Si cap layer is a heavily doped layer and it is used as a carrier injection layer to inject carriers to the core waveguide layer under a forward-bias condition. Based on the carrier injection, the device can switch the input optical wavelengths from the three input waveguides to any one of the two output optical waveguides. To make coupling compatible with a 10-µm diameter single-mode fiber and for easy coupling of light beam to the input waveguides, thickness and width of the single-mode ridge waveguides are chosen to be 2.5 and 10 µm, respectively. Based on the single-mode operation condition for ridge waveguides, etching depth of 1.0 µm is chosen for all waveguides corresponding to the 4% Ge content.

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Optical switches

The effective index method was used to analyze the behavior of mode propagation and to determine the power switching capability in the MMI section of the device. In our optimum design, the width of the MMI section is 32.2 µm and the length is 1209 µm. The input channel spacing is 25 µm and the output channel spacing is 30 µm. The total length of the device is 3500 µm. Theoretical results of the device for the three wavelengths of λ1 = 1545 nm, λ2 = 1550 nm, and λ3 = 1555 nm are as follows: 1 If only λ1 is coupled into the input waveguide 1, optical power is divided between the output waveguides 4 and 5. The simulated result is as shown in Plate VIIIa. In this case, the device functions as OPS. 2 If input λ1 = 1545 nm from the input waveguide 1 and λ2 = 1550 nm from the input waveguide 2 is applied simultaneously, the device couples optical power of λ2 to the optical power of λ1 and the combined power of λ1 and λ2 is outputted from the waveguide 5, whereas only λ1 is outputted at waveguide 4. The simulated result is as shown in Plate VIIIb. Similarly, if input λ1 = 1545 nm from the input waveguide 1 and λ3 = 1555 nm from the input waveguide 3 is applied simultaneously, the combined signal of λ1 and λ3 is available at the output waveguide 4, whereas only λ1 is available at output waveguide 5, as shown in Plate VIIIc. In these two cases, the device functions as an optical power combiner. 3 If inputs λ2 and λ3 are coupled simultaneously to waveguides 2 and 3, respectively, the output optical power due to these two inputs is available at the cross waveguides 5 and 4, respectively. The simulated result is as shown in Plate VIIId. In this scenario, the device functions as a passive OXC. When a forward-bias voltage is applied to the MMI section, the two wavelengths λ2 and λ3 are available at waveguides 4 and 5, respectively. In this case, the device functions as a switchable OXC. 4 When λ1, λ2, and λ3 are coupled simultaneously to the input waveguides 1, 2, and 3, respectively, without applying bias voltage to the central section, the output power at waveguide 4 is the combination of optical power due to λ1 and λ3. Output at waveguide 5 is an optical power due to λ1 and λ2 and is as shown in Plate VIIIe. Upon the application of bias voltage, optical power due to λ1 and λ2 is available at waveguide 4 whereas optical power due to λ1 and λ3 is available at waveguide 5. In these two cases, the device functions as a wavelength-selective OXC switch. It should be pointed out that, in this approach, four S-shaped waveguides are used at the inputs and the outputs of the MMI section to prevent significant interaction between the waveguides. The samples were grown in an UHV-CVD system and the waveguides were formed based on lithography and plasma etching of silicon. After etching, the device was passivated by SiO2 and then, the n+ and p+ ohmic contact electrodes were deposited by evaporating aluminum layer on the top of the sample and the bottom of the substrate, respectively, followed by alloying at 450°C. Figure 2.38

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Electro-optical switches

45

Al electrode

MMI section

Al electrode

2.38  Part of MMI section with two output waveguides of the fabricated 3 × 2 optical switch.

shows the microscope image of the fabricated switch. The chips were diced and the input/output facets of a device were mechanically polished. During the measurement, first we aligned the pigtailed fiber of the tunable laser source with the input waveguide 1 and tuned the laser wavelength to 1545 nm, and measured the output powers from the output ports 4 and 5. Second, we aligned the pigtailed fiber with the input waveguide 2, then tuned the laser wavelength to 1550 nm, and measured the output powers from the output ports 4 and 5 for different bias voltages. Lastly, we aligned the pigtailed fiber to the input waveguide 3, then tuned the laser wavelength to 1555 nm and measured the output powers from the output ports 4 and 5 for different bias voltages. Based on this experimental procedure, insertion losses and crosstalks of the device were calculated as follows: 1 When coupling λ1 = 1545 nm into the input waveguide 1 with a power of Pin-1, this power was divided between the two output ports 4 (Pout-4) and 5 (Pout-5). As a result, two bright output spots were observed by an infrared vidicon and the respective powers were measured by an optical power meter at the output ports 4 and 5. The calculated total insertion loss, which is defined as 210 log10[(Pout-4 + Pout-5)/(Pin-1)], is about 5.25 dB. This insertion loss includes 3.0 dB Fresnel-losses at both waveguide facets. In this work, the modal mismatch loss is ignored because the spot size of 2.5 µm from the tapered single-mode fiber matches the SiGe core layer thickness of 2.5 µm.

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Optical switches

2 When coupling λ2 = 1550 nm into the input waveguide 2 with a power of Pin-2, the optical power was mainly available at cross-waveguide 5 when no bias voltage is applied. However, it is outputted from the bar-waveguide 4 with a bias voltage. By measuring the output powers Pout-4 and Pout-5 from the output ports 4 and 5 without a bias voltage, the calculated crosstalk, which is defined as 10 log10(Pout-4/Pout-5), is around 217.7 dB. The calculated total insertion loss, which is defined by 210 log10[(Pout-4 + Pout-5)/(Pin-2)], is about 5.1 dB. With a bias voltage of 4.7 V, the calculated crosstalk and the total insertion loss are 216.5 and 5.4 dB, respectively. In this case, the operation current was 370 mA with the injection current density of 950 A/cm2 because the area of the MMI p-n+ junction is 1209 µm × 32.2 µm. 3 Similarly, when coupling λ3 = 1555 nm into the input waveguide 3 with a power of Pin-3, without applying bias voltage, the calculated crosstalk and total insertion loss are 217.6 and 5.2 dB, respectively. However, with the application of a bias voltage of 4.7 V, the calculated crosstalk and total insertion loss are 216 and 5.3 dB, respectively, for the same values of operating current and injection current density as given in the preceding point (2). The device can serve network functions other than a traditional MMI 1 × 2 power splitter or a traditional MMI 2 × 2 switch.

2.4.6 3 × 3 switch To match the requirements of optical digital information processing in future highcapacity optical networks and to avoid cumbersome optical-electrical-optical conversion, all-optical logic switches in any manner are necessary. To combine the advantages of Si-based optical waveguide device and the MMI principle, an alloptical logic switch has been proposed and investigated theoretically. Figure 2.39 shows a schematic diagram of the proposed all-optical logic switch26. It consists of three sections: input section, central section, and output section. The input section consists of three waveguides A, B, and C, the central section is a MMI coupler and the output section consists of three waveguides 1, 2, and 3. All input and output waveguides are single-mode ridge waveguides for operation at a wavelength of 1550 nm. Figure 2.40 shows the cross-section of the 492μm

4980 μm

A B

1 38.4 μm

C

2 3

6500 μm

2.39  Schematic diagram of the proposed all-optical logic switch.

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Electro-optical switches

47

W d

n1 h

Si1–xGex Si substrate

n2 n3

2.40  Cross-section view of the waveguide.

single-mode ridge waveguide, where w, h, and d represent the width, height, and etching depth of the ridge waveguide, respectively. To easily couple light beam from a single-mode fiber with a diameter of 10 µm to each of the input waveguides, thickness (h) and width (w) of the all single-mode ridge waveguides are chosen to be 2.5 and 10 µm, respectively. Based on the large cross-section single-mode operation principle, an etching depth (d) of 1.0 µm is chosen for all waveguides corresponding to a 4% Ge content. To get multifunctional operation, the refractive index of the waveguides B and C is chosen to be 3.504 which is lower than the refractive index of other waveguides (n = 3.507 for 1550 nm). The effective index method and the beam propagation method (BPM) method were used to analyze the behavior of the mode propagation and to determine the performance of the device. The width of the MMI section is 38.4 µm and the length is 4980 µm. Spacing between the two single-mode waveguides is 4 µm. The total length of the device is 6500 µm with an input length of 492 µm. Plate IX shows the simulated results based on the MMI principle and the BPM method. All incident light beams including optical control pulse with the same wavelength (1.55 µm), phase, and polarization are used in the simulations of the devices. It can be seen from Plate IX that the device can operate the following four logic switches: 1 OR logic switch: When the incident light beams are coupled into the input waveguides A, and/or B, and/or C individually or simultaneously, there are always optical signals in waveguide port 2, as shown in Plate IX. In this case, the device operates as OR logic switch with an average insertion loss of 0.1dB. Table 2.1 shows the output signals in port 2 and the comparison with the simulation patterns in Plate IX. It should be pointed out that 0 and 1 in Table 2.1 indicate without output signal and with output signal, respectively. 2 NOT logic switch: When the incident light beam B is the input optical signal while the incident light beams A and C are the control pulses to control the output signals of output port 1 and port 3, respectively, the device operates as NOT logic switch, as shown in Plates IXb, d, f, and g. Tables 2.2a and 2.2b show the output signal in port 1 and 3, respectively, and the comparison with © Woodhead Publishing Limited, 2010

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Optical switches

Table 2.1  OR logic switch Input signal A 0 B 0 C 0 Comparison in Plate IX Output signal in port 2 0

1 0 0 (a) 1

0 1 0 (b) 1

0 0 1 (c) 1

1 1 0 (d) 1

1 0 1 (e) 1

0 1 1 (f) 1

1 1 1 (g) 1

Table 2.2a  NOT logic switch Control pulse A Input signal B Comparison in Plate IX Output signal in port 1

0 1 (b),(f) 1

1 1 (d),(g) 0

0 1 (b),(d) 1

1 1 (f),(g) 0

Table 2.2b  NOT logic switch Control pulse C Input signal B Comparison in Plate IX Output signal in port 3

the simulation patterns in Plates IXb, d, f, and g. Similarly, when the incident light beam C is the input optical signal while the incident light beams A and B are the control pulses to control the output signals of output port 1 and port 3, respectively, the NOT logic operations of the device are as shown in Plates IXc, e, f, and g. Tables 2.2c and 2.2d show the output signal in port 1 and 3, respectively, and the comparison with the simulation patterns in Plates IXc, e, f, and g. 3 NAND logic switch: When the incident light beam A is the input optical signal and the incident light beams B and C are the control pulses, respectively, the device operates as NAND logic switch as shown in Plates IXa, d, e, and g. Table 2.3 shows the output signals in port 3 and the comparison with the simulation patterns in Plates IXa, d, e, and g. 4 NOR logic switch: When the incident light beam A is the input optical signal and the incident light beams B and C are the control pulses, respectively, the device operates as NOR switch, as shown in Plates IXa, d, e, and g. Table 2.4 shows the output signals in port 1 and the comparison with the simulation patterns in Plate IXa, d, e, and g.

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Electro-optical switches

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Table 2.2c  NOT logic switch Control pulse A Input signal C Comparison in Plate IX Output signal in port 1

0 1 (c),(f) 1

1 1 (e),(g) 0

0 1 (c),(e) 1

1 1 (f),(g) 0

Table 2.2d  NOT logic switch Control pulse B Input signal C Comparison in Plate IX Output signal in port 3

Table 2.3  NAND logic switch Input signal Control pulse

A B

Control pulse C Comparison in Plate IX Output signal in port 3

1 0

1 1

1 0

1 1

0 (a) 1

0 (d) 1

1 (e) 1

1 (g) 0

1 0

1 1

1 0

1 1

0 (a) 1

0 (d) 0

1 (e) 0

1 (g) 0

Table 2.4  NOR logic switch Input signal Control pulse

A B

Control pulse C Comparison in Plate IX Output signal in port 1

By switching the optical signal to different input waveguide ports, the designed 3 × 3 optical switch can function as OR, NOT, NAND, and NOR logic states simultaneously or individually. It is a kind of switch for next generation logic optical circuits, ultrahigh speed signal processing, and future Si-based all-optical integrated circuits. The 3 × 3 optical switch can also be realized using hyperbolic MMI structure. Figure 2.41 shows the schematic structure of 3 × 3 hyperbolic multi-mode switch in SOI 27. The switch consists of three parts: three single-mode input waveguides,

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Optical switches Port 4

Port 1

Port 2

n3

n1

n4

n2

θ

Port 3

Port 5

Port 6

2.41  3 × 3 SOI multimode interference optical switch.

a hyperbolic MMI with four refractive index modulated regions, and three single-mode output waveguides. The index-modulation regions denoted by n1, n2, n3, and n4 are placed at 1/8 LMMI and 1/2 LMMI of the MMI waveguide, respectively, where LMMI is the length of the MMI region. The emitting port of the output light can be manipulated by tuning the refractive index of n1, n2, n3, and n4. By changing the refractive index of one or two parts within the hyperbolic MMI, the switch can realize on and off functions between any pair of input and output ports.

2.4.7 1 × 4 switch Figure 2.42 shows a versatile 1 × 4 optical switch based on InGaAsP MQWs28. It can be divided into two primary sections; the beam steering section and the output waveguides. The beam steering section is composed of the input waveguide and a steering region. A laser beam entering the 4-µm wide input waveguide is launched into the exact center of the steering region, which consists of a slab waveguide with two parallel Ti/Au/Zn/Au top layer contact stripes, separated by 22 µm, as measured from the inner edge of each stripe. The contact stripes are both 800 µm long and 10 µm wide. The output waveguides comprise the second section of the device. Each one is 4 µm wide and has a length of 500 µm. Initially, each output waveguide is separated by 2 µm, but as they spread out from the beam steering region, this value increases to 5 µm at the output facet. The wafer structure is grown on an n+-InP substrate. It is composed of a 1-µm thick n-type InP buffer layer which is grown on 14 pairs of 10-nm thick, undoped, InGaAsP (Eg = 0.816 eV) quantum wells interspaced with 10-nm InGaAsP (Eg = 1.08 eV) barriers. The MQWs are topped by a 1.6-µm, n-type, InP cladding layer and a 100-nm InGaAs capping layer. The resulting slab waveguide is essentially symmetric, with all layers grown by metal organic chemical vapor deposition (MOCVD). The device operates using the carrier-induced refractive index change in semiconductors. When no current passes through the contact stripes, a 1.51-µm

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Electro-optical switches

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Output waveguides 4-μm wide separated by 5-μm

Ti/Au/Zn/Au contacts

22 μm separation

Ni/Ge/Au contact

2.42  Schematic of the 1 × 4 InP-based optical switch.

wavelength laser beam, launched into the steering region by the input waveguide, will expand into a slab mode. Lateral control and confinement of this beam is achieved by the application of an electrical current to each stripe. Zinc is previously diffused underneath the contact stripes in order to control the carrier spreading within the active region of the device. These diffused regions act to channel the electrons into the MQW layer and consequently enhance their efficiency in providing optical confinement and waveguiding. As current is applied, electrons are injected into the MQW layer where they then spread laterally through carrier diffusion. The areas within the active region that become saturated with electrons experience a corresponding decrease in refractive index. Carrier concentration is highest in the areas directly underneath the stripes, and decreases with lateral distance. This effectively results in the formation of a graded index channel waveguide between the two stripes in the steering region. Careful adjustment of the ratio between the currents applied to the two parallel contact stripes allows the waveguide to be shifted across the entire available range, thereby steering the signal beam. The input optical beam can then be directed to any of the output waveguides. Device fabrication began with the deposition of a silicon nitride diffusion mask for creating the zinc-diffused regions. Plasma-enhanced chemical vapor deposition (PECVD) was used to deposit a 200-nm-thick silicon nitride film on the substrate surface. For each device, two 10 µm × 800 µm diffusion windows were defined in the nitride film using conventional photolithography and CF 4 plasma-based reactive ion etching. The zinc in-diffusion process was carried out using a semisealed open-tube diffusion technique. After the diffusion process was complete, the

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52

silicon nitride mask was removed and Ti/Au/Zn/Au p-type contacts were patterned on top of the zinc-diffused areas by evaporation and lift-off. Photolithography was then used to pattern the input waveguide and output waveguide structures. Wet chemical etching with an H3PO 4:H2O2:DI water (1:1:38) mixture was employed to selectively remove the InGaAs top layer. The remaining portions of the InGaAs layer were then used as a mask for the selective wet etching of the InP using an HCl:H3PO 4:CH 3CHOHCOOH (2:5:1) mixture. A 10-nm InGaAsP etch stop layer, located 190 nm above the MQW layer, allowed precision control of the etch depth and yielded InP ridges of constant height, in addition to a smoothly etched surface. After etching was finished, the substrate was lapped to a thickness of 150 µm and polished to a mirror finish. Finally, the n-type contact consisting of Ni/Ge/Au was deposited via thermal evaporation and annealed in. The device sample was then cleaved and mounted on a copper header for testing. Figure 2.43 shows the experimental output near-field spots28. The fabricated 1 × 4 switch exhibits a 214 dB crosstalk between channels over a wavelength range of 30 nm, while maintaining low electrical power consumption and allowing the switch to be operated uncooled and under dc current conditions.

2.4.8 2 × 4 switch Based on MMI principle and free-carrier plasma dispersion effect, a 2 × 4 optical switch can be designed. The 2 × 4 optical switch is also called 2 × 4 decoder switch. It consists of two input single-mode ridge waveguides, an MMI section, and four output single-mode ridge waveguides. In the MMI section, two index-modulation regions are introduced. The device can divert input optical signals to any one of the

8.5

38

18.2

35.2

23.3

15.8

30.2

8.4

2.43  Experimental result of the near-field output facet at different applied currents.

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Right stripe current (mA)

Left stripe current (mA)

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four output waveguides when a forward-bias voltage is applied to the two indexmodulation regions. As a decoder switch, both input and output signals should be optical ones. Its control signals can be some kind of modulations applied to the index modulation regions, such as forward-bias voltages applied to p-n junctions of the index-modulation regions. Compared with other kinds of multi-port switches, the decoder switch generally possesses fewer control signals and modulation regions. This is because the output signals can be controlled by control signals and their logic states. n control signals are sufficient to control 2n output ports. Figure 2.44 shows a schematic structure of the proposed 2 × 4 decoder switch29. Figure 2.44a is the top view of the decoder switch while Figures 2.44b and c are cross-section views of the index-modulation regions I and II, respectively. As shown in Figure 2.44a, the switch consists of three sections: an input section, a central section, and an output section. The input section consists of two input waveguides A and B, while the output section consists of four output waveguides 1, 2, 3, and 4. All the input and output waveguides are single-mode waveguides. The central section consists of a multi-mode waveguide with two electronically controlled index-modulation regions (I, II) and a rectangular air groove. Indexmodulation regions I and II are designed as p-n junctions as shown in Figures 2.44b and c, respectively. In this design, the optical signals are input from the two input waveguides A and B, and are outputted from the four output waveguides 1, 2, 3, and 4. The control signals VCI and VCII are the forward-bias voltages applied to the p-n junctions of the two index-modulation regions I and II, respectively. When a control signal, i.e. a forward-bias voltage is applied to any one of the two p-n junctions, a decrease of refractive index in the index-modulation regions I and II will be caused due to the plasma dispersion effect by injected minority carriers. This decrease in the refractive index will lead to a change in propagation of the input optical signals. This will divert the input optical signals to any one of the four output waveguides and the device functions as a 2 × 4 decoder switch. In design, physical parameters of the decoder switch are determined by the following considerations. In order to be compatible with single-mode fiber operation in 1.55 µm, all the input waveguides and output waveguides are set to be 6 µm in width and 2.5 µm in thickness. Based on a large cross-section singlemode operation principle, an etching depth of 1.0 µm is chosen for all waveguides corresponding to a Ge component of 4%. Considering proper spacing between the single-mode waveguides, the width of the multi-mode waveguide is set to be 36 µm. To achieve decoder switch function, the length of the multi-mode waveguide is set to a little larger than 3L π, and a rectangular air groove is introduced at the end side of the multi-mode waveguide, as shown in Figure 2.44a. The purpose will be discussed in the following paragraph. In the simulation, the decrease of the refractive index is chosen to be ∆n = 0.3%. The calculated forward-bias voltages VA and the current density J applied to the p-n junction in the regions I and II are 0.95 V and 27.7 kA/cm2, respectively. The

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Optical switches (a)

1.7 cm 1 A

10 mm

II

2 24 mm

I

6 mm

36 mm

3 10 mm

B

4 1000 mm

14150 mm

500 mm

1850 mm

(b)

SiO2

VCI

+

n , 1 × 10

18

–3

cm

p-SiGe, 2 × 1016 cm–3 5 nm, p-Si cladding, 2 × 1016 cm–3 p+-Si sub, 1 × 1018 cm–3

(c)

VCII

n+, 1 × 1018 cm–3

SiO2

p-SiGe, 2 × 1016 cm–3 p+, 1 × 1018 cm–3 5 nm, p-Si cladding, 2 × 1016 cm–3 p+-Si sub, 1 × 1018 cm–3

2.44  Schematic structure of the proposed 2 × 4 switch: (a) top view, (b) cross-section view of the index-modulation region I and (c) crosssection view of the index-modulation region II.

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optimal widths of the two regions (I, II) and air groove are 24, 18, and 6 µm, respectively, while the optimal lengths of them are 700, 258, and 400 µm, respectively. The optimal length of the multi-mode waveguide is 14,150 µm. Thus, two control signals VCI, VCII are sufficient to control the four different output ports. Fundamental characteristics of the 2 × 4 switch are demonstrated theoretically by the beam propagation method. Because the switch will work if and only if the inputs are phase coherent, so all the input light beams are assumed to be of same wavelength, i.e. of 1.55 µm and with original phase, amplitude. For simplification, we use ‘0’ and ‘1’ to indicate without forward-bias voltages and with forward-bias voltages VA, respectively. So the four different binary logic states of the two control signals (VCII , VCI ) are (0, 0), (0, 1), (1, 0), and (1, 1), respectively. Simulated results show that the insertion loss of the decoder switch is less than 0.36 dB and the crosstalk is less than 219.7 dB. The device can divert input optical signals to any one of the four output waveguides when a forward-bias voltage is applied to the two index-modulation regions. Optical signal input from input A When an optical signal is input from the input waveguide A, it will be output from the four different output waveguides according to binary logic states of the two control signals (VCII , VCI ), as shown in Plate VI. 1 When (VCII, VCI) = (0, 0), the input optical signal will be output from the output waveguide 3 (Plate VIa). The normalized output power in the output waveguide 3 is P3 = 95.2% while the normalized output powers in the output waveguides 1, 2, and 4 are P1 = 0.03%, P2 = 0.03%, and P4 = 1%, respectively. The calculated insertion loss is 210 log (Ptotal-out/Pin) = 0.17 dB while the calculated crosstalks are 10 log (P1/P3) = 235.0 dB, 10 log (P2/P3) = 235.0 dB, and 10 log (P4/P3) = 219.8 dB for the output waveguides 1, 2, and 4, respectively. 2 When (VCII , VCI ) = (0, 1), the optical signal will be output from output waveguide 4 (see Plate VIb). The normalized output power in the output waveguide 4 is P4 = 93.2% while the normalized output powers in the output waveguides 1, 2, and 3 are P1 = 0.06%, P2 = 0.03%, and P3 = 0.5%, respectively. The calculated insertion loss is 0.28 dB while the calculated crosstalks are 231.9 dB, 234.9 dB, and 222.7 dB for the waveguides 1, 2, and 3, respectively. 3 When (VCII, VCI) = (1, 0), the optical signal will be output from output waveguide 2 (see Plate VIc). The normalized output power in the output waveguide 2 is 95.0% while the normalized output powers in the output waveguides 1, 3, and 4 are 1%, 0.03%, and 0.06%, respectively. The calculated insertion loss is 0.17 dB while the calculated crosstalks are 219.8 dB, 235.0 dB, and 232.0 dB for the waveguides 1, 3, and 4, respectively. 4 When (VCII, VCI) = (1, 1), the optical signal will be output from output waveguide 1 (see Plate VId). The normalized output power in the output © Woodhead Publishing Limited, 2010

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Optical switches

Table 2.5  Output states of the decoder switch when optical signal input from input A Input A

Control signals (VCII , VCI )

Outputs Port 1

Port 2

Port 3

Port 4

1 1 1 1

(0, 0) (0, 1) (1, 0) (1, 1)

0 0 0 1

0 0 1 0

1 0 0 0

0 1 0 0

waveguide 1 is 92.3% while the normalized output powers in the output waveguides 2, 3, and 4 are 0.6%, 0.03%, and 0.03%, respectively. The calculated insertion loss is 0.32 dB while the calculated crosstalks are 221.9 dB, 234.9 dB, and 234.9 dB for the waveguides 2, 3, and 4, respectively. Table 2.5 shows the output states of the 2 × 4 decoder switch when optical signal is input from input waveguide A. It should be pointed out that in the ‘Input’ column of this table, 1 indicates the optical signal coupled from the input waveguide A, while 0 and 1 in the ‘Outputs’ columns indicate without output signal and with output signal in the output waveguides, respectively. Optical signal input from input B Similarly, when an optical signal is input from the input waveguide B, it will be outputted from the four different output waveguides according to the binary logic states of the two control signals (VCII , VCI ), as shown in Plate X. 1 When (VCII , VCI ) = (0, 0), the input optical signal will be output from the output waveguide 2 (see Plate Xa). The normalized output power in the output waveguide 2 is P2 = 95.0% while the normalized output powers in the output waveguides 1, 3, and 4 are P1 = 1%, P3 = 0.03%, and P4 = 0.03%, respectively. The calculated insertion loss is 210 log (Ptotal-out/Pin) = 0.17 dB while the calculated crosstalks are 10 log (P1/P2) = 219.8 dB, 10 log (P3/P2) = 235.0 dB and 10 log (P4/P2) = 235.0 dB for the waveguides 1, 3, and 4, respectively. 2 When (VCII, VCI) = (0, 1), the optical signal will be output from output waveguide 1 (see Plate Xb). The normalized output power in the output waveguide 1 is P1 = 92.3% while the normalized output powers in the output waveguides 2, 3, and 4 are P2 = 0.6%, P3 = 0.03%, and P4 = 0.06%, respectively. The calculated insertion loss is 0.32 dB while the calculated crosstalks are 221.9 dB, 234.9 dB, and 231.9 dB for the waveguides 2, 3, and 4, respectively. 3 When (VCII , VCI ) = (1, 0), the optical signal will be output from output waveguide 3 (see Plate Xc). The normalized output power in the output waveguide 3 is 95.2% while the normalized output powers in the output

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waveguides 1, 2, and 4 are 0.03%, 0.02%, and 1%, respectively. The calculated insertion loss is 0.17 dB while the calculated crosstalks are 235.0 dB, 236.8 dB, and 219.8 dB for the waveguides 1, 2, and 4, respectively. 4 When (VCII, VCI) = (1, 1), the optical signal will be output from output waveguide 4 (see Plate Xd). The normalized output power in the output waveguide 4 is 93.2% while the normalized output powers in the output waveguides 1, 2, and 3 are 0.03%, 0.03%, and 0.6%, respectively. The calculated insertion loss is 0.28 dB while the calculated crosstalks are 234.9 dB, 234.9 dB, and 221.9 dB for the waveguides 1, 2, and 3, respectively. Table 2.6 shows the output states of the 2 × 4 switch when optical signal is input from input waveguide B. Optical signals input from inputs A and B simultaneously The 2 × 4 decoder switch can also operate when optical signals are input from input A and input B simultaneously, as shown in Plate XI. 1 When (VCII , VCI ) = (0, 0), optical signal A will be output from output waveguide 3 while optical signal B will be outputted from output waveguide 2 (see Plate XIa). The normalized output powers in the output waveguides 1, 2, 3, and 4 are 0.5%, 46.8%, 46.8%, and 0.5%, respectively. The calculated insertion loss and average crosstalk are 0.24 dB and 219.7 dB, respectively. 2 When (VCII , VCI ) = (0, 1), optical signal A will be output from output waveguide 4 while optical signal B will be outputted from output waveguide 1 (see Plate XIb). The normalized output powers in the output waveguides 1, 2, 3, and 4 are 45.9%, 0.3%, 0.3%, and 45.9%, respectively. The calculated insertion loss and average crosstalk are 0.34 dB and 221.8 dB, respectively. 3 When (VCII , VCI ) = (1, 0), optical signal A will be output from output waveguide 2 while optical signal B will be output from output waveguide 3 (see Plate XIc). The normalized output powers in the output waveguides 1, 2, 3, and 4 are 0.5%, 47.3%, 45.2%, and 0.3%, respectively. The Table 2.6  Output states of the decoder switch when optical signal input from input B Input B

Control signals (VCII , VCI )

Outputs Port 1

Port 2

Port 3

Port 4

1 1 1 1

(0, 0) (0, 1) (1, 0) (1, 1)

0 1 0 0

1 0 0 0

0 0 1 0

0 0 0 1

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Optical switches

Table 2.7  Output states of the decoder switch when optical signal input from input A and B simultaneously Inputs A

B

Control signals (VCII , VCI )

1 1 1 1

1 1 1 1

(0, 0) (0, 1) (1, 0) (1, 1)

Outputs Port 1

Port 2

Port 3

Port 4

0 1(B) 0 1(A)

1(B) 0 1(A) 0

1(A) 0 1(B) 0

0 1(A) 0 1(B)

calculated insertion loss and average crosstalk are 0.30 dB and 220.8 dB, respectively. 4 When (VCII , VCI ) = (1, 1), optical signal A will be output from output waveguide 1 while optical signal B will be output from output waveguide 4 (see Plate XId). The normalized output powers in the output waveguides 1, 2, 3, and 4 are 46.5%, 0.3%, 0.3%, and 45.0%, respectively. The calculated insertion loss and average crosstalk are 0.36 dB and 221.8 dB, respectively. Table 2.7 is the truth table of the 2 × 4 decoder switch when optical signals are input from input A and input B simultaneously. It should be pointed out that in the column ‘Outputs’ of Table 2.7, 1(A) and 1(B) indicate that the output signals in the output waveguide are optical signal A and optical signal B, respectively.

2.5

Performance and challenges

Optical switches are key components for applications in 1.31–1.55 µm optical communications, networks, and microsystems. They can reduce the cost of the network and increase fiber transmission capacity and at the same time, distribute optical signals to different subscribers. Optical switches in Si-based waveguides make use of changes in the refractive index induced by carrier injection and offer advantages of small device size, polarization independence, and capability of integrating with other Si-based optoelectronic devices. By adding Ge into Si, the bandgap of SiGe shifts towards the optical communication wavelength while the refractive index increases, which is good for wave guiding. The p-n junction can also be formed easily during SiGe epitaxy. In this chapter, six switching approaches at the optical communication wavelength are introduced. Optical switches utilizing carrier-induced refractive index change have tremendous potential for the application of optical processing because of their small size, single-mode operation, and polarization independence. According to the types of carrier injection, optical waveguide switches can be divided into vertical injection and lateral injection. For the former, one electrode is located on the top of the switch and the other at the bottom of the substrate, and

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the substrate must be n+ or p+ type for good ohmic contact. For either n+ type or p+ type substrate, a large amount of free carriers will be present at the interface between the substrate and the waveguide layer. Hence, carrier absorption loss will be very large at the interface when the switch is in operation. However, the injection current and injection current density cannot be reduced by any other means. With lateral injection, the carrier absorption loss at the interface between the substrate and the waveguide layer can be eliminated due to the avoidance of either n+ or p+ substrate. However, the injection current and injection current density still cannot be reduced.

2.6

References

  1 R. A. Soref, J. Schmidtchen and K. Petermann, ‘Large single-mode rib waveguides in GeSi-Si and Si-on-SiO2,’ IEEE J. Quantum Electron. 27, 1971–74 (1991).   2 R. People, ‘Physics and application of GexSi12x/Si strained-layer heterostructures,’ IEEE J. Quantum Electron. 22, 1696–1710 (1986).   3 B. J. Li, S.-J. Chua, C. W. Leitz and E. A. Fitzgerald, ‘1 × 2 optical waveguide filters based on multimode interference for 1.3- and 1.55-µm operation,’ Opt. Eng. 41(3), 723–7 (2002).   4 L. B. Soldano and E. C. M. Pennings, ‘Optical multi-mode interference devices based on self-imaging: principles and applications,’ J. Lightwave Technol. 13, 615–27 (1995).   5 M. Yagi, S. Nagai, H. Inayoshi and K. Utaka, ‘Versatile multimode interference photonic switches with partial index-modulation regions,’ Electron. Lett. 36, 533–4 (2000).   6 B. J. Li, J. Wan, G. Z. Li, Z. Jiang, E. K. Liu and X. Wang, ‘Y-branch 1.3/1.55 µm wavelength demultiplexer based on the plasma dispersion effect,’ Thin Solid Films 369(1–2), 419–22 (2000).   7 C. Z. Zhao, E. K. Liu, G. Z. Li and L. Guo, ‘Silicon-on-insulator optical intensity modulator based on waveguide-vanishing effect,’ Electron. Lett. 32(18), 1667–68 (1996).   8 H. Okayama, T. Ushikubo and M. Kawahara, ‘Low drive voltage Y-branch digital optical switch,’ Electron. Lett. 27, 24–26 (1991).   9 J. F. Vinchant, M. Renaud, A. Goutelle, M. Erman, P. Svensson and L. Thylen, ‘Low driving voltage or current digital optical switch on InP for multiwavelength system applications,’ Electron. Lett. 28, 1135–7 (1992). 10 Y. L. Liu, E. K. Liu, S. L. Zhang, G. Z. Li and J. S. Luo, ‘Silicon 1 × 2 digital optical switch using plasma dispersion,’ Electron. Lett. 30, 130–1 (1994). 11 M. N. Khan, B. I. Miller, E. C. Burrows and C. A. Burrus, ‘Crosstalk-, loss-, and lengthreduced digital optical Y-branch switches using a double-etch waveguide structure,’ IEEE Photon. Technol. Lett. 11, 1250–2 (1999). 12 S. Abdalla, S. Ng, P. Barrios, D. Celo, A. Delâge, S. El-Mougy, I. Golub, J.-J. He, S. Janz, R. McKinnon, P. Poole, S. Raymond, T. J. Smy and B. Syrett, ‘Carrier injectionbased digital optical switch with reconfigurable output waveguide arms,’ IEEE Photon. Technol. Lett. 16, 1038–40 (2004). 13 B. J. Li, Y. Zhang, L. Teng, Y. Z. Zhao, S.-J. Chua and X. Wang, ‘Symmetrical 1 × 2 digital photonic splitting switch with low electrical power consumption,’ Opt. Express 13(2), 654–9 (2005).

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14 B. J. Li, G. Z. Li, E. K. Liu, Z. M. Jiang, C. Pei and X. Wang, ‘1.55 µm reflection-type optical waveguide switch based on SiGe/Si plasma dispersion effect,’ Appl. Phys. Lett. 75(1), 1–3 (1999). 15 J. Nayyer and H. Hatami-Hanza, ‘Optical intersecting-waveguide switches with widened angle of deflection,’ IEEE Photon. Technol. Lett. 4, 1375–7 (1992). 16 B. J. Li and S.-J. Chua, ‘2 × 2 optical waveguide switch with bow-tie electrode based on carrier-injection total internal reflection in SiGe alloy,’ IEEE Photon. Technol. Lett. 13(3), 206–8 (2001). 17 B. J. Li and S.-J. Chua, ‘Reflection-type optical waveguide switch with bow-tie electrode,’ J. Lightwave Technol. 20(1), 65–70 (2002). 18 C. F. Janz, B. P. Keyworth, W. Allegretto, R. I. MacDonald, M. Fallahi, G. Hillier and C. Rolland, ‘Mach-Zehnder switch using an ultra-compact directional coupler in a strongly-confining rib structure,’ IEEE Photon. Technol. Lett. 6, 981 (1994). 19 C. Z. Zhao, E. K. Liu, G. Z. Li, Y. Gao and C. S. Guo, ‘Zero-gap directional coupler switch integrated into a silicon-on insulator for 1.3-µm operation,’ Opt. Lett. 21, 1664 (1996). 20 B. J. Li and S.-J. Chua, ‘Two-mode interference photonic waveguide switch,’ J. Lightwave Technol. 21(7), 1685–90 (2003). 21 B. J. Li and S.-J. Chua, ‘High carrier injection optical switch based on two-mode interference in SiGe alloy,’ Appl. Phys. Lett. 80(2), 180–2 (2002). 22 F. Sun, J. Yu and S. Chen, ‘A 2 × 2 optical switch based on plasma dispersion effect in silicon-on-insulator,’ Opt. Commun. 262, 164–9 (2006). 23 Z. W. Chen, B. J. Li, B. S. Chaudhari and S. J. Chua, ‘A 2 × 3 photonic switch in SiGe for 1.55 µm operation,’ Chin. J. Semicond. 27(3), 494–8 (2006). 24 B. J. Li, J. Li, Y. Z. Zhao, X. B. Lin, S.-J. Chua, L. Y. Miao, E. A. Fitzgerald, M. L. Lee and B. S. Chaudhari, ‘Ultracompact, multifunctional, and highly integrated 3 × 2 photonic switches,’ Appl. Phys. Lett. 84(13), 2241–3 (2004). 25 B. L. Li, S.-J. Chua, E. A. Fitzgerald, B. S. Chaudhari, S. Jiang and Z. Cai, ‘Intelligent integration of optical power splitter with optically switchable cross-connect based on multimode interference principle in SiGe/Si,’ Appl. Phys. Lett. 85(7), 1119–21 (2004). 26 Z. J. Li, Z. W. Chen and B. J. Li, ‘Optical pulse controlled all-optical logic gates in SiGe/Si multimode interference,’ Opt. Express 13(3), 1033–38 (2005). 27 X. Jia, S. Luo and X. Cheng, ‘Design and optimization of novel ultra-compact SOI multimode interference optical switch,’ Opt. Commun. 281, 1003–7 (2008). 28 D. A. May-Arrioja and P. LiKamWa, ‘Reconfigurable 1 × 4 InP-based optical switch,’ J. Microelectron. 39, 644–7 (2008). 29 Z. W. Chen, Z. J. Li and B. J. Li, ‘A 2 × 4 decoder switch in SiGe/Si multimode interference,’ Opt. Express 14(7), 2671–8 (2006).

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Optical field

Width (µm)

25.0 12.6 0.0 –12.6

0.00

0.23

0.46 0.68 Distance (mm)

0.90

(a) Optical field

Width (µm)

25.0 12.6 0.0 –12.6

0.00

0.23

0.46 0.68 Distance (mm)

0.90

(b) Optical field

Width (µm)

25.0 12.6 0.0 –12.6

0.00

0.23

0.68 0.46 Distance (mm)

0.90

(c)

Plate I Simulation results of the switch using beam propagation method: (a) switch-ON state on both branches, (b) switch-ON state on branch 2 and OFF state on branch 1, (c) switch-ON state on branch 1 and OFF state on branch 2.

15.0 Input Width (µm)

7.5

Output

0.0

y

z

–7.5 –15.0 0.0

609.5 1219.0 1828.5 2438.0 Length (µm)

x

Plate II Optical field intensity distribution of the SiGe photonic switch based on general resonance at zero injection current (OFF-state): (a) two-dimensional view and (b) threedimensional distribution.

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(a)

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Plate III Simulated switching states of the photonic switch when an input light is coupled into the input waveguide A: (a) switched to output 3, (b) switched to output 1, and (c) switched to output 2.

(b)

(a)

1

3

B

B

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Plate IV Simulated switching states of the photonic switch when input light is coupled into B: (a) switched to output 1, (b) switched to output 3, and (c) switched to output 2.

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Plate V Simulated switching states of the photonic switch when input lights are coupled into A and B simultaneously: (a) cross-state, (b) bar-state, and (c) combined-state.

A 3 4

(a)

(b)

1 2

(d)

(c)

Plate VI Simulated output results of the 2 × 4 decoder switch when optical signal is input from input A: (a) (VCII, VCI) = (0, 0), (b) (VCII, VCI) = (0, 1), (c) (VCII, VCI) = (1, 0), and (d) (VCII, VCI) = (1, 1).

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l1

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(a) l3 l3

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l1l3

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l1 l2

l3

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l1l2l3

l3

(i)

Plate VII Optical field profiles of the device and function descriptions: (a) optical power splitter, (b)–(d) one-wavelength switch, (e) optical crossconnect, (f)–(g) optical add multiplexer and two-wavelength switch, (h) optical combiner and optical add/drop multiplexer, and (i) wavelength division multiplexer and three-wavelength switch.

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Width (µm)

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(d)

(e)

Plate VIII Simulated optical intensity profiles of the device when light beams are coupled into the input waveguides: (a) λ1 = 1545 nm to waveguide 1, (b) λ1 = 1545 nm, and λ2 = 1550 nm to waveguides 1 and 2, respectively, (c) λ1 = 1545 nm and λ3 = 1555 nm to waveguides 1 and 3, respectively, (d) λ2 = 1550 nm and λ3 = 1555 nm to waveguides 2 and 3, respectively, and (e) λ1 = 1545 nm, λ2 = 1550 nm, and λ3 = 1555 nm to waveguides 1, 2, and 3, respectively.

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Plate IX (a)–(g) BPM-simulated optical fields with incident light beams having same wavelength and polarization.

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(b) 1

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Plate X Simulated output results of the 2 × 4 decoder switch when optical signal is input from input B: (a) (VCII, VCI) = (0, 0), (b) (VCII, VCI) = (0, 1), (c) (VCII, VCI) = (1, 0), and (d) (VCII, VCI) = (1, 1).

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(a)

A 2 3 B

(b) 1

4

(c)

2 3

(d) 1

4

Plate XI Simulated output results of the 2 × 4 decoder switch when optical signals are input from input A and B simultaneously: (a) (VCII, VCI) = (0, 0), (b) (VCII, VCI) = (0, 1), (c) (VCII, VCI) = (1, 0), and (d) (VCII, VCI) = (1, 1).

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3 Thermo-optical switches L. SIRLETO, G. COPPOLA, M. IODICE, M. CASALINO, M. GIOFFRÈ and I. RENDINA, National Research Council – Institute for Microelectronics and Microsystems, Naples, Italy Abstract: In this chapter, we first introduce the physical principles of thermooptic effect, providing also some basic details on the thermodynamic operation of an integrated optic device. Then, in the third section, we present a wide review of the most common materials utilized for the fabrication of thermo-optical switches, providing for each material an updated literature reporting the value of the thermo-optic coefficient. Finally, in the fourth section, recent proposals of thermo-optic-based switches are reviewed and their performances compared. Key words: OCIS codes: (130.3120) integrated optic devices, (250.0250) optoelectronics, optical switching, thermo-optic effect.

3.1

Introduction

Thermo-optical switches are key elements in the construction of all-optical transmission and switching networks (Yao et al., 2000). They are very attractive due to their small size, large scalability and potentiality for integration with waveguide DWDM (de)multiplexers. They play an important role in optical telecommunication applications, such as optical cross-connection (OXC), protection switching and switch arrays for optical add-drop multiplexing (OADM). Historically, the most common examples of thermo-optical switches are based on polymers and silica. The polymer devices are fabricated and marketed starting from standard polymer materials or particular patented molecules, while the silica-based devices are derived from the well-established silica-on-silicon technology for passive waveguide components. In both cases the refractive index should be chosen in order to reduce the coupling losses with the optical fiber. Channel waveguides in both technologies are typically made by first depositing the bottom cladding layer, followed by the deposition of the core layer. Then, reactive ion etching is used to etch the core ridge while a following coating process realizes the upper cladding layer. Typically, in the commercial thermooptical devices based on polymeric or silica technology, silicon wafers are used as substrates because of their compatibility with standard IC process equipment, good surface quality and excellent heat conducting property. This last characteristic is very important for thermo-optical components because it allows the substrate itself to act as a good heat sink. Switching between output channels can be induced 61 © Woodhead Publishing Limited, 2010

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by creating a difference in their propagation characteristics or, in other words, in their effective refractive indices. This can be done by driving the resistive stripe heater electrodes deposited on the top of the cladding layer above the waveguides. Polymer and silica planar lightwave circuit designs having milliwatt thermal powers and millisecond switching times have been realized. Thermo-optical switches in these materials are commercially available. The thermo-optic effect (TOE) is present in all materials. The thermo-optic coefficient (TOC), given by dε/dT, is the fundamental optical parameter, describing the temperature dependence of the refractive index of materials, where ε is the complex dielectric function of the material at temperature T. Being the TOC of considerable interest in many optics and optoelectronics applications, such as guiding, coupling and modulation of radiation, it is highly desirable to obtain a theoretical prescription, i.e. its absolute magnitude and its sign, over a wide range of frequencies, based on, at most, a small number of known values at a few frequencies. Therefore, in the next section, the physical principles of the TOE are discussed. We review the most important models presented in the literature. The aim is to provide not all details but their potentiality and limitation, pointing out the main hypothesis and conclusions. We also introduce the basic physics of thermodynamic problem involved in thermo-optical switches. Again, a complete description is outside the scope of our work; therefore, considering a simple case, we introduce the fundamental parameters describing the basic tradeoff of thermo-optical switches. Finally, we conclude the section by pointing out the importance of thermal effects in nonlinear optical devices. Regarding materials, it is well known that in photonic devices a large number of materials have been investigated. Of course, their properties, i.e. mechanical, thermal and optical, influence the performances of devices. Many times, when we design a device, a major difficulty is caused by the lack of experimental data about the properties of materials. Therefore, in section 3.3, a wide review of the most common materials utilized for the fabrication of thermo-optical switches is reported, providing for each material an updated literature reporting the value of the TOC. A number of thermo-optical switches have been developed. They include interferometric devices, such as directional coupler and Mach–Zehnder, and digital optical switches based on modal evolution in the conventional Y-junction branch and based on total internal reflection (TIR), micro-electro-mechanical system (MEMS) technologies. In section 3.4, we describe the working principle of the most utilized configurations and from the recent literature we report the most interesting devices in terms of achieved performances.

3.2

Theory and principles of thermo-optic effect

Optical properties of any medium can be described by the complex index of refraction, n = N – ik, or the complex dielectric function, ε = ε1 – iε2. ε is related

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to n by ε = n2 so that ε1 and ε2 can be determined from a knowledge of N and k: ε1 = N2 – k2 and ε2 = 2Nk. N, k, ε1 and ε2, referred to as the optical constants, depend on the photon energy (E = h-ω), where ω represents the photon frequency. These functions are called optical dispersion relations (Ashcroft and Mermin, 1976; Yu and Cardona, 1996). For all isotropic materials in the transparent regime, the macroscopic Clausius– Mossotti formula is applicable: [3.1] where αm is the polarizability of a macroscopic small sphere with a volume V, large in comparison with the lattice dimensions. It is worth noting that the formula (3.1) provides a valuable link between macroscopic and microscopic theories. A microscopic theory is required to calculate polarizability, which gives the response of the ions to the actual field acting on them. Then, the resulting dielectric constant can be used, in conjunction with the macroscopic Maxwell equations, to predict the optical property of material. Finally, we note that when written in terms of refractive index n = √ε, the relation (3.1) is known as the Lorentz–Lorenz formula (Ashcroft and Mermin, 1976; Born and Wolf, 1999). For isotropic materials, the macroscopic Clausius–Mossotti formula permits a satisfactory description of TOC (Havinga, 1961; Bosman and Havinga, 1963). There are three effects contributing to the temperature dependence of dielectric constant: a direct volume expansion effect, the influence of volume expansion and of temperature on polarizability. Differentiation of formula (3.1) with respect to temperature at constant pressure gives:



[3.2]

The physical processes described by the terms A, B and C are: • A: due to an increase in specific volume as the temperature increases, a greater inter-atomic spacing in the lattice is obtained, which causes a decrease of dielectric constant. This is the direct effect of the volume expansion. • B: an increase of polarizability with the volume expansion. • C: the dependence of polarizability on temperature at constant volume. We also note that the sum of A and B, describing the total effect of volume expansion, can be written as:

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[3.3] This equation can be derived by differentiating the Clausius–Mossotti formula (3.1) with respect to pressure, at constant temperature. Here, e is the pressure p T 1 V dependence of dielectric constant, V ˙ T is the thermal expansion coefficient p 1 V and – is the compressibility. We note that both the thermal expansion ˙ V p p coefficient, describing the temperature-dependent volume expansion at constant pressure, and the compressibility, describing the pressure-dependent change of volume at constant temperature, are positive for most materials at most temperatures. Further, the derivative of volume with respect to temperature can be written in terms of the linear thermal expansion coefficient (TEC) α as:



 

  

 



[3.4]

Now, the equation (3.2) can be rewritten in a more meaningful form: [3.5] Starting from the Clausius–Mossotti formula, Bosman and Havinga (1963) found that: • The temperature dependence of polarizability at constant volume is mostly positive for materials, and its contribution to the temperature dependence of the dielectric constant is very important. • In ionic materials with a low melting point, thermal expansion is high and the TOC is negative. • In some nonlinear crystals having a high melting point, hardness and high elastic modulus, because of the small thermal expansion, the TOC is positive, being dominated by the volume change in polarizability. • In polymers the TOC has a large negative value because it is determined predominantly by density changes caused by the strong thermal expansion. • In silica the TOC is in absolute value an order of magnitude smaller than in polymers, but its sign is positive. In fact, the TOC in silica is due to the second term in equation (3.2), which originates from the thermal change in polarizability. In order to study the temperature dependence of the index of refraction n in the transparent regime of a fairly wide variety of crystals, Tsay et al. (1973) introduced

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a two-oscillator model based on variation with temperature of the fundamental transitions in crystals, which are energy gap Eg and fundamental phonon angular frequency ω0. The dielectric susceptibility χ is written as the sum of an electronic contribution due to band-to-band transitions and a lattice contribution: [3.6] Each of the latter undergoes a temperature variation consisting of contribution due to thermal expansion as well as contribution due to the explicit temperature dependence at constant volume:



[3.7]

where χe and χl are the electronic susceptibility and the lattice susceptibility, respectively, ωg is an average optical band gap and e* is the transverse effective charge (Tsay et al., 1973). We observe that two sorts of physical effects can influence dn / dT. One is the thermal expansion; when T increases the material expands becoming less dense. If this is the only mechanism operative, it would tend to make dn / dT negative. The other effect is the change in the thermal occupancies and spectra of the energy levels of the material as a function of temperature. For most materials Eg and ω0 decrease with temperature. For transparent materials, where ω of interest lies between these two energies, both these changes tend to increase n, therefore dn / dT is positive. Applying this model, it was found that for most semiconductors the lattice terms are negligible except in a very narrow frequency region near ω0. Therefore, electronic effects yield the dominant contribution throughout the transparent regime. The electronic contribution, in turn, is dominated by the temperature variation of the band gap at constant volume, as opposed to those resulting from thermal expansion effects. Therefore, for semiconductors dn/dT is positive and of the order of ≈ 10 – 4K– 1. Whereas, for highly ionic crystals thermal expansion terms dominate the electronic contribution and dn/dT takes on large negative values of the order of ≈ 10 – 5K– 1. However, applying this model, because many parameters are unknown, the evaluation of TOE is critical and the procedure is not straightforward. We note that both the refractive index and the TOC are dispersive in the transmission region. But the dispersions of these two important optical parameters are not the same. In order to analyze the thermo-optic frequency dispersion effect, a model based on only three parameters was proposed by Ghosh (1994). The

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parameters considered to affect dn / dT are: TEC, temperature coefficient of the excitonic band gap and a newly introduced isentropic band gap (Ghosh, 1995). The equation for representing TOC is given by:

[3.8] where (1/Eeg dEeg/dT) is the temperature coefficient of the excitonic band gap, E is the photon energy and Eig is the isentropic band gap. We note that the isentropic band gap is an energy gap corresponding to the band-to-band transition that is not affected by temperature variations (lying, for example, in the UV region of both crystalline silicon (c-Si) and amorphous silicon (a-Si)) (Ghosh, 1997). The above equation is rewritten in terms of the normalized dispersive wavelength R as:



[3.9]

where R = λ2 /(λ2 – λig2), K 2 = n2∞ – 1 and n∞ is the low-frequency refractive index in the IR region. We note that, because α is normally positive, the contribution from the first factor is negative but it is small. Regarding the second factor, we note that the temperature coefficient of the excitonic band gap is of the order of 10–4 eV/°C, and it is normally negative for optical materials. Therefore, the second factor contribution is, in general, positive. Additionally it is larger than the first one, yielding positive values of dn/dT for most of the optical materials. Therefore, the Ghosh model permits us to demonstrate that the electronic effect, particularly the temperature variation of the excitonic band, yields the dominant contribution. When a constant dn/dT is considered, equation (3.9) is transformed:

[3.10]

Equation (3.10) is a form of the Sellmeier relation that represents the product of the refractive index and the TOC. It is used to satisfactorily characterize the TOC of optical material, and G and H are called Sellmeier coefficients for the TOC. They are related to the TEC and the temperature coefficient of the excitonic band gap, 1 dEeg 2 K . This model is respectively, by the relations G = –3α K 2 and H 5 2 Eeg dT physically meaningful because it takes into account both the physical parameter α and dEeg/dT which are measured with greater accuracy. We note that most published papers on semiconductors usually assume that dn / dT data are nearly constant and independent of temperature over a fairly wide range of temperature. But, in practice, this assumption is not acceptable. In order

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to overcome this limitation, the TOC of c-Si in the temperature range 300–600 K at the fiber optic communication wavelength of 1.5 micron was analyzed by Della Corte, Esposito Montefusco et al. (2000). The temperature dependence, given by ∂n/∂T = 9.48 × 10 –5 + 3.47 × 10–5 × T – 1.49 × 10 –10 × T2, where the temperature T is expressed in degrees Kelvin, was attributed to the variation of the interband transition energies at some critical points of the silicon band structure, and the experimental data were fitted using single- and double-oscillator models. An important figure of merit for a thermo-optical switch is switching time. In order to evaluate this parameter, the thermal transient and steady-state response, the heat conduction equation, describing the quantity of heat transported per unit time and unit volume, has to be solved: [3.11] where Q(x, y, z, t) is the heat generation rate per unit volume, ρ is the material density, cp is the specific heat and k is the thermal conductivity, considered constant. Of course, the higher the value of k the better the material conducts heat. This equation is derived by applying Fourier’s law, which claims that the heat flow proceeds along the steepest decrease of temperature. We assume that the initial temperature distribution is:

[3.12]

and the boundary conditions are: on the top surface on the lateral surfaces – T(t) = T (heat sink) on the bottom surface

[3.13] [3.14] [3.15]

Condition (3.13) establishes natural convection as heat transfer mechanism between the device and air; in this equation s is the surface outward normal, h is the natural convection heat transfer coefficient, TS is the surface temperature and TA is the air temperature. The boundary condition in equation (3.14) states that the lateral surfaces are adiabatic, i.e. the ends of the waveguide are isolated so that there is no passage of heat through them. Equation (3.15) assigns a fixed temperature at the bottom of the device, i.e. the substrate is considered a perfect heat sink. We are interested in two kinds of solutions. The first one is the steady-state temperature distribution, which is independent of time t and the initial conditions. It is obtained considering that the first member of equation (3.11) equals zero. After that, the transient part of the solution of the original problem has to be

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found. We note that the boundary conditions are non-homogeneous; therefore a numerical, solution is often considered. The switching time τ is defined as the 1 time to reach 12 e  of the steady-state temperature and it can be determined by: [3.16] k where γ 5 p.c is the thermal diffusivity and L is the thickness of waveguide p layer stack. Another important figure of merit for a thermo-optical switch is the switching power, i.e. the power dissipation per unit length that is required to get a certain temperature difference. It is possible to demonstrate that this power is proportional to the thermal conductivity: Ps ≈ k∆T

[3.17]

where ∆T is the temperature difference between the initial and final stationary states. Therefore, considering a thermo-optical switch, a trade-off between the switching time and the power dissipation per unit length has to be taken into account. If a thermo-optical switch is realized using a material with a high thermal conductivity, a short switching time but a high switching power per unit length is obtained. On the contrary, using a material with a small thermal conductivity, a long switching time but a low switching power per unit length is achieved. Thermal processes can lead to large (and often unwanted) nonlinear optical effects (Boyd, 1992). The origin of thermal nonlinear optical effects is that some fraction of the incident light power is absorbed passing through an optical material. The temperature of the illuminated portion of the material consequently increases, which leads to a change in the refractive index of the material. We note that thermal nonlinear optical effects are nonlocal, because the change in refractive index at some given point will in general depend on the laser intensity at other nearby points. The time scales for changes in the material temperature can be quite long (in the order of seconds) and, consequently, thermal effects often lead to strongly time-dependent nonlinear optical phenomena. It is possible to demonstrate that the response time associated with the change in temperature due to a circular laser beam of intensity P0 and radius r, which falls onto a slab of optical material, is given by: [3.18]



We note that this quantity is geometry-dependent (through the r2 factor). Even for tightly collimated beam with r = 10 µm, one finds that τ ≈ 100 µs. These response times are much longer than the pulse duration tp produced by most © Woodhead Publishing Limited, 2010



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pulsed lasers. One thus reaches the conclusion that, in the consideration of thermal effects, the power P is the relevant quantity for continuous-wave laser beams and that the pulse energy Qp = P0·tp is the relevant quantity in the consideration of pulsed lasers.

3.3

Materials for thermo-optical switches

In order to be selected for waveguide technology and for thermo-optical device fabrication, a material has to satisfy the following set of requirements: • low optical losses (no more than 0.1 dB/cm) in the communication spectral windows around 850, 1300 or 1550 nm; • low wavelength dispersion; • low birefringence; • low polarization-dependent losses; • thermally stable mechanical properties; • resistance to humidity; • good mechanical properties such as flexibility and toughness; • low cost; • high TOC.

3.3.1  Polymeric materials for thermo-optical switches Polymer waveguide technology has a great potential for economic mass production of complex planar photonic circuits that comply with the severe requirements imposed by applications in communication systems. The most appealing characteristic of polymer waveguide technology is the simplicity and flexibility of waveguide fabrication methods. Polymer thin films can be deposited in a wide thickness range by spin or dip coating using relatively simple equipment. A variety of channel waveguide fabrication methods exist, ranging from existing micro-technology techniques, such as etching, to mass production methods developed especially for polymers, including molding and laser delineation. The low-cost prospect arises from the availability of a wide range of cheap optical polymers, which have shown excellent optical, chemical and mechanical properties. For instance, most of these polymers are transparent in the wavelength range 400–2000 nm, and losses of polymer-based waveguides can be as low as 0.1 dB/cm in the three telecommunication windows around 850, 1300 and 1550 nm. Moreover, the refractive indices of a variety of polymer materials can be tailored and precisely controlled to suit a specific design purpose in a broad refractive index range from n = 1.3 to 1.7. This additional characteristic is rarely found in other waveguide technologies (with the exception of SiOxNy technology). Classes of polymers for use in integrated optics include acrylates, polyimides and olefins (e.g., cyclobutene). Companies that developed such polymers include AlliedSignal, Amoco, Dow Chemical, DuPont, General Electric,

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Hoechst Celanese, JDS Uniphase Photonics (formerly Akzo Nobel Photonics), NTT and ChemOptics. In transparent polymers, the TOC is about –10 – 4 K–1 at room temperature. Although this strong effect is favorable for actuating devices that use moderate power (well below 100 mW per functional element), its drawback is that environmental temperature changes may affect device operation. Therefore, the design should aim at an operation where the differential temperature is more relevant than the absolute temperature. In this way, overall thermal drift has little effect. Since high temperature may affect the physical and chemical stability of polymer structures, designers should observe strict thermal limits, depending on the particular material (e.g., well below 100°C for PMMA-like materials or up to 300°C for polyimides). Macroscopic thermal processes tend to be slow. However, in integrated optic polymer devices this low speed is mitigated by the relatively small volume involved and by the fact that a substrate such as silicon or aluminum may act as a very good heat sink. The switching time of devices made on such substrates can be well below 1 ms, which is sufficient for many applications in optical telecom, such as protection switches and tunable filters. The following discussion details the thermo-optic properties of primary optical polymers in accordance with the requirements just given. Benzocyclobutene (BCB) BCB is a benzene ring fused to a cyclobutane ring. It has chemical formula C8H8. BCB is frequently used to create photosensitive polymers. BCB-based polymer dielectrics may be spun on various substrates for use in MEMS and microelectronic processing. BCB is transparent and has relatively high TEC (26.5 × 1025 K21). Ellipsometric measurements returned a value for TOC ranging from 22.5 × 1025 K21 (Guo et al., 1996; Nikolajsen et al., 2004) to 21.15 × 1024 K21 (The Dow Chemical Company, 1999; Sun et al., 2005). Bisphenol A-aldehyde (BPA) BPA is an organic compound acting as building block of several important plastics and plastic additives. Polymers based on BPA precursor have high transparency, relatively high refractive index, low birefringence and large TOC at optical telecommunication wavelengths. The TOC values at 1310 nm and 1550 nm (TE mode) ranged from –0.97 × 1024 K21 to 21.33 × 1024 K21 and from 20.96 × 1024 K21 to 21.29 × 1024 K21, respectively (Y. Song et al., 2008; Zhou et al., 2006). Fluoroacrylate (FA) The FA is composed of pentafluorostyrene (PFS), trifluoroethylmethacrylate (TFM) and glycidylmethacrylate (GMA). Its refractive index, which is a linear

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function of PFS concentration, can be varied over a relatively large range from 1.444 to 1.456 (Keil et al., 2000). Optical characterization of thin polymer films on silicon substrates returns a value for TOC of –2.8 × 1024 K21 at 1550 nm and 20°C (Zou et al., 2003). Fluorinated poly(arylene ether sulfide) (FPAE) FPAE, developed for interlayer dielectric materials, has a number of advantages: low moisture absorption in the near-IR region, excellent long-term thermal stability of optical and mechanical properties, easy control of the refractive index and good processability (Kang et al., 2001; Kim et al., 2001). However, the material has some drawbacks: poor adhesion to substrates and large birefringence ∆n = ne – no, in the range from 0.0040 to 0.0045, which can induce significant polarization-dependent losses (PDL). The TOC was estimated to be about 21.0 × 10 – 4 K21 (Oh et al., 1998). Poly(methyl methacrylate) (PMMA) PMMA is a thermoplastic and transparent plastic, a synthetic polymer of methyl methacrylate, sold by several trade names and is commonly called acrylic glass, perspex or plexiglas. The transparent regime of PMMA is from about 300 to 2800 nm. Its refractive index for visible wavelengths is in the range 1.4899–1.4893. It has excellent environmental stability compared to other plastics such as polycarbonate and poor resistance to solvents, as it swells and dissolves easily. A multi-wavelength ellipsometer was used to measure its refractive index as a function of temperature (Zhang et al., 2006). For PMMA the TOC is –1.3 × 1024 K21 while the volume coefficient of thermal expansion is 2.2 × 1024 K21. More accurate prism-based measurements of the TOC in the visible and in the IR range, for undoped and doped PMMA and for both TE and TM polarization are reported in X. Li et al. (2006). The reported values for ∂n/∂T are in the range –1.17 × 1024 K21 to 21.26 × 1024 K21. Polyimide (PI) PI is a polymer commercially available under several names and is known for thermal stability, good chemical resistance and excellent mechanical properties. Recently, PI-based materials have been developed for waveguide applications. In particular, fluorine-containing polyimides (FPI) have favorable properties for waveguide applications. Terui and Ando (2003) reported the results of the optical characterization of several PI-based layers spin coated on silicon substrate. The measured refractive index variations at 1320 nm return a TOC which is in the range from 24.6 × 1025 K21 to 21.04 × 1024 K21. These values are slightly smaller than those of conventional optical polymers, and the values for TE polarization are significantly larger than those for TM polarization.

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Polyurethane (PUR) PUR is a versatile polymer; it has excellent abrasion resistance and some properties of both rubber and plastics. Unfortunately, the conventional PUR exhibits poor thermal stability that limits its applications. The improvement of thermal stability of PUR is obtained with a chemical modification in the structure by introducing thermally stable heterocyclic polymers such as polyimides. A recent paper (Qiua et al., 2009) presents TOC measurements conducted in an attenuated-total-reflection setup equipped with a temperature-controlled prismbased apparatus. The ∂n/∂T was estimated in the range 23.9302 × 1024 K21 to 24.0981 × 1024 K21, for λ = 546–632.8 nm. Pure PUR exhibits even greater TOC, of about 25.3 × 1024 K21 (Akkari et al., 1995). ChemOptics Exguide™ resins (ZPU, ZP and LFR) In the last few years ChemOptics, a Korean company, has produced and commercialized optical waveguide resins (polymer materials) with a wide range of refractive index for industrial purposes and research activities, for the fabrication of nano- and micro-optical elements using UV-imprinting technique (ChemOptics, 2006). These resins are useful for optical waveguides and optical thin film applications due to their low optical loss, high thermal and environmental stability and small birefringence. Precise and continuous control of the refractive index can be achieved by blending techniques. Both UV (ZPU and LFR series) and thermally (ZP series) curable polymers are available. These patented polymers are widely used in research activities (Noh et al., 2008; Hu et al., 2007; Kim et al., 2004; Chen et al., 2005; Noh et al., 2006; Yu et al., 2006; Al-Hetar et al., 2008) for the fabrication of thermo-optical devices. The TOC ranges from 26.5 × 1025 K21 for ZP series to 22.5 × 1024 K21 for LFR series.

3.3.2  Amorphous materials for thermo-optical switches Silica (SiO2) Silica is extensively used in the silicon IC industry and for the manufacture of optical fibers. It has a stable, well-controlled refractive index and is highly transparent. Silica-based single-mode waveguides have a low propagation loss and an extremely low-fiber coupling loss because of their compatibility with singlemode optical fibers. Therefore, silica-based waveguides are expected to be used in various low-loss guided-wave devices for single-mode optical fiber transmission systems or fiber sensor systems (Li and Henry, 1996). Silica-based single-mode waveguides can also be used for active optical devices such as optical switches, though not very fast, by using the temperature dependence of the refractive index. For silica glass, the TOC is universally assumed to be in the range from 0.62 × 1025 K21 to 1.28 × 1025 K21 (Malitson, 1965). Despite the low value, if

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compared to some polymers or silicon, a large number of active thermo-optical devices have been proposed in the literature, mostly based on original design, for the operation efficiency enhancement (Lai et al., 1998; Kasahara et al., 1999). Amorphous silicon Hydrogenated amorphous silicon (a-Si:H) is a suitable material for the realization of planar waveguides to route and modulate the optical signal. a-Si:H can be deposited by plasma-enhanced chemical-vapor deposition (PECVD) on almost any substrate at temperatures below 230°C, thus preserving compatibility with any microelectronic technology. Furthermore, low-temperature PECVD makes it possible to realize heterostructures in conjunction with a-SiGe:H or with a-SiC:H, with the refractive index n continuously varying between 3.6 and 2.6. Interferometric measurements performed on a-Si-based waveguide (Cocorullo et al., 1996) show that this material has a strong TOE. The TOC at 1300 nm is 2.1 × 1024 K21 at room temperature. The characterization results of the TOC for a-Si:H and a-SiC:H at 1550 nm and from room temperature up to 230°C are also reported in the literature (Della Corte et al., 2001). The TOC of a-Si:H at room temperature was estimated to be 2.3 × 1024 K21. This value is about 20% higher than that of c-Si.

3.3.3 Semiconductor and crystalline materials for ­ thermo-optical switches Silicon The characterization of the TOE in c-Si reported in the literature is based on the measurement of temperature variation necessary to induce a complete optical detuning in a Fabry–Perot filter. These measurements were performed at room temperature (Cocorullo and Rendina, 1992) and up to 280°C (Cocorullo et al., 1999). The value of the TOC for c-Si at room temperature was found to be 1.86 × 1024 K21. Measurements indicate no dependence from sample doping or crystal orientation. Poly-silicon was also characterized and its TOC is about 2.25 × 1024 K21 [Park et al., 2005]. Silicon nanocrystals Silicon nanocrystals dispersed in silicon-rich silicon oxide (SRSO) (Seo et al., 2004) or silicon nitride (Torres-Torres et al., 2008) matrix have been characterized. The refractive indices of all SRSO films increased with increasing temperature, with the TOC increasing from 1.0 to 6.6 × 1025 K21 as the silicon content is increased from 37 to 45 at.%. The TOC of nc-Si, obtained by correcting the volume fraction of nc-Si, also increased with increasing silicon content from 1 to 2.5 × 1024 K21. The results indicate that the TOE of nc-Si is size dependent, and

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that it must be taken into account when interpreting the luminescence data from SRSO films with high density of nc-Si. Self-diffraction experiments, under pulsed illumination regime, made it possible to determine, for nc-Si embedded in a silicon nitride film, a TOC of 1.0 × 1024 K21. III–V semiconductors The III–V semiconductors and related alloys (GaAs, AlGaAs, InP, etc.) are widely used in microelectronic and optoelectronic fields because of their outstanding electronic and optical properties. Practically all kinds of passive and active devices working in the communication spectral windows around 850 (GaAs), 1300 or 1550 (InP) nm can be fabricated with III–V semiconductors: waveguides, switches, modulators, lasers and photodetectors. Despite the fact that typical applications of such materials are in the realization of advanced devices, the TOE has been characterized and exploited (Della Corte, Cocorullo et al., 2000; Green et al., 2005), usually for the steady-state control of the operation point of the device. The reported values of the TOC are 2.35 × 1024 K21 for GaAs and 2.01 × 1024 K21 for InP. Lithium niobate (LiNbO3) LiNbO3 is the most utilized material for the fabrication of active optoelectronic devices. It is a birefringent crystal, transparent for wavelengths between 350 and 5200 nm, and has a bandgap of around 4 eV. It is an excellent material for manufacture of optical waveguides. Its crystal structure lacks inversion symmetry and displays ferroelectricity, Pockels effect, piezoelectric effect, photoelasticity, nonlinear optical polarizability and also TOE. Direct measurements (Moretti et al., 2005) of both ordinary and extraordinary TOCs in LiNbO3 return ∂no /∂T = 3.7 × 1026 K21 and ∂ne /∂T = 4.1 × 1025 K21 at 632 nm and ∂no /∂T ≈ 0 (below experimental uncertainties) and Τne /∂T = 3.3 × 1025 K21 at 1523 nm, at room temperature. Tantalum pentoxide (Ta2O5) Ta2O5 has promising potential to be one of the best optical coating materials because of its high index and low absorption, for near-UV and near-IR antireflection, multilayer filter design and optical thermometric sensing applications. Ta2O5 films can be deposited by various techniques such as chemical vapor deposition, electron beam evaporation, ion beam and dual-ion beam sputtering, reactive RF sputtering, DC sputtering, ion-assisted deposition and anode oxidation. Values of refractive index and extinction coefficient may vary depending on the fabrication technique used. Optical characterization (Inci, 2004) of the reflection response of a Ta2O5 film deposited by electron beam evaporation

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on the cleaved end-face of a single-mode optical fiber made it possible to measure a TOC of about 1.21 × 1024 K21 around 1550 nm. Aluminum oxide (Al2O3) Al2O3, commonly referred to as alumina, is the most common naturally occurring aluminum-based material and its major utilization is in the manufacture of aluminum metal. In recent years, the high potential of sputter-deposited amorphous Al2O3 for applications in passive and active (rare-earth-ion-doped film) integrated optics has been demonstrated. Recently, Al2O3 layer deposition and dry etching processes have been successfully optimized. Thin films with optical losses as low as 0.11 dB/cm at 1522 nm wavelength have been fabricated by a reproducible reactive co-sputtering process. The possibility to design and fabricate a class of Al2O3-based integrated optic devices generates interest to exploit the TOE. Characterization in the near-UV and visible range of the TOC returns a value ranging between 1.0 × 1025 K21 and 2.0 × 1025 K21, for the ordinary refractive index, and between 0.9 × 1025 K21 and 1.4 × 1025 K21, for the extraordinary refractive index (Tropf and Thomas, 1998).

3.4

Device structures of thermo-optical switches

For optical switches we need to introduce three main figures of merit: the crosstalk, the electrical power consumption and the switching time. The crosstalk is the ratio in dB between the optical power which passes through the output port during the ‘on’ state and the optical power which is present at the output port in the ‘off’ state. The electrical power consumption is the electrical power needed to achieve and maintain the switching between the off (on) state and the on (off) state. The switching time is the time to pass from the 10% (90%) of the optical power in off (on) state to the 90% (10%) of the optical power in on (off) state. Moreover, there are some secondary figures of merit, such as propagation loss, insertion loss, polarization- and/or wavelength-dependent loss (PDL and/or WDL), i.e. the measure of the peak-to-peak difference in transmission of the switch with respect to the possible states of polarization and/or wavelengths. The acceptable values for these parameters depend on the particular application where the optical switch has to be employed, whereas the optimization of each single parameter depends on the configuration of the optical switch. Many configurations have been reported in the literature; in the following a bird’s eye view of the main and recent developments is reported.

3.4.1 Adiabatic mode coupler The first structure is the digital optical switch (DOS), which has become, since its invention, a very attractive component for space switching in multi-wavelength

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Optical switches Output branches Opening angle = 0.5° Length = 3mm Electrodes Isolation gap

Cladding layer Guiding layer Substrate

Common ground

3.1  Schematic view of a DOS.

optical communication system applications. The most commonly used form of DOS is the linear Y-junction branch consisting of an input tapered waveguide, which adiabatically adapts (i.e. slowly varying) the launched fundamental mode in the bimodal DOS input region, followed by two single-mode symmetric output branches (see schematic diagram in Fig. 3.1). The operation principle of the 1 × 2 DOS is based on the modal effective index variation induced by waveguide heating, which can modify the beam propagation pattern inside the structure itself. The heating can be induced by means of an electrode over or close to the optical waveguide; when an electric current is sent through the heater, the Joule effect induces a temperature change and, as a consequence, a refractive index variation of the waveguide material. So, if the two arms have the same temperature, the DOS is geometrically symmetric and acts as a –3 dB power divider. Therefore, the optical input power has to be evenly divided into the two output ports. On increasing the temperature of one arm, the Y-junction is made asymmetric; the light is guided by adiabatically evolving the input mode to the mode of destination arm with the increased refractive index (Keil et al., 1996; Eldada, 2007; Diemeer et al., 1989; Noh et al., 2004; Hoekstra et al., 2001). This situation is represented by the ‘on’ state in the increased refractive index arm and the ‘off’ state in the other arm. In more details: at the branching point, where the gap between the waveguides is small, equal amounts of lights are launched into each single-mode waveguide in phase to excite the local normal mode of the branching waveguides. At the end of the branching structure, where the waveguide gap is large, most of the power of the zero-order normal mode is in the waveguide in which refractive index has been enhanced. The field of the zero-order mode changes its shape as it propagates along the branch structure; this effect is called

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modal evolution. Such ideal behavior is encountered as long as the geometric transition represented by the branch is sufficiently adiabatic, so that mode coupling between the normal modes does not occur. Switching between the two output arms is provided by reversing the sense of asymmetry. The division of the modal power over the two branches is related to the DOS angle, the effective indices of the output branch and the difference between the effective index and the index of the background. The main characteristics of DOS configuration are switch response, high insensitivity to both wavelength and polarization, large fabrication tolerance and no precise control of the driving electrical power. This advantage is due to the step-like response of the DOS to applied electrical signal, which allows the light to remain in a higher index branch, notwithstanding an increase in the applied electrical signal beyond the switching threshold. Thus, when a Y-branch DOS operates above the switching threshold, variations in polarization and wavelength do not impact significantly the switching capacity of the DOS. On the other hand, in order to ensure adiabatic mode coupling, the angle between the branching waveguides (vertex angle) needs to be very small. Controlled fabrication of such parameter is quite difficult and error-prone; in addition, a small vertex angle leads to a long device length. Moreover, adiabatic coupling also requires that the waveguide structure be weakly guiding so that its index step can be modified by a small index change induced by the driving electrical power. Both the long length and weakly guiding imply high propagation losses. The small Y-branch angle causes many problems in the fabrication process, with the result that crosstalk is usually worse than expected. By means of conventional DOS configuration, it is difficult to achieve a crosstalk lower than –20 dB, which is generally insufficient for network applications. Many authors have reported wide-angle DOSs that are easy to fabricate and achieve a low switching power without adversely affecting the crosstalk (Liu et al., 1994; Nelson et al., 1994; Siebel et al., 2000; Sun et al., 2005; Iodice et al., 2006). However, to further improve the crosstalk level, optical attenuators are connected to the ends of the Y-branch arms (Siebel et al., 2001). In particular, activating the attenuator of the waveguide in off state, the residual optical power can be further reduced enhancing the optical crosstalk. Different approaches are reported in the literature; in the following, the more interesting configuration is described. In particular Yang et al. (2001) and Noh et al. (2000) proposed the configuration shown in Fig. 3.2. The device consists of input and output single-mode waveguides, tapering regions, multimode supporting waveguide region and electrodes at an angle β. At first, light is expanded adiabatically into the multimode supporting waveguide region following the Y-branch. When electrical power is applied along the electrode, the refractive index of the heated arm is lowered by TOE. Therefore, the propagating light is partially reflected at an angle of 2β with respect to the propagating direction. If the angle is larger than the fundamental mode of the multimode supporting

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Optical switches Tapering region 1× 2 DOS Electrode

Attenuator Electrode

Multi-mode supporting waveguide

3.2  Wide angle DOS with optical attenuators.

region, then the reflected light is coupled back into the higher-order modes after passing the heated electrode. These higher-order modes are successively filtered out through the output tapered region and the output single-mode waveguide. As the applied power increases, so does the amount of reflected light, which then leads to larger attenuation. This configuration allows a crosstalk of about –40 dB with an electrical power consumption of about 170 mW. However, to optimize the overall thermo-optical device the heating electrode used for the switch section and the attenuator are connected and controlled by a single current source. However, for optimum operation, each section requires different heating power because the attenuator requires higher temperature change than the switch. Noh et al. (2006), with the aim to adjust the resistance of the attenuator region, gradually changed the width of the heater from a value of Wa to a value of Wb as illustrated in Fig. 3.3(a). This configuration allows a crosstalk of about –70 dB with an electrical power consumption of about 200 mW and a switching time of about 10 ms with a PDL of about 0.1 dB. Han et al. (2008) realized the attenuation section by using tapered waveguides and slightly shifting the heater electrodes aside from the top center of the waveguides, as illustrated in Fig. 3.3(b). The taper structure is introduced to weaken the confinement of the guiding modes in the straight waveguides connected to the ends of the Y-branch arms, which can be easily radiated away from the straight waveguides when the heater electrodes are turned on. In addition, the heater electrodes in the attenuators are slightly shifted aside from the top center of the arm waveguide. Thus, the modes propagating along the straight arms can be efficiently deflected away from the tapered waveguide to free spaces due to the slant-inclined and distorted index distribution caused by the non-symmetric thermal gradient induced by the shifted heater electrodes. By means of this configuration, a crosstalk of about –45 dB with an electrical power consumption of about 60 mW can be achieved.

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Thermo-optical switches (a)

Attenuator

Switch

Waveguide

q

(b)

Electrode

Wb

Wa

79

Wb

Modified radiation-type attenuator Heater electrode

Port 2

q Port 1

Y-branch waveguide

Y-branch switch

Port 3

3.3  DOS and attenuators configuration proposed by (a) Noh et al. (2004) and (b) Han et al. (2008).

In order to have an additional degree of freedom in the design of the DOS, three-dimensional (3D) integrated optic switches have been investigated. This technology, even if it presents practical fabrication difficulties, has allowed the realization of different configurations. In particular, Kim et al. (2004) described a digital thermo-optical switch with an electrode deposited on a slant waveguide (Fig. 3.4(a)). This 3D optical switch allows a crosstalk of about –13 dB for a power consumption of about 800 mW and a switching time of about 7 ms. Keil et al. (2001) reported a thermo-optical vertical coupler switch, in which the lower waveguide is made of SiO2 whereas the upper one is made of polymer. Thus, this structure requires the combination of two technologies – the SiO2 technology for the lower waveguide and the polymer technology for the upper waveguide. The crosstalk achieved is about –32 dB, obtained with an electrical power consumption lower than 80 mW. In order to use a single technology on a unique material, Chen et al. (2005) proposed an optical switch made of two vertically coupled polymer waveguides. A sketch of the structure is shown in Fig. 3.4(b). The switching action is induced by means of an electrode built into one of the waveguides; current is applied to the electrode to generate heat which

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Optical switches Output 1

(a) h1

Electrode

Output 2

h2 h1>h2 Waveguide

Input (b)

Electrode

Waveguide 1 1’

2 1 Waveguide

2’

3.4  Vertical DOS configuration proposed by (a) Keil et al. (1996) and (b) Chen et al. (2005).

in turn changes the refractive index of the waveguide, hence initiating the switching action. The performances of this structure are a crosstalk of –23 dB, electrical power consumption of about 50 mW and switching time of about 2 ms.

3.4.2  Interferometric configurations Interferometer structures are based on mode interference effect, and the Mach– Zehnder interferometer (MZI) is perhaps the most extensively studied thermooptical switch so far. The conventional and simplest form of the switch (Okuno et al., 1995) is made up of one 3 dB splitter and one 3 dB combiner connected by two channels; a thermo-optic phase shifter is placed in one arm of the interferometer (Fig. 3.5(a)). However, the 3 dB splitter and the 3 dB combiner are frequently replaced with a multi-mode interference (MMI) coupler (Fig. 3.5(b)), based on self-imaging effect (Tsao and Peng, 2001; Mukai, 2004; Wang et al., 2006). MMI couplers have many advantages, such as compactness, tolerance for the fabrication parameters and wide bandwidth (Soldano and Pennings, 1995). A thermo-optic phase shifter can be simply a heater deposited over the waveguide. Several works have been reported on thermal MZI optical switches using different materials (Kasahara et al., 1999; Lai et al., 1998; Min-Cheol et al., 1998; Treyz, 1991; Espinola et al., 2003; Harjanne et al., 2004; Geis et al., 2004; Chu et al., 2005a). It is interesting to note that Harjanne et al. (2004), utilizing a multi-step voltage circuit to overdrive a thermo-optical switch, obtained response

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Heater A In A

Out A

In B

Out B

(a)

Heater B Arm 1 Path 1 Phase shifter Path 2 Arm 2

(b)

2×2 MMI splitter

3.5  Schematic view of a MZI with a 3 dB splitter (a) and with a MMI coupler (b).

times <1 µs. Geis et al. (2004), in order to obtain sub-µs speeds with sub-mW switching power, employed a doped strip waveguide as heater, i.e. the waveguide is directly heated by passing current through it. In order to achieve a drastic reduction of the size of the active region of the MZI, and thus of the power consumption, silicon photonic wire waveguides with a nanometer-order crosssectional-size silicon core have been considered (Chu et al., 2005a; J. Song et al., 2008). In particular, Song et al. (2008) in order to further reduce the operating power consumed in a Michelson interferometer configuration, designed a folded MZI employing only one splitter and half-length arms (see Fig. 3.6). Since light shuttles in the arms and the length of the tuning region is shorter (by half) than that of a MZI, a low operating power was achieved. The Michelson interferometer has been realized by positioning at the end of the arms optical waveguide loops that act as mirrors. This solution has been preferred to a Bragg grating mirror (Tsao and Peng, 2001), in order to extend the bandwidth of the device. The light in port A (see Fig. 3.6) will be the reflection of the light at the input port. The performances of the described configuration are switching power of 10 mW and switching time of about 35 µs. Recently, very short devices have been realized employing photonic crystal (PhC)-based structures. The unique proprieties of this new material system, such as photonic bandgap and slow light group velocity (Joannopoulos et al., 1995), have allowed very short MZI thermo-optical switches (Chu et al., 2005b; Gu et al., 2007). In particular, Gu et al. (2007) described a PhC waveguide-based MZI characterized by switching power of 78 mW, switching time of about 20 µs with a length of the active region of 80 µm.

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A

B

3.6  Schematic view of a Michelson interferometer reported by J. Song et al. (2008).

A short active region is possible due to the slow light group velocity of the PhC structure, which is equivalent to a longer effective interaction length between the optical light and the region of the phase shifter. To make this interaction length longer, switches based on high Q-factor resonator have been developed. The Q-factor of an optical resonator is defined as the ratio between the wavelength of the optical beam and the full width at half maximum (Q = λ0/FWHM) of the optical spectral response of the structure. Fast response time and low power are demonstrated by utilizing different resonator configurations. In particular, Kiyat et al. (2006) developed a thermo-optical switch based on a racetrack resonator (see Fig. 3.7(a)). The designed structure exhibits a Q-factor of about 38 000, which realized a switching time of about 4.8 µs with a power consumption of 17 mW. In spite of high Q-factor of this thermo-optical switch, the speed and power consumption appeared limited by the large thermal mass. Good trade-off between thermal mass and performances of the device has been demonstrated by using a Fabry–Perot cavity (Barrios et al., 2004; Pruessner et al., 2007). In the work of Pruessner et al. (2007) the Fabry–Perot cavity consists of a silicon-on-insulator (SOI) rib waveguide between two deep-etched silicon/air distributed Bragg reflector mirrors. Due to the large index contrast between silicon and air, the © Woodhead Publishing Limited, 2010



Thermo-optical switches 3 mm

Air Ni

Bus waveguide

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0

R

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40

SiO2 Si

60°

mm

1 mm 1 mm

(a)

0.3 mm 0.58 mm

Ni SiO2 100 mm

1 mm

Si substrate

(b) LC

Aluminum electrode

DBR

Current flow Isolation trench wC Aluminum electrode

Rib waveguide

3.7  Resonator configuration with a racetrack cavity (Kiyat et al., 2006) (a) and Fabry–Perot cavity (Pruessner et al., 2007) (b).

mirrors were constructed with few periods. The experimental results carried out on this device reported a Q-factor of about 4600 and switching time of about 640 ns for power consumption of about 10 mW.

3.4.3 MOEMS configurations The recent applications of Si-CMOS compatible technologies to the field of microoptical-electro-mechanical system (MOEMS) structures (Senturia, 2001) have allowed physical alteration of the propagation of the light beam by the mechanical rotation of a reflector (mirror or grating) (Lin and Goldstein, 2002; Marxer and de Rooij, 1999; Li et al., 2003). Such systems have successfully miniaturized and integrated active light switching components onto a single chip; however, they are limited by slow response time and mechanical instability. Thus, also in this context, thermo-optical approach seems to win an edge over others due to its easy engineering implementation and relatively large TOC. The more interesting approaches are © Woodhead Publishing Limited, 2010

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based on TIR (J. Li et al., 2006; Zhong et al., 2007; Zhu et al., 2007; Zhu et al., 2008). In particular, Zhong et al. (2007) employed the TOE to drive light transition between the transmission and the TIR of a micromachined silicon prism; Fig. 3.8(a) shows an image obtained by a scanning electron microscope (SEM). The principle of a thermo-optical switching is illustrated in Fig. 3.8(b) and (c). The micromachined structure allows an easy alignment between the optical fibers and the micrometric prism. Without any externally applied electrical signal, the input light impinges on the second interface with an angle θ, slightly smaller than the critical angle, and the transmitted light, through a mirror placed next to the prism, is steered in the output 1 fiber (Fig. 3.8(b)). Increasing the temperature of the prism and thus increasing its refractive index, the critical angle decreases. As a result, the critical angle becomes lower than the incident angle θ, and the incident light is totally reflected into output 2 (Fig. 3.8(c)). In the transmission state, the optical power at output 1 is –17.3 dB (normalized relative to the input power) while the reflected power at output 2 is -32.9 dB. As the prism is heated by a DT = 69 K, the reflected power at output 2 rises up to – 6.7 dB, while that at output 1 decreases dramatically to –46.8 dB. Thus, optical switching with isolations of 15.6 and 40.1 dB for the two switching states is achieved. Finally, it is worth noting that the optical switch design includes a pair of MEMS actuators in addition to the prism key element to compensate for common deviation of the incident angle from the design, due to several uncertain factors.

(a)

Mirror

Output 1

Output 2 Actuator

Input

10 kV

×160

100 mm

34

37

SEI

3.8  MEMS configuration proposed by Zhong et al. (2007): (a) SEM image; (b) transmission without heating; (c) heated configuration.

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Thermo-optical switches Mirror

(b)

45° I

Output 1

3rd

q 2nd n0 1st

a

Input

(c)

Output 2 3rd

q 2nd n0 + Dn

1st Input

3.8  Continued.

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Optical switches

Another micromachined structure has been realized by Zhu et al. (2007, 2008). The switching operations have been performed employing the resonant tunneling effect in the optical single-well (Zhu et al., 2007) or double-well structure (Zhu et al., 2008). An optical well has been obtained inserting a thin layer of lower refractive index material (air) between two thick layers of higher refractive index materials (silicon). The behavior of the optical well structure can be explained in first approximation by the TIR effect; however, for a more accurate treatment a quantum mechanical approach has to be adopted. In fact, this approach also allows a description of the two optical wells reported in (Zhu et al., 2008). In this work, the double-well structure is composed of two hemi-cylindrical silicon prisms, two air gaps and a silicon rib with a micro-heater on the top for adjusting its refractive index (see Fig. 3.9(a)). The shape of the prism allows collimating the light from optical fiber to be an exact plane wave when hitting the working region and vice versa (Zhu et al., 2007). In the ‘on’ state (Fig. 3.9(b)), the transmission reaches its maximum by choosing the proper parameters. Increasing the rib’s refractive index by heating up the rib region, the resonance condition of the optical well is broken, thus the light transmission is shut off and the switch is in the ‘off’ state (Fig. 3.9(c)). Figure 3.9(b) and (c) illustrate the optical potential diagrams relative to the on and off state, respectively. The configuration allows achievement of an extinction ratio larger than 30 dB, and a response time with a fall time (on to off state) of about 1 µs, and a rise time (off to on state) of about 25 µs when a short current pulse is used to drive the heater. In Zhu et al., (2008) the power consumption is not explicitly reported; however, the optical single-well structure described in Zhu et al., (2007) reported comparable values: extinction ratio of 30 dB, fall time of 2.2 µs and rise time of about 23 µs, utilizing a heating power of 119.2 mW. The performances of the afore-described structures are summarized in Table 3.1.

3.5

Conclusions

Nowadays, the SOI technology is the most promising approach for the realization of integrated passive and active optical devices based on TOE. The inherent highindex contrast between Si-air (∆n = 2.5) and Si-SiO2 (∆n = 2.0) strongly confines the electric field within the core. High confinement permits sharp waveguide bends and ultra-small device sizes; thus, allowing the miniaturization of passive optical devices such as waveguides, bends, splitters and interferometers, leading to highdensity photonic integrated circuits. Bonded silicon technology process (BESOI) can achieve a sufficiently uniform and high-quality buried oxide to prevent substrate coupling loss present in other SOI types. Moreover, SOI-based optical devices offer the potential for integration with SOI-based CMOS electronics in order to achieve monolithically integrated optoelectronic systems-on-a-chip. In addition, there has been increasing interest in OXC switches and modulators based

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(a) Micro-heater Output 1st Prism

Reflection 2nd Prism

Input ×180

10kV

(b)

Air gap

100 mm

10

31

SEI

Output wavefront

1st Prism 2nd Prism

Input wavefront

Micro-heater Resonance tunneling

3.9  MEMS configuration proposed by Zhu et al. (2008): (a) SEM image; (b)–(c) optical potential diagrams relative to on state and off state.

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Optical switches (c)

Reflected wavefront

Input wavefront

Rib

Dn

RI change

3.9  Continued.

on the TOE for telecom applications. SOI is an attractive alternative material system because it has a large TOC (∂n/∂T), which, in combination with small device dimension and high thermal conductivity of silicon, allows low power and fast response time. In fact, sub-micron SOI waveguide can be heated very quickly, with an optimal uniformity of temperature distribution inside the cross-section and with reasonable power requirements of a few milliwatts (Pruessner et al., 2007). On the other hand, the reduced lower oxide cladding thickness, due to the highindex contrast with the core, allows a fast heat flux toward the substrate, permitting a switching time of a few microseconds (Espinola et al., 2003; Gu et al., 2007), or even lower (Geis et al., 2004), utilizing a smart drive design, with the aim to emphasize the natural heat convection mechanism, thus reducing the cooling time (Iodice et al., 2003; Harjanne et al., 2004).

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Table 3.1  Recent thermo-optical switches Reference

Configuration (material)

Switch power Switch time [mW]

Crosstalk [dB]

Adiabatic mode coupler Noh et al. (2004) Yang et al. (2001) Han et al. (2008) Kim et al. (2004) Keil et al. (2001) Chen et al. (2005) J. Song et al. (2008) Gu et al. (2007) Kiyat et al. (2006) Pruessner et al. (2007) Zhu et al. (2008)

3.6 a-Si a-Si:H BCB BESOI BPA CMOS c-Si DOS DWDM FA FPAE FPI FWHM GMA IC IR MEMS MMI

DOS (polymer) 200 DOS (polymer) 165–180 DOS (polymer) 60 Vertical coupler 800 Vertical coupler <80 (polymer/silica) Vertical coupler 50 (polymer) Michelson 100 interferometer (SOI) Photonic crystal 78 (SOI) Racetrack 17 resonator (SOI) Fabry–Perot 10 cavity (SOI) Optical well — (silicon)

List of abbreviations Amorphous silicon Hydrogenated amorphous silicon Benzocyclobutene Bonded and etched-back silicon-on-insulator Bisphenol A-aldehyde Complementary metal oxide semiconductor Crystalline silicon Digital optical switch Dense wavelength division multiplexing Fluoroacrylate Fluorinated poly(arylene ether sulfide) Fluorine-containing polyimides Full width at half maximum Glycidylmethacrylate Integrated circuits Infrared Micro-electro-mechanical systems Multi-mode interference

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10 ms — — 7 ms —

270 240 245 213 232

2 ms

223

35 µs

—

20 µs

—

4.8 µs

—

640 ns

—

On→off: 1 µs 230 Off→on: 25 µs

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Optical switches

MOEMS MZI OADM OXC PDL PECVD PFS PhC PI PMMA PUR SEM SOI SRSO TE TEC TFM TIR TM TOC TOE UV WDL

3.7

Micro-optical-electro-mechanical system Mach–Zehnder interferometer Optical add-drop multiplexing Optical cross-connection Polarization-dependent loss Plasma-enhanced chemical-vapor deposition Pentafluorostyrene Photonic crystal Polyimide Poly(methyl methacrylate) Polyurethane Scanning electron microscope Silicon-on-insulator Silicon-rich silicon oxide Transverse electric Thermal expansion coefficient Trifluoroethylmethacrylate Total internal reflection Transverse magnetic Thermo-optic coefficient Thermo-optic effect Ultraviolet Wavelength-dependent loss

List of symbols

α αm β cp ε = ε1 – iε2 E Eeg Eg Eig G, H γ h – h k L N = n – ik ne

Linear thermal expansion coefficient Polarizability Adiabatic mode coupler splitting angle Specific heat at constant pressure Complex dielectric function Photon energy Excitonic band gap Energy gap Isentropic band gap Sellmeier coefficients for TOC Thermal diffusivity Reduced Planck constant Natural convection heat transfer coefficient Thermal conductivity Waveguide thickness Complex refractive index Extraordinary refractive index

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[1/K] [C·m2/V] [J/(kg·K)] [F/m] [eV] [eV] [eV] [eV] [1/K] [m2/s] [J·s] [W/(m2·K)] [W/(m·K)] [m]

no n∞ Q Q P ρ P0 Ps R r τ T TA tp TS θ V W ω ω0 χ χe χl λ0

3.8

Thermo-optical switches Ordinary refractive index Low-frequency refractive index Heat generation rate per unit volume Optical resonator figure of merit Pressure Density Laser beam power Switching power Normalized dispersive wavelength Laser beam radius Switching time Temperature Air temperature Laser pulse duration Surface temperature Incident light angle Volume Heater width Photon angular frequency Fundamental phonon angular frequency Dielectric susceptibility Electronic susceptibility Lattice susceptibility Switch operating wavelength

91

[W/m3] [Pa] [kg/m3] [W] [W] [m] [s] [K] [K] [s] [K] [m3] [m] [1/s] [1/s]

[m]

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4 Magneto-optical switches J. TIOH, R.J. WEBER and M. MINA, Iowa State University, USA Abstract: This chapter introduces the reader to magneto-optical switching for optical communication and in particular for fiber optics applications. The chapter first reviews the main concepts and families of magneto-optical switches. It then further details current work on magneto-optical switching, including newly developed formulations for characterizing magneto-optical effects that will hopefully provide a useful and practicable device-level foundation for the reader. Key words: Faraday effect, Faraday rotation, magneto-optical switching, magneto-optical devices, optical communication.

4.1

Introduction

The main purpose of this chapter is to introduce the reader to magneto-optical switching for optical communication and in particular for fiber optics applications. The goal is to explain the main concepts, and then introduce the main families of magneto-optical switching devices. In order to explain the family of devices one needs to have a general understanding (historical, as well as physical) of optical fiber, magnetism, electromagnetic wave propagation as well as the interaction between electromagnetic waves and materials. The optical fiber sections will not include extensive formulation to facilitate a deeper examination of magnetic switching. We will elaborate in more detail work on the subject of magneto-optical switching, and in particular we will introduce one of our newly developed formulations regarding the magneto-optical effect known as Faraday rotation. While other books and published material have addressed this formulation and introduced the main concepts and governing equation, few practical developments are available to researchers. We hope that our presentation and developments provide a useful and practicable device-level foundation for the reader. Since most microwave, millimeter wave and optical devices are characterized based on their port scattering and transmission parameters, we will be introducing methods to incorporate this approach to Faraday rotation at optical frequencies. Finally the chapter provides an extensive and comprehensive reference list. The list includes historically important, application-based, as well as new references for researchers and students of the field.

4.2

History of optical communication

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thrive when isolated from others and thus his sermon underscores the immense importance of communication. Thus, it is no surprise that optical communication dates back to antiquity; from fire and smoke signals to signaling lamps, flags and semaphores. One of the earliest documented optical networks is the Roman smoke signal telegraph, which dates back to c. ad .150. To keep pace with the rapid expansion of their empire the Romans developed a highly sophisticated network of towers within visible range of each other. This first optical network spanned a total distance of 4500 km and used smoke signals to relay military messages. Another famous optical network is the Chappe optical telegraph network of the eighteenth century. The French revolution, epoch of the rise of capitalism, required France to defend itself from enemies both within and without. This situation highlighted the importance of high-speed, long-distance communications, and Claude Chappe, a former priest, designed and built the first optical telegraph. Spanning a distance of 200 km, it linked Lille and Paris via a series of 15 towers spaced 12–25 km apart. Each tower was equipped with telescopes and mechanical semaphore arms, which could be reconfigured to display 196 distinct characters. These were manually relayed by operators from tower to tower and peak message speeds of 3000 km/h were achieved. This network was highly successful and continued to expand until 1846, spanning 5000 km with 556 stations. Many concepts in modern networks, e.g. flow control, error detection and synchronization, had their inception in the Chappe telegraph (Holzmann and Pehrson, 1994). Samuel Morse ushered in the era of electrical communications in 1837 with the invention of the telegraph (Morse, 1840). For the ensuing century, optical communications remained largely supplanted. Its comeback had roots in the 1870 demonstration of transmission of light by total internal reflection in a stream of water by John Tyndall, which marked the inauguration of research into the guided transmission of light. About a decade later, Alexander Graham Bell developed a voice transmission system, dubbed the photophone (Bell, 1880), which employed free-space optics and had a range of 200 m. Limited by line-of-sight requirements, it was not until the invention of both a powerful coherent optical source that could be modulated (lasers (Maiman, 1960)) and a flexible, sufficiently low-loss transmission medium (optical fibers (Kao and Hockham, 1966; Kapron et al., 1970)) that the tide was turned.

4.2.1 Modern optical communications Contemporary optical fibers are a far cry from their original counterparts (see Fig. 4.1). Due to their characteristics, modern optical communications utilizes the medium (1310 nm) and long (1550 nm) wavelength bands or transmission windows due to least dispersion and attenuation in those windows, respectively. The availability of sources and amplifiers at these wavelength windows is also an integral factor for communications purposes. The latest zero

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4.1  Optical transmission windows. The curve is for modern fiber with a water attenuation peak at 1383 nm.

water peak (ZWP) fibers manufactured using vapor-phase axial deposition remove the attenuation peak at 1383 nm due to optical absorption and scattering by water ions. This opens up a very broad wavelength window spanning 1285–625 nm with an attenuation constant of ≤0.33 dB/km at 1310 nm, ≤0.31 dB/ km at 1383 nm and ≤0.19 dB/km at 1550 nm (PureBand® Zero Water Peak Fiber Specification, 2008, Sumitomo Electric Lightwave Corp., Research Triangle Park, NC, USA). Commensurate with the wavelength windows identified above, optical fibers have an enormous potential transmission capacity. The amount of information that can be transmitted is directly related to the frequency range over which the carrier operates. An increase in the carrier frequency theoretically increases the transmission bandwidth. Referring to Fig. 4.1, it is seen that both the medium and long wavelength bands exhibit very low loss, around 0.4 dB/km for the second window (medium wavelength, 1250–350 nm) and 0.2 dB/km for the third window (long wavelength, 1450–600 nm). The useful wavelength range is therefore about 250 nm. Expressed in terms of analogue bandwidth, a 1 nm waveband translates to a bandwidth of 178 GHz at 1300 nm and 133 GHz at 1500 nm. Thus, optical fibers have a total usable bandwidth of approximately 30 THz. The information-carrying capacity depends on the modulation technique used. Assuming the widely used on–off keying format is employed, which has a maximum theoretical bandwidth efficiency of 1 bps/Hz, one can expect a digital bandwidth of 30 Tbit/s if fiber non-idealities are ignored. The removal of the water absorption peaks by ZWP fibers serves to further increase this already phenomenal figure.

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4.2.2 Contemporary networks Given the immense potential of optical fibers, it comes as no surprise that they are predominantly replacing copper as the transmission medium of choice, vastly increasing single-link bandwidth in the process. As shown in Fig. 4.2, the past decade has witnessed a networking paradigm shift from connection-oriented communication to high-bandwidth IP-centric packet-switched data traffic, driven by the influx of high-bandwidth applications (Cisco Systems Inc., 2008). Home entertainment applications such as HDTV are pushing the capabilities of current cable network technologies, video conferencing applications are challenging current commercial network technologies and the growing complexity of commercial and military aircraft, including multifaceted sensor arrays and highdefinition flight displays, is pushing avionics networks to their limit. The availability of such applications relies heavily upon the ability to transport data in a fast and reliable manner without significantly increasing operating and ownership costs. As these applications are pushing current network technologies to their capacity limits, researchers are being forced to create high-speed networks capable of supporting the varied bit-rates, protocols and formats required by these applications in a highly scalable manner. A communications network is essentially an arrangement of physical links in which messages may be passed from one part of a network to another using either a single or multiple links. As modern networks continue to evolve in both size and complexity, new technologies have emerged to facilitate the most basic networking Internet traffic (TB/month)

14000000 12000000 P2P Web, E-Mail Gaming Video to PC Video comms VolP IPTV

10000000 8000000 6000000 4000000 2000000 0

2005 2006 2007 2008 2009 2010 2011

4.2  Forecasted growth of global IP traffic.

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functions to efficiently utilize the potential of optical fibers – routing, switching and multiplexing. Wavelength division multiplexing (WDM) WDM technology was initially developed to increase the capacity of point-topoint fiber links. WDM-enabled networks allow multiple opaque point-to-point connections to be established where the optical signal must undergo opticalelectronic-optical (OEO) conversion at each intermediate node in the network. Network designers are able to occupy multiple wavelengths leading to increased bandwidth and fault tolerance while decreasing congestion and blocking. A pressing concern in current commercial WDM implementations is the lack of network transparency. A network is considered opaque if it requires its constituent nodes to be aware of the underlying packet format and bit-rate. The need to handle data streams in the electrical domain with respect to the aforementioned factors engenders a large optical-electronic bandwidth mismatch. The bandwidth on a single wavelength is 10 Gbps today and is likely to increase to 100 Gbps in the near future. The enabling technologies for electronic processing of data at such high speeds are both costly and underdeveloped. Advances are being made to make electronic switches more scalable by adding additional ports to the switching fabric. However, electronic switches will still be hard-pressed to keep pace with the optical data rate as it continues spiraling upwards. An additional concern associated with the requirement of high-speed electronics is the prohibitive cost of infrastructure upgrades. For instance, if a legacy switch operating at 2.5 Gbps is present anywhere in the core network, all data passing through that switch is limited to data rate of 2.5 Gbps. Thus, any network upgrade requires the replacement of all legacy equipment, which is termed a ‘forklift upgrade’ in industry involving the massive overhaul of existing infrastructure. All optical networks avoid this problem in that data rate is only limited by the capabilities of the end stations. Thus, upgrading a connection does not require an upgrade in the core network and this enables metro operators to scale their networks to meet customer requirements and enhance their services. Finally, current WDM implementations lack sub-wavelength granularity. Once a lightpath is established the entire wavelength is used exclusively by the connection’s source–destination pair; no sub-wavelength sharing between nodes along the lightpath is allowed. Given the bursty and highly variable nature of IP traffic, under-utilization of wavelength capacity tends to occur unless the source and destination nodes efficiently aggregate traffic.

4.2.3 Next-generation networks With the advancement of device implementation technologies such as optical cross-connects and micro-electro-mechanical systems, it is possible to design

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transparent optical networks in which optical signals on an arriving wavelength can be switched to an output link of the same wavelength without conversion to the electronic domain. Signals on these all-optical lightpaths can be of different bit-rates and formats as they are never terminated inside the core network. This bit-rate, format and protocol transparency are of vital importance in nextgeneration optical networks.

4.3

All-optical switches

Researchers have been exploring different ways of supplanting the electronic switch fabrics present in current commercial optical networks. Successful all-optical switching technologies should ideally demonstrate superiority in power consumption, scalability, insertion loss (IL), polarization-dependent loss (PDL), wavelength dependency, switching speed and crosstalk. The technologies discussed below have individual niche areas, and it is highly likely that they will co-exist on networks as each type represents different engineering trade-offs. The main contemporary switch technologies are micro-electro-mechanical systems (MEMS), acousto-optical (AO), electro-optical (EO), thermo-optical (TO) and magneto-optical (MO). MEMS switches are either free space (membranes, micro-mirrors) or based on planar moving waveguides that redirect light beams to the desired output port (Fan et al., 2002; Huang and Shen, 2006; Ji et al., 2004; Lin et al., 1999; Patterson et al., 2002; Ryf et al., 2001; Yano et al., 2005). An example of a 2D MEMS switch is shown in Fig. 4.3. They also vary in the actuation mechanism used – electrostatic vs. magnetostatic, latching vs. non-latching. Free-space variants suffer from higher ILs due to beam divergence (~3 dB), slower switching times (ms),

Mirrors

Outn Outn–1 Output fibers Out1

Lenses

Inn

In1

Input fibers

4.3  Example of a crossbar 2D MEMS switch.

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Acoustic absorber Incident light

Diffracted light

Ultrasonic waves Acoustic transducer

4.4  Creation of diffraction grating by ultrasonic waves.

high actuation voltage/current requirements and higher power dissipation for nonlatching configurations (~80 mW). Waveguide variants offer faster switching times (100 ns) and lower ILs (~1 dB) at the cost of higher crosstalk (~-30 dB). AO switches are based on ultrasonic waves traveling within a crystal or planar waveguide that deflect light from one path to another (Aubin et al., 2004; Birks et al., 1996; Koh et al., 1998; Park et al., 2001) as illustrated in Fig. 4.4. When a mechanical vibration is present in a material it causes regular zones of compression and tension within the material. In most materials this causes changes in the refractive index. This periodic pattern of refractive index changes forms a diffraction grating that causes the incoming light to be diffracted. Control of the ultrasonic wave amplitude and frequency enables control of the amount and wavelength of light that is diffracted. AO switches are able to handle high power levels and offer reasonable ILs (~3 dB) and switching times (~40 µs) but suffer from poor isolation (~-20 dB) and power efficiency as well as inherent wavelength dependency. EO switches are among the most mature available and have been implemented using semiconductor optical amplifiers (SOAs), LiNbO3, liquid crystal, electroholography and switchable waveguide Bragg gratings (d’Alessandro and Asquini, 2003; Ertel et al., 2006; Fatalocchi et al., 2005; Kondo et al., 1982; Silberberg et al., 1987; Yuan et al., 2004). An EO modulator using a LiNbO3 crystal to impart a change in the refractive index of the material that varies linearly with field strength is shown in Fig. 4.5. Depending on the variant, they offer ILs ranging from < 1 dB to 10 dB, switching times from 10 ns to 1 ms and isolations of -10 dB to -40 dB. However, the majority of them have a strong wavelength dependency and those that do not are typically subject to higher ILs. SOA-based switches also potentially suffer from a limited dynamic range.

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Optical switches Vertical polarizer

Electrical contacts

Horizontal polarizer

Electro-optic material

4.5  EO modulator using LiNbO3 crystal. Output 1 h1

Electrode

Output 2

h2

θ

h1>h1 Waveguide Input

4.6  Digital thermo-optical waveguide switch.

TO switches are based on either the thermal behavior of materials or the waveguide thermo-optic effect (Espinola et al., 2003; Kim et al., 2004; Wang et al., 2006; Yamagata et al., 2005; Zhong et al., 2007). Interferometric TO switches heat the material in one of the interferometer legs to generate a phase shift relative to the other leg, leading to interference effects between the two light beams when they are recombined. Digital TO switches generally utilize the interaction of two silica waveguides on silicon as shown in Fig. 4.6. Heating the material changes the refractive index of the waveguide, imparting a phase difference and thereby altering the selectivity of the output ports. While having excellent PDL, they typically consume more power due to the heating process (~70 mW) and have a slow switching time (~10 ms). Several comprehensive reviews of optical switching technologies and commercially available devices are given in Jajszczyk (2005), Ma and Kuo (2003) and Papadimitriou et al. (2003).

4.4

Magneto-optical switches

MO switches are based on the Faraday rotation of polarized light when it passes through an MO material in the direction of an applied field. There has not been as much work done investigating these types of switches due to the lack of sufficiently high quality MO materials. Recent advances in bismuth-substituted iron garnets

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and orthoferrites have yielded materials with a high MO figure of merit, giving low ILs, ultrawide bandwidths and a higher degree of rotation for less applied field.

4.5

Theory and principles of magneto-optical switches

To understand Faraday rotation which affects the state of polarization (SOP) of an electromagnetic (EM) wave, we first define the polarization of an EM wave. A brief explanation of the relationship between the SOP and magnitude as well as relative phase of the components of the EM wave is first given. Next the theoretical background and overview for the existence of domains in a magnetically structured material such as ferro and ferrimagnetic materials is discussed. This is followed by an overview of the theory of Faraday rotation.

4.5.1 Polarization Polarization is a property ascribed to EM waves that describes the orientation of their field vectors. Thus, a study of the polarization of light denotes an investigation into how the field vector associated with the wave temporally evolves at a fixed point of space. If this evolution is the same in every point of space, the field is said to be polarized. The standard convention is to only consider the electric field vector as the magnetic field vector is both proportional and perpendicular to it. As a 3D object, an EM wave can be considered to be a superposition of its two orthogonal components orientated in a plane perpendicular to the propagation direction (Pedrotti et al., 2006). If propagation in the z direction is assumed, the single frequency (or time harmonic) wave can be expressed as:

[4.1]

These components are oscillating in time with the same frequency. Depending on the X and Y component magnitudes and the value of the relative phase shift φ between the two orthogonal components, the propagating wave is said to be either linearly, circularly or elliptically polarized. A linear polarization results if the two components are in phase, i.e. φ = 0° ± n180°. They add at every plane to give an electric vector that has a fixed direction determined by the relative amplitudes of the components. The designation linear stems from the fact that the electric vector maintains its direction in space. A circular polarization results if the relative phase is an odd multiple of 90°, i.e. φ = 90° ± n180° and the component amplitudes are equal. Looking in the direction of propagation, φ < 0° leads to a clockwise sense of circular polarization, which is also termed right circularly polarized (RCP). Conversely, φ > 0° leads to a counter-clockwise sense of circular polarization, which is also termed left circularly polarized (LCP). The most general case, which occurs for all other combinations of component amplitudes and relative phase, is an elliptical polarization. Figure 4.7 illustrates the

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4.7  Examples of Lissajous figures for an amplitude ratio of 0.75 and various relative phases.

influence of the relative phase on the resulting Lissajous figures for fixed, unequal amplitudes. Linear and elliptical polarizations are shown. Circular polarization would have occurred when φ = 90° if the component amplitudes were equal. Pragmatically speaking, changing the polarization of an EM wave is an indirect method of controlling the relative phase of its constituent orthogonal components. One method of achieving this is via the exploitation of the Faraday effect in MO materials.

4.5.2 Faraday effect The Faraday effect was first experimentally observed by Michael Faraday in 1845 in a piece of glass placed between the poles of the magnet where the plane of polarization of light was rotated (Krauss and Carver, 1973). This was the very first MO effect to be discovered. The microscopic origin of the MO effects is the interaction between light passing through or reflecting from a medium and the electron spin due to the spin–orbit coupling. It is manifested in a different response of the electrons to LCP and RCP light. This is further detailed in Kahn et al. (1969). The macroscopic theory of MO effects originates from the use of Maxwell’s equations for material media, which are approximate in that spatial averages are taken over volume elements that are large compared to the inter-atomic dimensions (Lorentz, 1916; Van Vleck, 1932). The propagation of EM waves through an

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optically isotropic insulator is described by the solutions of the Maxwell equations and use of the constitutive relations:

[4.2]



[4.3]



[4.4]



[4.5]

From a mathematical point of view, MO effects could be derived from either a permeability or permittivity tensor. However, due to the inertia of the magnetization process at optical frequencies, the gyromagnetic effects described by the permeability tensor are negligible, i.e. µ is very close to unity and assumed to be a simple scalar (Landau and Lifschitz, 1984). Thus, ε now becomes a second-rank tensor containing both the gyrotropic and birefringent properties of the material:



[4.6]

where the direction of either the applied magnetic field or material magnetization is taken as the z-axis. Generally speaking, all the permittivity tensor elements are complex and completely describe the optical behavior of the material; the imaginary portion describing its absorption behavior and the real portion describing the effect of such behavior on the refractive index. The diagonal elements (εxx, εyy and εzz) are usually weak functions of the applied magnetic field (or magnetization) and assumed to be constants. Thus, they represent the optical properties of the material in the absence of MO effects. The justification for treating the diagonal elements as constants stems from their origin due to small distortions in cubic symmetry. The symmetry perturbations induce additional birefringence, which has the effect of making the material biaxial and this is phenomenologically described by changes to the diagonal tensor elements. However, this birefringence is small and can usually be ignored, depending on the specific material being considered. Conversely, the off-diagonal elements have a first-order linear dependence on the applied magnetic field (or magnetization) and are the source of the majority of MO effects that are considered in switch design. Depending on the boundary conditions set by the specific application, the Maxwell equations combined with (4.6) describe the transmission and reflection characteristics of the material (Hunt, 1967). The influence on the intensity and polarization of light impinging on the material constitutes the MO effects and are classified as either the Faraday effect (light transmission) or Kerr effect (light

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reflection). As a first step to obtaining the magnitude of the MO effects, the plane wave solutions to Maxwell equations using the permittivity tensor as defined in (4.6) are desired. The solutions can be expressed as specific polarization states (eigenmodes) that will propagate through the MO material unchanged and the eigenvalues are the effective refractive indices for these polarizations. In the case of the rare earth iron garnets, the light is assumed to propagate in a direction parallel to the applied magnetic field, which forms the polar geometry of the Faraday effect as shown in Fig. 4.8. Additionally, the cubic symmetry perturbations mentioned earlier are neglected and it is assumed that εxx = εyy. Taken together, these assumptions imply that the garnets are being treated as having a single optical axis and that this axis coincides with both the direction of the applied magnetic field and the light propagation vector. The eigenmodes for this case are the LCP and RCP states with complex effective refractive indices given by:

[4.7]

where n+ and n– are the effective refractive indices for the RCP and LCP states, respectively. Faraday rotation arises from the real (dispersive) component of the difference in n+ and n–, where a phase difference develops between the RCP and LCP components of linearly polarized light that is transmitted through the MO material. This manifests itself as a rotation of the linear polarization by the Faraday rotation angle θF with the overall process illustrated in Fig. 4.9.

X Linearly polarized light Z Y M

4.8  Polar geometry of the Faraday effect. Propagation through material =

+

+

4.9  Illustration of the Faraday rotation mechanism.

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For light having a wavelength λ and an MO material of thickness t, the Faraday rotation is defined as:

[4.8]

where positive rotation is defined according to the right-hand rule as shown in Fig. 4.10. This rotation is non-reciprocal, which distinguishes it from optical activity. The sign of the rotation angle (with respect to a fixed reference frame) does not change when the direction of propagation is reversed stemming from a sign change of the off-diagonal elements in (4.6). This means that linearly polarized light making two passes in opposite directions accumulates 2θF of rotation rather returning to its incident state, which is the principle relied on in optical isolators. Although only the Faraday rotation is of interest as the working basis of an alloptical switch, it should be mentioned that the imaginary (absorptive) components of the difference delinearize the polarization of the incident light due to differing degrees of absorption for the RCP and LCP components. This manifests itself as Faraday ellipticity, which is defined as the imaginary components of the expression in (4.8) and illustrated in Fig. 4.11. Given garnets with sufficiently high MO figures of merit, they can be assumed sufficiently transparent that the absorptive components can be reasonably ignored. Additionally the analysis above neglects reflections at the MO material boundaries, which has a similar effect as the optical loss due to material absorption (Grzegorczyk and Kong, 2005). Finally, different geometries give rise to the other MO effects such as the Kerr effect, magnetic linear birefringence and magnetic circular dichroism.

y x

z

H

4.10  Right-hand rule for Faraday rotation, defined as the electric field vector rotation of light propagation in the positive z direction.

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+

+

=

4.11  Illustration of the Faraday ellipticity mechanism.

Phenomenological model In this model a magnetic material is replaced by an array of magnetic dipoles that are aligned to each other forming the magnetic structure of the material as shown in Fig. 4.12 (see Balanis, 1989 or Collin, 1966 for a more detailed development). When an external field is applied the dipoles turn in the direction of the field. Similar to the precession of a top in the gravitational field, an isolated single magnetic dipole rotates with a precession frequency, known as the Larmor precession frequency, in the presence of an external magnetic field. Like the torque exerted by the external r gravitational field on a spinning top, an r magnetic field exerts a torque (T ) on a single magnetic dipole moment m given as: [4.9] r r r r where m = nIds, H0 is the applied magnetic field and B0 is the applied magnetic flux density.rThis torque causes the dipole to precess around the z-axis, which is parallel to B0 as shown in Fig. 4.13. The magnetic dipole moment of a single electron of mass me moving with a velocity v in a circle of radius a with angular momentum P can be written as:

[4.10]

P = meva

[4.11]

Ba

Ba ni (a)

Ii

(b) dsi

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111

B0 Z

ω0

φ m

+

X P

Y

γ B0 x B = T

4.13  Torque on a single dipole moment m causing it to precess about the applied field, the angular momentum P is directed antiparallel to the dipole moment.

The ratio m/P is the gyromagnetic ratio γ and is equal to:


=

m e = m=
P P 2me

[4.12]

The angular momentum P is anti-parallel to the magnetic dipole m due to the negative electron charge. The rate of change of the angular momentum is equal to the torque; thus,






dP


[4.13] = T = μ0 m  H 0 =
μ0  P  H 0 =
P   0 =  0  P dt µ0|γ| PH0 sin φ = ω0P sin φ = – µ0mH0 sin φ

[4.14]

where ω 0 has the unit of frequency and is termed the Larmor precession frequency. ω0 = |γ|µ0H0 = |γ| B0

[4.15]

When light travels through the material in the presence of B0, the AC magnetic field of the EM signal is superimposed on B0. This AC field perturbs the Larmor precession. Now consider a linearly polarized optical signal propagating throughout the material in the z direction. The AC magnetic field (of theroptical signal) is a superposition of two rcircularly polarized waves denoted by B±1. The r right-hand (CW) polarized field, B1+, and the left-hand (CCW) polarized field, B1–, are directed perpendicular to the z-axis as shown in Fig. 4.14. These fields can be expressed as:
[4.16] B1+ = aˆ x
jaˆ y B1+ e
j z

(

)

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Optical switches


B1
= aˆ x + jaˆ y B1
e
j z

(

)

[4.17]

r The resultant B ±t field is at an angle θ± from the z-axis and is given by (as shown in Fig. 4.15):

 B±   ± = tan
1  1   B0 

[4.18]

r r B±t also rotates about the z-axis in the CW and CCW directions for B+1 and B 1–, respectively. Under the steady-state conditions the magnetic dipole will be forced to precess about the z-axis at the same rate. It follows from (4.13) that:





dP +
+
+
+ = T = m  Bt =
P +  Bt+ =  + aˆ z  P + dt



dP






= T = m  Bt =
 P
 Bt
=

aˆ z  P
dt

[4.19] [4.20]

Or
 P + Bt+ sin ( +
 + ) =  + P + sin  +

[4.21]


 P
Bt
sin (


) = 
P
sin 


[4.22]


 ( Bt+ sin  + ) cos  +
( Bt
cos  + ) sin  +  =  + sin  +

[4.23]


 ( Bt
sin 
) cos 

( Bt
cos
) sin  +  = 
sin 
With reference to Fig. 4.15, we can write the following:

[4.24]

Or

B+t sin θ + = B+1

[4.25]

B+t cos

[4.26]

θ+

= B0 x

+ B 1 cos

ωt

+

B1

+

x

ωt

ωt

pa

Pro

B 1 sin ωt

on

ti ga

dir

z

y

–

B1

dir

– B 1 cos ωt gation z

pa

Pro

y

–

B 1 sin ωt

(a)

(b)

4.14  Rotation of magnetic field as a function of time, for (a) clockwise and (b) counter-clockwise polarizations.

© Woodhead Publishing Limited, 2010



Magneto-optical switches +

z

B1

B0

Bi

θ+ φ

–

z

+

B1

m– φ–

B0

m+

–

B1

θ– y

y

x

ω+

P+

113

x

P–

ω–

(b)

(a)

4.15  Precession of a spinning electron caused by applied magnetic field (a) clockwise and (b) counter-clockwise.

Bt– sin θ – = B1–

[4.27]

Bt– cos θ – = B0

[4.28]

which can be reduced to: tan  + =

 B1+  B1+ =  B0
 +  0
 +

[4.29]

tan 
=

 B1
 B1
=  B0 + 
 0



[4.30]

Thus, components of m± that rotate in sync with B±1 are: mt+ = m0+ tan  + =

m0+  B1+ 0
 +

[4.31]

mt+ = m0
tan 
=

m0
 B1
0 + 


[4.32]

Consider N orbiting electrons per unit volume and uniform density, the total magnetization is M = Nm. B is related to the magnetic field intensity H and the magnetization vector as:

 Nm0+
B1+ 
=
μe+ H1+ B + = μ0 ( H1+ + M 1+ ) = μ0 ( H 1+ + Nmt+ ) = μ0  H1+  0 –  +  

[4.33]

 Nm0
g B1

=
me
H1
[4.34] B
= m0 ( H1
+ M 1
) = m0 ( H1
+ Nmt
) = m0  H1
w0 + w
 

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Optical switches

Thus, µ+e and µe– are the effective permeabilities for clockwise and counterclockwise circularly polarized waves, respectively, and are given by:

 μ  M 0+  μe+ = μ0  1+ 0  0
 +  

[4.35]

 μ  M 0
 μe
= μ0  1+ 0  0 + 
 

[4.36]

It is known that the phase velocity and the phase constant are influenced by the permeability; thus, the phases associated with (4.35) and (4.36) will be different. This feature of MO materials such as ferrites is utilized for the design of optical devices. It is important to note that the development above is for historical purposes only; since ω0 is very large at optical frequencies, the bracketed terms in equations (4.35) and (4.36) are ≈1. Atomic model The single-ion model is a good approximation for describing MO properties (Leycuras et al., 1982; Leycuras et al., 1984). Thus, free-ion theory is useful in describing the microscopical origin of MO effects in terms of quantum mechanics (Condon and Shortley, 1959). The Hamiltonian of a free ion can be written as: Hˆ = Hˆ 0 + Hˆ ee
+ Hˆ so

[4.37]

where H0 is the sum of single-particle Hamiltonians with potential energies corresponding to the self-consistent central-field approximation, Hee is the energy related to the electrostatic interaction and Hso is the Hamiltonian describing the spin–orbit interaction. The state of the ion is determined by the distribution of the electrons among the single-electron state, which is in turn determined by the Pauli–Fermi principle and by the minimum energy of the ion. Most MO materials are governed by ions that deal with unfilled 3d and 4f shells. For these the Russell– Saunders coupling (L-S coupling) i.e. the spin–spin and orbit–orbit couplings is the strongest. Here the orbital angular momenta of the electrons are strongly coupled such that states with a different total L and/or different total S have different energy. Thus, the ground configuration, defined by H0, splits into terms with particular values of the total (spin) S and (orbital) L momenta, and the state is described by the wavefunctions |κ; S, L, Ms, ML〉, where κ is the configuration index. The splitting of the configuration κ is shown in Fig. 4.16 where all the important interactions are considered. The interaction of the ion with the external field (the Zeeman interaction) takes the following form:

H z = mB


(l + 2s ) • H i

i

i

© Woodhead Publishing Limited, 2010

[4.38]



Magneto-optical switches

115

K 2S+1L 2S+2L J

H’ee

H0

HSL

HCF

Hex + Hz

4.16  Interactions splitting the configuration κ in the corresponding hierarchy.

where li and si are the angular and spin momenta of the i-th electron, respectively, and the summation is taken over all of the electrons of a particular configuration. Projection of the Zeeman Hamiltonian onto the functional space of the particular term 2S+1L and the multiplet 2S+1LJ provides other approximate forms of the Zeeman Hamiltonian: HZ = mB (L + 2S)•H

[4.39]

HZ = gL µB J•H

[4.40]

where gL is the Lande factor of the ion, which depends on the quantum numbers S, L and J. The Hamiltonian of the Heisenberg exchange between the ions:

H ex =


I S •S ij i

j



[4.41]

ij

where Iij, the exchange integral of the i-th and the j-th ions, is the spin Hamiltonian, which parameterizes the low-energy part of the energy spectrum of the electrostatic interaction in the system of ions. In materials having a substantial concentration of magnetic ions, this interaction accounts for the development of the magnetic order. It is important to note that the exchange field affects only the spin of the ion and is therefore not equivalent to a magnetic field, which also affects the orbital momentum. This difference is substantial for magneto-optics. In a simple model the state of an ion is given by J and M, which are the eigenvalues of the angular momentum operator of the ion and its z projection. A magnetic field removes the degeneracy of the multiplet in M, splitting each J multiplet into 2J + 1 levels. The selection rule for the electric dipole transitions can be easily obtained by taking into account that the angular momentum and its z projection for photons of CW (CCW) polarization are j = 1 and m = +1 (–1), respectively. Thus, the CW (CCW) polarized photon will induce a transition of the ion to the state with ∆ J = ± 1, 0 and ∆M = +(1)(–1). These different transitions result in different dispersion values for the CW and CCW polarized light that gives rise to the Faraday effect.

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116

Optical switches

4.6

Material

The material primarily considered for switch design is bismuth-substituted rare earth iron garnets. The unique properties of garnets that find use in engineering applications originate from their constituent elements and the interactions of these elements in the unit cell (Paoletti, 1978). The general formula for garnets is denoted as X3Y2Z3O12, where X, Y and Z represent ions on dodecahedral, octahedral and tetrahedral sub-lattices, respectively, as shown in Fig. 4.17. Each oxygen ion is surrounded by either X, Y or Z sites and the unit cell is cubic, surrounded by 160 atoms – 96 O, 24 X, 16 Y and 24 Z sites. Naturally occurring garnets are always solid solutions of several different ions in each sub-lattice. Synthetic garnets can be made in an almost stoichiometrically pure form with only one ion species at X, Y or Z sites, respectively, and the number of possible constituents is large. A typical rare earth garnet contains iron and rare earth metal ions generally having the chemical formula RE 33+Fe23+Fe33+O12, although other elements are introduced as dilutants to fine-tune its properties. The rare earth ions are located at the dodecahedral sites and iron in the other two sub-lattices. In the garnet structure the magnetic properties are directly affected by the kinds of ions which occupy a certain sub-lattice. The lattices couple with each other anti-ferromagnetically with the strongest interaction between the octahedral and tetrahedral sites and the weakest between the octahedral and dodecahedral sites. Thus, the total magnetic moment of the garnet is the moment contributed by one iron ion minus that from the rare earth ions.

O2–

Y3+

Fe3+

Fe3+

4.17  Structure of an yttrium iron garnet (Y3Fe5O12) unit cell.

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Magneto-optical switches

117

Additionally, garnets have several easy magnetization axes due to their cubic structure. However, engineering applications call for uniaxial anisotropy perpendicular to a surface of interest. This is achieved by growth anisotropy, which comes about from the phenomenon that rare earths prefer some lattice positions over others depending on material growth direction. The magnetic moments of these rare earths add up and effectively create anisotropy (Eschenfelder, 1980). In principle, the creation of a suitable material for MO applications merely involves choosing the proper species and amount of ions to occupy the different sub-lattices. As alluded to by the derivations in the previous section, MO applications require a high MO figure of merit, which is a measure of efficiency of a Faraday rotator defined as:

MOFM =

 Rotation / Length = F Absorption / Length


[4.42]

This encapsulates the need for a strong specific Faraday rotation at the application wavelength range coupled with low absorption. Early work indicated that the rotation found in garnets is enhanced by the introduction of bismuth and that this does not negatively affect its absorption characteristics for wavelengths longer than 600 nm (Aichele et al., 2003; Takeuchi et al., 1973). Thus, the addition of bismuth is a way of increasing the MO figure of merit, and this is usually done for the commonly used yttrium iron garnets (YIGs), where yttrium is mainly diluted by bismuth to achieve higher rotations – the more bismuth, the stronger the rotation. This material synthesis strategy is not straightforward to implement, and it was first believed that more than two bismuth ions per formula unit of YIG were out of the question due to the large ionic radius of bismuth. This causes the lattice parameter of Bi:YIG to increase with increasing bismuth content and form a nonthermodynamically stable configuration. What makes this strategy possible is the use of synthesis methods that do not require thermodynamic equilibrium as well as substrates with higher lattice parameters. High Faraday rotations (20°/µm) and even completely bismuth-substituted iron garnets using these methods have been reported in the literature (Boudiara et al., 2004; Okuda et al., 1990). However, the non-equilibrium nature of BIG engenders difficulties in scaling up its production to an industrial scale. Bismuth-substituted garnet Faraday rotators are industrially manufactured by at least three firms – Garnetec Ltd. in Russia, GranOpt Co. Ltd. in Akita, Japan (GranOpt Co. Ltd., 2007, http://www.granopt.jp) and Integrated Photonics Inc. in Hillsborough, NJ, USA (Integrated Photonics Inc., 2007, http://www. integratedphotonics.com). Rotators from all three firms are comparable in terms of design wavelengths (1300–600 nm), external field magnitudes (0–700 Oe) and isolation ratios (35 dB). Models requiring either external bias fields or built-in magnetization are available and absorptive losses are of the order of 0.1 dB.

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118

Optical switches

4.7

Characterization of Faraday rotation

In order to analyze a structure consisting of fibers, films, birefringent and magnetic materials, it is helpful to have an analysis structure. Often Jones Calculus is used to do such an analysis. However, standard Jones Calculus does not account for the effect of reflections. The reader may find it useful to peruse the appendices for an introduction to S and T parameters.

4.7.1 Non-reciprocal transmission line Consider a hypothetical transmission line made from a material that has different propagation constants and characteristic impedances for each direction of propagation on the line (Weber, 2009a). Assuming a and b variables as shown in Fig. 4.18, one can define the T parameters as: b  T  a  1 11 T12 2   =     a1   T21 T22   b2 

a  T  2 11 T12   =   b2   T21 T22 


1

b  1    a1 

[4.43]

When normalized to an external characteristic impedance Z 0E, it can be shown that:


( Z 0+
Z 0E ) ( Z 0

Z 0E ) e j z + ( Z 0+ + Z 0E ) ( Z 0
+ Z 0E ) e
j  +

T11 =


z

[4.44]

2 ( Z 0+ + Z 0
) Z 0E

( Z0+
Z0E ) ( Z0
+ Z0E ) e j z
( Z0+
Z0E ) ( Z0
+ Z0E ) e
j z 2 ( Z 0+ + Z 0
) Z 0E


+

T12 =


( Z 0+ + Z0E ) ( Z 0

Z 0E ) e j  z + ( Z 0+ + Z 0E ) ( Z 0

Z 0E ) e
j +

T21 =

[4.45]


z

2 ( Z 0+ + Z0
)

[4.46]

a1CW

b2CW

a1CCW

b2CCW

b1CCW

a2CCW

b1CW

a2CW S

Z–> Zero external length Z=0

Z=0

4.18  Transmission line block labeled in its own a and b variables.

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Magneto-optical switches

( Z0+ + Z0E ) ( Z0
+ Z0E ) e j z
( Z0+
Z0E ) ( Z0

Z0E ) e
j z 2 ( Z 0+ + Z 0
) ( Z +
Z0E ) ( Z0
+ Z0E )
( Z0+
Z0E )( Z0
+ Z0E ) e
j(  +  )z S11 = 0 ( Z0+ + Z0E ) ( Z0
+ Z0E )
( Z0+
Z0E )( Z0

Z0E ) e
j(  +  )z 2Z 0E ( Z 0+ + Z 0
) e
j  z S12 = ( Z0+ + Z0E ) ( Z0
+ Z0E )
( Z0+
Z0E ) ( Z0

Z0E ) e
j( +  )z 2Z 0E ( Z 0+ + Z 0
) e
j  z S21 = ( Z0+ + Z0E ) ( Z0
+ Z0E )
( Z0+
Z0E ) ( Z0

Z0E ) e
j( +  )z ( Z

Z0E ) ( Z0+ + Z0E )
( Z0

Z0E ) ( Z0+ + Z0E ) e
j(  +  )z S22 = 0 ( Z0+ + Z0E ) ( Z0
+ Z0E )
( Z0+
Z0E ) ( Z0

Z0E ) e
j(  +  )z

119




+

T22 =

[4.47]

+




+




[4.48]

+




[4.49]

+




[4.50]

+




+







+

[4.51]

are the transmission parameters and scattering parameters for a non-reciprocal transmission line where Z 0+ and Z 0– are the characteristic impedances and β+ and β– are the propagation constants for waves in the positive direction and negative direction, respectively. Notice that normalization to one or the other of the characteristic impedances does not minimize the complexity of the equations. One could develop a scattering matrix normalized to these individual impedances but that matrix could not be converted to a transmission matrix to be used for cascading since the adjacent port impedances would not be the same. The differences between the characteristic impedances might be small. The differences between the propagation constants would then also be small but the accumulated phase shift difference is important. Note that if the direction of bias magnetism is changed S12 becomes S21 and S22 becomes S11 and vice versa.

4.7.2 Wave formulation for non-reciprocal medium One might attempt to directly extend the formulas for a non-reciprocal transmission line for use with plane wave EM fields (Weber, 2009b). However, in plane wave characterizations, there are two orthogonal axes for the electric field. For a particular device under test (DUT), there might also be coupling between the two orthogonal electric fields at the ports. The following analysis assumes that the waves in the fiber and in the material are plane waves. In reality, the waves would be Gaussian profile waves. However, the guiding behavior of fibers keeps the wave profile from changing allowing one to use a plane wave as a first approximation. The longitudinal distance through the magnetic material is assumed to be very short so that the wave profile does not substantially change. In addition, it is assumed that the modes generated in the interface between the magnetic material and a fiber do not propagate down the fiber. Those modes are

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Optical switches

assumed to be insignificant in the following analysis. For physical arrangements where the higher order modes are appreciable, the formalism would need to be extended for higher order modes. A superposition of circularly polarized waves (a clockwise and a counter-clockwise circularly polarized wave) will be used to describe wave propagation in the ferrite material and in the interconnecting fibers since in the anisotropic magnetic medium, linearly polarized waves do not exist. The exciting waves for the medium would often be linear polarized waves. Rectangular linear polarized plane waves can be considered as a superposition of two circularly polarized plane waves. Waves which have frequencies and phases described with non-random variables, would be in general elliptically polarized. Elliptically polarized waves can be decomposed into a superposition of a clockwise and a counter-clockwise circularly polarized traveling wave. Assuming that the waves under consideration consist only of plane waves traveling in the z direction, there are four different circularly polarized (eight linearly polarized) waves traveling in the magnetic region. An assumption is made that the modes under consideration are ‘plane’ in the central limit such as the lowest order Gaussian profile waves in weakly guiding fibers. In magnetic material, the two forward waves travel with different phase velocities and different characteristic impedances. There are also two reverse traveling waves with different phase velocities and different characteristic impedances. The characteristic impedance and phase velocity of a clockwise wave traveling in the forward direction are the same as the characteristic impedance and phase velocity of a counter-clockwise wave traveling in the reverse direction. An analysis of the electro-magnetic waves via Maxwell’s equations shows that plane waves entering from an isotropic magnetic material into a magnetic material generate circularly polarized waves in the magnetic material with the coupling taking place at the boundary. There are many possible formulations for the superposition of the waves in each region. For port-to-port phase delay calculations, the input boundary is considered to be at z = 0. The output boundary is also considered to be at z = 0 with the DUT being described as inserted into a fiber at z = 0 as shown in Fig. 4.19. Let the four composite waves shown be described as given below. The coupling of the waves at the input will be described in terms of the continuity of tangential electric and magnetic fields. The characteristic impedance and propagation constants of the material have a subscript 1 for forward CW waves and reverse CCW waves. Likewise the characteristic impedance and propagation constants of the material have a subscript 2 for forward CCW waves and reverse CW waves. Waves in the material will have subscripts 1, 2, 3 and 4. A four-port transmission matrix will be formed. The input physical port will consist of two EM ports (one for each of the two orthogonal circularly polarized waves), and the output physical port will consist of two EM ports. The normalization impedance for the tranmission matrix will be chosen as the fiber material impedance. It is necessary to use the same normalization impedance on both ports to allow cascading of the matrices

© Woodhead Publishing Limited, 2010



Magneto-optical switches + ECW1M

+ ECW1 = E5 (ax– jay )

+ ECW 2 = E9 (ax– jay )

+ + ECCW1M ECCW2M

+ ECCW1 = E6 (ax+ jay ) – ECCW1 = E7 (ax– – ECW1 = E8 (ax+

+ ECW2M

– ECW1M – ECCW1M

jay ) jay )

121

+ ECCW 2 = E10 (ax+ jay )

– ECW2M – ECCW2M

– ECCW 2 = E11 (ax– jay ) – ECW 2 = E12 (ax+ jay )

T Z–> Zero external length

Z=0

Z=0

4.19  Notation for port-to-port phase delay calculations.

for analysis. Throughout this analysis, characteristic impedance will be used to mean material impedivity. The tangential electric and magnetic field values at the input boundary for the first 8 of the 16 waves are: + ECW 1M = E1 (a x
jay )

+ H CW 1M =

E1 ( jax + ay ) 1



+ ECCW 1M = E2 (ax + ja y )

+ H CCW 1M =

E2 (
jax + ay ) 2


ECCW 1M = E 3 (a x
jay )


H CCW1M =

E3 (
jax
ay ) 1


ECW 1M = E4 (a x + jay )


H CW 1M =

E4 ( jax
ay ) 2

+ ECW 1 = E5 (a x
ja y )

+ ECCW 1 = E6 (a x + jay )

+ H CW 1=

E5 ( jax + ay ) f


H CCW 1=


ECW 1 = E8 (a x + jay )


H CW 1=



[4.53]



[4.54]

[4.55]

[4.56]

E6 (
jax + ay ) f

[4.57]

E7 (
jax
ay ) f

[4.58]

E8 ( jax
ay ) f

[4.59]

+ H CCW 1=


ECCW 1 = E 7 (a x
jay )

[4.52]

r where unit vectors a are in the x or y direction and ηf = fiber material characteristic impedance η 1 = magnetic material characteristic impedance in the forward and CW direction η2 = magnetic material characteristic impedance in the forward and CCW direction.

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Optical switches

Applying the boundary conditions at the input gives the following result: E  1   1  E2   E  = 2 f  3 E   4

  +f  1  0   
1
 f   0 

(

(


1
 f

0

)

0

(

2 +  f

0

0

1 +  f

0

0

2 +  f

) (


2
 f

)


2
 f

)

        

E  5    E6  E   7 E   8

[4.60]

The tangential electric and magnetic field values at the output boundary for the second 8 of the 16 waves are: E1
j 1 L + + ECW H CW ( jax + ay ) e
j1L [4.61] 2 M = E1 (a x
jay ) e 2M = 1
j 2 L + ECCW 2 M = E2 (a x + jay ) e

j 1 L
ECCW 2 M = E 3 (a x
jay ) e

E2 (
jax + ay ) e
j 2 L 2

[4.62]

E3 (
jax
ay ) e j 1 L 1

[4.63]

+ H CCW 2M =


H CCW 2M =

j 2 L
ECW 2 M = E4 (a x + jay ) e


H CW 2M =

E4 ( jax
ay ) e j2 L 2

E9 ( jax + ay ) f

+ ECW 2 = E9 (a x
jay )

+ H CW 2 =

+ ECCW 2 = E10 (a x + jay )

+ H CCW 2 =

E10 (
jax + ay ) f


ECCW 2 = E11 (a x
jay )


H CCW 2 =

E11 (
jax
ay ) f


ECW 2 = E12 (a x + jay )


H CW 2 =



[4.64]

[4.65]

[4.66]



[4.67]

E12 ( jax
ay ) f

[4.68]

where L is the length of the magnetic material. Applying the boundary conditions at the output port gives the following result: E  1   1  E2   E  = 2 f  3 E   4

   +  f e+ j 1 L 0
1
 f e+ j 1L 0  1   + j2 L + j 2 L  0  +  e 0

 e   2 f 2 f  
j 1 L
j 1 L 0 1 +  f e 0 
1
 f e   
j  L
j  L  0
2
 f e 2 0 2 +  f e 2  

(

)

(

(

(

)

)

(

)

(

(

)

)

)

(

)

 E  9    E10  E   11  E   12

[4.69]

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Magneto-optical switches

123

The relationship between the input waves and output waves using forward T parameters is:



 E E   5  9  E 
T  E    6   =  ( UL )(TUR )    10   [4.70]       
E 7    (TLL ) (TLR )  
E11         E8     E12     1 +  f  (TUL ) =    

(

)

2

(

e j1 L
1
 f

2

)(

)(

2

)

+f

)

2

(

e j2 L
2
 f

)

2

e
j  2 L

4 f 2

 2 
 2f e j1 L
e
j 1 L  1  4 f 1 =    0  

 
1
 f   =   

0

(


12
 2f e j1 L
e
j 1 L   4 f 1 =   0  

(

(TLR )

e
j1 L

0

(

(TLL )

2

4 f 1

(

(TUR )

)

(

e j 1 L + 1 +  f

)

2

)

0

(

)(


22
 2f e j2 L
e
j 2 L 4 f 2

)

0

(

2 2

)(


 2f e j 2 L
e
j 2 L 4 f 2

e
j1L

      

(


2
 f

)

2

(

e j2 L + 2 +  f 4 f 2

)

2

[4.71]

[4.72]

      

[4.73]

0

4 f 1 0

)

)

       

e
j  2 L

       

[4.74]

When there are terminations on the output port and the source is on the input:

 E   
1 E5 9  = (TUL )     E10   E6 

[4.75]

E   
1 E5 7  = (TLL ) (TUL )     E8   E6 

[4.76]

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Optical switches

When there are terminations on the input port and the source is on the output:

 E  E 
1 9 11  =
(TUL ) (TUR )     E10   E12 

[4.77]

E 
1 7  =
(TLL )(TUL ) (TUR ) + (TLR )   E8 

(



  12  

)  EE

11

[4.78]

The relationship between the input waves and output waves using reverse T parameters is:   E   9  E   T    10   =  ( R
UL )(TR
UR )        E11    (TR
LL ) (TR
LR )      E12  

  E  5 E    6       E7       E8  

[4.79]

[4.80]

(TR
UR )


12
 2f e
j1 L
e j1 L  0  4 f 1 = 
22
 2f e
j2 L
e j 2 L  0  4 f 2 

(TR
LL )

 2 
 2f e
j 1 L
e j 1 L  1  4 f 1 =    0  

(

)(

)

(

(

)(

)(

)

)

0

(

2 2

)(


 2f e
j 2 L
e j2 L 4 f 2

)

      

[4.81]

      

[4.82]

  22 22

j j LL j j LL  

1 1

f f ee 1 1 ++ 1 1++f f ee 1 1 00   44f   f 11   (T(TR
R
LRLR) )==  22 22

2 2

f f ee

j j 2 L2 L++ 2 2++f f eejj2 L2 L  00   44f     f 22

((

))

((

))

((

))

((

))

[4.83]

© Woodhead Publishing Limited, 2010



Magneto-optical switches

125

When there are terminations on the output port and the source is on the input:

E  E 
1 7 5  =
(TR
LR ) (TR
LL )     E8   E6   E  9  =   E10 

( (T

[4.84]

E 

R
UL

)
(TR
UR ) (TR
LR )
1 (TR
LL ))  E5  

6



[4.85]

When there are terminations on the input port and the source is on the output:

 E  
1 E11 7  = (TR
LR )     E8   E12 

[4.86]

  E  
1 E11 9  = (TR
UR ) (TR
LR )     E10   E12 

[4.87]

The scattering matrix for an x-axis polarized wave can be determined from adding the appropriate wave pairs. Likewise the scattering matrix for a y-axis polarized wave can be determined from subtracting the appropriate wave pairs. However, since a single-axis-oriented input can give a dual-axis output, the concept of the scattering matrix would need to be extended based on its use similar to the concept of common mode and differential mode scattering matrices. Likewise, a single-axis-oriented input can give reflections back from the input on both the x-axis and y-axis. Using a and b variables from scattering parameter notation, the (T) matrix can be used to relate these variables. However, an additional comment is necessary. In the scattering parameter notation, a variables represent quantities incident on a port while b variables represent quantities reflected from a port. Since the preceding analysis has been developed using modes and direction, it is necessary to point out that the top part of sub-matrices represent one mode pair and the bottom part of sub-matrices represent the other mode pair. Therefore some a and/or b variables will have opposite subscripts (CW and CCW) because some of them are in the positive z direction and some in the negative z direction. Three different formulations for the multi-mode scattering matrix are given below:   b   1CCW      b1CW    (TLL )(TUL )
1 (TLR )
(TLL ) (TUL )
1 (TUR )      =  
1
1
(TUL ) (TUR )  (TUL )    b2CW     b     2CCW  

  a   1CW     a1CCW         a2CCW      a   2CW  

© Woodhead Publishing Limited, 2010

[4.88]

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Optical switches

  b   1CCW  
1    b1CW  
(TR
LR ) (TR
LL )  (TR
LR )
1    =   
1
1   (TR
UL )
(TR
UR ) (TR
LR ) (TR
LL )(TR
UR ) (TR
LR )    b2CW     b    2CCW  

  b   1CCW  
1
1    b1CW   
T    ( R
LR ) (TR
LL ) (TR
LR )     =  
1
1
(TUL ) (TUR )  (TUL )   b2CW       b   2CCW     b   1CCW  
1   b1CW    T    ( LL ) (TUL ) (TR
LR )
1     =  
1
1
(TUL ) (TUR )    b2CW    (TUL )    b   2CCW  

(

22

)

44  ff  11

(

ee jj11LL

 11

  ff

[4.90]

  a   1CW     a1CCW         a2CCW      a   2CW  

[4.91]

  a   1CW     a1CCW         a  2CCW     a   2CW  

[4.92]

00

22 e

jj11LL

)

[4.89]

  a   1CW     a1CCW         a  2CCW     a   2CW  

b    1CCW  
1   b1CW   
T    ( R
LR ) (TR
LL ) (TR
LR )
1     =     b (TUL )
1 (TR
UR ) (TR
LR )
1     2CW    b   2CCW       +  1 +  ff
1
1 =   1 (( SS2121 )) == ((TTUL ) = )  UL   

  a   1CW     a1CCW         a2CCW      a    2CW  

e

00

(

22

+ +  ff

44  ff  22

22

)

(

ee jj22 LL

 22

  ff

22

jj  22 L L

) ee

      

[4.93]  12
 2f e j 1 L
e
j1L  2 2  j 1 L
1
 f e
j 1 L  1 +  f e
1 ( S11 ) = (TLL ) (TUL ) =   0   

(

(

)(

)

)

(

0

)

(

)(


 2f e j 2 L
e
j 2 L

2 2

(

2

+ f

)

2

(

e j2 L
2
 f

2

)

) e
j  2 L

       

[4.94]

© Woodhead Publishing Limited, 2010



Magneto-optical switches  12
 2f e j1 L
e
j 1 L  2 2  j 1L
1
 f e
j1 L  1 +  f e =   0   

(


1

( S22 ) =
(TUL ) (TUR )

(

)(

)

)

(

127

0

)

(

2 2

(

2

+f

)(


 2f e j2 L
e
j2 L

)

2

e

j 2 L

(


2
 f

)

2

) e
j 2 L

       

[4.95] The scattering parameter matrix of a non-reciprocal transmission line with two different circularly polarized waves is given below. Notice that transmission is between the same senses of polarization but reflection is between opposite senses of polarization.

b    1CCW     b1CW  
S  ( )(S )    11 12  =     
b2CW    ( S21 )( S22 )      b2CCW    12
 2f e j 1 L
e
j1 L  2 2  j 1 L
1
 f e
j 1 L  1 +  f e =  0   

(

( S11 )

( S12 )

( S21 )

)(

(

(

    + 1 f =     

(

(

)

(

2 2

(

2

+f

)(


 2f e j2 L
e
j2 L 2

)

(

e j 2 L
2
 f

4 f 1

)

2

(

e j1L
1
 f

)

2

(

+ f

)

2

(

e j1L
1
 f

)

2

(

)

2

e
j  2 L

0

e
j1 L

4 f 2

0

(

2

)(

)

e
j  2 L

e j2 L
2
 f

4 f 1

)

2

4 f 2 2

2

)

)

       

0

e
j1 L

0

(

( S22 )

0

 12
 2f e j 1L
e
j1 L  2 2  j 1 L
1
 f e
j 1 L  1 +  f e =  0   

(

[4.96]

)

)

    + 1 f =     



a    1CW     a1CCW       
a2CCW       a2CW  

+ f

)

2

(

e j2 L
2
 f

)

(

)

2

e
j  2 L

0

)

(

2 2

(

2

+f

)(


 2f e j2 L
e
j2 L

)

2

e

j 2 L

(


2
 f

© Woodhead Publishing Limited, 2010

)

2

) e
j 2 L

[4.97]

      

[4.98]

      

[4.99]

     [4.100]   

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Optical switches

The effect of an air gap or other inserted material having a different characteristic impedance from that of the fiber can be accommodated using the four-port transmission matrix for two circularly polarized traveling waves shown here. This matrix can be generated from the magnetic material matrix by assuming the material constants are the same in both directions.

(TUL ) =

  2 2 0 1  2 f m cos (
m L ) + j m +  f sin (
m L )  2 f m  2 +  2 sin
L    cos
L + j  0 2 ( ) ( )   f m m m f m

(

)

(

)

[4.101]

(TLR ) =

(TUR ) =

  2 2 0 1 
j m
 f sin ( m L )  2 f m  2
 2 sin  L   0
j ( )   m f m

[4.102]

(TLL ) =

  2 2 0 1  + j m
 f sin (  m L )   2 f m  0 + j m2
 2f sin ( m L )  

[4.103]

(

)

(

(

)

)

(

)

  2 2 0 1  2 f m cos ( m L )
j m +  f sin ( m L )   2 f m  0 2 f m cos (  m L )
j m2 +  2f sin (  m L )  

(

)

(

)

[4.104]

(TR
UL ) =

  2 2 0 1  2 f m cos (  m L )
j m +  f sin (  m L )  2 f m  2 +  2 sin  L    cos  L
j  0 2 ( ) ( )   f m m m f m

(

)

(

)

[4.105]   j m2
 2f sin (  m L ) 0     0 j m2
 2f sin ( m L )  

(

)

(TR
UR ) =

1 2 f m

(TR
LL ) =

  2 2 0 1 
j m
 f sin ( m L )  2 f m  2
 2 sin  L   0
j ( m )   m f

(TR
LR ) =

  2 2 0 1  2 f m cos (  m L ) + j m +  f sin (  m L )   2 f m  2 2 0 2 f m cos (  m L ) + j m +  f sin ( m L )  

(

(

[4.106]

)

)

(

(

[4.107]

)

)

(

)

[4.108] where the subscript m refers to material parameters of the inserted section and the subscript f refers to material parameters of the fiber, the reference material. For an air gap, the subscript m would refer to free space while for a matching section such as index matching material, the subscript m would refer to the parameters of that material.

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Magneto-optical switches

129

When using a linear excitation, e.g., an x-axis excitation, E5 and E6 (a1CW = a1CCW ) would be set equal to each other. For a y-axis excitation, E5 and E6 would be set to the negative of each other (a1CW = –a1CCW ). Similarly, when determining the axis of rotation for the polarization, the tangent ratio of the appropriate values of the port excitations would be used.

4.8

Summary

The need for all-optical switching in next-generation optical networks has been discussed. An overview of several different switch implementation technologies was given with each representing a different set of engineering trade-offs and having different performance characteristics in terms of power consumption, scalability, insertion loss, polarization-dependent loss, wavelength dependency and switching speed. Recent advances in suitable materials that have made switches based on MO effects more viable are highlighted.

4.9

Appendices

The formalism of matrix notation is one of the most useful means of characterizing an optical component or device. The matrix representation greatly aids in analyzing optical structures since a matrix equation can be readily solved with modern computing tools. Two representations that are particularly useful in characterizing structures at optical frequencies are the scattering and transmission matrices (Kurokawa, 1965; Matthews, 1955), which are more commonly and conventionally employed in microwave and millimeter wave analyses.

4.9.1 Scattering matrix The relevant variables for a two-port element as illustrated in Fig. 4.20 are designated a1, a2, b1, and b2 and are defined as follows: a1a1* = time-averaged power flowing into port 1 b1b1* = time-averaged power flowing out of port 1 a2a2* = time-averaged power flowing into port 2 b2b2* = time-averaged power flowing out of port 2

a1 Port 1

(s)

a2 Port 2

b1

b2

4.20  Scattering matrix model and variables.

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Optical switches

These variables are related as follows:

b  S S  1 11 12 
  =   b2   S21 S22  b2 = S21a1 + S22 a2 b1 = S11a1 + S12 a2

a  1  
(b) = (S )( a)  a2 

[4.109]

where the matrix (S) is termed the scattering matrix. Physically, a1,2 and b1,2 represent the actual physical beams that travel ‘into’ and ‘out of’ an optical element. The output beams can be the result of many signal transformations within the optical element (e.g. reflections, diffraction, etc.), which could be an interface between two different optical media, a waveguide, lens or grating. In essence the optical element operates on the input to produce an output and this operation is represented by the scattering matrix (S). Several significant advantages are afforded by the scattering matrix representation of a structure at optical frequencies, chief among which is the ease of empirically determining the matrix elements. For instance, connecting a matched termination at port 2 means that all the energy flowing out of port 2 is dissipated in the termination. Thus, no energy is returned to the structure and a2 = 0, which can be utilized to simplify the scattering matrix equations. Another advantage lies in the fact that most optical sources are constant power sources as opposed to most low-frequency sources, which have either constant voltage or constant current. A variable that maintains a constant magnitude (e.g. the square root of time-averaged power) is helpful in characterizing an optical element. It is important to note that the matched termination at a given port may be chosen at liberty and for this reason is also termed the reference termination. The elements of the scattering matrix will be different for different values of reference termination and thus this reference must be specified together with the scattering matrix in order for the matrix representation to be useful in designing optical devices and networks. The variables a and b are complex quantities that can be derived using the Poynting vector (S, not to be confused with the scattering matrix), which represents the rate of energy transport per unit area in an EM field and is defined as follows for a polarizable medium:
*
1

1

*
1
E 1
2
* S = ExH = E H z = E E z z= 2 2 2 2μ 0 v μ0v

(

)

[4.110]

With propagation in the +z direction, the total power through an xy plane surface is given by:

P = p(z)p* (z) = dx dy z • S =



n
c 1
1 2 2 E = E = 0 E(z)E * (z) 2 μ0 2μ 0 v 2

[4.111]

Thus, it is seen that the complex power amplitude, p(z), can be written as follows and is the same as those denoted by a1,2 and b1,2.

© Woodhead Publishing Limited, 2010



Magneto-optical switches

p(z) =

n
0 c E(z) = 2

n
0 c E0 e jkn z 2

131 [4.112]

Since these are complex quantities, one must be more specific when referring to power flow into and out of the ports. The elements of the scattering matrix are measured at different points along the optical structure being tested and these are termed reference planes. The phase term (denoted by ejknz in equation (4.112)) of the variables a1,2 and b1,2 depends upon the position of reference planes. Thus, a given scattering matrix characterizes a structure which encompasses everything between the reference planes. Two interesting properties of the scattering matrix that render it invaluable are: 1 If a structure is reciprocal, the scattering matrix is symmetrical. 2 If a structure is lossless, the scattering matrix is unitary, i.e. (S*)t (S) = (I), where the subscript t means ‘transposed’ and (I) is the identity matrix. The first statement can be easily proved by showing that the scattering matrix (S) is equal to its own transpose. The second statement can be substantiated by applying the conservation of energy and forcing the net input of output power flow of the structure to be zero, as it must be if there the structure is lossless. Only two-port structures have been considered in the foregoing discussion. However, it should be recognized that the analysis can be extended to a higher number of ports with little difficulty. A final point of interest is that the scattering matrix representation inherently assumes a linear system, which means that the output of a structure being analyzed must be linearly related to its input. While it may not be immediately obvious, it should be noted that the matrices can be dependent upon other external parameters (e.g. frequency/wavelength) and that they may be in fact nonlinear with respect to these other parameters.

4.9.2 Transmission matrix The scattering matrix utilizes the physical inputs and outputs of an optical element, i.e. the beams that travel ‘into’ and ‘out of’ this element. These are not the most convenient quantities to utilize when analyzing multiple elements in a given optical path. Thus, an alternate matrix representation is the transmission matrix, which uses mathematical rather than physical inputs and outputs. With reference to Fig. 4.20, we redefine the time-averaged power variables as being related as follows:

a1 = T11b2 + T12 a2 b1 = T21b2 + T22 a2

a  T T  1 11 12 
  =   b1   T21 T22 

b  2    a2 

[4.113]

Comparing equation (4.113) to (4.109), it is seen that rather than having both physical input variables on the right and both physical output variables on the left, we now have a mathematical ‘input’ column vector on the right and an ‘output’

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Optical switches (T1)

(T2)

(T1) (T2)

4.21  Transmission matrix for cascaded elements.

column vector on the left. This is mathematically legitimate since the system being examined is linear. The advantage of the transmission matrix is the ease with which cascaded devices or structures can be characterized. For instance, the transmission matrix of a pair of stacked two-port optical elements (Fig. 4.21) is simply the matrix product of their individual transmission matrices. Since the transmission matrix variables are the same as those of the scattering matrix, we should expect some relationship between the elements of the scattering matrix representation of a structure and its transmission matrix representation. Thus, the elements of (T) are related to those of (S) as follows:

 1 S
22  T T  S21 11 12  S21   =   T21 T22  S S S  11 S12
11 22  S21 S21

     

[4.114]

Conversely, the elements of (S) are related to those of (T) as follows:

T T T  21 T22
12 21   S S  T11  11 12  T11   =    S21 S22  T12 1  
  T11 T11

[4.115]

The elements of the scattering matrix are relatively easy to determine empirically or to deduce on the basis of the properties of the scattering matrix. Thus, knowledge of the scattering matrix elements enables one to formulate the transmission matrix, which is helpful when analyzing cascaded optical elements. One final note is that while another matrix representation exists (the general circuit parameter or ABCD matrix), its usefulness is limited to non-optical applications as it utilizes the voltages and currents at a structure’s ports as its variables.

4.10 References Aichele, T., Lorenz, A., Hergt, R. and Gornert, P. (2003), ‘Garnet layers prepared by liquid phase epitaxy for microwave and magneto-optical applications – a review’, Crystal Research and Crystal Technology, 38: 575–87. Aubin, G., Sapriel, J., Molchanov, V.Y., Gabet, R., Grosso, P., Gosselin, S. and Jaouen, Y. (2004), ‘Multichannel acousto-optic cells for fast optical crossconnect’, Electronics Letters, 40: 448–9.

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133

Balanis, C.A. (1989), Advanced Engineering Electromagnetics. New York: Wiley. Bell, A.G. (1880), ‘Apparatus for signaling and communicating, called “photophone”  ’, U.S. Patent 235199. Birks, T.A., Culverhouse, D.O., Farwell, S.G. and Russel, P.S.J. (1996), ‘2×2 single-mode fiber routing switch’, Optics Letters, 21: 722–4. Boudiara, T., Payet-Gervya, B., Blanc-Mignona, M.F., Rousseaua, J.J., Le Berreb, M. and Joistenc, H. (2004), ‘Magneto-optical properties of yttrium iron garnet (YIG) thin films elaborated by radio frequency sputtering’, Journal of Magnetism Magnetic Materials, 284: 77–85. Cisco Systems Inc. (2008), ‘Global IP traffic forecast and methodology 2006–11’. Collin, R.E. (1966), Foundations for Microwave Engineering. New York: McGraw-Hill. Condon, E.U. and Shortley, G.H. (1959), Theory of Atomic Spectra. Cambridge: Cambridge University Press. d’Alessandro, A. and Asquini, R. (2003), ‘Liquid crystal devices for photonic switching applications: state of the art and future developments’, Molecular Crystals Liquid Crystals, 398(1): 207–21. Ertel, J., Helbing, R., Hoke, C., Landolt, O., Nishimura, K., Robrish, P. and Trutna, R. (2006), ‘Design and performance of a reconfigurable liquid-crystal-based optical add/ drop multiplexer’, Journal of Lightwave Technology, 24(4): 1674–80. Eschenfelder, A.H. (1980), Magnetic Bubble Technology (2nd ed.). New York: Springer Verlag. Espinola, R.L., Tsai, M.C., Yardley, J.T. and Osgood Jr., R.M. (2003), ‘Fast and low-power thermo-optic switch on thin silicon-on-insulator’, IEEE Photonics Technology Letters, 15(10): 1366–8. Fan, L., Gloeckner, S., Dobblelaere, P.D., Patra, S., Reiley, D., King, C., Yeh, T., Gritters, J., Gutierrez, S., Loke, Y., Harburn, M., Chen, R., Kruglick, E., Wu, M. and Husain, A. (2002), ‘Digital MEMS switch for planar photonic crossconnect’, Proceedings of Optical Fiber Communication Conference 2002, 93–4. Fratalocchi, A., Asquini, R. and Assanto, G. (2005), ‘Integrated electro-optic switch in liquid crystals’, Optics Express, 13: 32–7. Grzegorczyk, T.M. and Kong, J.A. (2005), ‘Visualization of Faraday rotation and optical activity at oblique incidence’, IEEE Antennas and Propagation Magazine, 47(5): 23–33. Holzmann, G.J. and Pehrson, B. (1994), The Early History of Data Networks (1st ed.). Hoboken, NJ: Wiley–IEEE Computer Society. Huang, Z. and Shen, J. (2006), ‘Latching micromagnetic optical switch’, Journal of Microelectromechanical Systems, 15(1): 16–23. Hunt, R.P. (1967), ‘Magneto-optic scattering from thin solid films’, Journal of Applied Physics, 38: 1652–71. Jajszczyk, A. (2005), ‘Optical networks – the electro-optic reality’, Optical Switching and Networking, 1(1): 3–18. Ji, C.H., Yee, Y., Choi, J., Kim, S.H. and Bu, J.U. (2004), ‘Electromagnetic 2×2 MEMS optical switch’, IEEE Journal of Selected Topics in Quantum Electronics, 10(3): 545–50. Kahn, F.J., Pershan, P.S. and Remeika, J.P. (1969), ‘Ultraviolet magneto-optical properties of single-crystal orthoferrites, garnets and other ferric oxide compounds’, Physical Review, 186(3): 891–918. Kao, K.C. and Hockham, G.A. (1966), ‘Dielectric surface waveguide for optical frequencies’, Proceedings of IEEE, 113: 1151–8. Kapron, F.P., Keck, D.B. and Maurer, R.D. (1970), ‘Radiation losses in glass optical waveguides’, Applied Physics Letters, 17: 423–5.

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Kim, K.H., Kwon, M.S., Shin, S.Y. and Choi, D.S. (2004), ‘Vertical digital thermo-optic switch in polymer’, IEEE Photonics Technology Letters, 16(3): 783–5. Koh, Y.W., Yun, S.H., Kim, Y.K., Seo, H.S., Han, S.R., Oh, K., Paek, U.C. and Kim, B.Y. (1998), ‘Broadband polarization-insensitive all-fiber acousto-optic modulator’, Proceedings of Optical Fiber Communication Conference 1998, 239–40. Kondo, M., Ohta, Y., Fujiwara, M. and Sakaguchi, M. (1982), ‘Integrated optical switch matrix for single-mode fiber networks’, IEEE Journal of Quantum Electronics, 18(10): 1759–65. Krauss, J.D. and Carver, K.R. (1973), Electromagnetics (2nd ed.). New York: McGrawHill. Kurokawa, K. (1965), ‘Power waves and the scattering matrix’, IEEE Transactions on Microwave Theory and Techniques, 13(2): 194–202. Landau, L.D. and Lifschitz, E.M. (1984), Electrodynamics of Continuous Media (2nd ed.). Pergamon Press. Leycuras, C., LeGall, H., Desvignes, J., Guillot, M. and Marchand, A. (1982), ‘Magnetooptic and magnetic properties of praseodymium substituted garnets’, Journal of Applied Physics, 53: 8181–3. Leycuras, C., LeGall, H., Desvignes, J., Guillot, M. and Marchand, A. (1984), ‘Magnetic and magneto-optical properties of a cerium YIG single crystal’, IEEE Transactions on Magnetics, 21(5): 1660–2. Lin, L.Y., Goldstein, E.L. and Tkach, R.W. (1999), ‘Free-space micromachined optical switches for optical networking’, IEEE Journal of Selected Topics in Quantum Electronics, 5(1): 4–9. Lorentz, H.A. (1916), The Theory of Electrons (1st ed.). G.B. Teubner. Ma, X. and Kuo, G.S. (2003), ‘Optical switching technology comparison: optical MEMS vs. other technologies’, IEEE Communications Magazine, 41(11): 16–23. Maiman, T.H. (1960), ‘Stimulated optical radiation in ruby’, Nature, 187: 493–4. Matthews, E.W. (1955), ‘The use of scattering matrices in microwave circuits’, IRE Transactions on Microwave Theory and Techniques, 3(3): 21–6. Morse, S.F.B. (1840), ‘Telegraph signs’. U.S. Patent 1647. Okuda, T., Katayama, T., Kobayashi, H. and Kobayashi, N. (1990), ‘Magnetic properties of Bi3Fe5O12 garnet’, Journal of Applied Physics, 67: 4944–6. Paoletti, A. (1978), Physics of Magnetic Garnets. Amsterdam: North-Holland Publishing. Papadimitriou, G.I., Papazoglou, C. and Pomportsis, A.S. (2003), ‘Optical switching: switch fabrics, techniques, and architectures’, Journal of Lightwave Technology, 21(2): 384–405. Park, H.S., Song, K.Y., Yun, S.H. and Kim, B.Y. (2001), ‘All-fiber wavelength tunable acousto-optic switch’, Proceedings of Optical Fiber Communication Conference 2001, 3: 1–3. Patterson, P.R., Dooyoung, H., Nguyen, H., Toshiyoshi, H., Chao, R.M. and Wu, M.C. (2002), ‘A scanning micromirror with angular comb drive actuation’, Proceedings of 15th IEEE Conference on Micro Electro Mechanical Systems, 544–7. Pedrotti, F.L., Pedrotti, L.M. and Pedrotti, L.S. (2006), Introduction to Optics (3rd ed.). Addison-Wesley. Ryf, R., Kim, J., Hickey, J.P., Gnauck, A., Carr, A., et al. (2001), ‘1296-port MEMS transparent optical crossconnect with 2.07 petabit/s switch capacity’, Proceedings of Optical Fiber Communication Conference and Exhibit 2001, 4: 1–3. Silberberg, Y., Perlmutter, P. and Baran, J.E. (1987), ‘Digital optical switch’, Applied Physics Letters, 51(16): 1230–2.

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Takeuchi, H., Shinagawa, K. and Taniguchi, S. (1973), ‘Faraday effect of bi-substituted rare-earth iron garnet’, Japanese Journal of Applied Physics, 12(3): 465. Tioh, J. (2008), ‘All-optical switch for next generation networks: development and integration’. Van Vleck, J.H. (1932), The Theory of Electronic and Magnetic Susceptibility (1st ed.). New York: Oxford University Press. Wang, X., Howley, B., Chen, M.Y. and Chen, R.T. (2006), ‘4 × 4 Non-blocking polymeric thermo-optic switch matrix using the total internal reflection effect’, IEEE Journal of Selected Topics in Quantum Electronics, 12(5): 997–1000. Weber, R.J. (2009a), ‘ABCD and S matrices for a non-reciprocal transmission line’. Weber, R.J. (2009b), ‘Faraday rotation characterization’. Yamagata, S., Kato, T. and Kokubun, Y. (2005), ‘Non-blocking wavelength channel switch using TO effect of double series coupled micro-ring resonator’, Electronics Letters, 41(10): 593–5. Yano, M., Yamagishi, F. and Tsuda, T. (2005), ‘Optical MEMS for photonic switching’, IEEE Journal of Selected Topics in Quantum Electronics, 11(2): 383–94. Yuan, W., Kim, S., Sadowy, G., Zhang, C., Wang, C., Steier, W.H. and Fetterman, H.R. (2004), ‘Polymeric electro-optic digital optical switches with low switching voltage’, Electronics Letters, 40(3): 195–7. Zhong, T., Zhang, X.M., Liu, A.Q., Li, J., Lu, C. and Tang, D.Y. (2007), ‘Thermal-optic switch by total internal reflection of micromachined silicon prism’, IEEE Journal of Selected Topics in Quantum Electronics, 13(2): 348–58.

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5 MEMS-based optical switches L.L.P. WONG and J.T.W. YEOW, University of Waterloo, Canada, and A.A. GOLDENBERG, University of Toronto, Canada Abstract: The optical switch is one of the most important components of an optical network. Microelectromechanical systems (MEMS)-based optical switches have been a popular research topic and have shown a lot of promise. This chapter is a comprehensive review of MEMS-based optical switch architectures, actuating principles and fabrication process. The challenges that MEMS face as an enabling technology for optical switch implementation will also be discussed. Key words: microelectromechanical system (MEMS), optical switch, optical cross-connect (OXC).

5.1

Introduction

The development of microelectromechanical systems (MEMS) has emerged as an important research area in engineering in recent years. MEMS refer to very small devices that consist of micrometer-sized mechanical and electrical components. The ability to create tiny machines opens up a lot of opportunities for new applications. Typical MEMS applications include sensors, actuators, switches, gyroscopes and accelerometers. Commercially successful MEMS devices are already shipped in consumer products such as cellphones and digital cameras. MEMS are fabricated using mature semiconductor processes, making them highly reliable. Moreover, their compact sizes, low power consumption and high performance make them an ideal candidate for optical switches.

5.2

Optical systems

In order to appreciate the benefits of using MEMS to implement optical switches, one must understand the wavelength division multiplexing (WDM) system. At the transmitter side, optical signals of different frequencies first need to undergo gain adjustment by variable optical attenuators (VOA) so that signals from various sources will have comparable power. An optical multiplexer (MUX) is then used to combine the signals into a single optical fiber. Transporting optical signals of different wavelengths increases the capacity of the system and reduces implementation cost per unit bandwidth. The reverse process occurs at the receiver. An optical de-multiplexer (DEMUX) is utilized and signals of different wavelengths are separated into different optical fibers and routed to their destinations. 136 © Woodhead Publishing Limited, 2010



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5.2.1 Optical cross-connect The optical system that is described so far has only fixed connections. In other words, signals from one source can only be hard-wired to a certain destination. An optical cross-connect (OXC) is used to redirect optical beams from one input fiber to multiple output fibers. In essence, OXC serves as a routing matrix that allows any input port to connect to any output port. Originally, the switching system is implemented by converting the incoming optical signal into an electrical signal by a photo-detector before being switched electrically to a defined output, and subsequently the electrical signal is converted back into an optical signal. This optical-electrical-optical (OEO) conversion process is expensive to implement and maintain. An increase in the encoding speed of the optical signal will require the electronic switching system to be upgraded. As a result, OEO-based optical switches could easily become the bottleneck of the network. Moreover, the unnecessary conversions will burn more power and add noise to the signal. MEMS technology, on the other hand, allows the processing of optical signals without the OEO conversion. Micromirrors can be built, as can be seen in the next section, to redirect the optical signals into pre-defined optical fiber outputs.

5.2.2 Wavelength selective cross-connect Combining the WDM and OXC would result in a useful system called wavelength selective cross-connect (WSXC). As shown in Fig. 5.1, a WSXC consists of N OXCs, where N is the number of wavelengths in each optical fiber. At the input side, the signals are split into different wavelengths using optical DEMUXs, the same process as in the receiver side of a WDM system. Those signals of various wavelengths can then be switched to the desired outputs and combined with optical MUXs just like a WDM transmitter. The WSXC allows signals from multiple sources to be routed to the same destination as long as those signals have distinct wavelengths.

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2D optical cross-connect switches

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5.1  Wavelength selective cross-connect (WSXC) (De Dobbelaere et al., 2002) © 2002 IEEE.

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5.2  Optical add/drop multiplexer (OADM): implementation using (a) 2 × 2 optical switches and (b) 2D OXC (De Dobbelaere et al., 2002) © 2002 IEEE.

5.2.3 Optical add/drop multiplexer Besides re-routing signals, another application for OXC is in an optical add/drop multiplexer (OADM). Figure 5.2(a) shows the basic principle of an OADM. After the incoming signal is de-multiplexed, each channel, which corresponds to a different wavelength, can be added or dropped by means of a 2 × 2 optical switch. The same can be achieved using an OXC. As illustrated in Fig. 5.2(b), signals of different wavelengths come in from the left, while the channels are added from the bottom. If a switch, for example in row y and column x, is turned on, the signal coming in along column x from the bottom will be added to the output while the original signal from row y will be dropped to column x at the top. OADM serves as an important component of an optical network.

5.3

Optical switch architectures

Numerous architectures of MEMS switches for OXCs have been proposed over the past decade. The basic mechanism for routing optical signal has mostly been reflection off micromirrors. Nevertheless, the configurations of the micromirrors and how they are actuated have a significant impact on the switching performance. In this section, different architectures such as two-dimensional (2D) and threedimensional (3D) optical switches will be discussed in detail. Moreover, the pros and cons of these architectures will also be examined.

5.3.1 Two-dimensional optical switches These switches are called 2D because the optical signals inside the switch only travel on a 2D plane, even though the switch is actually a 3D structure as the micromirrors move in and out of the signal plane.

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N × N crossbar switch Toshiyoshi and Fujita (1996) reported a 2 × 2 architecture that consists of four micromirrors. Each mirror can reflect light when it is in the ON position or let the light through when it is in the OFF position. When the mirror is in its OFF position, it lies horizontally above the signal path. To turn the mirror ON, one side of the mirror drops down such that it is now in a vertical position. The mirror then forms a 45° inclination to both the input and output signal paths. In order to keep the light on a 2D plane, the angle of the mirror must be well controlled. This can be easily achieved by a stopper that is implemented on the substrate below. The stopper prevents the mirror from turning more than 90°. The switch architecture is illustrated in Fig. 5.3. This architecture can be extended to N × N, with N2 micromirrors. The binary nature of the mirror position simplifies the control circuitry. However, this architecture does have its disadvantages. For instance, the micromirror count can go up very quickly as the port count increases. A 16 × 16 switch will require 256 mirrors, which take up a lot of space and hence making the size of the switch bigger. In addition, the crossbar switch also suffers from insertion loss variation due to the fact that the optical path length inside the switch is different along different input/output paths. Polygon switch A polygon OXC was proposed by Lin et al. (1998). This architecture takes advantage of the connection symmetry property of the switch. In other words, if we have Input X going to Output Y and Input Y going to Output X in the N × N crossbar switch, the mirror arrangement will always be symmetric across the diagonal. The mirror count could be reduced if the two mirrors on each side of the axis of the symmetry can be combined into one. The polygon switch, as shown in Torsion mirror chip

Output CBF6 Mirror is ON M4 Mirror is OFF

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5.3  2D 2 × 2 crossbar switch (Toshiyoshi and Fujita, 1996) © 1996 IEEE.

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5.4  Polygon OXC (Lin et al., 1998) © 1998 IEEE.

Fig. 5.4, uses a double-sided mirror for each set of input-output pairs. For example, when the mirror at the intersection between the B2 and C2 path is in the ON state, optical signal coming in from port B2 will reflect off the mirror to output C2, while the signal from input C2 will use the other side of the same mirror to get to output B2. The control circuitry is further simplified with a reduced number of mirrors, in exchange for a more complicated fabrication process to produce double-sided micromirrors. However, this architecture requires a specific traffic connection, one that is symmetric, in order to function. For example, if input port B2 is routed to output port C1, it would be impossible for the signal from input port C2 to get to output port B2. Re-arrangeable non-blocking switch Non-blocking means that a new path can be set up even though an old path is already established (without affecting the existing connections). Re-arrangeable non-blocking, on the other hand, suggests that a new path can be formed but the old path might need to be rearranged. In comparison, a strictly non-blocking switch allows a new path to be created without any change in the existing paths. Shen et al. (2002b) introduced a re-arrangeable non-blocking architecture that reduces the mirror count from N2 to N(N+1)/2. Similar to the polygon switch, this architecture also utilizes double-sided micromirrors. An optical signal might need to be reflected more than once, up to seven for a 4 × 4 switch in the worst case, before it can reach the output port. As a result, the variation in propagation delay might be a concern.

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L-switching matrix Yeow et al. (2003) came up with the L-switching matrix architecture that reduces the micromirror count of a conventional crossbar switch by one quarter. For an N × N ports switch, it requires N2/4 double-sided and 2N2/4 single-sided mirrors. Figure 5.5 presents a 4 × 4 architecture of the L-switching matrix. The top left quadrant is the input quadrant. It is where the double-sided mirrors are located. The single-sided mirrors are situated in the other two quadrants, top right and bottom left, with the output ports on the bottom right corner of the structure. To illustrate how the L-switching matrix works, imagine mirrors (1, 1), (3, 1) and (1, 4) are turned on. Optical signal from IN1 will reflect off mirrors (1, 1) and (1, 4) to reach x In3

In4

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5.5  L-switching matrix (Yeow et al., 2002) © 2002 IEEE.

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OUT1. Similarly, signal from IN3 will utilize the other side of double-sided mirror (1, 1) and mirror (3, 1) to reach OUT3. If mirror (1, 1) is switched off, only one reflection will occur for each signal, as a result signal from IN1 and IN3 will go to OUT3 and OUT1, respectively. One major advantage of the L-switching matrix is the difference between most distance and least distance path, as it is important to minimize the free-space propagation of optical beam. For a 32 × 32 port switch, the maximum path difference is only 30 pitches for the L-switching matrix while the number is over 60 pitches for the conventional crossbar architecture. A single pitch is defined as the distance between two adjacent mirrors. The optical signal will be reflected at most two times inside the switch. Care must be taken in the control scheme when establishing a path as multiple paths can be chosen in certain instances, as the L-switching matrix is re-arrangeably non-blocking but not strictly non-blocking. Yeow and Abdallah (2005) proposed a staircase-switching algorithm to minimize the occurrence of internal blocking conditions. Multistage 2D switch A typical commercially available OXC switch has a large number of input and output ports, for example, 256 × 256 in the Lucent LambdaRouter™ (Bishop et al., 2002). A switch of this size is generally not feasible using a 2D architecture as the number of mirrors, as well as the cost and the potential of having a faulty device, goes up very quickly. One solution to this problem is to construct a large matrix using multiple stages of smaller ones. Configurations such as the threestage Clos network, the Spanke–Beneš network and the Beneš network have been proposed (Shen et al., 2002a; Li et al., 2003). A Clos network implementation is illustrated in Fig. 5.6. This special case of Clos network consists of only m × m switches, but the network is connected in a unique way so that it can be used to route m2 inputs to m2 outputs. The switches are divided into three columns in a Clos network. The left column is called the ingress stage. The m2 input signals are

Input N = m2

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5.6  Multi-stage switch using Clos network implementation (Yeow et al., 2002) © 2002 IEEE.

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divided into groups of m, with each group going into a different switch of the ingress stage. Each input switch then routes its m outputs to the switches in the middle column, called the middle stage. Similar signal routing happens between the middle stage and the output stage, or egress stage. The key point here is that every switch in the first two stages has a physical connection with all the switches in the subsequent stage. There are in fact m paths that an input signal can take to reach each output. This is necessary as all m signals from the same input switch could be destined for the same output switch in the worst case. The Clos network ensures that the input signals can be routed to the outputs in any combination.

5.3.2 Three-dimensional optical switches To allow the optical signal to travel in 3D, the mirrors must be able to rotate about two axes. Both the fabrication process and the control scheme are more complicated for the 3D switches, yet the switch can be made smaller compared to a 2D switch with the same number of ports because the number of micromirrors required is greatly reduced. For an N × N port switch, a typical 3D implementation would require N or 2N switches. Lucent Microstar In 2000, Lucent Technologies announced a 3D OXC, the Microstar™ micromirror array, which consists of micromirrors that can rotate about two axes (Aksyuk et al., 2000). A picture of the micromirror can be seen in Fig. 5.7. It achieves two-axis rotation by using a gimbal ring, which is able to rotate about one axis, and have the mirror turn about another axis with respect to the gimbal ring. The angular range of each axis is greater than ±6°. The product, LambdaRouter, uses the 2N configuration for 3D switch and contains 2 × 256 mirrors in a single OXC switch. There is one micromirror associated with each input port and also one for each output port. Optical signal from the input port reflects off the input mirror to the output mirror and then to the output port. As a result, a 256 × 256 strictly non-blocking switch can be achieved. In addition, a 1024 × 1024 OXC using a similar approach was also demonstrated in a later publication (Aksyuk et al., 2003). Since the introduction of the Lucent LambdaRouter, several 3D MEMS OXCs have been reported that use similar architecture but different micromirror structures or actuation methods (Bernstein et al., 2004; Fernandez et al., 2004). Microlens scanner Instead of using mirrors, Takahashi et al. (2007) proposed a microlens structure that could be potentially used in an OXC switch. Two sets of microlens array are used, with one each on the input and the output sides. Each lens in the input array can be moved to direct the incoming light beam to any of the output optical fiber

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5.7  Image of 3D micromirror used in the Lucent LambdaRouter (Aksyuk et al., 2003) © 2003 IEEE.

through the corresponding lens in the output array. A 9 × 9 OXC with coupling loss of 13.8 dB was reported.

5.4

Actuating principles of MEMS-based optical switches

The movement of MEMS micromirrors needs to be controlled accurately and reliably in order to be used in an OXC. Moreover, other properties such as small size, easy to fabricate and low power consumption are also desirable. As a result, the actuating principles of the MEMS micromirrors have attracted a great deal of attention in research. This section will present several actuating principles that have been applied to micromirror switching.

5.4.1  Electrostatic One of the earliest implementations of MEMS optical switches, proposed by Toshiyoshi and Fujita (1996), uses electrostatic force to drive the micromirror movement. The basic principle of electrostatic relies on the attractive forces between opposite charges. If there is an electric potential difference between two objects, an electrostatic field will be developed which pulls the two objects

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towards each other. In the Toshiyoshi design, the mirrors are normally held at an OFF position. When the underlying electrode is charged to a voltage different from that of the mirror, the mirror will be attracted to the electrode and be tilted to the ON position. Electrostatic actuation is very reliable as it is highly repeatable. It has also been well researched and modeled. However, one major disadvantage of using electrostatic force for micromirror actuation is the requirement of very high driving voltages. Toshiyoshi and Fujita reported that they required a voltage in the range of 100 to 150 V to turn on the mirror. Given the high driving voltage requirement of electrostatic micromirror actuation, several researchers have come up with ways to relax this constraint. Yoon et al. (2002) reported that they were able to lower the driving voltage to about 50 V by introducing an extra vertical electrode. Kuo et al. (2004) proposed a structure using stress-induced bending micromirrors, as illustrated in Fig. 5.8. Electrostatic force was used to attract a bending beam downward, moving the mirror into its ON position. Driving voltages of 25 and 18 V, depending on the side of the beam that was attracted, were reported. It should be noted that the 3D optical switch developed by Lucent Technologies also employed electrostatic actuation (Aksyuk et al., 2003). Four electrodes, two for each axis, were used. The voltage required was less than that of a 2D switch, so as to avoid the ‘snap-down’ point of the mirror.

5.4.2 Electromagnetic Using electric current to generate magnetic field is another efficient way to actuate MEMS micromirrors. Electromagnetic actuation requires much lower driving voltage than that of the electrostatic mechanism. However, switches employing electromagnetic actuation suffer from crosstalk as the magnetic field can potentially affect adjacent micromirrors if shielding is not done properly. Miller et al. (1997) from the California Institute of Technology demonstrated an electromagnetic 2 × 2 fiber optic bypass switch. The double-sided mirror in the switch lies below the optical fibers in its OFF state to let the signal through. When the switch is activated, the mirror moves up and reflects the signal into neighboring optical fibers (see Fig. 5.9). It was reported that only 30 mA of current is needed to achieve a mirror displacement of 200–300 µm that is necessary to get the mirror completely out of the signal path in the OFF state. A team from LG Electronics Institute of Technology presented another electromagnetic MEMS optical switch (Ji et al., 2004). In their paper, they also proposed the use of an electro-permanent magnet to make a latchable optical switch. The nonzero remnant flux of the magnet helps maintain the mirror in a certain state even when the power is off. On the other hand, the magnetic field can be removed by applying an opposite current to the coil. Power consumption can be greatly reduced in this scheme as current is needed only during state changing of switches.

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5.8  Stress-induced bending micromirror: (a) OFF state, (b) ON state on the right side and (c) ON state on the left side (Kuo et al., 2004) © 2004 IEEE.

Electromagnetic actuation can be used in 3D optical switches as demonstrated by a number of researchers. The challenge lies in the fact that the angle of the micromirror has to be controlled precisely over two axes. Bernstein et al. (2004) demonstrated a mirror that can be tilted by more than 10° per mA in each axis,

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Magnetic field

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5.9  Electromagnetic fiber optic bypass switch (Miller et al., 1997) © 1997 IEEE.

with an accuracy of less than 0.01°. In 2006, Hsieh et al. (2006) reported an electromagnetic optical switch that requires a driving voltage of less than 0.5 V and a switching time of 5 ms. The low voltage makes integration of micromirrors and microelectronics, which can be used to implement the mirror control circuitry, a possibility.

5.4.3 Other actuating mechanisms Electrostatic and electromagnetic mechanisms are the two most widely used actuating methods in MEMS optical switches today. However, there are several other actuating mechanisms that are gaining popularity and are worth mentioning. Comb-drive The comb-drive could be grouped into electrostatic actuation but it does not use any electrodes to attract the micromirror directly. Instead, a voltage is applied between two combs to generate an electrostatic field and move the mirror (Jung et al., 2006). This more complicated structure provides a larger force density as the surface area between the two combs is bigger. Also, since

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the two combs will never be in contact with each other, the mirror’s range of motion can be made larger as it does not suffer from the typical snap-down or pull-in effect. Scratch drive actuator A team from AT&T research labs has utilized scratch drive actuator (SDA) to rotate the mirror (Lin et al., 1999). The mirror is connected to a translation plate through pushrods and micro-hinges. The SDA mechanism causes the translation plate to move, which in turn gets the mirror to rotate up if the translation plate is moving away from the mirror or vice versa. The basic idea is presented in Fig. 5.10. It has been shown that the tilt angle of the mirror can be controlled precisely using SDA. But the fact that the mirror can only rotate about one axis limits its application to 2D switches only. Thermal The micromirror movement can also be controlled thermally. Werber and Zappe (2006) proposed a thermo-pneumatically actuated micromirror that can be tilted up to 13°. This is caused by the expansion of the gas in a sealed cavity. Michael et al. (2005) fabricated a beam that can be expanded when heated. The beam, which can be buckled upward or downward, is part of a bi-stable microbridge that puts the micromirror either in and out of position. Since the heat can be conducted to nearby cells, crosstalk is a potential problem for thermally actuated mirror array. Switch mirror

Hinge joint

Translation plate

Hinge Pushrod

Scratch drive actuator

5.10  Switching of micromirror using scratch drive actuator (Lin et al., 1999) © 1999 IEEE.

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Micro-motor Finally, a micro-motor can be used to rotate the mirror as Oohira et al. (2004) demonstrated. As in a normal motor, an AC signal is needed to power it. It requires low driving voltage as a 5 V peak-to-peak sinusoidal signal at 80 kHz is required. However, the angular resolution of the motor was limited to 0.5°.

5.4.4 Closed-loop versus open-loop control Pointing accuracy is an important performance parameter for micromirrors in 3D switches. Unlike their 2D counterparts, which operate only in binary states, mirrors in 3D switches must have their tilt angle controlled precisely. The easiest control scheme is by means of an open-loop control system. As the driving voltage goes up, the mirror is moved by a larger angle. Each driving voltage corresponds to a different tilt angle. If the relationship between the driving voltage and the mirror angle is consistent, the desired tilt angle can be achieved using a look-up table. On the other hand, factors such as process variation, temperature and aging could change the actuation relationship. If that is the case, a closed-loop system is preferable as the driving voltage is adjusted continuously until the desired mirror tilt angle is reached. However, some form of sensing mechanisms must be present, which makes the switch bigger and more complicated. Also, the servo control algorithm must be carefully designed to ensure good stability.

5.5

Materials and fabrication of MEMS-based optical switches

MEMS optical switches must be reliable, repeatable and robust. Fortunately, modern fabrication processes have greatly improved the performance of MEMS devices. A combination of different fabrication techniques, along with different materials, has realized micromirrors and movable structures such as springs and hinges. Important performance parameters such as the lifetime of a switch depend on the physical characteristic of the mirrors, which is a function of the fabrication processes and materials used.

5.5.1 Fabrication techniques MEMS, like most integrated circuits in the market today, use silicon as the primary substrate material because of its excellent mechanical properties (Petersen, 1982). Moreover, a lot of MEMS fabrication techniques are the same as the integrated circuit processes. As a result they are well understood and can be efficiently controlled in production. There are two basic micromachining technologies: (1) bulk micromachining and (2) surface micromachining.

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Bulk micromachining Bulk micromachining, sometimes called the etching process, involves removal of silicon from a substrate. Silicon substrates are patterned with photoresist through a mask using photolithography techniques. The layer of patterned photoresist exposes the silicon that should be etched and protects the portion that should remain. There are two types of etchants: (1) anisotropic and (2) isotropic. The difference between the two is illustrated in Fig. 5.11. For anisotropic etchants, the etch rate depends on the silicon crystallographic direction. On the other hand, isotropic etchants react with silicon evenly in all directions. This is sometimes referred to as wet etching because of the involvement of chemical etchants. With normal etching process it is difficult to create deep trenches with vertical sidewalls because of the orientation of the crystal planes. Deep reactive ion

(100) Surface orientation (111) 54.74°

Silicon substrate (110) Surface orientation (111)

Silicon substrate (a) SiO2

Silicon substrate (b)

5.11  Bulk micromachining: (a) anisotropic etching of (100) and (110) silicon substrate and (b) isotropic etching of silicon (Yeow et al., 2001) © 2001 IEEE.

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etching (DRIE) was invented by the Robert Bosch Corp. to create deep anisotropic etches (Lärmer and Schilp, 1994). DRIE is considered a type of dry etching as it uses ions to attack the substrate. DRIE can create trenches with depth-to-width ratio higher than 20:1. Micromirrors are usually fabricated parallel to the substrate, but Marxer et al. (1997) were able to create vertical mirrors using DRIE. Surface micromachining Only simple mechanical structures can be built by bulk micromachining techniques. Surface micromachining includes a series of deposition and etching of materials to create complex microstructures. Layers of thin-film materials are added to or removed from the wafer. The layer that will be removed, usually by means of chemical etching, is called the sacrificial layer. Whereas the layers that are deposited after a sacrificial layer to form part of the final structure are called the structural layers. When the sacrificial layer is removed, the structural materials are left over a void created by the etching, thus forming a free-standing structure. Figure 5.12 shows the surface micromachining process. In this case, silicon dioxide (SiO2) is used as the sacrificial material. The structural layer is deposited on top of SiO2. The structural pattern is then etched on the top layer, leaving only Structural layer

Sacrificial layer

5.12  Surface micromachining process to construct MEMS device. Removal of the sacrificial layer results in a free-standing structure (Yeow et al., 2001) © 2001 IEEE.

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useful structural material on, as well as exposing the sacrificial layer. Finally, SiO2 is then etched away, resulting in the free-standing mechanical structure. Wafer bonding Sometimes the mechanical structure is so complicated that instead of developing each layer one by one it makes more sense to develop different parts of the structure on separate wafers. The silicon wafers can then be bonded together at a high temperature. Greywall et al. (2003) constructed a 3D optical switch using the wafer bonding technique. They fabricated the mirrors in one wafer and the electrodes in another. Without a doubt, the alignment between the two wafers is critical in the bonding process. Wafer bonding is especially useful when an enclosed space is needed. The thermally actuated mirror by Werber and Zappe (2006) requires a sealed cavity that is constructed by bonding the silicon wafer onto a glass substrate.

5.5.2  Materials Like the fabrication techniques, the materials used in the process are critical to the MEMS device performance. Very often, materials with different properties are exploited in different structures of a MEMS device. Some examples of material characteristics include thermal and electrical conductivities, elasticity and roughness. Substrate Silicon is the most popular choice for MEMS substrate. However, a lot of MEMS-based optical switch researchers opt to start with a silicon-on-insulator (SOI) substrate (Yeow et al., 2003; Bernstein et al., 2004; Fernandez et al., 2004). A SOI substrate is basically a silicon device layer over a buried oxide layer, on top of a thicker silicon substrate. The SOI option is attractive because both the thickness and the flatness of the top silicon layer, which can be used to build the micromirror, are very well characterized. Moreover, the insulator layer in the SOI substrate can act as a sacrificial layer. One disadvantage of SOI is its high cost. However, the need for SOI substrate is often justified. Occasionally other materials such as glass (Werber and Zappe, 2006) can be used as substrate but a silicon-based substrate is still the choice for most MEMS-based optical switches. Micromirror In a MEMS optical switch, the only element that interacts with the optical signal is the mirror. Therefore, the mirror surface quality is an important consideration. One of the most important characteristics of the mirror surface is its reflectivity. Any portion of the incoming light that is not reflected to the output port will translate to an insertion loss of the switch. Silicon, as a metalloid, has low reflectivity compared

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to metals and thus is not a good candidate for mirror surface material. In fact, micromirror surfaces are typically coated with metals to improve their reflectivity. Common metals that are used for this purpose include gold (Au) (Aksyuk et al., 2000; Ji et al., 2004; Lin et al., 2000), aluminum (Al) (Li et al., 2004), a combination of chromium (Cr) and gold (Toshiyoshi and Fujita, 1996; Yeow et al., 2003) or titanium (Ti) and gold (Bernstein et al., 2004). Lin et al. (2000) reported that a reflectivity of 97% for commercial mirror was achieved when a gold coating with a thickness between 500 and 5000 Å was applied to the mirror surface. Because of the metal coating, the bulk mirror layer cannot be too thin or the flatness of the mirror might be affected by the internal stress that exists between the two layers. On the other hand, a heavy mirror increases actuation driving voltage and switching time so a thick layer is also undesirable. Bulk micromirrors are usually made of polysilicon or silicon. To mitigate the curvature problem, a layer of phosphosilicate glass (PSG) can be sandwiched between two polysilicon layers to create a more rigid structure (Lin et al., 2000; Chen et al., 1999). Single crystal silicon (SCS) is the material of choice in a few designs because of its low intrinsic stress and smoothness characteristics (Pu et al., 2004; Chu et al., 2002). Mirror spring/torsion beam When the switch is in the ON state, the part that undergoes the most stress is the mirror spring or the torsion beam. It is therefore important to use materials that are elastic for the mirror spring. It turns out that polysilicon and silicon, materials that are used to build the mirror core, have good flexibility. Hence, they are good candidates for the spring materials. To obtain the required flexibility, the spring or torsion beam must be long and thin. For example, the Lucent Microstar uses polysilicon beams in a folded serpentine arrangement to construct the spring (Aksyuk et al., 2003).

5.6

Challenges surrounding MEMS-based optical switches

Among all the various technologies that can be used to implement optical switches, MEMS seems to show the most promise. However, there are still important issues that need to be addressed before the MEMS approach can become the technology of choice for the next-generation optical switches.

5.6.1  Reliability MEMS-based optical switches must be able to function in adverse conditions, as well as over an extended period of time. The fact that there are mechanical moving parts inside the switch makes reliability a bigger concern in MEMS than in traditional solutions such as integrated circuits. As mentioned in section 5.4, the

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relationship between mirror displacement and driving voltage could change due to factors such as temperature and aging. The closed-loop approach helps combat that problem. The change, if any, in the switching performance after millions of switching cycles also needs to be studied. In order to gain wide acceptance in optical networks, the reliability and repeatability of MEMS optical switches must be well researched and understood.

5.6.2 Scalability The demand of network bandwidth is always increasing. An optical switch installed today, even with port redundancy built in, could potentially run out of port count in the next few years. Instead of replacing the switch with a bigger one, using a switch with the ability to expand port count is a more cost-effective approach. A software-based switch might be able to increase its port count by a firmware upgrade. But MEMS-based optical switches are physically limited by the number of mirrors. A combination of clever algorithm and novel architecture is needed to overcome this disadvantage.

5.6.3 Manufacturability Performance of MEMS-based devices could change from wafer to wafer. A large process variation means a low yield, which will ultimately drive the cost high. A simpler process is highly desirable because fewer fabrication steps means less variation. Process control for MEMS is difficult because unlike normal integrated circuit process, in which there are a fixed number of layers with pre-defined dimensions, the MEMS fabrication steps are different among designs. Research in novel fabrication processes and materials will advance the MEMS manufacturing process. Moreover, standardization of processing steps will allow a better process control.

5.6.4 Packaging Because of the mechanical nature of MEMS-based optical switches, packaging has a huge impact on their performance. MEMS packaging is more demanding than integrated circuit packaging since MEMS devices have a closer interaction with the outside world. In addition, MEMS external signals are not just electrical in nature. Standardization of packaging, as in the case of the integrated circuit industry, would reduce MEMS design cost.

5.6.5 Competing technologies MEMS-based optical switch has an advantage over traditional OEO switch, mainly because it is future-proof. However, it is not the only all-optical switch technology.

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MEMS-based optical switches must be able to outperform other switch technologies in performance parameters such as noise figure and switching speed.

5.7

Conclusions

This chapter has provided an overview of MEMS-based OXC switches. MEMS technology is recognized as one of the best candidates for optical switch application because it enables an all-optical solution, which is cheaper to maintain and implement. Different architectures of MEMS-based switches, including 2D and 3D versions, were evaluated. Actuating principles involving electrostatic or electromagnetic, as well as their advantages and disadvantages, were discussed. The fabrication techniques of MEMS and the materials used in MEMS-based optical switches were reviewed. There are several challenges that MEMS technology has to overcome before it can become the solution of choice for optical switches. We hope that in future more research and studies will be done on the architectural design, fabrication and packaging of MEMS-based optical switches.

5.8

List of abbreviations

2D 3D DEMUX DRIE MEMS MUX OADM OEO OXC PSG SCS SDA SOI VOA WDM WSXC

5.9

Two-dimensional Three-dimensional De-multiplexer Deep reactive ion etching Microelectromechanical system Multiplexer Optical add/drop multiplexer Optical-electrical-optical Optical cross-connect Phosphosilicate glass Single crystal silicon Scratch drive actuator Silicon-on-insulator Variable optical attenuator Wavelength division multiplexing Wavelength selective cross-connect

References

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micromirrors for large optical cross-connects’, IEEE Journal of Lightwave Technology, 21: 634–42. Bernstein, J.J., Taylor, W.P., Brazzle, J.D., Corcoran, C.J., Kirkos, G., Odhner, J.E., Pareek, A., Waelti, M. and Zai, M. (2004), ‘Electromagnetically actuated mirror arrays for use in 3D optical switching applications’, IEEE Journal of Microelectromechanical Systems, 13: 526–35. Bishop, D.J., Giles, C.R. and Austin, G.P. (2002), ‘The Lucent LambdaRouter: MEMS technology of the future here today’, IEEE Communications Magazine, 40: 75–9. Chen, R.T., Nguyen, H. and Wu, M.C. (1999), ‘A high-speed low-voltage stress-induced micromachined 2 × 2 optical switch’, IEEE Photonics Technology Letters, 11: 1396–8. Chu, P.B., Lee, S.S. and Park, S. (2002), ‘MEMS: the path to large optical crossconnects’, IEEE Communications Magazine, 40: 80–7. De Dobbelaere, P., Falta, K., Fan, L., Gloeckner, S. and Patra, S. (2002), ‘Digital MEMS for optical switching’, IEEE Communications Magazine, 40: 88–95. Fernandez, A., Staker, B.P., Owens, W.E., Muray, L.P., Spallas, J.P. and Banyai, W.C. (2004), ‘Modular MEMS design and fabrication for an 80 × 80 transparent optical cross-connect switch’, Proceedings of SPIE, 5604: 208–17. Greywall, D.S., Busch, P.A., Pardo, F., Carr, D.W., Bogart, G. and Soh, H.T. (2003), ‘Crystalline silicon tilting mirrors for optical cross-connect switches’, IEEE Journal of Microelectromechanical Systems, 12: 708–12. Hsieh, H., Chiu, C., Tsao, T., Jiang, F. and Su, G.J. (2006), ‘Low-actuation-voltage MEMS for 2-D optical switches’, IEEE Journal of Lightwave Technology, 24: 4372–9. Ji, C., Yee, Y., Choi, J., Kim, S. and Bu, J. (2004), ‘Electromagnetic 2 × 2 MEMS optical switch’, IEEE Journal on Selected Topics of Quantum Electronics, 10: 545–50. Jung, I.W., Krishnamoorthy, U. and Solgaard, O. (2006), ‘High fill-factor two-axis gimbaled tip-tilt-piston micromirror array actuated by self-aligned vertical electrostatic combdrives’, IEEE Journal of Microelectromechanical Systems, 15: 563–71. Kuo, J., Lee, G. and Pan, W. (2004), ‘A high-speed low-voltage double-switch optical crossconnect using stress-induced bending micromirrors’, IEEE Photonics Technology Letters, 16: 2042–4. Lärmer, F. and Schilp, P. (1994), ‘Method of anisotropically etching silicon’. German Patent DE 4241045. Li, C.Y., Li, G.M., Li, V.O.K., Wai, P.K.A., Xie, H. and Yuan, X.C. (2003), ‘Using 2 × 2 switching modules to build large 2D MEMS optical switches’, Proceedings of IEEE Global Telecommunications Conference, 5: 2789–802. Li, J., Liu, A.Q., Zhong, W.D., Zhang, Q.X. and Lu, C. (2004), ‘MEMS switch based serial reconfigurable OADM’, Optics Communications, 230: 81–9. Lin, L.Y., Goldstein, E.L., Simmons, J.M. and Tkach, R.W. (1998), ‘High-density micromachined polygon optical crossconnects exploiting network connect-symmetry’, IEEE Photonics Technology Letters, 10: 1425–7. Lin, L.Y., Goldstein, E.L. and Tkach, R.W. (1999), ‘Free-space micromachined optical switches for optical networking’, IEEE Journal on Selected Topics of Quantum Electronics, 5: 4–9. Lin, L.Y., Goldstein, E.L. and Tkach, R.W. (2000), ‘On the expandability of free-space micromachined optical cross connects’, IEEE Journal of Lightwave Technology, 18: 482–9. Marxer, C., Thio, C., Grétillat, M., de Rooij, N.F., Bättig, R., Anthamatten, O., Valk, B. and Vogel, P. (1997), ‘Vertical mirrors fabricated by deep reactive ion etching for fiber optic switching applications’, IEEE Journal of Microelectromechanical Systems, 6: 277–85.

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Michael, A., Yu, K., Mackenzie, M. and Kwok, C.Y. (2005), ‘Out-of-plane electrothermally actuated bistable buckled microbridge actuator’, Proceedings of IEEE Sensors 2005, 596–9. Miller, R.A., Tai, Y., Xu, G., Bartha, J. and Lin, F. (1997), ‘An electromagnetic MEMS 2 × 2 fiber optic bypass switch’, Proceedings of International Conference on Solid-State Sensors and Actuators (TRANSDUCER ’97), 1: 89–92. Oohira, F., Iwase, M., Matsui, T., Hosogi, M., Ishimaru, I., Hashiguchi, G., Mihara, Y. and Iino, A. (2004), ‘Self-hold and precisely controllable optical cross-connect switches using ultrasonic’, Journal on Selected Topics of Quantum Electronics, 10: 551–7. Petersen, K.E. (1982), ‘Silicon as a mechanical material’, Proceedings of IEEE, 70: 420– 57. Pu, C., Park, S., Chu, P.B., Lee, S., Tsai, M., Peale, D., Bonadeo, N.H. and Brener, I. (2004), ‘Electrostatic actuation of three-dimensional MEMS mirrors using sidewall electrodes’, IEEE Journal on Selected Topics of Quantum Electronics, 10: 472–7. Shen, G., Cheng, T.H., Bose, S.K., Lu, C. and Chai, T.Y. (2002a), ‘Architectural design for multistage 2D MEMS optical switches’, IEEE Journal of Lightwave Technology, 20: 178–87. Shen, G., Cheng, T.H., Lu, C., Chai, T.Y. and Bose, S.K. (2002b), ‘A novel rearrangeable non-blocking architecture for 2D MEMS optical space switches’, Optical Networks Magazine, 3: 70–8. Takahashi, K., Kwon, H.N., Mita, M., Saruta, K., Lee, J., Fujita, H. and Toshiyoshi, H. (2007), ‘A silicon micromachined f-θ microlens scanner array by double-deck device design technique’, IEEE Journal on Selected Topics of Quantum Electronics, 13: 277– 82. Toshiyoshi, H. and Fujita, H. (1996), ‘Electrostatic micro torsion mirrors for an optical switch matrix’, IEEE Journal of Microelectromechanical Systems, 5: 231–7. Werber, A. and Zappe, H. (2006), ‘Thermo-pneumatically actuated, membrane-based micro-mirror devices’, Journal of Micromechanics and Microengineering, 16: 2524– 31. Yeow, J.T.W and Abdallah, S.S. (2005), ‘Novel MEMS L-switching matrix optical crossconnect architecture: design and analysis-optimal and staircase-switching algorithms’, IEEE Journal of Lightwave Technology, 23: 2877–92. Yeow, J.T.W., Law, K.L.E. and Goldenberg, A. (2001), ‘MEMS optical switches’, IEEE Communications Magazine, 39: 158–63. Yeow, J.T.W., Law, K.L.E. and Goldenberg, A. (2002), ‘Micromachined L-switching matrix’, IEEE ICC 2002, 5: 2848–54. Yeow, J.T.W., Law, K.L.E. and Goldenberg, A. (2003), ‘SOI-based 2D MEMS L-switching matrix for optical networking’, IEEE Journal on Selected Topics of Quantum Electronics, 9: 603–13. Yoon, Y., Bae, K., Kim, J. and Choi, H. (2002), ‘An optical switch with newly designed electrostatic actuators for optical cross connects’, Proceedings of IEEE/LEOS International Conference on Optical MEMS, 121–2.

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6 SOA-based optical switches A. ASSADIHAGHI, H. TEIMOORI and T.J. HALL, University of Ottawa, Canada Abstract: The application of semiconductor optical amplifiers (SOAs) is not limited only to the amplification of optical signals. It is also used as an important element in optical switching, all-optical signal processing, demultiplexing, regeneration and wavelength conversion. The goal of this chapter is to present design criteria, improvement of the performance and integration of the SOA-based switches. We begin with a description of SOA-based switch architectures in the second section. The physical structure of an SOA, its features and characteristics are presented in the third section. Some techniques for SOA design improvements and a study of the noise in SOA-based switches are presented in the fourth section. Key words: semiconductor optical amplifier (SOA), multi-quantum well, wavelength conversion, space switch, gain recovery.

6.1

Introduction

The application of semiconductor optical amplifiers (SOAs) is not limited only to the amplification of optical signals. It is also used as an important element in optical switching (Tanaka et al., 2009), all-optical signal processing (Teimoori et al., 2008), demultiplexing (Tangdiongga et al., 2007), regeneration (Ezra et al., 2009) and wavelength conversion (Banchi et al., 2010). The well-known benefits of SOAs include fast switching times, high extinction ratio (especially in switching applications), sizable operating gain bandwidth and a relatively compact footprint. SOAs, however, introduce amplified spontaneous emission (ASE) noise added to the input optical signal as an inevitable side effect of the amplification. The goal of this chapter is to present design criteria, improvement of the performance and integration of the SOA-based switches. We begin with a description of SOA-based switch architectures in section 6.2. The physical structure of an SOA, its features and characteristics are presented in section 6.3. Some techniques for SOA design improvements and a study of the noise in SOAbased switches are presented in section 6.4.

6.2

SOA-based switching strategy

In this section, we investigate the possibility of using the SOA as an active element in the switch application. The outcome of this study is to highlight different features of the SOA to be optimized for switching application (section 6.4). Essentially, the 158 © Woodhead Publishing Limited, 2010



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SOA functionality as a switch (gate) can be controlled either electrically or optically.

6.2.1 Electrically versus optically controlled switch Table 6.1 represents a brief comparison between the electrical and optical control techniques of SOA-based switches. Electrical control of the SOA gates is achieved by modulation of the SOA bias current. Therefore, operating the SOA in the linear regime is preferred (no saturation or pattering effect). Likewise, low threshold current and fast switching time are important issues in design. These gates are interesting in optical burst switching or optical packet switching networks; the intermediate nodes in the network are transparent and opaque in terms of data and control, respectively; the packet control data (header information) is converted and processed within the electronic control plane. A vast amount of data, however, remains in the optical domain (optical transmission plane). Thus, an optical space switch can establish transparent connections between multiple inputs and outputs. The bottleneck of this approach is the lack of all-optical memory to hold the data in the optical domain, while waiting for the electronic routing task to complete (Srivastava et al., 2009). In the all-optical switching approach, the SOA is operated in the nonlinear regime where one or more auxiliary light beams are injected into the SOA. Nonlinear effects in an SOA can be classified into two different phenomena: new frequency component generation inside the SOA medium, i.e. four-wave mixing (FWM), and alteration of a property of the input light, i.e. polarization (crosspolarization modulation), phase (cross-phase modulation) or amplitude (crossgain modulation). An element sensitive to the property altered by the SOA provides the gating action. For example, an arrayed waveguide grating (AWG), polarization beam splitter (PBS) or SOA-assisted interferometric architecture Table 6.1  Comparison between electrically and optically controlled SOA-based switches Feature

Electrically controlled switches

Optically controlled switches

Operation regime of SOA Power consumption Complexity Scalability Switching time Extinction ratio Interference Application Controlling circuit

Linear Low Low High ~Nanosecond High Low Space switch Simple

Nonlinear High High Low ~Picosecond Moderate Moderate Space/wavelength switch Delicate

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(e.g. Mach–Zehnder) can, respectively, act upon variation in wavelength, polarization and phase in the SOA. Table 6.2 provides diagrams of schemes that exploit different nonlinear effects in the SOA in order to perform an all-optical gating function. The schemes listed in Table 6.2 have been extensively studied for 1 × 2 and 2 × 2 switches (Table 6.3). A straightforward application for such switches is in transparent optical switching and all-optical logic gates (Teimoori et al., 2008). Although all-optical control of SOA gates is faster than the electrical ON_OFF gating of SOAs, the performance limitations of the various schemes prevent scaling to large port counts.

6.2.2 Switch fabric architectures For the packet switching application in dense wavelength division multiplexing (DWDM)-based telecommunication systems, among various proposals there are two prevalent approaches to scalable switch fabric architecture: route and select Table 6.2  Schematic diagram of the application of nonlinear effects in the SOA for switching application. SOA: semiconductor optical amplifier, PBS: polarization beam splitter Nonlinear effect

Scheme

Features

Cross-gain  Considerable ASE noise and Input (λ) modulation crosstalk, high power λ SOA (XGM)  consumption, limited λ CW (λ) transparency, small extinction ratio, complementary output 2f2–f1 Four-wave  Transparent, sensitive to Input (λ) f1 λ mixing  the input beam polarization SOA λ (FWM)  states, low efficiency for large f CW (λ) 2f1–f2 2 ∆λ between input beams, moderate extinction ratio Input (λ) Cross-phase  Higher efficiency via modulation interferometric SOA (XPM)  architectures, sensitive to the input beam polarization, high Output (λ) CW (λ) extinction ratio, possible integrability

Cross-  High sensitivity to Input (λ) 2 λ polarization polarization, high PBS SOA 3 modulation extinction ratio, low power 1 (XPolM) consumption (in comparison CW (λ) with XGM), fast, bit pattern dependency, transparent

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(R&S) architectures based on wavelength switching via AWG (Vlachos et al., 2003) and broadcast and select (B&S) architectures based on space switches (Dittmann et al., 2003). Wavelength switch The wavelength switch relies on passive wavelength components to route a light path from input to output port and consequently does not suffer from splitting losses of a broadcast architecture (Fig. 6.1(a)). The path is selected by changing the wavelength of the input light. The wavelength conversion function (Fig. 6.1(b)) is attractive in its own right for DWDM networks. Although such an advanced functionality requires a more complex switch control unit, it permits the use of idle wavelength channels to increase throughput and provides an additional means of packet contention resolution by deflection in wavelength rather than the standard practice of deflection in time (optical buffering) (Srivastava et al., 2009). Wavelength tuning ability, tuning range, dynamic range, transparency, noise, jitter and polarization-dependent loss are the most important parameters for evaluating

Wavelength switch

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Wavelength converter

(b)

6.1 (a) Simplified wavelength routing and (b) reconfigurable wavelength switch with wavelength conversion possibility.

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162

the performance of wavelength converters. The exploitation of nonlinear effects in the SOA is still one of the most promising approaches to broadband tunable wavelength converter (TWC) implementation (Srivastava et al., 2009). As a TWC, an SOA is operated in the saturation regime and its design should be toward higher saturation output power. Space switch Space-division switches can be classified according to their connectivity properties as blocking, re-arrangeably non-blocking (Duthie and Wale, 1988), wide-sense non-blocking (Zhou and Yang, 2002) and strictly non-blocking (Hamza and Deogun, 2007) configurations. The objective is the ability to connect every input to any output with minimum control and cross-points. As illustrated in Fig. 6.2, a well-known solution for performing N × N switching is the B&S architecture. In this architecture, each input is fanned out by a 1 × N splitter to deliver the optical signals to N outputs. A bank of N SOAs can be employed at each output of the switch fabric to select an input before fan-in via an N × 1 coupler. Upon switch control unit decision, multicasting and broadcasting is achieved by adjusting the electrical currents of the SOA. Such a switch system offers the following features: • Increased extinction ratio which arises from the large amount of absorption that a signal undergoes as it passes through an SOA in the OFF state. • The insertion loss may be compensated by the SOA gain in the OFF state. The switch may therefore be operated over the broad operating bandwidth of SOA switch array 1xN splitter

1 2 … N 1 2 … N … 1 2

…

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N

6.2  Broadcast and select switch architecture.

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Nx1coupler



SOA-based optical switches

163

the SOA and used in the linear regime. The SOA is transparent to the modulation and is therefore compatible with ultra-high-capacity networks. To be non-blocking an SOA is deployed at each of the N2 cross-points and hence the number of SOA gates required for the switching system raises rapidly with increasing port count. The good news in terms of power consumption is that among the N2 SOAs only N SOAs should be biased-on at any time. However, losses should be compensated within the switch. These include the fundamental 1/N loss associated with both the fan-out and fan-in loss of wavelengthindependent splitters and any additional excess loss. These losses even when compensated impact negatively on the noise performance of the switch. Table 6.3 compares the performance of SOA-based B&S integrated switches reported in the literature. In a WDM-based system in which the channels are separated in wavelength domain, one may adapt the B&S architecture using wavelength multiplexers and demultiplexers where appropriate. To achieve linear amplification of a signal, especially in the form of a WDM multiplex with a correspondingly increased total power, a high saturation output power is required for these SOA gates. In addition, the noise figure (NF) of the gates must be as low as possible Table 6.3  Comparison between reported SOA-based integrated switch matrices Reference

Dimension Wavelength Insertion loss

Extinction Technique ratio

Janson 2×2 et al. (1992)

1550 nm

12 dB

40 dB

InGaAsP substrate. Polarization sensitive

Song et al. (2004)

4×4

1550 nm

21 to 26 dB (gain)

40–60 dB

Undoped uppercladded waveguides with buried ridge stripe SOAs

Gustavsson 4 × 4 et al. (1992)

1550 nm

26 dB (gain)

40–50 dB

Multiple stages of SOAs, single polarization, variable bias current

Dorgeuille 8 × 8 et al. (2000)

1550 nm

14–16 dB

32–38 dB

Using 8 × 1 switch gates based on gain-clamped SOA as the basic building block

Burmeister 2 × 2 and Bowers (2006)

1550 nm

4–26 dB

>40 dB

In-Ga-As-P/In-P amplifier gate matrices

Kato et al. (1999)

1550 nm

5–9 dB

40 dB

Hybrid electrical/ optical multi-chip integration

4×4

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164

because it causes optical signal to noise ratio (OSNR) degradation repeatedly with every SOA stage. In more sophisticated, but complex, solutions both reconfigurable wavelength conversion and space switches can contribute to the switch fabric. Figure 6.3 gives an example of a schematic (Eramo and Listanti, 2009). Finally, in both the B&S and R&S approaches, port count scalability issues will arise. A fundamental limit (especially in B&S architecture) is splitting loss within the fabric. Only a part of the input power is routed to the desired output while the remainder is discarded. This directly results in rapid OSNR deterioration as ASE noise from the SOA is added to the signal. This problem can be alleviated by a staged fabric with input booster SOAs to compensate for the splitting loss. In general for large switch fabrics several parameters should be taken into account: total switching time, insertion loss, extinction ratio (OSNR), crosstalk, polarization dependency, monitoring, number of SOAs, number of waveguide crossovers (an integration issue) and reliability. Figure 6.4 presents examples of switch fabric architectures that have been considered for SOA-based implementation (Prucnal et al., 2006). In multistage architecture, the cascadability of SOAs should be considered in terms of power budget and pattern effects. In the former, the ASE of the SOA may be amplified in the chain of the SOA (e.g. in crossbar or Benes) which Tunable laser

Tunable laser

· · N

· ·

· Wavelength · conversion ·

Optical space switch

· · ·

M.N. x M.N.

1 · · · N

· · ·

6.3  Wavelength/space switch architecture.

© Woodhead Publishing Limited, 2010

Mux

Demux

Input M

Output 1

Mux

Input 1

1

Demux

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Optical switches

Output M



SOA-based optical switches Stage 1 (n×p)

N

Stage 3 (p×n)

n

1

1

1

n

n

2

2

2

n

n

k

p

k

n

1×N

1×N

1×N

1×N

1×N

Stage 2 (k×k)

2×2

N

165

N

N/2 × N/2

N/2 × N/2

2×2

N

1×N ×

×

6.4  Schematic diagram of the optical switch matrix.

deteriorates the OSNR quality in the switch fabric. Likewise, pattern effects appear when the SOA gain gradually saturates with successive 1’s in the input signal and recover with successive 0’s. This is more critical when the SOA is operated in nonlinear regime (Xu et al., 2010). Increasing the number of switch fabric ports increases the number of SOAs deployed regardless of single or multistage implementation of the switch fabric. Consequently, the remainder of this chapter is focused on the performance and low power consumption of the SOA as the building brick of the switch fabric. In the following sections the SOA structure and the techniques for improvement of SOA performance, in terms of low threshold current, fast gain recovery and low noise, are discussed.

6.3

SOA structure

The generic structure of the SOA is composed of a gain media (active region) embedded in cladding material of lower refractive index (heterostructure) in order to confine the optical wave in the gain media and form an optical waveguide (Fig. 6.5). Population inversion of the gain media can be achieved directly by electrical current injection. By using the appropriate bias current together with doping the various layers, the SOA overcomes the inherent absorption and loss of the active media and acts as an amplifier. Coating the input and output facets of the active waveguide with some anti-reflective material helps suppress feedback (Barnsley et al., 1990).

6.3.1 Active region (gain media) In the SOA, the gain media can be realized in the form of bulk semiconductor, multi-quantum wells (MQWs), quantum wires and quantum dots (Connelly,

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166

6.5  Generic structure of the SOA.

2001; Bimberg et al., 1999). Typical dimensions of the bulk structure are many times larger than the ‘de Broglie’ wavelength of electrons which ranges from 0.05 to 0.5 µm. The carriers in bulk gain media are free to move in all three directions. The density of states (ρ3D(E)), which describes the distribution of electron energy states in the conduction and valance bands, in a bulk structure has a parabolic profile (Fig. 6.6(a)). Nanostructured gain materials, such as quantum well (QW), wire and dot, have one, two and three dimensions, respectively, comparable with the magnitude of the ‘de Broglie’ wavelength of electrons, ranging typically between 2 and 12 nm. In QW structures, the movement of the carriers is confined in one direction. As a result, the density of states (ρ2D(E)) has a staircase pattern with parabolic envelope (Fig. 6.6(b)). Further confinement in two and three dimensions in quantum wire and quantum dot materials, respectively, (as depicted in Fig. 6.6(c) and (d)) results in a delta-like distribution of states about discrete energy levels.

6.3.2 Inter-band versus intra-band transition

r(3D)

r(1D)

r(2D)

Energy (a)

r(E)

r(E)

r(E)

There are two classes of electron transitions in the SOA gain media: inter-band and intra-band (Fig. 6.7). The inter-band dynamic refers to the exchange of carriers between the conduction band and the valence band which affects the carrier density. In the absence of carrier injection, the majority of the electrons are in the valence band. Carrier injection increases the density of electrons in the Density of state, r(E)

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Energy (b)

r(0D)

Energy

Energy (c)

(d)

6.6  Density of the states for (a) bulk, (b) multi-quantum well, (c) quantum wire and (d) quantum dot material versus energy.

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SOA-based optical switches

Conduction band

167

e2 e1

e2

Phonon emission via intra-band transition

e1

E= v E= v

Photon emission via inter-band transition

Valence band

hh1

hh1

Momentum

(a)

(b)

6.7  Schematic of the inter-band and intra-band transition in SOA active region: (a) bulk and (b) quantum well.

conduction band and holes in the valance band. The electrons in the conduction band spontaneously recombine with holes in the valence band on sub-nanosecond time scales and release the energy by emitting photons. The emission process can be stimulated by incident photons with energy slightly higher than the band gap. The stimulated emission process leads to an effective reduction in free-carrier lifetime to the picosecond range in SOAs in non-quiescent operation. Increasing the current injection above a threshold leads to population inversion between valence band and higher conduction band levels. Injected hot carriers rapidly relax to lower levels in the conduction band through intra-band collision process dissipating their excess energy by the emission of phonons. The intra-band mechanism may also be observed through the photogeneration of hot electrons by pumping with very short optical pulses, typically tens of femtoseconds in duration and with photon energies that exceed the band gap (Manning et al., 2007). The intra-band mechanism occurs on subpicosecond time scales much faster than the picosecond time scales of inter-band mechanisms. The electron distribution within bands may therefore be assumed as in quasi-equilibrium even though the distribution between bands is not in equilibrium. Auger recombination is an important mechanism of non-radiative intra-band transitions which are observed either due to the presence of doping-induced defects or at high carrier density. For example, it may happen that an electron/hole recombination (possibly mediated by defect) excites another electron/hole to a higher energy level (Vorob’ev, 2000). The excess energy then being dissipated by intra-band transitions results in phonon generation.

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6.3.3 Transparency threshold The transparency current density is defined as the minimal current for which the material becomes transparent for the incident photon (E>Eg). SOA gain is proportional to the density of electrons and holes at the energy levels of the transition. Using the first-order approximation, the gain can be defined as: g(N) = a(N – Ntr)

[6.1]

where a is the differential gain coefficient, N is the carrier density and Ntr is the transparency carrier density. Carriers tend to fill the lower energy states first as carrier injection increases. Therefore, the population inversion condition for optical gain is realized for smaller carrier densities for QW materials which have a sharp step-like onset to their density of states with increasing energy compared to bulk materials where the density of states increases parabolically. In QW structure, a lower transparency current can be achieved than in bulk materials by the enhanced confinement of the electrons/holes in a smaller volume by the heterojunctions formed between the QW and its adjacent layers leading to a higher concentration of carriers for the same current. The efficacy of the confinement depends on the size of the band offsets (∆Ec, ∆Ev). Deep QWs are required. However, too deep wells bring other problems such as non-uniformity in carrier distribution across multiple wells.

6.3.4 Gain nonlinearity Gain nonlinearities in an SOA can be classified as inter-band nonlinearities due to a change in the carrier concentration in conduction or valence band and intra-band nonlinearity caused by changes in the energy distribution of the carriers within each band. Inter-band nonlinearities When an optical beam (with a photon energy of E = hv > Eg) is incident to the SOA, it will simulate electron transitions between the conduction and valence bands, which adds new identical photons resulting in light amplification in the SOA. If the beam is sufficiently intense then the amplification will be saturated due to a shortage of the carriers able to participate in the transition. The saturation power in inter-band recombination is in a range of milliwatts for typical amplifiers operating at telecommunication wavelengths. In the absence of an input beam, the carrier concentration and hence SOA gain is determined by a carrier lifetime governed by spontaneous emission. The presence of an input beam in the SOA reduces the carrier lifetime as a result of additional stimulated recombination. An effective carrier lifetime can be calculated in a small signal analysis (Adams et al., 1995):

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SOA-based optical switches

τ e−1 = τ s−1 + vg .

G + 1 dg . .S G dN out

169

[6.2]

−1

 dRsp  where τ s =   is the differential carrier lifetime, Rsp is the spontaneous  dN  recombination rate, G is the SOA gain, vg is the group velocity and Sout is the output photon density. The differential carrier lifetime can be measured by the frequency response of light intensity modulation in response to small modulation of the current of an SOA under the condition that spontaneous emission is dominant. In an MQW SOA, depending on the bias current and the active media characteristics, the differential carrier lifetime varies typically between 200 and 800 ns. The effective carrier lifetime can be reduced by modifying the active media internally, for example using doped QW, or externally, for example through assist pumping (Zhang et al., 2007). Intra-band nonlinearities For sub-picosecond incident light pulses to the SOA, the pulse width is much shorter than the carrier lifetime. Therefore, the saturation power is governed by two intra-band processes: carrier heating (CH) and spectral hole burning (SHB). These mechanisms alter the distribution of the carrier but not the numbers. Spectral hole burning. Incident light interacts with the carrier distribution only in a narrow range of electron energy dependent on the photon energy distribution (determined by the center frequency and the spectral width of the pulse). On short time scales, the pulse causes a reduction in the carrier distribution at the particular photon energy (a deviation from the Fermi distribution) and hence a reduction in the gain experienced by a weak optical probe at that energy (a ‘spectral hole’). The time τshb, which is the time needed to restore the Fermi distribution by scattering processes (mainly carrier–carrier scattering), is typically several tens of femtoseconds, 50–80 fs for InGaAsP. Carrier heating. The photoinduced transitions including stimulated emission and absorption (Willatzen et al., 1991) tend to increase the temperature of the carrier distribution above the lattice temperature. The distribution cools down to the lattice temperature through phonon emission. The CH recovery time τch ranges from several hundreds of femtoseconds to a few picoseconds, e.g. 200–700 fs for InGaAsP.

6.3.5 Strained gain media The growth of material layers without strict lattice matching to the substrate induces strain. In SOA, strained gain media can offer lower polarization dependency, higher differential gain and reduced threshold current. The strain can be either compressive or tensile. In the former case, the natural lattice constant of

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the strained layer is larger than the lattice constant of the substrate (Fig. 6.8(a)). In the latter case the lattice constant is smaller than the lattice constant of the substrate (Fig. 6.8(b)). The epitaxial design must ensure that the accumulated lattice mismatch does not cause misfit dislocations at any interface. The maximum of the effective strain in an epitaxial system can be defined by the critical thickness parameter (Vawter and Myers, 1989). Tensile and compressive strains have different effects on the characteristic of the structure. For example, structures with tensile strain have lower spontaneous emission rates, while those with compressive strain have a lower Auger (nonradiative) recombination rate. Both tensile and compressive strains have a significant effect on differential gain due to the modification of the fine structure of the valence band. Tensile strain also increases the energy spacing between the first two valence sub-bands which for InP-based SOAs results in the improvement of differential gain compared to devices under compressive strain (Houghton et al., 1993).

6.3.6 Polarization-insensitive SOA When an optical beam passes through an SOA, it experiences an optical gain of G = exp[(Γgm – α)L]

[6.3]

where gm is the material gain, Γ is the confinement factor of the active media, L is the SOA length and α is the total internal loss. Due to the asymmetric waveguide structure in an SOA, the TE and TM confinement factors (ΓTE and ΓTM ) are different. Anisotropic material properties are also introduced by quantum confinement and lead to different values of gm and α for TE and TM modes. Therefore, polarization-independent SOA gain can be realized by careful balancing of material gain, confinement factor and loss of TE and TM modes:

Compressive strain

Tensile strain

(a)

(b)

6.8  Schematic of (a) compressive and (b) tensile quantum wells.

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SOA-based optical switches TE − α TM g TM − α Γ TE g m TE = Γ m TM

171 [6.4]

In cases where the internal loss is much smaller than modal gain (Γgm), the equation can be reduced to: TE = Γ TM g TM Γ TE g m m

[6.5]

Strict SOA polarization independency also requires TE and TM modes to experience the same phase retardation, i.e. the absence of birefringence. This originates from a difference in the effective index of TE and TM modes and consequently variation in differential refractive index (dn/dN). In this case, the polarization insensitivity can be achieved by satisfying the following equation: Γ TE

dn TE dn TM = Γ TM dN dN

[6.6]

6.3.7 Noise in SOA Noise in the SOA is mostly manifested by the beating in a detector between the signal and the spontaneous emission and the spontaneous–spontaneous beat. In the case of high power the latter is negligible in comparison with the former. A definition of NF suitable for direct detection systems is given by the detected electrical signal to noise ratio at the input divided by the detected electrical signal to noise ratio at the output. NF =

OSNRinput

[6.7] There are also other definitions such as OSNR or a NF definition based on the noise energy (Haus, 2000). However, in the case of signal-spontaneous dominancy, both optical and electrical noise ratios are equivalent (Tucker and Baney, 2001). The NF can be determined as following (Dreyer et al., 2002): OSNRoutput

  2P NF =  ASE   GhvB0 

[6.8]

where PASE is amplified spontaneous emission, G is the gain at the specific optical frequency of interest v, B0 is filter bandwidth and h is Planck constant. There are some methods for decreasing the noise (ASE) such as limiting the angular aperture at the output (Kogelnik and Yariv, 1964) or non-uniform carrier injection (Ratowsky et al., 1998).

6.4

SOA design criteria

Based on the linear (gating) or nonlinear (wavelength conversion) operation task of the SOA in the switch fabric, different features of the SOA should be

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highlighted. For example, in the gating application, the threshold current should be reduced. Besides, short amplifiers with low confinement factors are optimal. On the other hand, for wavelength conversion (nonlinear regime) structures are required with high confinement factors and long active regions to improve the conversion efficiency. Polarization-independent gain is a common criterion for all applications.

6.4.1 Reducing the threshold carrier density The threshold carrier density depends on the transparency carrier density and the excess loss (α0), which is the loss in the absence of carrier injection (N = 0): N th =

α0 + N tr .dg / dN dg / dN − d α / dN

[6.9]

where dg/dN is the differential gain and dα/dN is the rate of the absorption in the active region with carrier density N. The main contribution to α0 is from the scattering from the interfaces and defects resulting from poor fabrication. A few mechanisms contribute to absorption loss in the active region such as free-carrier absorption and inter-valence band absorption. Inter-valence band absorption increases as hole density increases, while the free-carrier absorption increases with the free-carrier density. The undoped graded index separate confinement heterostructure (GRINSCH) structure (Fu et al., 2004) for the confinement layer around the active media reduces considerably free-carrier absorption and intervalence band absorption loss. Therefore, QW SOA has a lower transparency carrier density which results in lower threshold carrier density. Doped barrier QW decreases the transparency carrier density further, with a greater reduction for n-doping than p-doping at the same doping concentration (Uomi, 1990). However, doping concentration should not pass a certain value (typically around 5 × 1018 cm23) due to diminishing returns and increasing internal loss leading to a net increase in the threshold carrier density.

6.4.2 Gain dynamic for SOA As discussed earlier, the gain media can be designed in bulk, MQW (symmetric/ asymmetric) or quantum dot materials. The quantum dot based switches are studied in the next chapter (chapter 7). Here we compare the gain dynamics of bulk and MQW SOA. In bulk gain media, carriers are directly injected in the gain region in order to interact with the photons (Fig. 6.9(a)). On the other hand in MQW, carriers are injected in the confinement layers and then they should be transported into the QWs, before interacting with the photons (Fig. 6.9(b)). Thereby, the carrier distribution is affected by the characteristics of well, barrier and separate confinement heterojunction (SCH) layers, such as dimensions, material and doping.

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SOA-based optical switches

173

J

J

Nsch

Nb

e c NBw1

NUB

Optical mode

NUB w1

Stimulated and spontaneous recombination

(a)

NUB w2 e c NBw2

…

Optical mode (b)

6.9  Schematic of the carrier distribution in (a) bulk and (b) MQW. NUwiB : unbounded/quasi-2D carrier, NBwi: bounded/2D carrier, e: escaped carrier, C: captured carrier.

For the switching application the MQW SOA is preferable than bulk SOA in terms of the following characteristics: • Hot carriers with undesirable effects on both gain and modulation are mainly generated by Auger recombination. The released energy from the cooling process of this hot carrier vibrates the crystal. The Auger coefficient decreases in QW structure compared to bulk active media due to the decreased valence effective mass in QW structure. • MQW SOA has higher differential gain (∂g/∂n, n is carrier injected, g is gain coefficient) in comparison with bulk SOA. The more the QWs, the more the enhancements in differential gain due to reduced dedicated carrier density in each single QW. • Complete carrier recovery in bulk gain media is limited by carrier lifetime with hundreds of picoseconds. However in MQW, the barrier layer has a significant effect on modulation bandwidth due to its action as a carrier reservoir. • QW tunneling prevents heating effects in MQW. Carrier depletion in MQW can be compensated much faster from tunneling from barrier confinement layers. • In MQW, a larger number of QWs can provide larger differential gain.

6.4.3 Carrier uniformity in MQW All the reported MQW advantages in terms of carrier dynamics and bandwidth modulation are valid for uniform distribution of the carriers. Otherwise, each QW has its own differential gain which results in degradation of dynamic performance and chirping. Thermionic emission and carrier tunneling are two important parameters in carrier uniformity design, influencing the choice of the barrier material (band gap) and thickness.

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• Low valence offset increases carrier uniformity and thermionic emission transport; however, too low valence offset leads to a shallow well. A shallow well increases the free-carrier absorption and carrier temperature (CH) and reduces optical gain. • Thin barrier width increases carrier tunneling but too thin barrier causes coupling of the adjacent wells which results in lower gain.

6.4.4 Impact of strain on gain dynamics A strained QW in comparison with a lattice-matched QW provides lower effective mass and consequently a higher differential gain. However, large strain percentages require narrow wells, and a narrower QW has shallower effective depth. Moreover, for high percentages of the strain, the number of QWs and the thickness of both well and the barrier should be considered in a way that the effective strain of the active region does not exceed the critical strain value.

6.4.5 Effect of doping on gain dynamics The doping of barriers is another parameter which can be optimized in order to increase SOA modulation bandwidth. Differential gain increases with the increase in the p-doping concentration. However, n-doping has an inverse effect on differential gain. Doping should be selected precisely; otherwise it increases the imaginary part of the effective refractive index of gain media which leads to additional loss.

α int = α i-undoped + αdope Pdope

[6.10]

where Pdope is the hole number due to doping. For each specific barrier material and dimension, the maximum doping should be evaluated in order to minimize internal loss and increase gain.

6.4.6 Rate equations Rate equations are the simplest method to analyze both the spectral and dynamic processes of the SOA structures. As we explained before, the carrier dynamic in a bulk active region is limited to the carrier lifetime and can be determined as follows (Castrejón and Duelk, 2006): ∂N ( z, t ) I [6.11] = − Rspon ( N ) − Rstim,sig ( N , S, λ ) − Rstim,ASE ( N ) ∂t qV Here, N is the total carrier density in the active region, S is the total photon density at the input, V is the volume of the active region, I is the injected current, vg is the group velocity and q is the elementary charge. Rstim,sig and Rstim,ASE refer to stimulated recombination rates by signal and ASE photons:

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SOA-based optical switches Rstim,sig ( N , S, λ ) = vgG( N )

S 1 + εS

175 [6.12]



Rstim,ASE (N ) = vg G(N )  sp (N,
) +  ASE (N,
)  d
where G = (Γgmat(N, λ) – α) is the optical gain, Γ is the confinement factor, α is the waveguide loss and gmat is the material gain. The ASE is given by the spectral integration of two stimulated recombination rates: one spontaneous spectral photon density σsp and ASE spectral photon density σASE . Rspon refers to several losses which can be determined as below: Rspon = A + BN + CN2

[6.13]

where A is leakage coefficient, B is bimolecular and radiative coefficient and C is Auger recombination coefficient. The rate equation for an MQW is more complex than for bulk. Two approaches can be used to simulate the transport characteristic of MQW structures. The first one is based on a rate equation with the same carrier density and gain for all QWs contributing in the active region (Colden and Corzine, 1995). Since the electrons and holes have different mobility, they may not uniformly participate in the gain dynamics. This causes different phase and gain contributions by different QWs. The non-uniformity of the carrier distribution increases in MQW with large number of QWs and asymmetric MQW (AMQW), where QWs near the first cladding or QWs with lower band gap trap more carriers. Such an MQW requires a high bias current to achieve quasi-uniform carrier distribution. Exact modeling of carrier dynamics in an MQW requires a series of coupled differential equations for hole, electron, carriers and photon density for SCH layers and QWs considering both 2D and 3D confined and unconfined carriers in QWs. However, due to the complexity of this modeling, a series of the approximation can be considered: 1 A quasi-neutrality assumption allows one to use the same carrier density equation for both electrons and holes. 2 Inter-sub-band transitions are neglected. 3 Carrier transport is modeled with ambipolar diffusion. Carrier populations considered in MQW rate equation are: 1 Carriers transported by diffusion (Nb, Nsch) are located in the inter-well barrier and the SCH layers. 2 The unbounded/quasi-2D carrier (NUB wi   ) above the wells which are prepared to be captured into the QW (2D carriers) or released from the QW into the 3D barrier. B ) which interacts with the gateway 3 Bounded/2D carrier inside the QW (Nwi states through local carrier capture and thermionic emission.

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dN sch1 ηI N (N ) = i − sch1 − Rspon ( N sch1 ) + 1 sch1 dt qVsch τ d τd dN sch2 N (N ) = − sch2 − Rspon ( N sch2 ) + M sch2 dt τd τd = (1 − ξ )

dt

+ξ

sch2

N sch1 τd sch1

N sch1 UB ( N ) − Rspon ( N w1 ) − w1 w τd τc

( N w1 )

UB

−

τd

sch1

dN bi (N ) = − Rsponi N bi + wi τd dt

UB

+

( N wi+1 ) τd

i b

d( N wi )

UB

= − Rspon ( N wi )

dt

UB

1

+

UB

−

i+1 b

N bi-1 τd

+

B

dt

= − Rspon ( N wi ) 1

d( N wM )

UB

dt

B

(N ) + wi

UB

τc

τd

τd

bi-1 (wi) 3

d( N wi )

dS = dt

UB

M  sch2

sch2

UB

N sch2 τd

+ξ

1 sch1

sch1

d( N w1 )

UB

−

b  (wi) UC

N bi

(N ) − wi τe

N bi

+

(N ) + wi+1 τt

N ) UB ( N = (1 − ξ ) sch2 − Rspon ( N wM ) − wM M τs τc

M

∑R

B stim,sig ( N wi , S , λ ) −

i=1

β S + τp M

τe

UB

B

−

( N w1 ) τd

1 sch1

UB

+

( N w1 )

1 b

B

τe

N bi

[6.14]

b  (wi+1) UC

( N wi )

bi (wi) 3

B

τd

UB

B

−

( N wi )

UB

τc

(N ) − wi

B

τt

(N ) + wM τe

B , S, λ ) − Rstim,sig ( N wi

B

+

τd

N bi (wM-1) UB  b

M

∑R

spon ( N wi )

B



i=1

B , S, λ) is stimulated recombination for i-th QW, η is internal quantum Rstim,sig(Nwi i efficiency, Vsch is SCH volume, ε is the gain suppression which accounts for SHB, τp is photon lifetime, τd is carrier diffusion time, τn is carrier lifetime, τc is capture time, τe is thermionic escape time and τt is tunneling time. Photon life depends on the mirror loss and intrinsic absorption. In SOAs, due to the use of coated facet mirror, loss is negligible, so photon life can be determined by:

τ p = vgα th = vg Γw g th

[6.15]

Ambipolar thermionic emission time (Tesc) is the average of the electron and hole emission times and ambipolar diffusion time of SCH layer is proportional to the ambipolar diffusion constant Da:

τ diff =

2 t sch 2 Da

[6.16] where the diffusion length is tsch (the single-sided SCH width). Typical values for the constants used in the rate equation have been listed in Table 6.4.

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Table 6.4  Typical parameters for MQW SOAs Parameter

Value

Units

Description

A

10

m21

Internal loss

Ξ

0.2

Leakage coefficient

τp

2.47

ps

Photon lifetime

τ n

1e29

s

Non-radiative carrier lifetime

τcap

1

ps

Ambipolar capture time

τesc

5.6

ps

Ambipolar escape time

E

1e223

m23

Gain compression coefficient

H

0.8

Injection efficiency

T

300

K

Lattice temperature

Csch × 10229

1.3

Cm3/s

Auger recombination in SCH

Cw × 10229

0.3

Cm3/s

Auger recombination in QW

Bsch ×

1.4

Cm3/s

Bimolecular recombination in SCH

0.8

Cm3/s

Bimolecular recombination in QW Shockley–Read–Hall coefficient in SCH

10210

Bw × 10210 Asch ×

108

1.3

s21

A w × 108

27

s21

Shockley–Read–Hall coefficient in QW

τtunn

0.07

ps

Tunneling time

6.4.7 Gain/carrier recovery in MQW The gain recovery of SOAs is limited by the carrier lifetime, which itself depends on the applied current and the optical intensity in the active layer. A high current provides a large carrier density and also a high ASE power, both of which tend to shorten the carrier lifetime. Therefore, to obtain a fast gain recovery, a high current must be applied. Another way to enhance the gain recovery is by increasing the optical intensity in the active layer. This leads to a higher stimulated recombination rate and, therefore, a shorter carrier lifetime. The optical intensity can either be generated inside the SOA, or injected into the SOA from an external laser. The first case is the so-called gain-clamped SOA (GCSOA). The gain of GCSOAs is fixed by the device design and is lower than that for an SOA. The GCSOAs can have high optical intensities and, therefore, fast gain recovery, but the internal lasing mode leads to relaxation oscillations in the gain recovery. The second case, where the optical intensity is injected into the SOA by an external laser, is more flexible as the gain of the SOA is not fixed by the design, and the wavelength of the external laser can be changed. The gain recovery of the externally injected SOAs exhibits an exponential recovery without oscillations.

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6.5

Summary

In this chapter, we have described two main classes of SOA-based optical switches which are space based or wavelength based. The switching can be performed either in the linear (electrically controlling) or in the nonlinear regime (all-optically controlling) of SOAs. SOA itself is suitable for high-capacity WDM optical networks. Thus, for applications that merit high band rate all-optical switches and rapidly reconfigurable switching, an SOA is still one of the promising candidate technologies. Some design techniques for enhancing switching speed, polarizationdependent gain, stability, reliability, power consumption and operation bandwidth are reviewed in this chapter. However, scalability, cost and power consumption and integrability of the SOA-based switch when the number of input/output ports are increased are still challenges for large-scale commercial deployment.

6.6

References

Adams, M.J., Davies, D.A.O., Tatham, M.C. and Fisher, M.A. (1995), ‘Tutorial review – nonlinearities in semiconductor laser amplifiers’, Journal of Optical and Quantum Electronics, 27(1): 1–13. Banchi, L., Presi, M., D’Errico, A., Contestabile, G. and Ciaramella, E. (2010), ‘All-optical 10 and 40 Gbit/s RZ-to-NRZ format and wavelength conversion using semiconductor optical amplifiers’, Journal of Lightwave Technology, 28(1): 32–8. Barnsley, P.E., Isaac, J.J. and Elton, D.J. (1990), ‘Ultra-low reflectivity broadband 1.5 µm GaInAsP semiconductor optical amplifier’, Electronics Letters, 26(12): 825–7. Bimberg, D., Grundmann, M. and Ledentsov, N.N. (1999), Quantum Dot Heterostructures. New York: Wiley. Burmeister, E.F. and Bowers, J.E. (2006), ‘Integrated gate matrix switch for optical packet buffering’, IEEE Photonics Technology Letters, 18(1): 103–5. Castrejón, R.G. and Duelk, M. (2006), ‘Uni-directional time-domain bulk SOA simulator considering carrier depletion by amplified spontaneous emission’, IEEE Journal of Quantum Electronics, 42(6): 581–8. Colden, L.A. and Corzine, S.W. (1995), Diode Lasers and Photonic Integrated Circuits. New York: Wiley. Connelly, M.J. (2001), ‘Wideband semiconductor optical amplifier steady-state numerical model’, IEEE Journal of Quantum Electronics, 37(3): 439–47. Dittmann, L., Develder, C., Chiaroni, D., Neri, F., Callegati, F., Koerber, W., Stavdas, A., Renaud, M., Rafel, A., Solé-Pareta, J., et al. (Eds.) (2003), ‘The European IST project DAVID: a viable approach towards optical packet switching’, IEEE JSAC: Special Issue on High-Performance Optical/Electronic Switches/Routers for High-Speed Internet, 21(7): 1026–40. Dorgeuille, F., Noirie, L., Faure, J.P., Ambrosy, A., Rabaron, S., Boubal, F., Schilling, M. and Artigue, C. (2000), ‘1.28 Tbit/s throughput 8 × 8 optical switch based on arrays of gain-clamped semiconductor optical amplifier gates’, Proceedings of Conference on Optical Fiber Communications. Paper PD 18–1. Dreyer, K., Joyner, C.H., Pleumeekers, J.L., Burrus, C.A., Dentai, A., Miller, B.I., Shunk, S., Sciortino, P., Chandrasekhar, S., Buhl, L., Storz, F. and M. Farwell. (2002), ‘High-

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gain mode-adapted semiconductor optical amplifier with 12.4 dBm saturation output power at 1550 nm’, Journal of Lightwave Technology, 20(4): 718–21. Duthie, P.J. and Wale, M.J. (1988), ‘Rearrangeably nonblocking 8 × 8 guided wave optical switch’, Electronics Letters, 24(10): 594–6. Eramo, V. and Listanti, M. (2009), ‘Power consumption in bufferless optical packet switches in SOA technology’, Journal of Optical Communication Networks, 1(3): B15–29. Ezra, Y.B., Lembrikov, B.I. and Haridim, M. (2009), ‘Ultrafast all-optical processor based on quantum-dot semiconductor optical amplifiers’, Journal of Quantum Electronics, 45(1): 34–41. Fu, L., Schweizer, H., Zhang, Y., Li, L., Baechle, A.M., Jochum, S., Bernatz, G.C. and Hansmann, S. (2004), ‘Design and realization of high-power ripple-free superluminescent diodes at 1300 nm’, IEEE Journal of Quantum Electronics, 40(9): 1270–4. Gustavsson, M., Lagerstrom, B., Thylen, L., Janson, M., Lundgren, L., Morner, A.C., Rask, M. and Stoltz, B. (1992), ‘Monolithically integrated 4 × 4 InGaAsP/InP laser amplifier gate switch arrays’, Electronics Letters, 28(24): 2223–5. Hamza, H.S. and Deogun, J.S. (2007), ‘Design and analysis of strictly nonblocking WDM optical-switching networks’, Journal of Optical Networking, 6(4): 322–40. Haus, H.A. (2000), ‘Noise figure definition valid from RF to optical frequencies’, IEEE Journal of Selected Topics in Quantum Electronics, 6(2): 240–7. Houghton, D.C., Davies, M. and Dion, M. (1993), ‘Limits of strain compensation in MQW InGaAsP-InP 1.5 µm lasers’, Proceedings of Fifth International Conference on Indium Phosphide and Related Materials, 187–90. Janson, M., Lundgren, L., Morner, A.C., Rask, M., Stoltz, B., Gustavsson, M. and Thylen, L. (1992), ‘Monolithically integrated 2 × 2 InGaAsP/InP laser amplifier gate switch arrays’, Electronic Letters, 28(8): 776–8. Kato, T., Sasaki, J., Shimoda, T., Hatakeyama, H., Tamanuki, T., Kitamura, S., Yamaguchi, M., Sasaki, T., Komatsu, K., Kitamura, M. and Itoh, M. (1999), ‘Hybrid integrated 4 × 4 optical matrix switch module on silica based planar waveguide platform’, IEICE Transactions on Electrons, E82-C(2): 305–12. Kogelnik, H. and Yariv, A. (1964), ‘Considerations of noise and schemes for its reduction in laser amplifiers’, Proceedings of the IEEE, 52(2): 165–72. Manning, R.J., Giller, R., Yang, X., Webb, R.P. and Cotter, D. (2007), ‘Faster switching with semiconductor optical amplifiers’, Photonics in Switching Conference, 145–6. Prucnal, P.R., Glesk, I., Toliver, P. and Xu, L. (2006), ‘Optical switching with SOAs’. In: Optical Switching. pp. 215–44. New York: Springer. Ratowsky, R.P., Dijaili, S., Kallman, J.S., Feit, M.D., Walker, J., Goward, W. and Lowry, M. (1998), ‘Modeling a distributed spatial filter low-noise semiconductor optical amplifier’, 1998 Victoria Meetings, Victoria, Canada. 29 March–3 April 1998. Song, J.H., Kim, H.S., Shim, E.D., Park, J.W. and Baek, Y.S. (2004), ‘Monolithically integrated 4 × 4 InGaAsP/InP laser amplifier gate switch matrix based on buried ridge stripe waveguides’, Japanese Journal of Applied Physics, 43(1 A/B): L18–20. Srivastava, R., Singh, R.K. and Singh, Y.N. (2009), ‘Design analysis of optical loop memory’, Journal of Lightwave Technology, 27(21): 4821–31. Tanaka, S., Jeong, S.H., Yamazaki, S., Uetake, A., Tomabechi, S., Ekawa, M. and Morito, K. (2009), ‘Monolithically integrated 8:1 SOA gate switch with large extinction ratio and wide input power dynamic range’, Journal of Quantum Electronics, 45(9): 1155–62.

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Tangdiongga, E., Liu, Y., de Waardt, H., Khoe, G.D., Koonen, A., Dorren, H.J.S., Shu, X. and Bennion, I. (2007), ‘All-optical demultiplexing of 640 to 40 Gbits/s using filtered chirp of a semiconductor optical amplifier’, Optics Letters, 32: 835–7. Teimoori, H., Apostolopoulos, D., Vlachos, K.G., Ware, C., Petrantonakis, D., Stampoulidis, L., Avramopoulos, H. and Erasme, D. (2008), ‘Optical-logic-gate aided packetswitching in transparent optical networks’, Journal of Lightwave Technology, 26(16): 2848–56. Tucker, R.S. and Baney, D.M. (2001), ‘Optical noise figure: theory and measurements’, Optical Fiber Communication, 3: WI1-1–WI1-3. Uomi, K. (1990), ‘Modulation-doped multi-quantum well (MD-MQW) lasers. I. Theory’, Japanese Journal of Applied Physics, 29(1): 81–7. Vawter, G.A. and Myers, D.R. (1989), ‘Useful design relationships for the engineering of thermodynamically stable strained-layer structures’, Journal of Applied Physics, 65: 4769–73. Vlachos, K., Zhang, J., Cheyns, J., Chi, N., Van Breusegem, E., Tafur Monroy, I., Jennen, J.G.L., Holm-Nielsen, P.V., Peucheret, C., O’Dowd, R., Demeester, P. and Koonen, A.M.J. (2003), ‘An optical IM/FSK coding technique for the implementation of a label-controlled arrayed waveguide packet router’, Journal of Lightwave Technology, 21(11): 2617–28. Vorob’ev, L.E. (2000), ‘Population inversion and IR amplification induced by intersubband electron transitions and resonant auger processes in quantum wells’, JETP Letters, 71(12): 511–5. Willatzen, M., Uskov, A., Mrk, J., Olesen, H., Tromborg, B. and Jauho, A.P. (1991), ‘Nonlinear gain suppression in semiconductor lasers due to carrier heating’, IEEE Photonics Technology Letters, 3(7):606–9. Xu, J., Zhang, X. and Mørk, J. (2010), ‘Investigation of patterning effects in ultrafast SOA-based optical switches’, Journal of Lightwave Technology, 46(1): 87–94. Zhang, R., Zhou, F., Bian, J., Zhao, L., Jian, S., Yu, S. and Wang, W. (2007), ‘A short carrier lifetime semiconductor optical amplifier with n-type modulation-doped multiple quantum well structure’, Semiconductor Science and Technology, 22: 283–6. Zhou, C. and Yang, Y. (2002), ‘Wide-sense non-blocking multicast in a class of regular optical WDM networks’, IEEE Transactions on Communications, 50(1): 126–34.

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7 Switching based on optical nonlinear effects M.P. FOK and P.R. PRUCNAL, Princeton University, USA Abstract: This chapter discusses the physical properties and performance of various types of nonlinear-effect-based optical switches. Nonlinear-effect-based optical switches are important because of the all-optical and high-speed characteristics of Kerr nonlinearities that will enable the realization of future all-optical high-speed networks. The chapter first reviews the nonlinear effects and nonlinear devices used in nonlinear-effect-based optical switches and then discusses their characteristics. The chapter further describes the development of various structures for nonlinear-effect-based optical switches and discusses their characteristics, advantages and applications. Key words: optical nonlinearity, Kerr effect, nonlinear fiber, semiconductor device.

7.1

Introduction

Optical Kerr nonlinearity is a useful tool to implement optical switches. The switching relies on the ultrafast light interaction in an optical nonlinear medium that can be a semiconductor device, a piece of nonlinear fiber, or a nonlinear waveguide. It switches out a signal either through self-switching or through the control of an optical signal. The use of optical nonlinearity allows all-optical processing of signals which will help realize future all-optical high-speed networks. Kerr nonlinearity has been widely used in many applications such as signal regeneration, wavelength conversion/multicasting, optical switching/ routing, optical demultiplexing and optical delay/buffering. Owning to the wide applications of Kerr nonlinearity, multiple functions can be implemented simultaneously in a single nonlinear device. In the early days of research on optical switching, switches based on optical nonlinear effects were built using fiber loop mirrors, as proposed and experimentally demonstrated by Doran and Wood (1988) and Doran et al. (1989), respectively. Early fiber-based optical switches required the use of extremely high power and long fiber length for switching, although these two requirements were not favorable for practical implementation and limited the performance of the system. Semiconductor devices are an attractive alternative to realizing compact optical switches which are suitable for integration. However, due to the slow carrier recovery in semiconductor devices, their usage has been very limited. In 1993, Sokoloff et al. experimentally demonstrated an ultrafast switching device named ‘terahertz optical asymmetric demultiplexer’ (TOAD), based on a semiconductor optical amplifier being placed off-center in a 181 © Woodhead Publishing Limited, 2010

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loop configuration, which operates at very high speed. Due to the compactness of semiconductor optical amplifiers, walk-off between the signal and control is negligible. The TOAD has a fast switching time with low switching power. Different types of nonlinear waveguides have been developed and fabricated to increase the compactness of nonlinear devices; however, such waveguides require free-space optical coupling, resulting in a relatively large device-to-fiber coupling loss. Due to the attractive advantages of nonlinear fibers including passiveness, robustness and ultrafast response, considerable research has focused on improving the dispersion characteristic, nonlinearity and compactness of nonlinear fiber, yielding significant improvements in performance. Over the past several years, the length of nonlinear fiber has shrunk from tens of kilometers to less than a meter. Due to recent developments in specialty nonlinear fiber, its usage in optical switching has become practical and is drawing continued attention from the research community. In this chapter, we describe different nonlinear effects and nonlinear media in sections 7.2 and 7.3 for the implementation of optical switches. In section 7.4, we display a series of optical switches with different structures, discussing their principles, characteristics and limitations. At the end of the chapter, the criteria for an ‘ideal’ optical switch are discussed, and open questions which are being explored by the research community are described.

7.2

Nonlinear effects for optical switches

Optical nonlinearity is an attractive phenomenon that will provide an all-optical solution for future optical networks. Various types of all-optical functionalities can be implemented, including wavelength conversion, signal regeneration, delay and switching based on different kinds of optical nonlinear effects. In this section, these nonlinearities are briefly explained, including self-phase modulation, crossphase modulation, four-wave mixing, cross-absorption modulation and spatial soliton trapping and dragging.

7.2.1 Self-phase modulation Self-phase modulation (SPM) occurs when a strong signal with time-varying intensity (e.g. a short pulse) propagates in a nonlinear medium. Under the influence of a strong optical signal with carrier frequency ω, the refractive index of a nonlinear medium n is related to the linear refractive index n0 of the medium, the second-order nonlinear refractive index n2 and the intensity of the input signal |E|2 based on the optical Kerr effect, as shown in equation (7.1): n(ω) = n0(ω) + n2(ω)|E|2

[7.1]

resulting in a phase shift of the pulse itself. When the intensity of the input pulse is unchanged during propagation, the nonlinear phase shift is governed by equation (7.2):

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Switching based on optical nonlinear effects


=

 n2 2 E z c

183 [7.2]

where z is the propagation distance of the pulse. Therefore, as the input signal propagates through the nonlinear medium, the input signal experiences a nonlinear phase shift that is related to the intensity of the signal itself. At the same time, new frequency components are generated since the instantaneous frequency is the time derivative of phase, resulting in spectral broadening.

7.2.2 Cross-phase modulation Cross-phase modulation (XPM) is similar to SPM, except that the process involves two signals, where the nonlinear phase shift is induced on a probe signal by a strong pump signal. The input probe signal is phase-modulated by the time-varying strong pump signal. The nonlinear phase shift depends on the intensity of the two input light beams as shown by equation (7.3):


probe =

 probe n2  2 2 E + 2 Epump  z  probe  c

[7.3]

where ωprobe is the frequency of the probe light and Eprobe and Epump are the electric fields of the probe and the stronger pulsed pump, respectively. The first term corresponds to SPM while the second term is for XPM. When the pump and probe signals are of the same polarization, the strength of XPM is twice as strong as SPM, as shown by the factor of two in the second term. The nonlinear phase shift of the probe signal depends only on the pump intensity when the probe signal is much weaker than the pump signal. It is worth noticing that XPM in optical fiber is directional, i.e. XPM only occurs in the signal that is co-propagating with the control signal. However, XPM in semiconductor devices is not directional because the control signal affects both the co-propagating and counter-propagating signal.

7.2.3 Four-wave mixing Four-wave mixing (FWM) involves the interaction between three input light beams with different wavelengths that are overlapping in time to produce a fourth wavelength. The beating between the input fields and the sum and difference frequencies gives rise to the generation of a fourth field. In optical switching and most signal processing applications, degenerate FWM is used, where two of the three input fields have the same wavelength. The degenerate FWM process is described by equation (7.4): Ec = (E2 • E 1*)E2γ(ω2 – ω1)exp[i(ωct + ∆Φc)]

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[7.4]

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where ω1, ω2, ωc, E1, E2 and Ec are the frequencies and the field amplitudes of input 1 (signal), input 2 (pump) and the converted signal, respectively. ∆Φc is the resulting phase of the FWM output, which is represented by (2Φ2–Φ1). As the wavelength separation ∆ω between the two input light beams increases, the complex coupling coefficient γ(ω2–ω1) of the two inputs decreases, resulting in a decrease in FWM efficiency. Here, the two input beams beat in the nonlinear medium, resulting in an index grating that scatters all other input beams that launched to the medium. As a result, sidebands with a frequency shift of ±∆ω near each of the pumps are generated. According to equation (7.4), FWM transfers both the amplitude and the phase information to the FWM output allowing FWM to operate in a data-format and bit-rate transparent manner.

7.2.4 Cross-absorption modulation Cross-absorption modulation (XAM) of an input light beam occurs in a negatively biased electro-absorption modulator (EAM) through the injection of a strong optical signal. By applying an electric field to the EAM, the absorption coefficient of the active region increases. This process, called the ‘Franz–Keldysh’ effect in bulk EAMs, is due to the effective bandgap of a semiconductor decreasing with increasing electric field. In the absence of an electric field, the active region bandgap in the EAM is just wide enough for the input light beam to pass through the device. When a sufficiently strong electric field is applied across the p-n junction, the effective bandgap is narrowed, such that the device starts to absorb the input light. In multi-quantum well EAM, the change in absorption coefficient is based on the quantum-confined Stark effect, which is a stronger absorption effect resulting from applying the electric field. XAM allows switching out the input light through the use of a strong optical signal. The strong optical signal is injected into the EAM, leading to a reduction of the absorption through electric field screening and absorption saturation. Therefore, the absorption coefficient is temporarily decreased, allowing the input signal to pass through the EAM, with minimal absorption. The strong optical signal and the input signal can be either co-propagating or counter-propagating in the EAM.

7.2.5 Spatial soliton trapping and dragging When the diffraction effect in a medium is cancelled by the Kerr nonlinearity, a fundamental soliton is formed. Here, an intense hyperbolic secant shaped light beam induces a refractive index distribution of a similar shape in the nonlinear medium, such that the center part of the light beam travels slower than the tails. This results in self-focusing of the beam, cancelling the light beam spreading due to diffraction. When two solitons are launched together into a nonlinear medium, the nonlinear refractive index is related to the coupling between the two beams. Spatial soliton trapping occurs when the relative propagating angle between the

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beams is small and the two input solitons are similar. If the two solitons are in phase, they tend to attract to each other, leading to a temporary superposition of the two solitons, followed by splitting into two fundamental solitons again as the solitons propagate. If the two solitons are out of phase, then they repel each other and separate further apart during their propagation. The attraction and repelling effects are weak if the two input solitons have a large separation. By controlling the relative phase between the two input solitons, the spatial position of the solitons at the output can be adjusted. Spatial soliton dragging occurs when the relative propagation angle is larger, and the two solitons are different. Here, the two solitons are orthogonally polarized so that their interaction is no longer phase-sensitive. The pump soliton needs to be more intense to maintain nonlinear propagation in the medium and propagate as a spatial soliton. The signal beam for dragging the pump soliton needs to be just strong enough to induce a nonlinear index change that can be sensed by the pump soliton. Since the signal beam only has to drag the pump soliton by the size of the pump beam, i.e. size of the spatial aperture, the signal beam does not need to be very intense. Due to the orthogonal nature of the signal and pump, the undesired coupling of the signal beam can be blocked easily using a polarizer at the output.

7.3

Nonlinear devices for optical switches

The nonlinear medium is the key element in any nonlinear process since it governs the types of nonlinear process supported and the efficiency, size, speed and power characteristics. A suitable nonlinear device should be chosen to fit the requirements of different applications. In this section, the characteristics of various kinds of commonly used nonlinear devices are described.

7.3.1 Semiconductor-based devices Semiconductor optical amplifiers The study of semiconductor optical amplifiers (SOAs) was first carried out in the 1960s, near the time when the semiconductor laser was invented. The invention of double heterostructure laser accelerated the use of SOAs in optical communication systems. SOA using InP/InGaAsP is designed to operate in the 1.3 and 1.55 µm windows. The active region in the SOA provides gain to the input signal, whereas the waveguide confines the optical signal to propagate in the active region. An external electric current provides carriers to the SOA which occupy energy states in the conduction band of the active region, leaving holes in the valence band. Stimulated recombination of carriers and holes occurs when an input light beam with suitable energy is launched into the SOA, resulting in the emission of photons with the same phase, frequency and propagation direction. When the stimulated emission is stronger than the stimulated absorption, signal amplification

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is obtained. Through the injection of current or an optical beam, the carrier density in the SOA changes, resulting in a change in refractive index of the device. In this way, various kinds of nonlinear effects can be obtained in SOAs, enabling them to be widely used as nonlinear devices. SOAs are very compact and can be cascaded easily and integrated monolithically with other functional devices on a single substrate. Compared with fiber-based nonlinear devices, the energy required to induce sufficient nonlinearity is lower, and the input signal also gets amplified in the SOA to produce an output with sufficient power to control a second stage device. Electro-absorption modulators The EAM is another type of semiconductor device that can be used for nonlinear optical signal processing. The absorption in EAMs depends on the applied electric field and is based on the Franz–Keldysh effect in bulk EAMs, i.e. the effective bandgap decreases with an increase in electric field. In multi-quantum well EAMs, the change in absorption coefficient is based on the quantum-confined Stark effect. Bulk EAMs consist of an active semiconductor region sandwiched between p-type and n-type-doped layers. Without applying a bias voltage across the p-n junction, the bandgap of the active region is just large enough for the input light to pass. Applying a reverse bias reduces the effective bandgap such that the input light is absorbed. Although high-speed EAM-based switches are usually implemented through electrical gating of the device, switching can also be achieved optically through an optical pump based on XAM. The absorption of the input signal decreases as the power of the optical pump increases. EAMs can be easily integrated monolithically on a single substrate with SOAs and laser diodes to implement various types of functions.

7.3.2 Optical fibers Optical fibers have been used for data transmission for many years, and their response in such applications is linear provided the optical power is low. Due to the anharmonic motion of bound electrons under the influence of an applied field, the response of optical fiber becomes nonlinear for strong optical beams. Most of the nonlinear effect in optical fiber originates from the intensitydependence of the refractive index through the Kerr effect, as described by equation (7.1). Unlike SOAs in which the operation speed is limited by the carrier recovery time, optical fiber has instantaneous and high-speed response to strong optical beams. Although optical fiber is suitable for high-speed processing, the long length of fiber required to produce sufficient nonlinearity makes it impractical to be used in real signal processing applications. With the development of specialty fibers, optical fiber-based nonlinear devices have become increasingly attractive and practical in recent years.

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Step-index fibers Standard single mode fiber (SMF) has a very low nonlinear coefficient, enabling it to be used as the transmission fiber in optical communication system. In the early days, SMF was used as a nonlinear medium, requiring tens of kilometers of fiber to provide a sufficient nonlinear effect. The invention of dispersion-shifted fiber (DSF) reduced the walk-off effect by moving the zero-dispersion wavelength to the 1550 nm regime, leading to a more effective nonlinear process. Although the nonlinear coefficient was slightly increased, the fiber length required to provide sufficient nonlinear strength was still in the range of kilometers. A number of specialty step-index fibers have been developed to improve the properties of nonlinear fiber; several kinds of them are introduced here. Germania-doped highly nonlinear fibers. Germania-doped silica is the most commonly used material for the core of optical fiber which has been used in telecommunications for over 30 years. Standard SMF has a GeO2 concentration that is usually below 10 mol.%. This fiber has not been replaced by any alternative due to its advantageous properties including excellent stability, strength and long operation life. Highly nonlinear fiber with a moderate doping of GeO2 was previously designed that included a deeply depressed ring surrounding a core doped with Fluorine, resulting in a nonlinear coefficient of 11 W21 km21. At this doping level, a couple of hundred meters of nonlinear fiber is required to induce various types of nonlinear effects. Nonlinear fiber that can be highly doped with Ge having up to 97% concentration has been recently developed (Dianov and Mashinsky, 2005). The high concentration of GeO2 results in a high ∆n of 0.142, as well as a large nonlinear coefficient of 50 W21 km21. The high nonlinear coefficient allows shortening the fiber length and induces a significant amount of nonlinearity. Only 15.5 m of 75 mol.% Ge-doped highly nonlinear fiber is needed to build an optical nonlinear switch. This kind of highly nonlinear fiber provides a reliable and homogeneous platform for various types of nonlinear effects. Highly nonlinear bismuth oxide fibers. Highly nonlinear bismuth oxide fiber (Bi-NLF) (Sugimoto et al., 2004) is based on a step-index structure, where the refractive indices of the core and the cladding glasses are 2.22 and 2.13, respectively. Bismuth oxide has a very nonlinear nature with n2 ranging from 30 × 10220 to 110 × 10220. The numerical aperture is 0.64, while the mode field diameter is 1.97 µm. The Bi-NLF exhibits an extremely large nonlinear coefficient of ~1100 W21 km21 at 1550 nm, resulting from the highly nonlinear nature of bismuth oxide and the small effective core size, where the nonlinear coefficient (γ) is governed by equation (7.5):


=

2 n2  Aeff

[7.5]

The nonlinear coefficient can go up to as high as 1360 W–1 km–1. With the development of Bi-NLF, the length of a nonlinear fiber is dramatically decreased

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to less than 1 m. Owing to the small fiber core size of Bi-NLF, it can be fusion spliced to a standard SMF with an ultra-high NA silica fiber (NA ~ 0.35) as an intermediate fiber. The total splicing loss for both ends is typically 4.3 dB, while the propagation loss is 0.8 dB m21. The Bi-NLF has a dispersion coefficient of 2300 ps nm21 km21 at 1550 nm. Due to the large nonlinear coefficient, only a very short piece of Bi-NLF is needed, and the effect of dispersion is very small. Chalcogenide fibers. Chalcogenide glasses are a composite of chalcogen elements (S, Se and Te) and other elements such as As, Ge, P, Sb and Si (Ta’eed et al., 2007), resulting in large refractive indices. The refractive index of a common chalcogenide glass As2S3 is about 2.4, which can be further increased by replacing the sulfur with selenium and tellurium. The zero-dispersion wavelength for chalcogenide glass is usually in the mid-IR range, while the 1550 nm regime experiences a large normal dispersion of a couple of hundred ps nm21 km21. A commonly used chalcogenide glass fiber – As2Se3 – has a dispersion coefficient of 2560 ps nm21 km21 at 1550 nm. Owing to the large nonlinearity of 1200 W21 km21 provided by As2Se3 fiber, only a short piece of fiber of about 1 m is needed to provide the required nonlinearity. The optical signals are usually butt-coupled to the As2Se3 fiber with Aeff = 37 µm2 through a short segment of higher numerical aperture fiber. The total loss in an As2Se3 fiber includes 0.6 dB of splicing loss, 2.2 dB coupling loss per facet and a propagation loss of 1 dB m21. For most nonlinear effects, As2Se3 fiber of 2 m or less is enough to provide sufficient nonlinearity. Photonic crystal fibers Photonic crystal fibers (PCFs) are mainly divided into two categories based on their guiding principles – index guiding and photonic bandgap guiding. Indexguiding PCF has a solid core and a microstructured cladding, and its guiding mechanism is similar to conventional fiber where light is guided based on total internal reflection. The microstructure in the cladding provides a much higher effective index contrast between the core and cladding, such that a stronger confinement of light results. For the photonic bandgap fiber, since the guiding mechanism is by a bandgap structure, the light can be confined in a lower-index core, or even a hollow core filled with air. Highly nonlinear PCF is usually based on index-guiding fiber and can be made to have a very small core (1.5–3.0 µm), which helps to increase the optical intensity within the fiber core, enhancing the effective nonlinearity of the fiber. PCFs provide a great deal of flexibility in terms of dispersion, nonlinearity and attenuation. One type of the commercially available highly nonlinear PCF has a dispersionflattened characteristic and a nonlinear coefficient of 11 W21 km21 (Hansen et al., 2003). The PCF has a small core that is suitable to be spliced to an SMF via a high NA fiber with 0.7 dB loss per facet.

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7.3.3 Other types of nonlinear devices In addition to optical fibers, various kinds of nonlinear waveguides can be used as the nonlinear medium for optical switches. Nonlinear waveguides can be made using the same material as nonlinear fibers, but with a more compact size and can be potentially integrated monolithically with multiple high-speed functional devices on a single chip. Typically, the insertion loss of waveguides is relatively large, due to both the fiber coupling loss and the propagation loss in the waveguide. The asymmetry of waveguide also leads to its being polarization-sensitive. An example of using nonlinear waveguides is the demultiplexing of a 640 Gb s21 signal using FWM in a 5 cm long chalcogenide glass waveguide (Galili et al., 2009). A 2.2 µm thick As2S3 layer is deposited onto a silica-on-silicon substrate, resulting in a 2 µm wide rib waveguide. Due to the high nonlinear refractive index of As2S3, a nonlinear coefficient of ~4100 W21 km21 is resulted in the waveguide with an effective mode area of 2.9 µm2. The insertion loss using the As2S3 waveguide is about 10 dB, which includes the fiber coupling loss and the loss inside the waveguide.

7.4

Structure of nonlinear-effect-based optical switches

Nonlinear-effect-based all-optical switches have promise in numerous applications including future all-optical networks. Optical switches having various configurations have been proposed and demonstrated. In this section, we cover several types of nonlinear-effect-based optical switches, including nonlinear optical loop mirrors with and without external control, Mach–Zehnder interferometers, ultrafast nonlinear interferometers, non-interferometric-based switches and spatial solitonbased 1 × N switches. There are many other types of switches that are not covered in this section including nonlinear directional couplers (Jensen, 1982), nonlinear polarization rotation based Keff shutters (Duguary and Hansen, 1969) and microring switches (Almeida et al., 2004).

7.4.1 Nonlinear optical loop mirror Nonlinear optical loop mirrors (NOLMs) were first proposed in 1989 (Doran et al., 1989) to demonstrate the self-switching of pulses. The principle of NOLMs is based on the fact that the two counter-propagating beams experience different nonlinear phase shift strengths in the nonlinear medium. The configuration of an NOLM is shown in Fig. 7.1; it consists of an optical coupler and a fiber loop for nonlinearity. An input is launched into the loop mirror, as indicated by E1, and the coupler at the fiber loop has an uneven splitting ratio (α), so that the power of the two counterpropagating beams, E2 and E3, is different. The stronger beam induces stronger SPM than the weaker beam, such that a phase difference results between the two counterpropagating beams after passing through the nonlinear fiber loop. Depending on the

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Fiber loop

E3

E2 Coupler Input

EO1

E1

α:1 – α

EO2 Output

7.1  Basic configuration of a nonlinear optical loop mirror (NOLM). α: coupling ratio.

strength of the input signal, the nonlinear phase modulation resulting from the stronger beam as well as the phase difference between the two counter-propagating beams changes. The two counter-propagating beams meet again at the coupler and interfere, converting the phase difference between them to a corresponding amplitude at the output of the loop mirror. The portion of signal that constructively interferes at the coupler comes out as Eo2; otherwise, it is reflected back to the input as Eo1. The transmission function of an NOLM is governed by equation (7.6): T = 1 – 2α(1 – α)[1 + cos(1 – 2α)φ]

[7.6]

where α is the coupling ratio of the coupler and φ is the nonlinear phase shift. The highest switching ratio is obtained when α is close to 0.5; however, either a stronger input signal or a stronger nonlinear medium is required to induce enough phase difference between the counter-propagating beams for switching. The switching of CW, pulse and soliton were demonstrated by Blow et al. (1989). To better utilize the nonlinearity in the NOLM, a modified NOLM using a loop amplifier (Fermann et al., 1990) or an attenuator (O’Neill and Webb, 1990; Striegler et al., 2005) is proposed and demonstrated, as shown in Fig. 7.2. Instead of using an uneven splitting ratio coupler to create different SPMs at the two counterpropagating beams, an in-loop amplifier/attenuator is used to change the amplitude of one of the beams (amplifying E2 into E2a in Fig. 7.2) before launching into the nonlinear medium. Thus, a difference in nonlinear phase between the two counterpropagating beams results. More recently, a low-power modified NOLM with only 15.5 m of highly Ge-doped silica-based fiber is demonstrated (Kravtsov et al., 2007). The modified NOLM uses a 90:10 splitting ratio and an optical isolator to balance the power of the two counter-propagating branches. The application of a self-switching NOLM including autocorrelation peak extraction (Kravtsov et al., 2007), noise reduction (Yamada and Nakazawa, 1994; Striegler et al., 2005; Cvecek et al., 2006) and pedestal suppression (Smith et al., 1990) are demonstrated.

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Fiber loop

E2a

Amplifier E3a

E2a E2

Input EO1

E1

E3

Output

Coupler 50:50

EO2

7.2  Configuration of a modified NOLM with an in-loop amplifier.

7.4.2 NOLM with external control signal In many optical switching applications such as demultiplexing in optical time division multiplexing (TDM), it is desired to have the switching operation determined by an external control signal. In 1990, Blow, Doran, Nayar and Nelson demonstrated an NOLM that switches out the input 1.5 µm CW light with a 1.3 µm control signal. The configuration is shown in Fig. 7.3. Since the coupler of the loop has a coupling ratio of 50:50 for the signal, but it is 100:0 for the control signal, the control signal only co-propagates with the clockwise signal but not with the counter-clockwise one. Since XPM in fiber is directional, only the clockwise

Fiber loop

Control at 1.3 µm

Signal only

Control + signal

Signal at 1.53 µm

50:50 (Signal) 100:0 (Control)

Transmitted signal

Reflected signal

7.3  Configuration of an NOLM with external optical control.

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signal experiences a nonlinear phase shift while the counter-clockwise signal does not, resulting in a phase difference between the two counter-propagating beams after propagating in the nonlinear medium. Optical demultiplexing from 491.8 to 75.66 MHz is achieved (Blow, Doran, Nayar and Nelson, 1990) using the above scheme, i.e. every 13th pulse is switched out. Using polarization-maintaining fiber and couplers, the NOLM was later modified (Avramopoulos et al., 1991) so that the same wavelength can be used for both the signal and the pump, while the signal and pump are separated based on their polarization difference. In 1992, Andrekson et al. performed a demultiplexing of 64 Gb s21 signal to 4 Gb s21 signal using 14 km of DSF, using the configuration as shown in Fig. 7.4. The signal is injected at the input of the loop as indicated by E1, while the control signal is injected inside the fiber loop through an optical coupler, instead of injecting it at the input of the fiber loop. In the fiber loop, the control signal only affects the co-propagating beam but not the counter-propagating one. Therefore, a nonlinear phase shift is induced in the co-propagating beam, resulting in a phase difference between the two counter-propagating beams. An optical bandpass filter is placed at the output of the loop to block the control signal while letting the demultiplexed signal pass through. The use of optical fiber as the nonlinear medium gives an instantaneous response to the induced nonlinearity and works well with high-speed signals. However, a long piece of fiber (kilometers long) is required and the required switching power is relatively high. The use of an SOA reduces the switching power and increases the compactness of the scheme. However, the slow recovery time of an SOA limits its usage in high-speed systems. Sokoloff et al. (1993) demonstrated an SOA-based NOLM demultiplexer with a tunable switching window that is capable of demultiplexing Tb s21 pulse trains. The configuration

Fiber loop Control

Dummy

E2 Signal E1

E3 Coupler 50:50

Demultiplexed output EO2

At λsignal BPF

7.4  Configuration of an NOLM with in-loop optical control. BPF: optical bandpass filter.

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∆x

A SO

Control

E3

E2 Signal E1

Coupler 50:50

Demultiplexed output EO2

At λsignal BPF

7.5  Configuration of a terahertz optical asymmetric demultiplexer (TOAD). SOA: semiconductor optical amplifier; BPF: optical bandpass filter.

of the SOA-based NOLM, called terahertz optical asymmetric demultiplexer, is shown in Fig. 7.5, where an SOA is offset by ∆x from the center of a short fiber loop. The control signal is injected into the loop through an optical coupler. Unlike nonlinear fiber, XPM in SOA is non-directional, i.e. the control signal affects both the co-propagating (E3) and counter-propagating (E2) signals. The operational principle is shown in Fig. 7.6. Due to the offset of SOA from the midpoint, the input signal from the clockwise direction arrives at the SOA earlier than the one from the counter-clockwise direction, where the delay is equal to two times the offset of the SOA from the middle of the loop. The control signal is adjusted such that it arrives in between the arrival time of the two counter-propagating signals at the SOA. The control signal induces a nonlinear phase shift into pulses B and C in the counter-clockwise beam, while only pulse C in the clockwise beam experiences the induced phase shift. As shown in Fig. 7.6, the offset in the SOA creates a switching window, where its width depends on the SOA offset in the TOAD. Only the pulse that falls inside the switching window (i.e. having different phase shift in the two counter-propagating branches) is directed to the output while the rest are being reflected back. The TOAD requires low switching energy and has the ability of switching at 50 Gb s21 or higher. Considerable effort has been made to improve the NOLM. To enhance the switching extinction ratio, Raman amplification is introduced to the NOLM (Starodumov et al., 1998). A long piece of fiber providing nonlinearity inside the loop is a perfect medium for Raman amplification, and the amplification increases as the control signal increases. A polarization-insensitive NOLM is also developed

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Optical switches ∆x

∆x

SOA Midpoint

Signal in CCW (E3)

A

B

C

C

B

A

Signal in CW (E2)

Induced nonlinear phase shift by control

Control in CCW

Optical nonlinear response in SOA Switching window

7.6  Operation principle of TOAD. SOA: semiconductor optical amplifier; CCW: counter-clockwise; CW: clockwise.

using twisted fiber (Liang et al., 1999) and polarization diversity (Olsson and Andrekson, 1997). In 2002, Sotobayashi et al. demonstrated demultiplexing of a 320 Gb s21 signal using 100 m of highly nonlinear DSF in NOLM. The supported data rate is almost three orders of magnitude higher compared with the first NOLM built in 1990. Due to the fast development of nonlinear fibers, the size of NOLM has shrunk quickly, offering a compact solution to optical switching. NOLM with just 11 m of highly Ge-doped fiber is demonstrated for the demultiplexing of a 160 Gb s21 signal (Kravtsov et al., 2009) and optical logic (Kostinski et al., 2009).

7.4.3 Mach–Zehnder interferometers A Mach–Zehnder interferometer (MZI) is a configuration that is suitable for integration on a small substrate. The switching principle is shown in Fig. 7.7. Both the input signals in the upper and lower arms of the MZI experience nonlinear phase shifts induced by the control signal. However, the time that the phase-shifted portions arrive at the output coupler to produce interference has a temporal delay

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ϕ Upper arm t ϕ Lower arm t Switching window

7.7  Operation principle of a Mach–Zehnder interferometer (MZI) switch. ϕ : nonlinear phase shift.

with respect to each other. The temporal delay leads to a phase difference in the two overlapping signals, resulting in a switching window. The relative temporal delay can be implemented using two control signals that are launched to the MZI in a synchronized manner, but with a certain time interval between them (Nakamura et al., 1994), or using one control signal while having the SOAs in the two branches located asymmetrically with respect to each other (Kang et al., 1995). The configuration of the MZI switch with two control signals is shown in Fig. 7.8. The MZI is built using two optical couplers (C1 and C4) and two nonlinear media in each of the arms of the interferometer. A CW signal is split into two branches at the input coupler, C1 of the MZI, while the control signal is split into two copies using a polarization beam splitter (PBS) and is injected into the two MZI arms separately through optical couplers (C2 and C3). The control signal going to the upper arm has a shorter path length than the one going to the lower arm, and the path difference (∆t) is adjustable for controlling the width of

Control 1 Output

C2 Signal Nonlinear

C1 t

C3

C4

Remained signal

Nonlinear

∇ Control 2

7.8  Schematic illustration of a MZI switch using two control signals. C1–C4: optical couplers; ∆t: time delay between two control signals.

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the switching window. Thus, the control signal arrives at the nonlinear medium in the upper arm of the MZI earlier than at the lower arm, resulting in a temporal offset between the nonlinear phase shifts induced in the two MZI arms. The MZI is biased such that the input signal is switched out only when there is a phase difference between the two MZI arms, i.e. the time interval between the arrival of control signal at the upper arm in the nonlinear medium and that at the lower arm. As mentioned earlier, optical switching based on the MZI configuration can also be achieved using just one control signal while having the nonlinear media in the MZI placed asymmetrically in the two branches, as shown in Fig. 7.9. The control signal is launched from the left coupler (C1), while the input signal is launched from the right coupler (C2), such that the control signal and input signal do not have to be separated by wavelength or by polarization. The two SOAs are placed asymmetrically in the upper and lower arms. Due to the asymmetry, the times when the control signal arrives at the nonlinear media in the upper arm and the lower arm are different. The input signal is switched out only when the signals in the two MZI arms have different nonlinear phase shifts, i.e. the time interval between the control signal arrival at the two nonlinear media. Therefore, the switching window is governed by the asymmetry of the two SOAs. This MZI switch was later integrated into an InGaAsP/InP SLA-MZI for 20 Gbit s21 all-optical add-drop multiplexer for optical time division multiplexing (OTDM) systems (Jahn et al., 1996).

7.4.4 Ultrafast nonlinear interferometers An ultrafast nonlinear interferometer (UNI) is a single-arm interferometer, in which the switching principle is based on XPM in a nonlinear medium. It was first demonstrated using an SOA and a 7.5 m of birefringent fiber (BiF) as a 40 Gb s –1 all-optical logic (Patel et al., 1996a) and a demultiplexer (Patel et al., 1996b). The configuration of a UNI is shown in Fig. 7.10. A UNI consists of two pieces of BiF that are aligned orthogonally; an SOA is placed between the two BiFs and a polarizer at the output. The input signal is aligned at 45° with respect to the BiF Signal

Control

Output

C1

Nonlinear

τ2

τ1

∇

∇

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C2

Nonlinear

7.9  Schematic illustration of a MZI switch with asymmetric nonlinear media. C1–C2: optical couplers; ∆τ1–∆τ2: time delays.

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Switching based on optical nonlinear effects

Signal

TM

BiF POL

BiF

197 Output

TE SOA POL Control

7.10  Configuration of an ultrafast nonlinear interferometer (UNI) switch. BiF: birefringent fiber; SOA: semiconductor optical amplifier; POL: polarizer.

axis, such that the two orthogonally polarized components having a time delay in between, named the TE signal and TM signal, result after passing through the first BiF. The time delay (∆τ) between the TE and TM signal pulses is governed by birefringence (B) and length of the BiF (L), represented by equation (7.7): ∆τ =

L⋅B c

[7.7]

A control signal is launched into the SOA through an optical coupler. When the control signal arrives at the SOA, it is temporally aligned with the TE signal – the delayed input signal component. Because the control signal and the TE signal are aligned in time, the presence of the control signal induces a  phase shift in the TE signal at the SOA, but not the TM signal, i.e. a  phase difference results between the TE and TM signal pulses. The two signal pulses are aligned in time again after propagating through the second piece of BiF. A polarizer is set to be orthogonal with the input signal polarization in the absence of a control signal. The phase shift between the TE and TM signals results in a polarization rotation of /2 when they are recombined at the polarizer. Thus, the input signal is switched out from the polarizer in the presence of a control signal, and it is blocked in the absence of a control signal. The UNI has been modified into various configurations to achieve different applications, including optical logic, demultiplexing and 2 × 2 switching. A configuration exploiting nonlinear fiber is also demonstrated using 0.8 m of highly nonlinear bismuth oxide fiber (Zouraraki et al., 2007). An example of a 2 × 2 switch is achieved by modifying the UNI as shown in Fig. 7.11 (Theophilopoulos et al., 2002) that consists of two inputs and two outputs. Data 1 enters the switch at port A and is launched to port 1 of PBS 1, while data 2 enters the switch at port B and is launched to port 1 of PBS 2. Both the PBSs are aligned at 45° with respect to the BiFs axis, such that each of the input signals splits into two orthogonal components with a half bit delay between them. The control signal is launched at port C to the switch through a fiber coupler such that

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Optical switches Data 1 A

Control C

BiF1 PBS1 1

Data 2 B

BiF2 PBS2

BPF SOA

1

BPF

2

2

D E

7.11  Configuration of a 2 × 2 UNI switch. PBS 1–PBS 2: polarization beam splitters; BiF1–BiF2: birefringent fibers; BPF: optical bandpass filter; SOA: semiconductor optical amplifier.

it is temporally aligned with the preceding polarization components of the two input signals. In the absence of the control signal, data 1 and data 2 are directed to the output through port 1 of PBS 2 and PBS 1, respectively. Data 1 eventually exits at port D of the switch, while signal 2 exits at port E of the switch. In the presence of a control signal, the phase of the two preceding polarization components is changed by . Therefore, the polarizations of the signals are rotated by /2. As a result, data 1 now passes through port 2 of PBS 2 and exits at port E of the switch, while data 2 passes through port 2 of PBS 1 and exits at port D. The control signal allows the switch to change from the bypass state to the exchange state.

7.4.5 Non-interferometric-based optical switch Four-wave mixing The principle of a FWM switch is based on the fact that a new wavelength component is generated only if two or more input wavelengths are present at the same time. FWM-based optical switches can be used for demultiplexing in time division multiple access system using an external control and extraction of autocorrelation peak in optical code division multiple access (CDMA) system through self-switching. FWM-based demultiplexing of a 16 Gb s21 signal using 14 km of DSF is demonstrated (Andrekson et al., 1991) as shown in Fig. 7.12. The input signal (λs) is combined with the control signal (λc) using an optical coupler, they are then amplified and launched to the DSF for FWM. The control signal is temporally aligned with the desired input signal pulses for demultiplexing. FWM occurs in the DSF in the presence of a control signal, and a new wavelength component at λf = (2λc–λs) is generated. It is then extracted using an optical bandpass filter. Switching based on FWM is a bit-rate and data-format transparent approach and works equally well for both non-return-to-zero (NRZ) and return-to-zero (RZ) signals. Research

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199

Control at λc

NLF Signal at λs

at λ f = (2 λc – λs)

Output

BPF

7.12  Schematic illustration of a four-wave mixing (FWM)-based switch. NLF: nonlinear fiber; BPF: optical bandpass filter.

has been done to solve the polarization-sensitivity issue of FWM, and polarization diversity using polarization-maintaining fiber (Calvani et al., 1999) is proposed. Several compact schemes that exploit FWM for 160 Gb s21 to 10 Gb s21 demultiplexing are demonstrated, including the use of 1 m of highly nonlinear bismuth oxide fiber (Scaffardia et al., 2006) and 5 cm of ultra-highly nonlinear As2S3 planar waveguide (Pelusi et al., 2007). Demultiplexing of a 640 Gb s21 signal is experimentally demonstrated using an As2S3 waveguide (Galili et al., 2009), which is 40 times faster than the FWM-based demultiplexing achieved in 1991. The self-switching of an autocorrelation peak in an optical CDMA system was proposed and experimentally demonstrated using 35 cm of highly nonlinear bismuth oxide fiber (Fok et al., 2009). The operation is based on the temporal alignment requirement of FWM, which is a condition that is inherently satisfied by the autocorrelation peaks of optical CDMA signal. Figure 7.13 illustrates the schematic and principle of self-switching in FWM. In a 2D optical CDMA system which uses wavelength-hopping time-spreading codes, an optical correlator typically decodes the signal by aligning all the wavelengths of the desired code in time, while all the wavelengths of the interfering users are spread over time, resulting in multiple access interference (MAI). Since the wavelengths of the MAI are not aligned in time, it cannot produce FWM, even if the total power level is high. Therefore, FWM occurs only for the autocorrelation peaks, which necessarily have all the wavelengths of the code aligned in time. By selecting the FWM output, only the autocorrelation peaks are obtained. The

OCDMA signals Decoder

t

t

t

NLF

Autocorrelation peak only BPF

7.13  Principle of self-switching of autocorrelation peak using four-wave mixing. NLF: nonlinear fiber; BPF: optical bandpass filter.

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scheme works well even when the cross-correlation peaks are of equal or higher amplitude than the autocorrelation peaks. Cross-absorption modulation in electro-absorption modulator An EAM is a very useful electro-optic switch where a very short switching window results by driving it with an electrical signal. Moreover, the EAM can be used as an all-optical switch through XAM using an optical control signal. The XAM-based switch using an EAM (Oxenlowe et al., 2001) is shown in Fig. 7.14. The input signal is combined with a strong control signal at lower repetition rate and is launched into the EAM. A negative voltage is applied to the EAM such that the input signal is absorbed. By injecting the strong control signal into the EAM, it screens out the electrically induced absorption for the input signal and creates a switching window for the input signal to pass through the EAM, i.e. the input signal is absorbed in the EAM unless a strong control signal saturates the absorption. An optical bandpass filter is placed after the EAM to block the control signal. The width of switching window depends on the strength of the electrical bias and the design of the device. A stronger electrical bias gives a shorter switching window, but also a stronger absorption of the input signal. Therefore, a stronger control signal is needed to switch out the desired input signal.

7.4.6 1×N switch based on spatial soliton interactions There are a number of ways that soliton interaction can be used for optical switching. Both temporal and spatial soliton phenomena are being studied. Temporal soliton switches are mainly represented by time-shift keying, while the spatial soliton phenomenon is represented using amplitude-shift keying and is easily cascadable. Optical switches based on spatial soliton trapping and dragging are described here. Through spatial soliton trapping, an attraction is induced between the two inputs when the two inputs are in phase. On the other hand, repulsion takes place when Control at λc

–ve Bias Signal at λs

Output at λs

EAM

BPF

7.14  Schematic illustration of a cross-absorption modulation (XAM)based switch. EAM: electro-absorption modulator; BPF: optical bandpass filter.

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the two inputs are out of phase (Reynaud and Barthelemy, 1990). The switching of solitons was experimentally demonstrated in a carbon disulfide (Shalaby and Barthelemy, 1991). A frequency-doubled Q-switched and mode-locked Nd:YAG laser with pulse width of 30 ps and power of 1 GW is launched to a MZI to create the two soliton beams for the switch. They are recombined at the MZI output at a slightly different direction but completely superimposed at the input of the carbon disulfide, the nonlinear medium. The sign of the phase difference between the two input solitons governs which soliton is going to trap most of the total initial power resulting in switching. The switching efficiency is higher if the initial light beams are set to be closer spatially. This structure is expanded to a 1×N switching device by modulating the phase of the spatial soliton (Wu, 2005), as shown in Fig. 7.15. The 1×N switch consists of an (i) asymmetric nonlinear Mach–Zehnder interferometer (AMZI) to generate a phase difference between the two solitons, (ii) a uniform nonlinear medium where the spatial solitons are excited and routed and (iii) a nonlinear output waveguide for coupling the spatial solitons that are excited. The phase difference of the two solitons is governed by the length of the MZI arms and the input power of the soliton, as shown in equation (7.8):  = n02 +  E 2

(

1 2

) k (L 0

2


L1 )

[7.8]

where n0 is the linear refractive index, L1 and L2 are the lengths of the two arms of the AMZI, α is the nonlinear coefficient and k0 is the wave number in free space. The phase difference depends on the input power and the optical path difference in the asymmetric NMZI arms and is used to control the routing of signal based on the attraction–repulsion properties of spatial solitons. Spatial soliton dragging is another way to achieve optical switching. When a strong beam and a weak signal beam are orthogonally polarized and are launched

L2

Input AMZI

Uniform nonlinear medium

Outputs

L1

7.15  Schematic illustration of a spatial soliton 1×N switch. AMZI: asymmetric nonlinear Mach–Zehnder interferometer; L1–L2: length of the AMZI arms.

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to a nonlinear medium at the same time but at a different propagation angle, the strong beam is dragged by the weak signal beam and its spatial position is changed over by a beam diameter (Blair et al., 1994). Since the spatial aperture is aligned with the strong beam in the absence of the weak signal, the strong beam is blocked by the spatial aperture when it is dragged by the weak signal. Therefore, the undragged strong beam is the output and can be used as a control of the following stage of the system. Optical switching based on spatial soliton dragging is experimentally demonstrated in AlGaAs waveguides (Kang et al., 1996), as shown in Fig. 7.16. Two overlapping orthogonally polarized optical beams, namely TE beam and TM beam, are launched at various angles with respect to each other. The TM control beam is used to drag the TE beam. The two beams are coupled into the 14 mm long AlGaAs waveguide through a cylindrical telescope. A polarizer is placed after the coupled output of the waveguide such that the TM control beam is blocked. The pin hole is set to block the TE beam in the absence of TM control beam. With the TM control beam, the TE beam is dragged and its lateral position changed and it is able to get out from the pin hole. The lateral shift of the TE beam varies as the power ratio of the two beams changes.

7.5

The ‘ideal’ nonlinear-effect-based optical switch?

Nonlinear-effect-based optical switches provide an ultra-fast option for processing signals due to their instantaneous nonlinear response. They also allow all-optical switching of signals that helps to realize future high-speed all-optical networks. Nonlinear-effect-based optical switches have been developed for the past 20 years since the proposal of the first NOLM in 1988. Research efforts have been made and significant improvement has been achieved in terms of compactness, switching time and switching energy. Semiconductor devices have several advantageous characteristics, including compactness, integration capability and low switching energy. Nonlinear-fiber-based devices stand out in terms of their ultra-high speed,

TM beam blocked by the polarizer

TE TM

TE output

TE beam dragged by the TM beam

7.16  Schematic illustration of a spatial soliton dragging switch.

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passiveness and insignificant pattern effects. Other types of nonlinear waveguides provide a compact and strongly nonlinear medium for performing switching. As seen from the development of the nonlinear-effect-based switches, both size and switching speed are the trends that researchers are working on. Since different types of nonlinear media have different properties and those properties are important in various applications, it is impossible to determine a single champion among them. Besides, other parameters such as the number of inputs and outputs, data format and the ability to cascade devices must also be considered when designing these devices. The choice of optical switch should be applicationoriented and must be determined by the nature of the application.

7.6

References

Almeida, V.R., Barrios, C.A., Panepucci, R.R. and Lipson, M. (2004), ‘All-optical control of light on a silicon chip’, Nature, 431: 1081–4. Andrekson, P.A., Olsson, N.A., Simpson, J.R., Digiovanni, D.J., Morton, P.A., Tanbun-Ek, T., Logan, R.A. and Wecht, K.W. (1992), ‘64 Gb/s all-optical demultiplexing with the nonlinear optical-loop mirror’, IEEE Photonics Technology Letters, 4: 644–7. Andrekson, P.A., Olsson, N.A., Simpson, J.R., Tanbun-Ek, T., Logan, R.A. and Haner, M. (1991), ‘16 Gbit/s all-optical demultiplexing using four-wave mixing’, Electronics Letters, 27: 922–4. Avramopoulos, H., French, P.M.W., Gabriel, M.C., Houth, H.H., Whitaker, N.A. and Morse, T. (1991), ‘Complete switching in a 3-terminal sagnac switch’, IEEE Photonics Technology Letters, 3: 235–7. Blair, S., Wagner, K. and McLeod, R. (1994), ‘Asymmetric spatial soliton dragging’, Optics Letters, 19: 1943–5. Blow, K.J., Doran, N.J. and Nayar, B.K. (1989), ‘Experimental demonstration of optical soliton switching in an all-fiber nonlinear Sagnac interferometer’, Optics Letters, 14: 754–6. Blow, K.J., Doran, N.J., Nayar, B.K. and Nelson, B.P. (1990), ‘Two-wavelength operation of the nonlinear fiber loop mirror’, Optics Letters, 15: 248–50. Blow, K.J., Doran, N.J. and Nelson, B.P. (1990), ‘Demonstration of the nonlinear fibre loop mirror as an ultrafast all-optical demultiplexer’, Electronics Letters, 26: 962–4. Calvani, R., Cisternino, F., Girardi, R. and Riccardi, E. (1999), ‘Polarisation independent all-optical demultiplexing using four-wave mixing in dispersion shifted fibre’, Electronics Letters, 35: 72–3. Cvecek, K., Onishchukov, G., Sponsel, K., Striegler, A.G., Schmauss, B. and Leuchs, G. (2006), ‘Experimental investigation of a modified NOLM for phase-encoded signal regeneration’, IEEE Photonics Technology Letters, 18: 1801–3. Dianov, E. and Mashinsky, V. (2005), ‘Germania-based core optical fibers’, Journal of Lightwave Technology, 23: 3500–8. Doran, N.J., Forrester, D.S. and Nayar, B.K. (1989), ‘Experimental investigation of alloptical switching in fibre loop mirror device’, Electronics Letters, 25: 267–9. Doran, N.J. and Wood, D. (1988), ‘Nonlinear-optical loop mirror’, Optics Letters, 13: 56–8. Duguary, M.A. and Hansen, J.W. (1969), ‘An ultrafast light gate’, Applied Physics Letters, 15: 192–4.

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Fermann, M.E., Haberl, F., Hofer, M. and Hochreiter, H. (1990), ‘Nonlinear amplifying loop mirror’, Optics Letters, 15: 752–4. Fok, M.P., Deng, Y. and Prucnal, P.R. (2009), ‘A compact nonlinear fiber-based optical autocorrelation peak discriminator’, Optics Express, 17: 9918–23. Galili, M., Xu, J., Mulvad, H.C., Oxenløwe, L.K., Clausen, A.T., Jeppesen, P., LutherDavies, B., Madden, S., Rode, A., Choi, D.Y., Pelusi, M., Luan, F. and Eggleton, B.J. (2009), ‘Breakthrough switching speed with an all-optical chalcogenide glass chip: 640 Gbit/s demultiplexing’, Optics Express, 17: 2182–7. Hansen, K.P., Folkenberg, J.R., Peucheret, C. and Bjarklev, A. (2003), ‘Fully dispersion controlled triangular-core nonlinear photonic crystal fiber’, Proceedings of the Optical Fiber Communication Conference. Paper PD2-1. Jahn, E., Agrawal, N., Ehrke, H.J., Ludwig, R., Pieper, W. and Weber, H.G. (1996), ‘Monolithically integrated asymmetric Mach–Zehnder interferometer as a 20 Gbit/s all-optical add/drop multiplexer for OTDM systems’, Electronics Letters, 32: 216–7. Jensen, S. (1982), ‘The nonlinear coherent coupler’, IEEE Journal of Quantum Electronics, 18: 1580–3. Kang, J.U., Stegeman, G.I. and Aitchison, J.S. (1996), ‘One-dimensional spatial soliton dragging, trapping and all-optical switching in AlGaAs waveguides’, Optics Letters, 21: 189–1. Kang, K.I., Glesk, I., Chang, T.G., Prucnal, P.R. and Boncek, R.K. (1995), ‘Demonstration of all-optical Mach–Zehnder demultiplexer’, Electronics Letters, 31: 749–50. Kostinski, N., Fok, M.P. and Prucnal, P.R. (2009), ‘Experimental demonstration of an alloptical fiber-based Fredkin gate’, Optics Letters, 34: 2766–8. Kravtsov, K., Huang, Y.K. and Prucnal, P.R. (2009), ‘All-optical 160 Gbits/s time-domain demultiplexer based on the heavily GeO2-doped silica-based nonlinear fiber’, Optics Letters, 34: 491–3. Kravtsov, K., Prucnal, P.R. and Bubnov, M.M. (2007), ‘Simple nonlinear interferometerbased all-optical thresholder and its applications for optical CDMA’, Optics Express, 15: 13114–22. Liang, Y., Lou, J.W., Andersen, J.K., Stocker, J.C., Boyraz, O., Islam, M.N. and Nolan, D.A. (1999), ‘Polarization-insensitive nonlinear optical loop mirror demultiplexer with twisted fiber’, Optics Letters, 24: 726–8. Nakamura, S., Tajima, K. and Sugimoto, Y. (1994), ‘Experimental investigation on highspeed switching characteristics of a novel symmetric Mach–Zehnder all-optical switch’, Applied Physics Letters, 65: 283–5. Olsson, B.E. and Andrekson, P.A. (1997), ‘Polarization independent demultiplexing in a polarization diversity nonlinear optical loop mirror’, IEEE Photonics Technology Letters, 9: 764–6. O’Neill, A.W. and Webb, R.P. (1990), ‘All-optical loop mirror switch employing an asymmetric amplifier/attenuator combination’, Electronics Letters, 26: 2008–9. Oxenlowe, L.K., Hilliger, E., Tersigni, A., Nik, A.M., Hojfeldt, S., Romstad, F., Yvind, K., Skovgaard, P.M.W., Hoppe, K. and Hanberg, J. (2001), ‘All-optical demultiplexing and wavelength conversion in an electroabsorption modulator’, Proceedings of the 27th European Conference on Optical Communication, 4: 604–5. Patel, N.S., Hall, K.L. and Rauschenbach, K.A. (1996a), ‘40-Gbitys cascadable all-optical logic with an ultrafast nonlinear interferometer’, Optics Letters, 21: 1466–8. Patel, N.S., Rauschenbach, K.A. and Hall, K.L. (1996b), ‘40-Gb/s demultiplexing using an ultrafast nonlinear interferometer (UNI)’, IEEE Photonics Technology Letters, 8: 1695–7.

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Pelusi, M.D., Ta’eed, V.G., Lamont, M.R.E., Madden, S., Choi, D.Y., Luther-Davies, B. and Eggleton, B.J. (2007), ‘Ultra-high nonlinear As2S3 planar waveguide for 160-Gb/s optical time-division demultiplexing by four-wave mixing’, IEEE Photonics Technology Letters, 19: 1496–8. Reynaud, F. and Barthelemy, A. (1990), ‘Optically controlled interaction between two fundamental soliton beams’, Europhysics Letters, 12: 401–5. Scaffardia, M., Fresia, F., Melonia, G., Bogonib, A., Potìb, L., Calabrettaa, N. and Guglielmuccic, M. (2006), ‘Ultra-fast 160:10 Gbit/s time demultiplexing by four-wave mixing in 1 m-long B2O3-based fiber’, Optics Communication, 268: 38–41. Shalaby, M. and Barthelemy, A. (1991), ‘Experimental spatial soliton trapping and switching’, Optics Letters, 16: 1472–4. Smith, K., Doran, N.J. and Wigley, P.G.J. (1990), ‘Pulse shaping, compression and pedestal suppression employing a nonlinear-optical loop mirror’, Optics Letters, 15: 1294–6. Sokoloff, J.P., Prucnal, P.R., Glesk, I. and Kane, M. (1993), ‘A terahertz optical asymmetric demultiplexer (TOAD)’, IEEE Photonics Technology Letters, 5: 787–90. Sotobayashi, H., Sawaguchi, C., Koyamada, Y. and Chujo, W. (2002), ‘Ultrafast walk-offfree nonlinear optical loop mirror by a simplified configuration for 320-Gbit/s timedivision multiplexing signal demultiplexing’, Optics Letters, 27: 1555–7. Starodumov, A.N., Barmenkov, Y.O., Martinez, A. and Torres, I. (1998), ‘Nonlinear optical switch based on stimulated Raman scattering’, Optical Fiber Technology, 4: 285–92. Striegler, A.G., Meissner, M., Cvecek, K., Sponsel, K., Leuchs, G. and Schmauss, B. (2005), ‘NOLM-based RZ-DPSK signal regeneration’, IEEE Photonics Technology Letters, 17: 639–41. Sugimoto, N., Nagashima, T., Hasegawa, T. and Ohara, S. (2004), ‘Bismuth based optical fiber with nonlinear coefficient of 1360 W-1 km-1’, Proceedings of Optical Fiber Communication Conference. Paper PDP 26. Ta’eed, V., Baker, N.J., Fu, L., Finsterbusch, K., Lamont, M.R.E., Moss, D.J., Nguyen, H.C., Eggleton, B.J., Choi, D.Y., Madden, S. and Luther-Davies, B. (2007), ‘Ultrafast all-optical chalcogenide glass photonic circuits’, Optics Express, 15: 9205–21. Theophilopoulos, G., Kalyvas, M., Bintjas, C., Pleros, N., Yiannopoulos, K., Stavdas, A., Avramopoulos, H. and Guekos, G. (2002), ‘Optically addressable 2×2 exchange/bypass packet switch’, IEEE Photonics Technology Letters, 14: 998–1000. Wu, Y.D. (2005), ‘All-optical 1×N switching device by use of the phase modulation of spatial solitons’, Applied Optics, 44: 4144–7. Yamada, E. and Nakazawa, M. (1994), ‘Reduction of amplified spontaneous emission from a transmitted soliton signal using a nonlinear amplifying loop mirror and a nonlinearoptical loop mirror’, IEEE Journal of Quantum Electronics, 30: 1842–50. Zouraraki, O., Bakopoulos, P., Vyrsokinos, K. and Avramopoulos, H. (2007), ‘2×2 bismuthoxide-fiber based crossbar switch for all-optical switching architectures’. In: I. Tomkos, F. Neri, J.S. Pareta, X.M. Bruin and S.S. Lopez, eds. Optical Network Design and Modeling. pp. 21–8. Berlin/Heidelberg: Springer.

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8 Liquid crystal optical switches C. VÁZQUEZ GARCÍA, I. PÉREZ GARCILÓPEZ and P. CONTRERAS LALLANA, Universidad Carlos III, Spain, and B. VINOUZE and B. FRACASSO, Telecom Bretagne, France

Abstract: This chapter describes the potential of liquid crystal (LC) in switching, from LC materials properties and principles to switching parameters and applications, including the advantages and limitations of LC technology in optical switching. A description of the main types of switches based on the mechanisms used for steering the light with LCs such as reflection, waveguide, polarization management or volume beam-steering is summarized with special emphasis on their parameters as switches and their applications. Apart from previous configurations, there are growing technologies such as ring resonators, holograms and microstructures fibers, which use LC as electro-optical elements for controlling optical switching status. These promising technologies will also be analyzed. Key words: liquid crystal switches, polarization management switches, wavelength-selective switches, liquid crystal photonic crystal fibers, LC ring resonators, holographic LC switches.

8.1

Introduction

Bandwidth demand in telecommunications is continuously growing; this makes necessary the use of all-optical switches without any conversion to electrical form. But it is important to delimit where to apply optical switching (Ferguson, 2006) and more specifically where to apply liquid crystal (LC) technologies. As they cannot respond faster than several microseconds, we shall focus in this chapter on space-switching, for telecom and sensor applications, in protection and recovery applications, and optical add/drop multiplexing, which need fewer restrictions about switching time. Protection and recovery refers to those networks in which an additional path is implemented in order to maintain the transmission when a failure is detected. Optical add/drop multiplexers (OADMs) residing in network nodes insert (add) or extract (drop) optical channels (wavelengths) to or from the wavelength division multiplexing (WDM) optical transmission stream. If they can be reconfigured, they are named ROADM. They can be used as building blocks for optical cross-connect (OXC), a switching matrix for provisioning lightpaths, where any input optical channel can be connected to any output. As an example, although the determination of the minimum response time required for WDM, transport network restoration or flexible bandwidth allocation depends on several network management and service-related issues, it is widely 206 © Woodhead Publishing Limited, 2010



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agreed that the switching time of an OXC should not exceed a few tens of milliseconds (MacDonald et al., 2000). On the other hand, packet switching applications require faster switches, in the nanosecond range (Liu et al., 2006). Hence, they will not be considered here. Before describing the basic principles of LC optical switching, it is important to discuss, in brief, the optical parameters which are taken into account when evaluating an optical switch (Papadimitriou et al., 2003). To define these parameters properly, one should consider only one active input (with incoming light) as shown in Fig. 8.1. • Insertion loss (IL): This is the fraction of the signal power that is lost between an input and an output-connected port of the switch. This loss is measured in decibels and must be as small as possible. IL value of a switch should be uniform over the input–output connections.

 Ps  IL =
10 • log  out  > 0  Pin 

[8.1]

• Crosstalk: ratio of the power leaked to the non-switched output to the input power. It is used to measure the signal interference between channels. This ratio should be low.

 P
 CT = 10 i log  out  < 0  Pin 

• •

• •

[8.2] Switching time: time elapsed from the switching command to the moment the IL of the switch path achieves 90% of its final value. Polarization-dependent loss (PDL): peak-to-peak difference in transmission for light with orthogonal states of polarization. Optical switches must have low PDL (typically <0.5 dB). Power consumption: electrical power that the switch requires for operation. Scalability: ability to obtain switches with a greater number of ports from a basic low-port count structure.

Other parameters which are only relevant in telecom applications are bit rate or amount of bits per second that the switch can manage and polarization mode

Optical switch Unactivated input port Active input

Pin

P 'out Non-switched output s

P out Switched output port

8.1  Optical switch schematic for parameters definition.

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dispersion (PMD), due to the fact that various states of polarization travel at slightly different speeds when they pass through the switch. In the framework of these parameters, LC switches in general terms have the following advantages: they have low IL – less than 6 dB to access 40–80 channels, outstanding uniformity, on the order of 1 dB across all the channels, good CT, better than 240 dB (Hardy, 1999), low PDL, low consumption, broadband capability. They use a mature technology and have no moving parts for switch reconfiguration. Others technologies could have lower IL or lower CT, but LC puts all these parameters together in a robust package. Also nowadays, having in mind recommendations of the Kyoto Protocol and Intergovernmental Panel on Climate Change, low power consumption is a key aspect and because of that there is a need to evaluate and reduce Information and Communication Technologies Impact on the Energy Footprint. In that respect, it is estimated that by 2020, the power consumption related to ICT will be 1/7th of total (Pickavet and Tucker, 2008) and LC in switching matrices might alleviate this. For a better understanding of the potential of LCs in switching, LC materials properties and principles are reviewed. Then a description of the main types of switches based on the mechanisms used for steering light, their parameters and specific applications are presented. Future trends and recent developments are discussed in the last section.

8.2

Liquid crystal theory and principles

Some organic compounds possess a degree of molecular order intermediate between that of crystalline solids and that of amorphous liquids, depending on the ambient temperature. These materials are referred to as thermotropic LCs. They are anisotropic in some of their properties (dielectric constant and refractive index, among others) like solids and, simultaneously, they are fluids like ordinary liquids. As a consequence, optical properties of LC cells can be easily controlled by applying low electric fields. Basically, a LC-based device consists of a thin layer of LC material sandwiched between two glass substrates. The thickness of the LC layer is kept uniform by using small calibrated spacers made of plastic microspheres or glass fibers. Transparent electrodes placed inside the substrates apply a voltage to the LC molecules, which controls light transmission through the LC cell. Rubbed alignment films are needed to force the LC molecular orientation on the substrates (Takatoh et al., 2005). Finally, in devices based on polarization control, polarizers are placed on both sides of the substrates for displays and other photonic applications (Fig. 8.2).

8.2.1 Liquid crystalline phases The intermediate phases, between the solid and the liquid states, found in LC materials are called liquid crystalline phases or mesophases (Chandrasekhar,

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Light transmission

Polarizer Glass substrate

Voltage

Spacers

Transparent electrode alignment layer Spacers

Liquid crystal Alignment layer transparent electrode

Glass substrate

Polarizer

8.2  Schematic drawing of an LC cell.

1992; De Gennes and Prost, 1994; Khoo, 2007). These mesophases appear when the molecules of the material are sharply anisotropic, normally like a rod (calamitic molecules) or a disc (discotic molecules). Most photonic applications use LC with rod-like molecules. These materials can exhibit a variety of mesophases as a function of the temperature (Fig. 8.3). At high temperature, the molecules are disordered (isotropic liquid state). When the temperature is decreased, the molecules show an orientational order, with the long molecular axis oriented in a preferred direction, but not a positional order (nematic phase, N). If the temperature is decreased further, the molecules show a partial positional order forming a layered structure, with the long axis of the molecules in the normal direction of the layer (smectic-A phase, SmA). A further decrease of the temperature produces a tilt of the molecular director with regard to the perpendicular of the layer (smectic-C phase, SmC). At low temperature the material shows both orientational and positional orders (crystalline solid). It should be noted that only some LC materials show all the mesophases. On the other hand, if a liquid crystalline substance is composed of optically active molecules, the chiral mesophases (denoted by putting a star after the Crystal solid

Smectic-C

Smectic-A

Nematic

Liquid

Temperature

8.3  Schematic representation of mesophases of thermotropic LC with rod-like molecules.

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mesophase labels: N* or cholesteric and SmC* or ferroelectric, FLC) appear. Cholesterics are similar to nematic LCs in all physical properties except that the molecules tend to align in a helical way. In the smectic-C* mesophase the director axis is tilted from the normal layer and rotates around the normal direction in successive layers, producing a spontaneous electric polarization.

8.2.2 Basic liquid crystal structures for optical switching In the past few years, LC cells appear as one of the promising technologies to achieve optical switching in telecommunications networks (Hardy, 1999; Cornwell and Albert, 2000). As previously reported, these devices do not need moving parts to switch, but a control voltage. Optical switches based on twisted nematic (TN) devices and surface-stabilized ferroelectric liquid crystals (SSFLC) have been demonstrated (Yamazaki and Yamaguchi, 1991; Pain et al., 1997; Vázquez et al., 2003; Riza and Yuan, 1998). More recently, systems based on polymer-dispersed liquid crystal (PDLC) have also been developed (Lallana et al., 2008). A brief review of the working principles and properties of the most extended LC technologies for optical switching is presented below. Twisted nematic (TN) cells Nematic LCs with positive dielectric anisotropy, for which the dielectric constant is greater in the long molecular axis than that in the other directions, are used in TN devices. Planar alignment (i.e., LC molecular axis parallel to glass substrate plane) and perpendicular rubbing directions in both alignment layers are used. As a consequence, the LC molecules perform a 90° twist through the thickness of the LC cell (PolRot cell). Through such a cell the polarization state of a linearly polarized light is modified and finally goes out at 90° of incoming direction, if the Mauguin condition is satisfied: d i
n >>  2

[8.3]

where d is the cell thickness, ∆n is the LC birefringence and λ is the light wavelength. LC molecules reorient if voltages greater than the threshold voltage, Vth, are applied to the electrodes. When a sufficient voltage value, named switching voltage Vsw > Vth, is applied (ON state), LC molecules align parallel to the electric field and the polarization rotation disappears. In most display applications, two crossed polarizers are placed on the outside of the substrates, with the transmissive axis of each polarizer parallel to the rubbing direction of each alignment layer. The basic operation of a TN device working in this mode, known as normally white (NW) mode, is roughly depicted in Fig. 8.4(a). When no voltage is applied

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(b) LC between parallel polarizers

Input light Polarizer

LC cell

Analyzer Output

8.4  Representation of a TN cell operation: (a) crossed polarizers and (b) parallel polarizers.

(OFF state) the incident light is transmitted. In the ON state, the light is blocked by the output polarizer and the device appears dark. From an electrical point of view, a LC cell acts, basically, as a capacitor with a non-ideal dielectric material (the LC). The electrical equivalent circuit (EEC) of the LC device can be obtained using an experimental procedure based on the impedance spectroscopy technique (Barsoukov and Macdonald, 2005). This method consists in measuring the complex impedance (magnitude and phase) of the device and fitting the EEC components from these impedance measurements (Pena et al., 2002; Pérez et al., 2007). Results for a TN device, filled with LC mixture E7 from Merck (Darmstadt, Germany), with an active area of 1 cm2 and a thickness of 5 µm, are summarized in Fig. 8.5. The EEC consists in a

–20

1E+005

–40

1E+004

–60 –80

1E+003

TN cell

RS

RP CLC

–100 1E+002 1E+002 1E+003 1E+004 1E+005 1E+006 Frequency (Hz)

(a)

4

RS = 120 W RP = 0.95 MW

Capacitance (nF)

Impedance magnitude Impedance phase

Impedance phase (º)

Impedance magnitude (W)

1E+006

3 2 1 Vsw

Vth

CLC = 0.8 nF

0

(b)

1

2 3 4 Applied voltage (Vrms)

(c)

8.5  (a) Impedance (magnitude and phase) measurement of TN cell in the OFF state, from 100 Hz to 1 MHz, (b) EEC in the OFF state, and (c) capacitance variation as a function of applied voltage (threshold and switching voltages can be estimated from this).

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voltage-dependent capacitor (CLC ) with series (RS) and parallel (RP) resistors. The variation of CLC with the voltage applied to the cell electrodes is due to the dielectric permittivity modifications linked to the molecular reorientations. Therefore, the threshold (usually 1–2 V) and switching (usually 3–5 V) voltages can be derived from this electrical modeling. At low voltages (below Vth) the capacitance is constant. A nonlinear variation of CLC is obtained if the voltage is increased. Finally, the device capacitance remains almost constant for voltages greater than Vsw. Power consumption (nW) and response time (usually in the 20–30 ms range) of the TN device can be also estimated from EEC simulation. The optical transmission factor, T, for polarized monochromatic light incident on a 90° TN cell sandwiched between parallel polarizers in the ON state (see Fig. 8.4(b)) can be calculated using Jones matrices (Gooch and Tarry, 1975):

 sin 2
i 1+ u 2 2 T = To  1+ u 2  

(

)

    

[8.4]

where To is ideal maximum transmission and u = 2∆nd/λ. – –– The minimum transmission occurs when u = √3, √15 … Since a smaller u gives – a smaller cell gap and a faster response speed, usually u = √3 is used in practical devices (Yang and Wu, 2006). Note that contrast is maximized for a λ value and optical transmission varies for other wavelengths. Wavelength dependence must be reduced by optimizing the fabrication parameters. Surface-stabilized ferroelectric liquid crystal (SSFLC) cells SSFLC cells are the most widely used devices based on FLC. In these devices, the FLC material is sandwiched between two substrates separated by a very thin LC layer (1–2 µm). Although a variety of molecular orientations have been applied in SSFLC (Lagerwall, 1999), the bi-stable bookshelf layer structure is the most employed (see Fig. 8.6). This material exhibits a macroscopic spontaneous polarization Ps, which must be stabilized by the surfaces in order to prevent the natural helix formation (Clark and Lagerwall, 1980). The electro-optical effect is a rotation of the smectic cone driven by coupling between the polarization and the electric field. This structure has response time of a few microseconds as well as a memory effect (bi-stability). In the presence of an electric field, the molecular orientation changes and the device remains in this state until a reverse polarity voltage is applied. An electrical modeling (Moore and Travis, 1999; Rep and Prins, 1999) of these devices allows obtaining switching voltage (a few volts), power consumption (nW) and response time (a few microseconds) in practical devices, as a function of fabrication parameters. When an SSFLC device is placed between crossed polarizers, with one of them parallel to the molecular axis of one of the stables states, one of the two states will

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P –V

θ

θ

+V P

8.6  Bi-stable bookshelf SSFLC cell. 

be black. Optical transmission for the other state can be calculated, using the Jones calculus, and is given by: [8.5] T = To sin 2 ( 4 ) i sin 2  
nd     where To is the maximum possible transmission in the device (i.e., transmission between parallel polarizers), θ is the cone angle of SmC* material (optimal value is 22.5°) and d is the thickness of the device (d = λ/2∆n for optimal working). As happens in TN cells, the device performance is optimal for a given λ, optical transmission for other wavelengths changes abruptly compared with TN cells. TN and SSFLC cells are just some of the possible configurations used in developing optical switches, other examples such as using PDLCs will be shown in section 8.3. Liquid crystal spatial light modulators (SLMs) A SLM is a device that modulates in one or two dimensions an optical beam in amplitude, phase or polarization, using the birefringence properties of the LC cell. Most two-dimensional (2D) LC SLMs are driven electrically; but optically addressed analog light valves are also proposed (Moddel et al., 1989). The spatial structure of an electrically addressed SLM is shown in Fig. 8.7. The pixel pitch p is defined as the center-to-center spacing between adjacent pixels. The interpixel gap i describes the edge-to-edge spacing between adjacent pixels. Assuming square-shaped pixels, the geometrical fill factor F is defined as the ratio (p/i)2 and this parameter puts an upper bound to the SLM optical efficiency. Most widespread LC SLMs are transmissive panels that consist of a LC layer aligned between two glass sheets, with a control circuitry added using the thin film transistor (TFT) technology. Such displays are rather large and used for laptop computers, TV sets and head-up displays. Major drawbacks are: rather large pixels, a moderate ‘fill factor’ (<60%) due to large dead areas between TFTs and a lack of flatness, which is a problem if the modulated beam is coherent. More compact SLMs can be obtained with the liquid crystal on silicon (LCOS) geometry (Underwood et al., 1986; Lelah et al., 2001). The device structure (see Fig. 8.8) is a LC layer sandwiched between a reflective silicon backplane and a

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214

Optical switches Pixel pitch

Pixel gap

Pixel size

m rows

n columns

8.7  2D pixellated SLM geometry.

Front glass

Mirror array

Spacer

Counter-electrode layer Liquid crystal

Bonding

Chip carrier

Silicon backplane

8.8  Liquid crystal on silicon (LCOS) SLM structure.

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transmissive counter-electrode. Very dense column/row circuits and pixel/mirror arrays can be built through a VLSI integration process, yielding compact, light and flat displays with high-definition (1920 × 1080 HDTV format), highresolution (pixel pitch as small as 8 µm) and high fill factor (>95%). The main applications for LCOS SLMs include high-definition rear-projection TV sets, portable video-projectors, wavefront control, adaptive optics, beam-steering for optical tweezers (Hossack et al., 2003), optical switching matrices or wavelengthselective switches (WSS) (Baxter et al., 2006). In that case, it is necessary to adapt the LCOS cell characteristics to optical fibers transmission constraints (Heggarty et al., 2003).

8.3

Liquid crystal switches and applications

Different types of switches can be distinguished, depending on the physical mechanisms used to steer the light with LCs, such as reflection, wave-guiding, polarization management or beam-steering (planar or volume). Some of them will be summarized in the following sections, with special emphasis on their switching parameters and applications.

8.3.1 Optical crystal switching architectures Broadly speaking, optical space-switches can be implemented according to two basic architectures: broadcast-and-select (BS) and space-routing (SR). Using BS, the light information from an input channel is split to the entire output channel array through an intermediate blocking stage, which then selects the desired output channel (Fig. 8.9). Using LC modulators, the blocking stage is implemented by amplitude modulators that must exhibit high contrast ratio to avoid crosstalk. Although strictly non-blocking, this scheme suffers from: (1) a large complexity of the intermediate stage when the number of channel increases (considering a 2D input array involves a 4D selecting mask) and (2) a very poor power budget which means that optical amplification is mandatory in this case. The SR scheme (Fig. 8.10) is generally more adapted to LC switching. It consists in driving the information from an input to an output channel using SR intermediate elements, usually arranged in different stages. This scheme limits the IL, but may be subject to connection blockings if the number of switching elements is limited, as for the Benes or Banyan topologies (Yu et al., 2006). Using LC devices to implement SR switches by individual light steering can be performed in two ways: a multi-stage planar topology using arrays of 2 × 2 binary polarization switches (Hogari et al., 1991) or single- and dual-stage schemes in which SR is performed by beam-steering in free-space (Fukushima et al., 1991). This will be illustrated in the next subsections (8.3.2–8.3.4).

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Optical switches Broadcast

On

Combine

Off A

A

Off Off

B

Off

C

On

C

Off

B

On Input channels

Off

Output channels

Select

8.9  Broadcast-and-select switching architecture. Connecting N inputs to N outputs requires 2N couplers and N 2 ON/OFF selectors.

8.3.2 Switches based on polarization management The polarization rotation (PolRot) is the first configuration used in LC switches (Wagner and Cheng, 1980) because the basic principles are widely used in the TN displays for tens of years. This switch is based on the change of polarization state of the incident light when applying an electric field over the LC cell (see Fig. 8.4). The change of polarization with a TN cell in combination with spatial polarization selective calcite crystals or polarization beam splitters (PBS) allows optical space-switching. In order to make the device polarization insensitive and to minimize losses, the polarization diversity method is used by treating each polarization mode in parallel. The input signal is decomposed into its TE and TM components, which are separately recombined at the switch output. A schematic of this generic polarization management LC optical switch can be seen in Fig. 8.11. A simple example of the previous description is the 1 × 2 LC switch structure shown in Fig. 8.12. The principle of operation is as follows (McAdams and

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2×2 switch 1 Bar state Input 2 channels Cross state

3

2 Output channels

1

3

8.10  Space-routing switching architecture, illustrated here by the so-called ‘Crossbar’ scheme (strictly non-blocking). Connecting N inputs to N outputs requires N 2 elementary 2 × 2 switching points.

LC

Inputs

Polarization separation

Polarization handling

Recombination

Outputs

8.11  Block diagram of a generic polarization management LC switch.

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Optical switches Port 3

POF

218

L3 s

PS L1 Port 1

Port 2

s

POF

POF p+s

s P–s

p

p

L2

PBS

8.12  Scheme of a 1 × 2 LC optical switch. POF: polymer optical fibers; PS: TN-LC cell; PBS: polarization beam splitter; P: polarizer; L: focusing/collimating lens.

Goodman, 1990; Vázquez et al., 2003): the first polarizer, P, at the input manipulates the polarization states to the desired one at the output. The liquid crystal, denoted as polarization switch PS, can shift the polarization of the input light depending on the voltage applied to it. The linear polarization of the input light is shifted 90° after passing through the TN-LC cell when no bias voltage is applied. This light passes though the PBS to output port 2. By applying a voltage greater than Vsw to the TN-LC cell, LC molecules align parallel to the electric field, and the polarization rotation does not occur. This light in the ON state of the switch is reflected by the PBS to the other output or port 3. Switch transmission is controlled by the voltage applied to the LC cells; lower voltages induce less polarization shifts. Then, these switches can also operate as variable optical attenuators (VOAs). By applying a voltage Vth < V < Vsw a VOA can be implemented, splitting the input signal at both outputs with a variable ratio depending on the applied voltage and consequently on the manipulation of the stage of light polarization by the LC. Performance of polarization management based liquid crystal switches There are different configurations based on the previous basic principles depending on the type and the number of elements used and the switch capacity. Evolution of the state-of-the-art showing different implementations and their characteristics are shown in Table 8.1. In these switches, in addition to LC cells, polarizing beam splitters and calcite plates, optional elements are: mirrors, half wave plates, quarter wave plates, halfangle prisms, right-angle prisms, beam displacement prisms, total internal

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© Woodhead Publishing Limited, 2010

TN–LC

(Wagner and Cheng, 1980) (Soref, 1981)

(Riza and Yuan, 1998) (Riza and Yuan, 1999) (Vázquez et al., 2003) (Riza and Madamopoulus, 2005) (Lallana et al., 2006)

1550

650–850

—

650–850

NLC

TN–LC

NLC

2×2

1300

FLC

6×6

820(670)

3×1

—

1×2

2×2

2×2

1×4

633

1300

2×2

—

2×2 2×2

632.8

633

1×2

Type

633

λ (nm)

FLC

(Soref and TN–LC McMahon, 1982) (McAdams et al., NLC–FLC 1990) (McAdams and FLC Goodman, 1990) (Grimes et al., FLC 1991) (Fujii, 1993) TN–LC

TN–LC

LC cell

Contribution

MM

—

MM

SM

SM

SM

MM

—

—

None

MM

MM

Fiber

223

220

222

240

234.1

243.3

221.6

220

232

227

220

CT (dB)

3

2

7

6.76

6.94

2.2

11.1

3.5

1.4

3

20–5 ms

—

ms

35.3µs

35.3µs

—

150µs

50µs

250µs

—

50/150ms

—

0.41 2.5

Turn–on/D. time

IL (dB)

Table 8.1  State-of-the-art and performance parameters of RotPol LC switches.

3V

—

8V

—

—

—

—

—

15Vrms

6V

5V

2.5V

Control voltage

2FO–Circulator, 2 PBS, 2 LC, 2 TIR, 2 BDP 2 PBS, 4 L, 6LC, 1 P

4PBS, 2M, 4LC, 2AP, 2HWP, 2QWP, 1LB 1PBS, 2LC, 2P, 1M, 1HWP, 1QWP, 1AP 1PBS, 1LC, 1P

2 PBS, 2 AP, 5 LC, 2 BR

6FLC, 6 GL

2 LC, 2 HWP, 3 Calcites 2 NLC 2 SS–FLC, 2 M, 4 HIEP 4 FLC, 4 PBS

4PBS, 2 LLC, 7 AP

2PBS, 2AP, 1LLC

Elements

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AP: BDP: BR: FLC: GL: HWP: HIEP: LB: LLC: M: MM: NLC: P: PBS: QWP: SM: TIR: TN–LC:

1 neglecting

650–850

808

NLC

NLC/FLC

2×2

Dual 3 × 2 —

MM 236.2

220 2.5

reflections, expected up to 1.2dB with MM fibers and GRIN–rod lenses Right–Angle Prism Beam Displacing Prism Birefringent Crystal Ferroelectric Liquid Crystal GRIN Lens Half Wave Plate High Index Equilateral Prism Leakage Block Large LC Cells Mirror MultiMode Nematic Liquid Crystal Polariser Polarizing Beam Splitter Quarter Wave Plate SingleMode Total Internal Reflection Prism Twist Nematic Liquid Crystal

(Lallana et al., 2007) (Yang et al., 2008) 60.6µs 35 µs

13,5ms ±15transient ±5V hold

5V 4 PBS, 2 HWP, 4 QWP, 4 M

3 PBS, 8 L, 6LC



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reflection prisms, birefringent crystals (see Table 8.1). Most of these are based on free-space optics bulk elements, using lenses for coupling light in optical fibers and only a few of them use fiber-optic devices (Sumriddetchkajorn and Riza, 2000). Better response times are obtained when FLC cells are used, near 35 µs (Riza and Yuan, 1998; Yang et al., 2008). Standard NLC cells exhibit higher response times, not less than 20 ms (Lallana et al., 2006). A reduced response time can be obtained in combining NLC cells and the transient nematic effect (TNE); 60 µs is reported in Yang et al. (2008). NLC cells have a slower response time than FLC, but NLC can operate in a wider wavelength range, because the FLC cell thickness, d, determines the wavelength in which the polarization shift is 90°. On the other hand, NLC cell thickness at first or second minimum can be optimized for a multiband operation fulfilling Mauguin’s regime in order to obtain the polarization switch. A broadband 3 × 1 reconfigurable optical multiplexer, from 650 to 850 nm, is presented in Lallana et al. (2006). With such TNLC, a 3 × 2 multiplexer based on graded index plastic optical fibers (GI-POF) can operate from 850 to 1300 nm (Lallana et al., 2007). Other TNLC systems give good uniformity in the C-band (1530–1560 nm) (Sumriddetchkajorn and Riza, 2000). Switch IL depends on the structure of the device. Simpler switches can manage only one polarization; thus, higher ILs are expected, 3.5 dB in McAdams and Goodman (1990). More complex switches using polarization diversity management (see Fig. 8.11) exhibit low IL, 1.4 dB (McAdams et al., 1990). Optical switches with IL less than 1 dB, crosstalk from 220 to 245 dB, low PDL (around 0.1–0.2 dB) and low PMD have been developed (see Table 8.1). Although not specifically reported in the papers, LC switches have very low power consumption, in nWs.

8.3.3 Liquid crystal materials for amplitude and phase modulation An engineer designing a LC switch aims at specific electro-optical performances (contrast, diffraction efficiency, transmission, voltages, speed) compatible with transmission parameter requirements (IL, crosstalk, PDL, PMD, response time, temperature dependence, power consumption, bit rate tolerance). These per­form­ ances are linked to the molecules’ chemical properties designed by chemists (refractive indexes, elastic constants, dielectric anisotropy and temperature range). LC manufacturers continue to synthesize hundreds of new molecules for displays or specific applications. The relatively slow speed of the LC switches, as already mentioned, could, however, limit their practical application in telecoms. The critical performances of the PolRot cells are contrast and bandwidth. The TN contrast is determined by the minima of Gooch and Tarry’s law (Gooch and Tarry, 1975) as previously reported in equation [8.4]. The first minimum solution

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222

Optical switches 1000000 100000

Contrast

10000 1000 100 10 1

3.5

3.6

3.7

3.8

3.9

4

4.1

LC thickness (mm)

8.13  LC cell contrast optimization for 1.5 µm wavelength (0.25 ∆n LC need a 3.8 ± .1 µm thickness for reaching 30 dB contrast in all the C band).

– is 2∆nd/λ = √3. The LC thickness management is demanding, a 3.8 µm ± .1 µm cell thickness for a 30 dB contrast at 1.5 µm wavelength (Fig. 8.13). Then, the 40 nm bandwidth is fully compatible with the C band for telecom applications. So, a clean cell assembly technology combined with the wide catalogue of nematic mixtures covers all the needs of PolRot switches (Pain et al., 1997). An interesting approach consists of using a PDLC as a variable switch between optical fibers (Lallana et al., 2008). The PDLC is composed of a polymer matrix with many LC droplets having a radius of the same size as the wavelength (Fig. 8.14(b)). Inside each droplet, the nematic is uniformly aligned; but from droplet to droplet, the nematic directors are randomly aligned and polarization is independent. Without voltage, the structure scatters light; the switch is in the OFF state. When voltage is applied, nematic molecules align parallel to the electric field, and the structure becomes transparent because the refractive index of the polymer is close to the LC refractive index (n0); so the switch is in the ON state. A common PDLC mixture uses 80% by weight TL205 LC with 20% of PN393 monomer (from Merck). The contrast adaptation to the wavelength is performed by adjusting the size of the LC droplets by: ∆n.a/λ = 0.3

[8.6]

where a is the droplet radius (Bosc et al., 1996). During the UV polymerization of the monomer, a higher power leads to smaller droplet radius. For the phase modulation used in beam deflection gratings, the main issues are diffraction efficiency and response time. Phase modulation can be easily realized with nematic LC in a parallel alignment configuration with a 5° pre-tilt standard

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(a) parallel nematic

223

(b) polymer dispersed LC

8.14  Other LC structures suitable for phase or amplitude switching: (a) parallel alignment nematic LC and (b) PDLC.

polyimide material (Fig. 8.14(a)). The parallel structure must be adjusted to reach the desired phase shift given by: ∆φ = 2π ∆n d/λ

[8.7]

A 2π phase shift at 1.5 µm wavelength is achieved with a 7.5 µm LC cell with a 0.2 ∆n. The diffraction efficiency reaches 40%, the theoretical limit of a twophase level grating. A reflective configuration allows dividing the response time by 4, because it is proportional to the viscosity, inversely proportional to the dielectric anisotropy and the square of the electric field. Note that LC birefringence – whatever the material – decreases with increasing wavelength according to Cauchy law nλ = n∞ + b/λ2 (b, specific for each mixture). For telecom wavelengths, the birefringence ∆n typically decreases around 20% with respect to the visible range value. The LC ∆n must be as high as possible (as an example, 0.29 for BL009 from Merck). The elastic splay constant (K11) must be as low as possible. Response time is relatively short (typically 20 ms) due to saturation voltages used there. FLC devices also display bi-level amplitude or phase gratings (Fig. 8.6). The main advantage is a symmetrical and fast response time (typically 10–100 µs). Because of a ∆n lower than 0.15, the FLC (like SCE13 from Merck) usually exhibits a diffraction efficiency in the range of 20%. An unstable LC alignment with time is observed due to thick cell for telecom wavelengths. Special fiber arrays with non-uniform pitch to prevent the coupling of the parasitic diffraction orders were proposed both in one and two dimensions (Fracasso et al., 2001; Letort et al., 2008). The gray scale capability of nematic LCs allows producing multi-phase gratings which lead to high diffraction efficiencies (Klaus et al., 1996). For a blazed grating (saw-tooth profile) with a number of phase levels N, we have a theoretical efficiency (Goodman and Silvestri, 1970), given by:

η (N) = [sinc(1/N)]2

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[8.8]

224

Optical switches

with sinc(x) = sin(πx)/πx. This gives η = 81% for four phase levels and η = 91% with N = 6. As a consequence, numerous gray levels are needed to achieve the smooth angle adaptations at each new routing configuration as also seen on DIMOS simulation (Fig. 8.15). Experimentally measured diffraction efficiency reaches 80% for six gray shades at 1.55 µm wavelength (Wolffer et al., 2000). The initial long response times can be reduced by a factor of 100 by using an over driving addressing scheme down to 50 ms rise time and 2 ms decay time (Tan et al., 2000). The reflective configuration in LCOS divides the rise time by a factor of 4. NLCs are easily implemented in LCOS due to their stable planar alignment (Lelah et al., 2001).

8.15  Simulation of NLC blazed grating with multi-electrode structure (DIMOS simulation).

To summarize, LC materials allow making amplitude as well as phase modulation switches. Ferroelectrics could be used in binary gratings, being potentially fast. Nematics are mainly used for PolRot switches and high diffraction efficiency blazed gratings. The wide range of nematic LCs is suitable to design devices from visible to IR wavelengths. Nevertheless, this technology, based on organic materials, still has to be accepted by telecom professionals who are used to manipulate mineral materials and components.

8.3.4 Switches based on beam deflection To overcome the capacity bottleneck inherent in planar switching architectures, free-space optics using programmable 2D beam-steering elements appear as the unique solution to implement large-capacity optical switches, with strictly nonblocking capabilities. The main application aimed here is transparent switching cores of OXCs for WDM networks (Smith et al., 1993). Two generic switching architectures between single-mode fiber arrays are the single-deflector 1 × N

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correlation setup (O’Brien et al., 1994) and the N × N dual-deflector scheme (Wolffer et al., 2000), depicted in Fig. 8.16. The light from a 2D fiber array is collimated by a 2D micro-lens array, and each collimated beamlet is locally deflected and coupled towards the proper output using a second deflector/lens combination. Although such large capacities as 256 × 256 channels with 1 dB average IL are obtained using reflective 3D micro-mirror arrays (Neilson et al., 2004), the liquid crystal SLM technology used in the refractive or diffractive type shows promising advantages such as: (1) no mechanical motion and hence high angle repeatability and stability, (2) low controlling voltages (a few volts) and (3) high reliability. The first option is refractive LC elements in the form of scanning Fresnel lenses (Sato et al., 1985) or LC micro-prism arrays (Hirabayashi et al., 1995). In the latter case, the steering structure is a homogeneously aligned nematic LC cell in which a micro-prism array (250 µm pitch) is deposited on one glass electrode. The local LC refractive index changes continuously with the voltage applied and an incident optical beam is refracted (and hence deflected) according to SnellDescartes’s law. High transmittance (95%) and high deflection angle are obtained at low driving voltage (2.8 V), but the steering is one-dimensional, the response time is very slow (~1 s) and the device is polarization dependent. 2D beamsteering could be obtained by crossing such 1D structures, to the detriment, however, of steering efficiency.

Micro-lens array 1

Input fiber array

Deflector array 1

Micro-lens array 2

Deflector array 2

Output fiber array

8.16  Two-stage N × N beam-steering switching architecture scheme, which is strictly non-blocking. The setup can be easily folded (and hence made more compact) using reflective deflector arrays.

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Optical switches

More flexible and truly 2D beam deflectors are provided by 2D LC SLMs displaying dynamic diffraction gratings, or more generally holographic optical elements (Fracasso et al., 1990; Fukushima et al., 1991; O’Brien et al., 1991) that can be configured for a wide variety of routing topologies. Since the response time of LC devices is related to the LC layer thickness, the prism function of the refractive case is replaced by a blazed grating (saw-tooth profile) with a phase amplitude of 2π. Figure 8.17 depicts two effects of a pixellated SLM: (1) the phase modulation distribution is approximated by a staircase grating profile with phase values φk = 2kπ/N and 0 ≤ k ≤ N–1 and (2) a spatial quantization of the grating periods (Px, Py) by the 2D pixel grid. Nematic LC cells allow quasi-linear phase ramps (i.e., very large N values) (Wolffer et al., 2000) but the 2π phase modulation depth is polarization dependent and exhibits rather slow reconfiguration times (a few tens of milliseconds). In contrast, SSFLC cells (Clark and Lagerwall, 1980) provide binary phase states (N = 2) with a purely polarization-insensitive scheme (Warr and Mears, 1995) and a fast reconfigurable time of a few tens of microseconds. From the so-called grating equation, a monochromatic plane wave with wavelength λ under normal incidence on a 2D SLM will be deflected at angles (θx, θy) such that:

sin  ym = m
Ny p

[8.9]

sin  xm = m
Nx p

[8.10]

and

where m is the diffraction order, Nx and Ny denote the number of pixels per period in the x and y dimensions and p is the pixel pitch. The first diffraction order Phase (rad)

Grating period Px

fN–1 = 2p fk x (m)

f0 = 0

Py

Pixel electrode (a)

Px (b)

8.17  (a) 1D quantized phase profile of a blazed grating (N = 4 phase levels). (b) Spatial quantization of a 2D grating displayed on a pixellated LC SLM. Px and Py are the grating period lengths and correspond to multiple values of the pixel pitch.

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(m = 1) is commonly used for beam-steering with a diffraction efficiency η as given in equation [8.8] with respect to the number of phase levels. We obtain a theoretical value η = 41% for two phase levels (FLC case) and η = 95% with N = 8. The maximum deflection angle is obtained for Nx = 2, yielding from equation [8.9] θmax = arcsin(λ/2p) which, for a typical pixel pitch p = 10 µm and λ = 1550 nm, gives θmax = 0.1 rad (5.7 degrees). In addition, steering resolutions lower than the micro-radian can be obtained using either pseudo-periodic diffraction gratings (Fracasso et al., 2003) or phase offsets (Engström, 2008). This great steering flexibility by monitoring the optical powers coupled in the output fibers permits adaptive beam positioning for possible mechanical tolerances in the packaging structure (Johansson et al., 2002). As an illustration, a 8 × 8 beam-steering spaceswitch using high-resolution transmissive nematic LC cell arrays (parallel-aligned) is presented in Wolffer et al. (2000). The device operates at 1550 nm using the twostage architecture shown in Fig. 8.16, embedded in a polarization diversity scheme. The basic LC cell is 9 µm thick and individual deflectors are made up with 309 electrodes over a 1.4 mm width, leading to a pixel pitch as small as 4.5 µm. The deflection efficiency of the two cell stages varies between 1 and 4 dB and the average measured fiber-to-fiber IL is 9 dB, with a PDL of 0.5 dB and an average crosstalk level of 243 dB. Finally, BER characterizations performed on switched connections through the device show that it is optically transparent at 10 Gbit/s.

8.3.5 Wavelength-selective switches (WSS) Recently, the emergence of devices such as wavelength blockers (Vasilyev et al., 2003) and WSS (Rhee et al., 2001) has made the design of efficient and integrated ROADM or OXC nodes possible. This solution is shown to be more flexible than using optical space-switching matrices in association with multiplexers and demultiplexer stages. Basically, WSS is an all-optical subsystem that can be viewed as an integrated 1 × N OXC. Its purpose is to switch any incoming wavelength from its input fiber to any of N output fiber ports. Figure 8.18 shows the particular switching configuration of a schematic WSS. Individual power attenuations of the routed channels can also be performed at that stage to compensate for possible power imbalance of the input WDM multiplex. A number of approaches to implementing WSS have been demonstrated (Bonenfant and Loyd, 2004), but the most mature technological solutions to date are MEMS arrays (Marom et al., 2005) and LCOS modulators (Baxter et al., 2006). LC-WSS based on polarization switching The generic optical scheme of a WSS is based on the structure of an ultra-short optical pulse shaper (Heritage et al., 1985), with micro-beam displacement between a pair of high-resolution wavelength dispersing diffraction gratings. Figure 8.19 shows the scheme for the first proposed solution, involving beam shifting by

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228

Optical switches λ1

λ8

O1 λ1

WSS

2 3 4 5 6 7 8

O2 Input port

O3

O4

Output ports

8.18  Schematic of a 1 × 4 wavelength selective switch operating on an input wavelength multiplexed with eight channels.

polarization rotation (Patel and Silberberg, 1995). The input fiber polychromatic channels (λ1, …, λN) are collimated, then angularly dispersed by the first grating and the beamlets are individually focussed by the first lens as separate spots on the LC pixels of a nematic twisted array. This element is followed by a polarizationselective deflective element like a Wollaston prism or a calcite plate. Without any polarization rotation (LC cell ON), the wavelength channels are recollimated by Positive Positive LC Polarization rotator array Polarization lens 2 lens 1 deflective plate

Diffraction grating 1

λ1

λ1

λ2

λ2

f

Collimator

Diffraction grating 2

f Output 1

Input fiber 1

Output 2

8.19  Beam-shifting LC-WSS structure. For clarity, the polarization diversity apparatus is not represented. The gratings are placed at the front and rear focal planes of the first and second lens, respectively.

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the second lens and recombined by the second grating into a single beam to the output fiber 1. When selected by the LC pixel (LC cell OFF), the wavelength channels are polarization rotated, then deflected off by the birefringent plate and recombined by the second grating into the second output fiber. The intermediate spatial shift is determined as a function of the lens focal length and the distance between the output fibers. In addition, a polarization diversity setup is used to make the device polarization independent, which involves a duplication of the number of beams crossing the optical setup. To achieve low CT and PDL values, the beamlets for the two polarizations of a given optical channel should experience the same polarization rotation and path length. To do this, one of the two eigen-polarizations of an input channel is halfwave-retarded, so that both beams share the same polarization and experience the same loss by crossing through the same optical components. First demonstration (Patel and Silberberg, 1995) on an eightwavelength switch with 4 nm separation exhibited 225 dB crosstalk and IL of about 10 dB. Further improvement of the architecture has led to devices operating on 80 channels with 5 dB IL and crosstalk values lower than 235 dB. System demonstrations have shown that this first WSS solution could be used to build ROADM structures operating at 10 and 40 Gbit/s. WSS based on LCOS beam-steering The configuration of Fig. 8.19 can be generalized to 1 × N wavelength switches, using beam-steering elements instead of polarization beam shifting elements. Figure 8.20 shows the generic and compact WSS configuration using a 4f imaging Diffraction grating

Input/output fiber array (1) (2) (4)

2

1

(3)

2

Beam deflector (LC or MEMS)

(5)

1 2

Fourier mirror

8.20  Compact and generic WSS architecture using beam steering at the intermediate plane.

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setup, where the same bulk diffraction grating is shared for both the demultiplexing and multiplexing around a dynamic beam-steering element, including MEMS, LCD and LCOS devices (Baxter et al., 2006). The wavelengths are dispersed along one dimension, and the orthogonal axis is used for port selection and possible amplitude control by beam shifts around the output fiber core center. The input light from a given fiber of the array is collimated and reflected by the concave mirror and then angularly dispersed by the diffraction grating in the horizontal plane. A second reflection on the mirror focuses each wavelength channel λi to a given area of the beam-steering SLM, which then reflects the channel back with a vertical angle θi (channel dependent). The path for the channel is then retraced through the device and the light is recoupled to a particular port of the vertical fiber array, depending on the initial θi value. Two-dimensional LCOS phase modulators can be employed as high-resolution, flexible and motionless reflective beam deflectors. In that particular case, the SLM area is divided into N vertical stripes corresponding to N input wavelength channels. Each pixellated stripe is configured to display a diffraction grating that does not interfere with the other stripes. Using 2D gratings, the diffraction efficiency can be adjusted to provide attenuation or power splitting functions (multi-cast). Using this technique with nematic LCOS SLMs and a polarization diversity setup (Baxter et al., 2006), a 1 × 9 WSS operating on 100 wavelengths and a 0.4 nm grid is demonstrated. Low IL (<4.6 dB) is measured, as well as high extinction for the blocked channels (<240 dB). The optical power was controlled on a per-channel and per-port basis with an amplitude resolution of 0.1 dB, providing additional VOA capabilities. Unlike MEMS-based devices, this controlled attenuation does not rely on mechanical beam displacement and hence does not necessitate feedback mechanisms. The device was successfully used to implement a multiport WSS with ‘drop and continue’ capabilities, where an input wavelength channel power is distributed over different output fibers (Frisken et al., 2006), showing no BER penalty at 10 Gbit/s. More recently, LCOS-based wavelength switching and processing platforms are designed and built not only to switch the wavelength channels, but also to shape them by generating a wide variety of phase filtering functions. Main applications were chromatic dispersion compensation and pulse temporal compression at rates up to 160 GHz (Roelens et al., 2008a; Roelens et al., 2008b).

8.4

Future trends

Apart from the previously mentioned configurations, there are growing technologies such as ring resonators, holograms and microstructured fibers which use LC as electro-optical elements for controlling the optical switching status. These promising technologies are described in this section. Different comparisons between optical switching technologies have been reported (Ma and Kuo, 2003; Papadimitriou et al., 2003; Tarek, 2006), and there are current

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projects around the world in optical switching technologies and architectures such as BONE (2008).

8.4.1 Liquid crystal photonic crystal fibers Photonic crystal fibers (PCFs) is a new class of optical waveguides, which have attracted large scientific and commercial interest during recent years. PCFs are microstructured waveguides, in silica or polymer, with a large number of air holes located in the cladding region of the fiber. In PCFs, light can be guided either by effective index mechanism related to the modified total internal reflection (TIR) or through light confinement by the photonic band gap (PBG) phenomenon. By introducing different gases, liquids or solid materials into the air holes, the propagation parameters of PCF can be finely tuned, providing new functionalities. LCs seem to be particularly interesting substances to infiltrate PCFs since their refractive indices can be relatively easily modified either by temperature or by an external electric field. Such a combination creates a new class of microstructured fibers that are known as photonic liquid crystal fibers (PLCFs). As previously reported, LC is composed of organic molecules that in their nematic mesophase are in average oriented along a particular direction, leading to macroscopic anisotropy in the optical properties of the material. The molecular orientation becomes random when the LC is heated beyond the threshold to the isotropic mesophase (see Fig. 8.3). The refractive index contrast of the liquid and the fiber material can be high and thus leads to strong light scattering. Thus, PLCFs allow for switching between a transparent and a scattering state. Tunable optical switches based on PLCF with a CT of 260 dB, an IL of 1 dB and thermally tuned have been demonstrated (Larsen et al., 2003). Temperature sensitivity of the wavelength is around 3 nm/°C. On the other hand, electrical tuning of PLCFs allows for switching between two different positions of PBGs which depends on ordinary refractive index of LC in off-voltage state and extraordinary index in the high-voltage state. This behavior significantly differs from temperature tuning of PLCFs characterized by smooth changes in PBG positions (Wolinski et al., 2006; Haakestad et al., 2005). More recently, optically controlled switches based on PLCF were reported showing a wide, continuous and fairly flat operating range from 600 to 1700 nm (Tuominen et al., 2007). The IL is 3 dB and the CT better than 220 dB. Relatively small power levels of 10 mW are sufficient to operate the compact switch. Hybrid structures incorporating LC in photonic crystal microcavities with compact size and consequently low power consumption are also developed. They can be made almost arbitrarily small with high cavity Q-factor by incorporating an electro-optical material that experiences a greater index contrast into the microcavity. In this way, photonic crystal microcavities are ideally suited for use in switch designs. The central transmission peak red shifts by approximately 4 nm, from 1514 nm at 0 V to 1518 nm at 15 V. Depending on the infiltrated LC

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being positive-anisotropy or negative-anisotropy (E7 and ZLI-4788 respectively, from Merck), the wavelength shift can be upwards or downwards (Anderson et al., 2007).

8.4.2 Ring resonators with LC Micro-ring resonators (mRRs) are very compact devices that have been used among others, as routers (Vázquez et al., 2000) and WSS (Heebner and Boyd, 1999). The resonant frequencies of these filters and switches have been shifted by changing the equivalent loop length by carrier injection (Djordjev et al., 2002), local heating and absorption (Little et al., 1998). Thermally changing the RR refractive index, by introducing a heater close to the resonator, may induce problems related to power dissipation when many resonators have to be integrated in a DWDM multiplexing system. This effect is reduced in ring resonators fabricated from silicon-on-insulator (SOI) wafers with an electrical tuning, by incorporating nematic LCs as the waveguide side cladding; achieving a tuning range of 0.22 nm (Maune et al., 2003). The high refractive index contrast available in SOI allows compact mRRs with low loss and high-Q. Vertically coupled mRR and cross-grid topology have also been demonstrated, and specific designs including mRR electrically tuned by driven LC embedded in the mRR loops are proposed (Vázquez et al., 2007). A schematic of a switch based on two vertically coupled mRRs in cross-grid topology can be seen in Fig. 8.21(a). LC effective refractive index with and without applying voltage are closed to those reported in Maune et al. (2003) for a NLC E63 mixture. Ideal waveguides in terms of attenuation coefficient are considered. Compact 100 mm2 devices with 28 nm free spectral range and 5 nm tuning range are designed. Numerical simulations of relative output power at three outputs versus wavelength, in the serial mRR configuration, can be seen in Fig. 8.21(b), being LCs in off-state, without applying any voltage. If a specific channel, for instance a TE field at λ = 1.541 mm, is launched it can be seen that maximum output power is on port 3. By changing the LC state, the output power at a specific wavelength in each output is shifted. New tuning capabilities can be achieved using compound Sagnac embedded in ring resonators as reported in Vázquez et al. (2005).

8.4.3 High-capacity holographic LC switches The distinction between beam-steering switches and holographic switches has not always been clear in the literature. Fundamentally, a diffraction grating is a particular and periodic case of a hologram whose optical reconstruction is not a 3D image, but is restricted to a sparse array of points (diffraction orders) whose relative intensities and locations are controlled by the period profile and period pitches, respectively. The holographic nature of an optical switch (LC-based or not) can have two origins:

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(a) Output 1

Output 3

5.0 mm

1.9 mm

1.9 mm LC2

Output 2

Z

Y

X

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2.0 mm 2.0 mm LC1

Input

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Monitor value (a.u.)

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LC1 = OFF LC2 = OFF 1

Output 2

0.6

Output 1

0.5

Output 3

0.4 0.3 0.2 0.1 0.0

1.50

1.52

1.54 1.56 Wavelength (mm)

1.58

1.60

8.21  (a) Schematic of two vertically coupled micro-RRs in a cross-grid topology. (b) Numerical simulations, using commercial FullWAVE software, of the relative output power versus wavelength at three ports with both LCs OFF. Maximum instantaneous electric field intensity pattern at λ = 1.541 µm at output 3.

• (H1): the addressing of the steering functions between the input and output ports is performed using a holographic two-beam interfering setup. • (H2): the grating phase profile used for beam-steering is merged with additional phase distributions (possibly non-periodic) to address other processing

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functions like wavelength filtering, chromatic dispersion compensation or high-resolution variable optical attenuation. For the first category (H1), optically addressed LC SLMs (OASLMs) are used to record the diffraction gratings. In Yamazaki and Yamaguchi (1992) a set of N inputs is connected to a set of N outputs using a matrix of interference patterns generated on a FLC OASLM. The 2D spatial frequencies of a given hologram are controlled by the positions of the two interfering beams emerging from a 2D LCD shutter array. Using this setup, a 4 × 1204 holographic switch is demonstrated in the visible range (Yamazaki et al., 1997). However, large IL (~30 dB) is measured due to the poor efficiency of the binary amplitude holograms recorded in the OASLM. The large average crosstalk values measured (~ 216 dB) are originated from light scattering in the OASLM structure. In the second category (H2), the switch employs electrically addressed SLMs in either the 1 × N scheme of Fig. 8.22 or the N × N scheme of Fig. 8.16. In that case, the holographic patterns can be more easily controlled and optimized than for the H1 case. The architecture of Fig. 8.22 is used to demonstrate 1 × N holographic switches (Crossland et al., 2000; Fracasso et al., 2003) operating at 1550 nm, using reflective FLC modulators providing fast switching times (a few tens of microseconds) and natural polarization independence if large cone angle materials can be used, without resorting to polarization diversity schemes. In Fracasso et al. (2003) a 1 × 14 switch with IL ranging from 8.5 to 10 dB over the 1520–70 nm is proposed. The measured PDL is less than 0.6 dB and the average output CT is 233 dB. Using computer-controlled holographic patterns instead of regular diffraction gratings, the device is also shown to offer 1+1 optical protection, multi-cast, wavelengthband selection and switch monitoring capabilities.

f

f

Input channel

Output channels Fiber array Positive lens

Beam deflector (reflective)

8.22  1 × N holographic switch architecture using a reflective telecentric setup. The input fiber core is imaged onto one (or several) output fiber core(s) with an intermediate beam deflection.

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8.4.4 Concluding remarks on liquid crystal devices In this chapter the potential of LC in switching, from LC materials properties and principles to switching parameters and applications, including the advantages and limitations of LC technology in optical switching, has been revised. Switches based on steering the light with LCs based on reflection, waveguide, polarization management or volume beam-steering have been developed in spatial and WSS for optical network and sensor applications. Apart from previous configurations, there are growing technologies such as ring resonators, holograms and microstructure fibers, which use LC as electro-optical elements for controlling optical switching status. These promising technologies are currently under development and main results have been analyzed. Telecom system makers need shorter response times and higher optical efficiencies for improving the global network performances. LC materials and cell configurations have an evolution strongly correlated to the evolutions of LCD-TVs, which are actually revolutionizing the display world by replacing CRT and the plasma display panels. Recent LCD-TVs have made extraordinary improvements on speed and viewing angles. Alignment stability, power consumption, temperature dependence are problems behind us. Response times of displays have been decreased by a factor of 10 in the last decade by using specific mixtures and suitable driving schemes. For telecom application, the LC makers still have to synthesize the ideal LC mixture bringing together a low rotational viscosity (γ), a high twist elastic constant and high birefringence ∆n. This work is going on, considering that some parameters like viscosity and birefringence have physico-chemical antagonisms. Some LC cells can be improved by using the perpendicular alignment techniques of the multivertical alignment (MVA) LCD-TVs. MVA with a negative dielectric LC exhibits faster response times than TN. To overcome the non-symmetrical response times (rise time linked to voltage and decay time due to LC relaxation), a possible way to design future LC cells could be to add co-planar electrodes like in the in-plane switching structure to the standard perpendicular ones. Then planar and perpendicular voltages will switch ON and OFF state leading to symmetrical and shortened (some milliseconds) ON and OFF times. To date, the OLED technology does not fulfill the telecom requirements concerning wavelengths, so LCs still have a clean horizon in competing with MEMS.

8.5

Acknowledgments

The content of this chapter was developed as a collaboration supported by the BONE project (‘Building the Future Optical Network in Europe’), a Network of Excellence funded by the European Commission within its seventh ICTFramework Programme and the Spanish Ministry of Science Project TEC200914718-C03-03.

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8.6

References

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Patel, J.S. and Silberberg, Y. (1995), ‘Liquid-crystal and grating-based multiple wavelength cross-connect switch’, IEEE Photonics Technology Letters, 7(5): 514–6. Pena, J.M.S., Pérez, I., Rodríguez, I., Vázquez, C., Urruchi, V., Quintana, X., De Frutos, J. and Otón, J.M. (2002), ‘Electrical model for thresholdless antiferroelectric liquid crystal cells’, Ferroelectrics, 271: 149–54. Pérez, I., Torres, J.C., Urruchi, V., Pena, J.M.S., Vázquez, C., Quintana, X. and Otón, J.M. (2007), ‘Voltage controlled square waveform generator based on a liquid crystal device’, XVII Conference on Liquid Crystals. Augustow, Poland. Pickavet, M. and Tucker, R. (2008), ‘Network solutions to reduce the energy footprint of ICT’, European Conference on Optical Communication 2008 Symposium. Available from: http://www.ecoc2008.org/programme.asp#tutorials [Accessed 4 May 2009]. Rep, D.B.A. and Prins, M.W.J. (1999), ‘Equivalent-circuit modelling of ferroelectric switching devices’, Journal of Applied Physics, 85(11): 7923–9. Rhee, J.K., Garcia, F., Ellis, A., Hallock, B., Kennedy, T., Lackey, T., Lindquist, R.G., Kondis, J.P., Scott, B.A., Harris, J.M., Wolf, D. and Dugan, M. (2001), ‘Variable passband optical add-drop multiplexer using wavelength selective switch’, Proceedings of European Conference on Optical Communication 2001. Amsterdam. 550–1. Riza, N.A. and Madamopoulus, N. (2005), ‘Compact switched-retroreflection-based 2 × 2 optical switching fabric for WDM applications’, Journal of Lightwave Technology, 23(1): 247–59. Riza, N.A. and Yuan, S. (1998), ‘Low optical interchannel crosstalk, fast switching speed, polarization independent 2 × 2 fiber optic switch using ferroelectric liquid crystals’, Electronics Letters, 34(13): 1341–2. Riza, N.A. and Yuan, S. (1999), ‘Reconfigurable wavelength add-drop filtering based on a banyan network topology and ferroelectric liquid crystal fiber-optic switch’, Journal of Lightwave Technology, 17(9): 1575–84. Roelens, M.A.F., Bolger, J.A., Williams, D. and Eggleton, B.J. (2008b), ‘Multiwavelength synchronous pulse burst generation with a wavelength selective switch’, Optics Express, 16: 10152–7. Roelens, M.A.F., Frisken, S., Bolger, J.A., Abakoumov, D., Baxter, G., Poole, S. and Eggleton, B.J. (2008a), ‘Dispersion trimming in a reconfigurable wavelength selective switch’, Journal of Lightwave Technology, 26(1): 73–8. Sato, S., Sugiyama, A. and Sato, R. (1985), ‘Variable-focus liquid-crystal Fresnel lens’, Japanese Journal of Applied Physics, 24: L626–8. Smith, P.J., Faulkner, D.W. and Hill, G.R. (1993), ‘Evolution scenarios for optical telecommunication networks using multiwavelength transmission’, Proceedings of IEEE, 81(11): 1580–7. Soref, R.A. (1981), ‘Low-cross-talk 2 × 2 optical switch’, Optics Letters, 6(6): 275–7. Soref, R.A. and McMahon, D.H. (1982), ‘Calcite 2 × 2 optical bypass switch controlled by liquid-crystal cells’, Optics Letters, 7(4): 186–8. Sumriddetchkajorn, S. and Riza, N.A. (2000), ‘Fiber-conectorized multiwavelength 2 × 2 switch structure using a fiber loop mirror’, Optics Communication, 175: 89–95. Takatoh, K., Hasagawa, M., Koden, M., Itoh, N., Hasegawa, R. and Sakamoto, M. (2005), Alignment Technologies and Applications of Liquid Crystal Devices. London: Taylor and Francis. Tan, A., Bakoba, A., Wolffer, N., Vinouze, B. and Gravey, P. (2000), ‘Improvement of response times of electrically addressed nematic liquid crystal blazed gratings’, Proceedings of SPIE: European Conference on Optical Communication 2000, 4089: 208.

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Tarek, S. (2006), Optical Switching. USA: Springer. Tuominen, J., Hoffren, H. and Ludvigsen, H. (2007), ‘All-optical switch based on liquidcrystal infiltrated photonic bandgap fiber in transverse configuration’, Journal of the European Optical Society – Rapid Publications, 2. Underwood, I., Vass, D.G. and Sillitto, R.M. (1986), ‘Evaluation of an nMOS VLSI array for an adaptive liquid-crystal spatial light modulator’, IEE Proceedings, Part J: Optoelectronics, 133: 77–83. Vasilyev, M., Tomkos, I., Rhee, J.K., Mehendale, M., Hallock, B.S., Szalabofka, B.K., Williams, M., Tsuda, S. and Sharma, M. (2003), ‘Broadcast-and-select OADM in 80 × 10, 7 Gbit/s ultra-longhaul networks’, IEEE Photonics Technology Letters, 15: 332–4. Vázquez, C., Lallana, P.C., Montalvo, J., Pena, J.M.S., d’Alessandro, A. and Donisi, D. (2007), ‘Switches and tunable filters based on ring resonators and liquid crystals’, Proceedings of SPIE: Photonic Materials, Devices, and Applications II, 6593: 65931F. Vázquez, C., Pena, J.M.S. and Aranda, A.L. (2003), ‘Broadband 1 × 2 polymer optical fiber switch using nematic liquid crystals’, Optics Communication, 224(1–3): 57–62. Vázquez, C., Vargas, S.E. and Pena, J.M.S. (2005), ‘Sagnac loop in ring resonators for tunable optical filters’, Journal of Lightwave Technology, 23(8): 2555–67. Vázquez, C., Vargas, S.E. and Pena, J.M.S. (2000), ‘Design and tolerance analysis of a router using an amplified ring resonator and Bragg gratings’, Applied Optics, 39(12): 1934–40. Wagner, R.E. and Cheng, J. (1980), ‘Electrically controlled optical switch for multimode fiber applications’, Applied Optics, 19(17): 2921–5. Warr, S.T. and Mears, R.J. (1995), ‘Polarisation insensitive operation of ferroelectric liquid crystal devices’, Electronics Letters, 31: 714–6. Wolffer, N., Vinouze, B., Lever, R. and Gravey, P. (2000), ‘8 × 8 Holographic liquid crystal switch’, Proceedings of SPIE: European Conference on Optical Communication 2000, III: 275–6. Wolinski, T.R., Szaniawska, K., Ertman, S., Lesiak, P. and Domanski, A.W. (2006), ‘Photonic liquid crystal fibers: new merging opportunities’, Proceedings of the Symposium on Photonics Technologies for 7th Framework Program, 95–9. Yamazaki, H., Matsunaga, T., Fukushima, S. and Kurokawa, T. (1997), ‘4 × 1204 Holographic switching with an optically addressed spatial light modulator’, Applied Optics, 36(14): 3063–9. Yamazaki, H. and Yamaguchi, M. (1991), ‘4 × 4 Free-space optical switching using realtime binary phase-only holograms generated by a liquid-crystal display’, Optics Letters, 16(18): 1415–7. Yamazaki, H. and Yamaguchi, M. (1992), ‘Experiments on multichannel holographical optical switch with the use of a liquid crystal display’, Optics Letters, 17(17): 1228–30. Yang, D.K., Wu, S.T. (2006), Fundamentals of Liquid Crystal Devices. Chichester: John Wiley. Yang, J., Su, X., Liu, X., He, X. and Lan, J. (2008), ‘Design of polarisation-independent bidirectional 2 × 2 optical switch’, Journal of Modern Optics, 55(7): 1051–63. Yu, C., Jiang, X., Horiguchi, S. and Quo, M. (2006), ‘Overall blocking behavior analysis of general banyan-based optical switching networks’, IEEE Transactions on Parallel and Distributed Systems, 17(9): 1037–47.

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9 Photonic crystal all-optical switches K. ASAKAWA, Y. SUGIMOTO, N. IKEDA and Y. WATANABE, National Institute for Materials Science, Japan, N. OZAKI, Wakayama University, Japan, Y. TAKATA, Kyocera Corporation, Japan, Y. KITAGAWA, Stanley Electric Co. Ltd, Japan, S. OHKOUCHI and S. NAKAMURA, NEC Corporation, Japan, A. WATANABE, Meijo University, Japan, and X. WANG, National Institute of Advanced Science and Technology, Japan Abstract: Ultra-small, ultra-low energy and ultra-fast SMZ-type all-optical analog switch, PC-SMZ and digital optical flip-flop, PC-FF, have been investigated by using GaAs-based photonic crystal (PC) waveguides and InAs-based quantum dots (QDs). For this purpose, a topology optimization design method for wide/flat-band PC waveguides and selective-area-growth of QDs using a metal-mask MBE method have been developed. This chapter reviews successful operation of an ultra-low energy (~100 fJ) PC-SMZ at an ultra-fast (~ps) speed with a high repetition-rate up to 40 Gb/s. It also reviews successful computational verification of optical bi-stability in the PC-FF with a high repetition-rate up to 40 Gb/s. Key words: photonic crystal, quantum dot, all-optical Mach–Zehnder switch, topology optimization, selective-area-growth.

9.1

Introduction

In recent communication networks, conventional electronic technologies have met with serious problems on account of tremendously increasing information and, as a result, novel photonic technologies have been attracting great interest, as shown in Fig. 9.1. In the datacom market, silicon photonics based on CMOScompatible nanophotonic components available in LSI have been urgent needs as key elements for high-speed interconnection (Izhaky et al., 2006; Analui et al., 2006). In the telecom market, on the other hand, an electronic router, currently as an inevitable high-speed digital signal processor, is predicted to encounter a serious problem of huge power consumption, say, in 2015–20 due to the degraded efficiency in the ultra-fast speed. Alternatively, all-optical devices capable at ultra-high speed, e.g., more than 40 Gb/s, and with ultra-low energy have been highlighted as an urgent target for development. We propose a nanophotonic-structure-based all-optical device that involves two-dimensional photonic crystal (2DPC) waveguides and quantum dots

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e rk

t

s b/

0G

16

~ 10Tb/s,

2015

Our work

t e rke k Bon a All-optical devices m ac

om ec ro

B

– Ultra-fast operation

g – Low energy sin es M c o r lp na E-router Huge power – Cisco ... consumption – 10-40Gb/s ...

et

Si g

G

40

ne

A cc

Co b 1G

n tio

k or tw b /s

Te l

ca

m Da /s t a c 10 m om Gb un LA i m /s N a

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es s

242

Si photonics in LSI Ethernet ...

– Intel – Luxtera – 10Gb/s ...

– High speed interconnection – CMOS compatible

9.1  Key roles of nanophotonics in recent and future telecommunication networks.

(QDs). The 2DPC exhibits wide band gap and strong dispersion (Yablonovitch, 1987), so that enhanced optical confinement and light/matter interaction in the tiny waveguide are expected, while the QD exhibits a high density-of-state (Arakawa and Sakaki, 1982) which induces large optical nonlinearity (ONL) with low optical energy. Our PC/QD-combined devices are two ultra-small and ultra-fast all-optical switches based on a symmetrical Mach–Zehnder (SMZ) configuration (Tajima, 1993). One is a time-differential phasemodulating analog switch, PC-SMZ (Sugimoto et al., 2002), having GaAsbased 2DPC slab waveguides and InAs-based ONL QDs. The other is an all-optical digital flip-flop switch, photonic crystal flip-flop (PC-FF) (Asakawa et al., 2006), combining two identical PC-SMZs connected by a feedback loop. Figure 9.2 shows key roles of these switches in the future energy-saving communication network. The PC-SMZ serves as an ultra-fast DEMUX element, while the PC-FF serves as a novel ultra-fast optical digital processor such as shift register and counter at the optical node in the network. As a result, we can owe most of the ultra-fast signal processing to these photonics and the remaining sophisticated but not so high-speed signal processing to the conventional electronics. In this chapter, theory and principles of nanophotonic structures and devices are described in the first half and, in the latter half, recent experimental results on these nanophotonics are reviewed.

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243

Most signal processing Photonic (ultra-fast)

PC-FF

Present NW

O/Signal : 100G

O

O/Signal : 40Gbps O/E, E/O Transfer Digital processing Electronic ~ 40Gbps

Vast power consumption

Digital processor MUX, DEMUX

PC-SMZ

O/E, E/O Transfer E

Digital processing Electronic : ~ 40Gbps Sophisticated processing Electronic (slow)

9.2  Key roles of all-optical analog switches, PC-SMZ, and digital flip-flop, PC-FF, in future optical networks.

9.2

Theory and principles of photonic crystal all-optical switches

9.2.1 Basic property of two-dimensional photonic crystal (2DPC) waveguide The 2DPC waveguide used in this chapter is composed of single-line defect in the air-bridge slab waveguide, as shown in Fig. 9.3(a). Figure 9.3(b) shows band diagrams in the 2DPC slab waveguide calculated by using the 3D finite-difference time-domain (FDTD) method. Two dotted-line curves appearing between upper and lower slab bands show even and odd modes specific to the 2DPC waveguide. Figure 9.3(c) shows measured transmittance spectra for straight and 60° double bends with 750 µm length. Air-bridge samples are fabricated by molecular beam epitaxy (MBE), electronbeam (EB) lithography, reactive ion beam etching (RIBE) and selective wetetching (Sugimoto et al., 2005), as shown in Fig. 9.4(a). The thickness of the GaAs core layer is 250 nm. Figure 9.4(b) and (c) are SEM photographs of the whole PC-SMZ and expanded waveguide patterns, respectively. A circle in Fig. 9.4(d) shows about 0.7 µm wide and 0.25 µm thick cross-section of the single-line defect in the 2DPC waveguide. Propagation loss was experimentally evaluated by a cut-back method to be as low as 0.76 dB/mm (Sugimoto et al., 2004), which is a record low for the GaAs 2DPC waveguide (Arentoft et al., 2002; McNab et al., 2003; Notomi et al., 2004).

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Single missing line (W1) Lattice constant : ~360 nm Air hole diameter : ~200 nm Slab thickness : ~250 nm

G–k (a)

1100

0.30 Odd

0.28 0.26

1300

Light line Single mode (line-defect)

0.24 0.22 0.0

0.1

1200

Even Slab bands

0.2 0.3 k (2p/a)

0.4

1400

Wavelength (nm)

Slab bands

0.32

1500

Normalized transmittance

0.3

0.34

a /l

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Optical switches

0.5

0.28

0.26

a /l 0.22

0.24

– 20 Straight W Bend

– 25 – 30

Dl = 40~50 nm

– 35 – 40 – 45 – 50 – 55 – 60 1100

1200

1300

1400

1500

1600

Wavelength (nm)

(b)

(c)

9.3  Properties of 2DPC waveguide used in the experiment. (a) Plan-view of 2DPC waveguide pattern with single missing line defect, (b) band diagram of the 2DPC slab waveguide and (c) measured transmittance spectra for the straight and 60° double bend waveguides. Whole pattern

Air-bridge slab

MBE growth EB & RIBE dry etching Wet etching GaAs core AIGaAs clad GaAs sub.

(b)

(c)

Single-line defect

EB lithography

CI2-RIBE

HF wet etching

(a)

(d)

9.4  (a) Fabrication processes of 2DPC slab waveguides (upper) and SEM photographs of the cross-sectional views (lower) and (b)–(d) SEM photographs of the whole PC-SMZ pattern, cleaved edge of the air-bridge 2DPC waveguide and its magnified photograph, respectively.

As shown in Fig. 9.5, the PC-SMZ requires two kinds of directional couplers (DCs), that is, DC S which divides and combines signal pulses and DC C which introduces the control pulse into the ONL arms. The DC S should be designed as 50% in coupling strength, while the DC C should exhibit 100% coupling for the

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Photonic crystal all-optical switches λ2 λ1 : 50%

4L(100%)

245

λ2 : 100% λ1 : 0%

λ1

L(50%)

Length : 4 × L

Length : L

9.5  Two kinds of DCs in the PC-SMZ waveguide pattern and their two different criteria. One is a 50% coupler for dividing/joining the signal pulse and another is a DC with 100% coupling for the control pulse and 0% coupling for the signal pulse.

control pulse and 0% (i.e., uncoupled) for the signal pulse. These requirements are achieved by controlling the DC length according to the relation, DC C = 4 × DC S, as indicated schematically at both sides in Tanaka et al. (2005a, 2005b). Figure 9.6(a) and (b) show plan-view SEM photographs of the fabricated DC patterns and output near-field spots observed with an infrared vidicon camera, respectively. Figure 9.6(c) shows measured transmission spectra for the sample

Normalized transmittance

50% coupling 1

1350

1300

1200

1250

l (nm)

W-Bend DC ac DC ad

10–1

0 x2 x x Dq x2 Even mode

10–2 a 10–3 0.26

(b)

b 0.27

Odd mode Dq=0

c d 0.28

0.29

0.30

0.31

Normalized transmittance

(a)

0.4

100% coupling

0.35

0.3

0.25

10–1 a

c

b

d

10–2

> 10 dB

a/l a-d a-c

10–3 10–4 900 1000 1100 1200 1300 1400 15001600

Frequency (a/l)

(c)

Wavelength (nm)

(d)

9.6  (a) Plan-view SEM photograph of the DC, (b) near-field pattern at the output ports of the DC observed by a vidicon camera and (c)–(d) measured transmittance spectra of 50% coupling DC and 100% coupling DC cascaded by two 50% DCs, respectively.

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246

designed as a 50% coupler. It is found that the uncoupled intensity DC ac and coupled intensity DC ad exhibit the same transmittance at a wavelength of 1320 nm. The result shows that the 50% coupling is achieved. A 100% coupler is achieved by cascading two identical 50% couplers. In Fig. 9.6(d), a coupled intensity, a-d, is more than 10 dB higher than an uncoupled intensity, a-c, showing the 100% coupler has been successfully achieved. For more details, refer to Tanaka et al. (2005a). Computer simulation of switching operation using DCs has been carried out, as shown by E-field distributions of optical beams in Fig. 9.7(a) and (b). When the phase difference, ∆φ, between two incident beams equals 0° or 180°, output intensities exhibit equally 1.0, while ∆φ equals 290° or +90°, they exhibit 2.0 or 0. The PC-SMZ is operated by the change of ∆φ between 0° or 180° in the ONL arms. Details are discussed later.

9.2.2 Optical nonlinearity in quantum dots (QDs) An ONL-induced phase-shift in the PC-SMZ is caused by the third-order ONL in the QD and resultant refractive-index change δn (Nakamura et al., 2002; Nakamura et al., 2003; Nakamura et al., 2004a). δn is attributed to the absorption saturation pumped by a control pulse, as shown in Fig. 9.8(a). The phase-shift depends on the pumping energy density, control/signal pulse detuning and inhomogeneous broadening of the QD absorption peak. Figure 9.8(b)

1.0

1.0

2.0 (0.0) 1.0

40

1.0

30 Z (µm)

30 Z (µm)

0.0 (2.0)

40

20

20

10

10

0

0 –10

1.0

0 X (µm)

10

1.0 φ = 0 or 180°

–1.0

–10

1.0 ∇

∇

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Optical switches

(a)

0 X (µm)

10

–1.0

1.0

φ = –90° (+90°) (b)

9.7  (a)–(b) Calculated E-field distributions of the 50% coupling DCs with phase differences of two incident beams by 0° or 180°, and +90° or –90°, respectively.

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Photonic crystal all-optical switches Signal pulse (probe)

Control pulse

Control pulse (pump)

QD absorption Saturation absorption

Dn

Transmission

Intensity

Df

Refractive index

Detuning energy (meV)

1

Signal pulse

PC+QD

247

TE, RT, 2ps-pulse

Transmittance



exp. cal.

Ps = 13 fJ/mm2

Absorptive

Transparent QD

TE 0.1 –1 10

Wavelength

(a)

(b)

10

0

10

1

2

10

10

3

Input pulse energy (fJ/mm2)

10

4

(c)

9.8  (a) Schematic absorption and refractive index spectra showing absorption saturation in QDs. Solid and broken lines indicate spectra before and after pumped by the control pulse, respectively, (b) wavelengths of the control and signal pulses in the transmittance spectrum of the 2DPC/QD waveguide and (c) measured transmittance of the 2DPC/QD waveguide as a function of pumping energy.

schematically shows a relationship between control and signal pulse wavelengths in the transmission spectrum for the QD-embedded 2DPC waveguide, as indicated by PC+QD. A sharp dip is caused by the QD absorption peak. As shown in Fig. 9.8(c), δn is determined by the transmittance change (∆T) from linear to nonlinear regimes when the pumping pulse energy increases in the QD-embedded 2DPC waveguide (Kanamoto et al., 2004). An absorption saturation power Ps of 13 fJ/µm2 suggests the optical switching energy as low as 100 fJ or less. As shown by the measured ∆T in Fig. 9.9(a), ∆T as large as 16 dB is obtained by an airbridge 2DPC waveguide, as shown by the upper diagram in Fig. 9.9(b), while a 10–1

DT

Transmission (arb. unit)

1290nm 1.5ps

Air-bridge PC/QD waveguide

16dB

G = 0.9 10

–2

3dB

x 1000

10–3 10–3

Ridge non-PC/QD waveguide

Ps

G = 0.1 10–2

10–1

100

101

102

103

Input pulse energy (pJ/pulse) (a)

(b)

9.9  (a) Comparison of transmittance changes ∆Ts in two different QD-containing waveguides and (b) schematic cross-sectional views of PC/QD waveguides (upper) and non-PC/QD waveguides (lower).

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conventional non-PC ridge waveguide exhibits only 3 dB in ∆T. The large ∆T is attributed to strong vertical confinement of the optical beam in the 2DPC waveguide as compared to weak confinement in the non-PC waveguide. The results in Fig. 9.8(c) and Fig. 9.9(a) are remarkable features of the PC/ QD-combined nanophotonics.

9.2.3 Principle of symmetrical Mach–Zehnder (SMZ) type all-optical switch: photonic crystal symmetrical Mach–Zehnder (PC-SMZ) Analog switch: PC-SMZ The principle of an ultra-fast operation of the PC-SMZ is shown in Fig. 9.10 (Tajima, 1993). A ‘switch-on’ control pulse incident in the upper ONL arm causes δn, leading to a phase-shift φ1 = 2π (δñ λ) lONL for a series of signal pulses, where λ is a wavelength of the signal pulse and lONL is an ONL arm length. The phaseshift φ2 is generated similarly in the lower ONL arm by the ‘switch-off’ control pulse. As a result, a phase-shift difference ∆φ = |φ1 – φ2| is generated at the combined Y-junction. Only when ∆φ = π/2 or π (depending on the junction structure), the signal pulses are switched spatially. Time response of the δn in the semiconductor is rapid (sub-ps) in rise but slow (sub-ns) in fall depending on a carrier lifetime in the semiconductor. However, since the δn is excited timedifferentially by the switch-on and switch-off control pulses, the δn in the tailing slow component are canceled, as shown in Fig. 9.10. Figure 9.11(a) shows a schematic of the PC-SMZ (Sugimoto et al., 2005), while Fig. 9.11(b) shows

Switch-on control pulse

Nonlinear phase shift Df

SOA... Tajima QD ... ours T

Nonlinear waveguide 1

Signal pulses

Nonlinear waveguide 2

T Df = p Switch-off control pulse

fNL in arm 1 fNL in arm 2

T Canceling

Differential phase modulation at ~ ps

9.10  Principle of time-differential phase modulator for the SMZ switch. SMZ: symmetrical Mach-Zehnder (K. Tajima, JJAP, 1993).

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Photonic crystal all-optical switches Output signal

Photonic crystal

Control pulse Signal pulse Control pulse

249

Control pulse DC with wavelength selectivity QDs

Signal pulse

QD-based large NL(χ3) media

Bar Cross 3 dB divider (CCDC) (b)

(a)

9.11  (a) Schematic diagram of the PC-SMZ and (b) schematic of PC-SMZ waveguide configuration.

2DPC patterns including several DC elements. Research items and numerical targets for achieving the PC-SMZ are summarized in Table 9.1.

9.2.4 Principle of SMZ type all-optical digital switch: photonic crystal flip-flop (PC-FF) The principle of the time-differential phase-modulator, mentioned above, reminds us that the PC-SMZ exhibits pseudo flip-flop (FF) operation, as shown in Fig. 9.12, because switch-on and switch-off control pulses serve as ‘set’ and ‘reset’ pulses. However, the decay of the ‘on’ state is remarkable due to the carrier relaxation in the QD when the reset pulse is far delayed from the set pulse, so that a complete bi-stable state is difficult to maintain. To remedy this drawback, we

Table 9.1  Research items and numerical targets of PC-SMZ Structure

Item

Numerical target

Photonic crystal

Band width 40 nm, Single mode Propagation loss 1 dB/mm Directional coupler Signal pulse: 50%, Control pulse: 100%

Quantum dot

Absorption peak Density Uniformity NLO phase shift

λ: 1.28 µm 4 × 1010/cm2 30 meV (PL peak: FWHM) π/2 (NLO arm length: 500 µm)

All-optical switches

Control pulse Signal pulse Switch speed Extinction ratio Chip size Insertion loss

λ: 1.28 µm, Energy: 500 fJ, Pulse width: 1ps λ: 1.30 µm, Pulse width: 10 ps (40 Gb/s) Rise/fall time: 1 ps, Window width: 25 ps 10 dB 0.5 mm × 0.5 mm 20 dB (Coupling to optical fiber)

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introduce clock pulses into the input control pulse for refreshment of the decaying ‘on’ state, as shown in Fig. 9.12. The proposed configuration for the optical FF is shown in Fig. 9.13(a). Two identical PC-SMZs (SW-1 and SW-2) (Asakawa et al., 2006) are coupled by a feedback loop supporting the bi-stability. This device is hereinafter referred to simply as PC-FF. In the PC-FF, the output (Q) is controlled by set pulses (S) and reset pulses (R). In SMZ-1, two beams divided from the CW light (B) experience ONL phase-shifts and finally form the output Q as a result of interference. The output Q equals 1 or 0 depending on the interference being constructive or destructive. A fraction of the output Q is used as light (A1) for exciting SMZ-2. In SMZ-2, clock pulses (or CW light) (A2) experience ONL phase-shifts and PC-SMZ:

For-bi-stable operation,

– Switching by set/reset pulses

– “on” state

decayed

when S/R period extended

Pseudo-FF

S

Set Reset SR Input

R

S

R

Relaxation

Output

New proposal : PC-FF

S

– Refreshment by clock pulse

Clock pulse R S

R

(feed-back loop)

Refreshed

Bi-stable, i.e., flip-flop 9.12  Principle of optical bi-stability in the PC-FF evolved from the PC-SMZ. Set SW-2 Feed-back loop S B R CW

S A2

A3

Clock pulse A1

R

t T Feed-back

A3

Q SW-1

t Reset

t Out

(a)

Q

t (b)

9.13  (a) Block diagram of the PC-FF with identical PC-SMZs coupled by feedback loop and (b) time chart of the PC-FF to show optical flip-flop operation.

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Photonic crystal all-optical switches

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then form output (A3) as a result of interference. Assume that the initial output Q is 0. The set pulse being input, the first arm of the SMZ-1 is excited and the output Q is switched to 1. This excites SMZ-2 and its output is input into the first arm of the SMZ-1 as feedback to make the state of Q = 1 maintained within the feedback delay time. When the reset pulse is input, the second arm of the SMZ-1 is excited, canceling out the ONL phase-shift in the first arm and thus the output is switched to Q = 0. The state of Q = 0 is also maintained until the set pulse is input. Figure 9.13(b) is a time chart showing the time responses at each port, S, R, A3 and Q. In this scheme, the delay time on the feedback loop should be reduced to as short as ps if the period of the ‘on’ state is less than 10 ps corresponding to more than 40 Gb/s switching speed. Since the total length of 2DPC waveguides in the PC-FF is as short as several 100 µm (corresponding to several ps delay time), the PC-FF is capable of operation at more than 40 Gb/s switching speed.

9.3

Design and fabrication of advanced 2DPC waveguide for PC-SMZ

For implementation of the PC-FF, a new design method called topology optimization (TO) has been developed for improving band-narrowed non-straight waveguides into wide/flat-band waveguides in the PC-SMZ, as shown in Fig. 9.14. The TO technology and experimental results are shown in the next section.

9.3.1 Principles of topology optimization (TO) design Recently, drastic improvement of bandwidth and transmittance in the 2DPC bends and branches has been reported by means of the TO design method (Jensen l-selective Y-junction

Intersection (X) 1

Control pulse (l2)

Signal pulse (l1)

3

Delay

2

Bend

4

Directional coupler (DC)

9.14  TO-design-adaptive waveguide in PC-FF: schematic of the PC-SMZ configuration with 2DPC intersection, bends, Y-junctions and DC.

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et al., 2005; Borel et al., 2005; Têtu et al., 2005). Figure 9.15(a) and (b) show a schematic picture and its simulated pattern of the Z-shaped waveguide designed by the TO method, respectively. Figure 9.15(c) shows calculated transmittance spectra for the TO method as compared to those for the conventional method. Drastic band broadening and flattening are easily found. A full computational model is shown in Fig. 9.16. TO procedure is performed to maximize transmittance in a straight line (a-b) by modifying the refractive-index distribution in the design domain indicated by the gray area. For more details, see other descriptions (Bendsøe and Sigmund, 2003).

Optimized shape

(b)

(a) Drastic broadening and flattening 1.0 Normalized transmission

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0.8 0.6

Standard Optimized

0.4 0.2 0.0 0.22

0.23

0.24

0.25

0.26

0.27

Normalized frequency

(c)

9.15  (a) Schematic picture of the TO-designed Z-shaped 2DPC waveguide and (b) pattern of actually TO-designed Z-shaped 2DPC waveguide; (c) calculated transmittance spectra.

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Rectangular elements

Maximize transmittance

~25nm a~360nm Refractive index Design variables (gray elements) X e

nGaAs Design domains

1

nair < ne < nGaAs 0

nair

9.16  Topology optmization for reverse problem: computational model of the TO design including the design domain.

9.3.2 TO-designed intersection First of all, numerical and experimental studies on the intersections with broad bandwidth, high transmittance and low crosstalk designed by the TO method are reviewed here (Ikeda et al., 2006; Watanabe et al., 2006). An intersection is formed by a line-defect waveguide composed of single missing row of air-holes, as shown in Fig. 9.17(a). The full computational model is shown in Fig. 9.17(b). TO procedure is executed to maximize transmittance in a straightforward line (a-d). Figure 9.18 shows designed intersection patterns and their experimental results, comparing the standard (upper part) and TO (lower part) designs. In the measured transmission spectra at the right-hand side, ‘ad’ means the transmittance between the a-d ports, while ‘ac’ means the undesirable crosstalk between the a-c ports. It is worth noticing that the crosstalk ‘ac’, which was equivalent to ‘ad’ for the standard design, is suppressed as low as 15 dB for the TO design and surprisingly the transmission spectra ‘ad’ is quite the same as

b

d

a

c

(a)

(b)

9.17  (a) Initial pattern of 2DPC intersection waveguides and (b) design domain (cross-spot), input port a, straightforward output port d and crosstalk output ports b and c.

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Standard design 15.0

Large cross-talk

Normalized frequency (c/a) 15.1 15.2 15.3

a

c

–20

–10

Transmittance (dB)

d

Transmittance (dB)

0

b

–20 –30 –40 –50 15.0

–40 ac

15.1 15.2 15.3 Wavelength (nm)

–50

15.4

a

d

c

–20

–10 –20 –30 –40 –50 –60 15.0

15.1 15.2 15.3 Wavelength (nm)

1350

1400

Equivalent to straight

Normalized frequency (c/a) 15.1 15.2 15.3 Transmittance (dB)

b

1300

Wavelength (nm)

TO design 15.0 0

Straight

ad

–30

–60

Transmittance (dB)

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15.4

ad

Straight

–30

–40 ac –50

1300

1350 Wavelength (nm)

1400

9.18  Comparison between standard (upper) and TO (lower) designs. (Left) 2DPC intersection pattern. (Middle) Calculated transmission spectra. (Right) Measured transmission spectra.

that for the straight waveguide in the whole wavelength region. The result clearly shows that the TO method is quite a desirable tool for designing nonstraight 2DPC waveguides.

9.3.3 TO-designed bend waveguide The TO design has been applied to the bend waveguide as well (Watanabe et al., 2007). Figure 9.19(a)–(c) show patterns of the 2DPC bend waveguides designed by the standard (STD) and optimized (TO1, TO2) methods, respectively, while Fig. 9.19(d) shows corresponding calculated transmission spectra. Arrows indicate ripples caused by multiple reflections between bends appearing only in the STD design. The TO design suppresses reflections at the bends and resultant ripples as well. Hence a wide and flat bandwidth within the non-leaky mode range is achieved. SEM photographs of the fabricated bends designed by the STD and TO2 methods are shown in Fig. 9.20(a) and (b), respectively. The measured transmittance spectra of the samples with four bends are shown in Fig. 9.20(c). The thin solid line shows that of a straight waveguide. An abrupt decrease of the transmittance in the long wavelength observed in the STD bend is not observed in the TO2 bend. These results verify dramatic improvement in the TO bend as well.

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Photonic crystal all-optical switches (a)

(b)

STD

(c)

TO1

255

TO2

Wavelength (nm) 1350

1300

1250

(d)

Transmittance (dB)

0

–10

–20

STD TO1 TO2

–30

–40 0.25

0.26

0.27

Frequency (c/a)

9.19  Design of (a) standard bend, (b) TO1 bend, (c) TO2 bend and (d) their calculated transmittance spectra for double bends.

9.3.4 TO-designed Y-junction The TO method is effective for designing the wavelength-sensitive Y-junction (Watanabe et al., 2008), as shown in Fig. 9.21(a). Optimization is performed on three conditions, as schematically shown in Fig. 9.21(b): (1) maximize the transmittances along AC and BC channels, (2) minimize the transmittance along unwanted AB and BA channels and (3) optimize them at the two separate (30 nm in this case) wavelengths. We used three target-wavelengths each around λ1 = 1275 nm and λ2 = 1305 nm for the design of PC-SMZ and PC-FF with 1300 nm operation wavelength. Before optimization, transmission spectra are unfavorably the same along the channels AC and BC because of their geometrical symmetry. After the TO

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–20 STD

Straight Transmittance [dB]

(a) TO2

TO2

–30

STD –40

–50 1250

1300

1350 1400 Wavelength [nm]

1450

(c)

(b)

9.20  (a)–(b) SEM images of fabricated bend waveguides designed with (a) standard and (b) TO methods, respectively and (c) their measured transmittance spectra.

Design domain

C

Control λ1 A

Transmittance

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Max

Max

AC

B Signal λ2 (a)

BC

Min

Min

λ1 λ2 Wavelength (b)

9.21  (a) Computational model of the TO-designed Y-junction indicating the design domain and (b) schematic picture of the required asymmetric transmission spectra.

design, asymmetric air-hole patterns appear, as shown in Fig. 9.22(a). That is, transmission spectra along the AC and BC channels cause ~20 dB selectivity at wavelengths apart by 30 nm, as shown in Fig. 9.22(c). Calculated mode patterns in Fig. 9.22(b) also show that the input lights smoothly propagate through the channels AC and BC at the Y-junction without entering the unwanted ports B and A. Figure 9.23(a) shows an optical microscope image of the fabricated TO-designed Y-junction waveguide with total length of 1 mm. Figure 9.23(b) shows SEM images of the TO-designed bend (left) and Y-junction (right), while Fig. 9.23(c) shows their transmission spectra. A dotted line shows that of the straight waveguide. Measured transmission spectra along the channels AC and BC show characteristics almost similar to those predicted by the calculation, as

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C

C

(a) A A

B

B

l1

l2

(b)

(c)

Transmittance [dB]

0 AC

BC

–10 20dB

20dB

–20 30nm –30 1240

1260

1280

1300

1320

1340

Wavelength [nm]

9.22  (a) Air-hole patterns of TO-designed bend and Y-junction, (b) calculated mode profiles of input lights along channels AC with λ1 and BC with λ2 corresponding to (a), and (c) transmission spectra of the channels AC and BC in the TO-designed Y-junction.

shown in Fig. 9.22(c), although isolation is not enough. The TO design has also been verified to be effective for the wavelength-selective Y-junction which is needed for the PC-FF.

9.4

Growth and characterization of optical QDs for PC-FF

Recent development of self-assembled InAs QDs has attracted a great deal of attention because of its potential for use in telecommunication systems. As indicated in Fig. 9.11(a), the PC-SMZ requires InAs QDs partially embedded in

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(a)

1mm

C

C (b) A

λ2

λ1 –25

(c)

Straight

AC

B

B

A

Transmittance [dB]

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Optical switches

–30

BC

4dB

10dB –35

–40 1240

30nm 1260

1280 Wavelength [nm]

1300

9.23  (a) Plan-view optical microscope image of the Y-junction waveguide, (b) SEM images of the bend (left) and Y-junction (right) parts in the fabricated TO-designed waveguides, and (c) measured transmission spectra of the channels AC and BC in the TO-designed Y-junction.

the GaAs 2DPC waveguide. For this purpose, selective-area-growth (SAG) of QDs has been developed by using a metal-mask (MM) method (Ohkouchi et al., 2006; Ozaki et al., 2007; Ohkouchi et al., 2007). The MM method enables us to grow the QD ensemble selectively in an area of several tens to several hundreds µm in size. In this section, recent results of this technique are reviewed.

9.4.1 MM method for selective-area-growth (SAG) of QDs Figure 9.24(a) and (b) show an MM configuration in the MBE growth chamber and open window fabricated in the MM, respectively. The position of the open

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Photonic crystal all-optical switches Open window

Cell

Intensity [arb. u.]

400μm~500μm

InAs QD

In-situ mask

300μm, 1000μm

GaAs

Unmasked region

0.90

Substrate holder

(a)

259

0.95

Masked region

1.00

1.05

1.10

Photon energy [eV]

(b)

(c)

9.24  (a) Schematic diagram of an MM method for selective-areagrowth of InAs QDs in the MBE equipment, (b) an open window pattern in the MM corresponding to ONL area in the PC-SMZ pattern and (c) PL spectra on the unmasked and masked areas in the MBEgrown wafer.

window is correctly assigned to the required ONL area in the PC-SMZ chip (Ozaki et al., 2007). SAG of QDs was confirmed by photoluminescence (PL) spectra observed only on the unmasked region, while no peak was found in a PL spectrum on the masked region, as shown in Fig. 9.24(c). These results indicate that the MM method is useful for SAG of the QD.

9.4.2 High density/uniformity in SAG-QDs Figure 9.25(a) shows a typical atomic force micrograph (AFM) image of the QD ensemble on the unmasked regions grown by the MM method. The QD density estimated from the AFM image is as high as 4 × 1010 cm22, which is almost equal to that of the conventional QDs without the MM. The mean lateral size and height of the QD were approximately 40 nm and 5 nm, respectively. By this technique, significantly excellent QD uniformity is estimated to be 28 meV in term of FWHM (full width at half maximum) in the PL peak, as shown in Fig. 9.25(b). These results show that the QD grown with the MM method exhibits rather high quality.

9.4.3 Wavelength-control technique in SAG-QDs Wavelength of the SAG-QD can be controlled by inserting an InGaAs layer as a stress-reducing layer (SRL) between the InAs-QD and GaAs-spacer layer (Ozaki et al., 2008). Figure 9.26(a) shows a sample structure. In the experiment, 3 nm thick In0.2Ga0.8 As layer was deposited on the QDs and 3 nm thick GaAs was grown for capping the QDs. Insertion of the SRL allows red-shifts in the PL peak energy. Figure 9.26(b) shows PL spectra as a function of wavelength, while Fig. 9.26(c) shows variation of the PL peak wavelength as a function of SRL thickness. This result indicates that insertion of the SRL is effective for controlling the PL wavelength of QDs without degrading the QD optical quality. Thus, combination of the MM method and insertion of the SRL is a promising way to achieve the PC-FF.

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High uniformity ΔE = 28 meV

High density 3 × 1010/cm2 AFM Image

2.00

PL Intensity (a. u.)

RT 5m W

1.00

0

1.0

0 2.00 μm

1.00

28m eV

1.1 1.2 1.3 Wavelength l (μm)

(a)

1.4

(b)

9.25  (a) AFM image showing high-density QDs and (b) PL spectrum showing highly uniform QDs. GaAs 10nm Al0.2Ga0.3As30nm

With MM

GaAs 50nm

GaAs 50nm

GaAs

3nm

In0.2Ga0.2As

3nm

InAs-QD

2.6 ML

Al0.2Ga0.3As 30nm GaAs 200nm

GaAs (001)

(a)

1200

0.95 2nm 3nm4nm 6nm

1250

1300

Wavelength [nm]

1350

PL peak wavelength [nm]

PL intensity [arb. units]

Photon energy [eV] 1 1.05 SRL thickness: 0nm

60

1340 1320

45

1300

30

1280 1260

15

1240 1220

0

1

2

3

4

5

SRL thickness [nm]

(b)

6

FWHM [meV]

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Optical switches

0

(c)

9.26  (a) Schematic cross-sectional picture of the MBE grown sample, (b) PL spectra of QDs capped with SRL as a function of the SRL thickness, and (c) PL peak wavelength shifts and their FWHM as a function of the SRL thickness.

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A schematic illustration of the PC-FF is shown in Fig. 9.27, where QD ensemble with the wavelength λ1= 1.29 µm in SW-1 is separately grown from the other QD ensemble with wavelength λ2 = 1.31 µm in SW-2. The MM mentioned above is designed to be 180° rotatable on the MBE wafer holder, as shown in Fig. 9.28(a). After the QD1 is grown, as illustrated in Fig. 9.28(b), an In0.2Ga0.8 As layer is deposited on the QD and capped with a GaAs layer. Then, the MM is rotated by 180° and the QD2 is grown in the same manner except for the thickness of SRL, as shown in Fig. 9.28(c). The SRL thickness is set to 2.7 nm for QD1 and 4.5 nm for QD2. Figure 9.29(a) shows a view of the PL intensity mapping for the neighboring QD1 and QD2, while Fig. 9.29(b) shows PL spectra from the corresponding QD1 and QD2. Thus, wavelength difference (∆λ) was controlled by ~ 20 nm as designed. These techniques are suitable for implementation of the PC-FF.

Selective-area-grown QD GaAs-sub. 1.31 µm-QD

SW-2 Monitor Clock

300 µm Set

Output

Input Reset

1.29 µm-QD

Feedback SW-1

500 µm

9.27  Configuration of the 2DPC waveguide and areas of QDs with different absorption wavelengths in the PC-FF. (a)

180° rotation

(b)

QD1

Metal mask

MM holder

GaAs-cap

InAs-QDs In0.2GaAs 2.7 nm

QD1

QD2

In0.2GaAs 4.5 nm

9.28  (a) Optical microscope photograph of the rotatable MM mounted in the substrate holder and (b) two different positions of the MM before and after 180º rotation of the MM for SAG of QD1 and QD2.

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(a)

(b) QD2

~820 mm

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Optical switches

QD1

1.1

1100 ~610 mm

Photon energy [eV]

1.05

1

0.95

0.9

Dl ~20 nm

PL intensity [arb. unit]

262

QD2

QD1

1200

1300

1400

Wavelength [nm]

9.29  (a) PL intensity mapping of QD1 and QD2 grown separately before and after 180º rotation of the MM and (b) PL spectra of QD1 and QD2 exhibiting different emission peaks at 1276 and 1296 nm.

9.4.4 Basic properties of phase-shift in QD-embedded 2DPC waveguide As mentioned in Fig. 9.10, the PC-SMZ is operated by the principle of the timedifferential phase-modulator, as shown in Fig. 9.30(a)–(c) (Nakamura et al., 2004b; Asakawa et al., 2006). The QD in the ONL arm exhibits absorption saturation when pumped by two time-differential control pulses (CPs) and consequently induces effective refractive-index changes ∆non,off, where ∆non,off show refractive-index changes of two identical ONL arms, ‘on’ (left) and ‘off’ (right) in Fig. 9.30(a). The ∆non,off result in the phase-shift ∆φon,off in each ONL arm, as shown in Fig. 9.30(b). The ∆φon,off exhibit time responses with ultra-fast rise times of the order of ps (indicated by trise in the figure), while they exhibit much slower relaxation times (indicated by tfall). As a result, the difference of the phase-shift ∆φ = ∆φon – ∆φoff between the two ONL arms provides phase modulation only at a time span τ on account of a phase-shift cancellation at the slow relaxation period, as shown in Fig. 9.30(c). Therefore, the SMZ device has a rectangle-like time response with sharp abruptness. Complete switching is achieved when ∆φ is 180°. Till now, we have demonstrated single switching of the PC-SMZ by single set of on/off CPs at 2 ps rise/fall times in as low as 10–100 fJ switching energy (Nakamura et al., 2004b). For sequential operation with repetitive sets of on/off CPs, we have to investigate the detailed relationship between the sequential CP rate and change of the ∆φ height. Otherwise, the ∆φ could be gradually decreased below 180° due to the decreasing ∆n, that is, due to the carrier accumulation effect caused by the following sets of on/off CPs before the carrier is relaxed sufficiently. Figure 9.31 schematically shows a sequential phase-shift excitation process in the QD/ PC waveguide. In the figure, left and right columns correspond to high-density

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263

Output signal Photonic crystal

Control pulse

Optical nonlinear QD

Signal pulse Control pulse

(b) Dfon,off

Dfon Dfoff

trise

(c)

ttall

t

Df = Dfon – Dfoff trise

9.30  (a) Schematic picture of the PC-SMZ, (b) time-differential phaseshift responses pumped by the on-CP and off-CP in the PC-SMZ and (c) phase-shift difference between two identical arms in the PC-SMZ, showing the ability of the rectangular-type switch.

QD ensemble leading to successful switching and low-density QDs leading to unsuccessful switching, respectively. Figure 9.31(a) and (b) correspond to large (high-density) and small (low-density) QD populations in the QD/PC waveguides, respectively. Figure 9.31(c) and (d) show accumulations of the phase-shifts for cases in Fig. 9.31(a) and (b), respectively. See Kitagawa et al. (2009) for detailed discussions. The resultant ∆φi defined by ∆φi-on – ∆φi-off remains constant or decays gradually depending on the relationship between the repetitive-rate of the pumping and the available number of the QDs, as shown in Fig. 9.31(e) and (f). Thus, the switching can be operated completely or incompletely.

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High-density QDs

Low-density QDs Deficient absorption leading to decayed Df

Efficient absorption for 180° Df

CP

CP

(a)

(b)

Dfi–on(i=1,2,...)

Phase shift

Dfi–on(i=1,2,...) fmax

Df1

Dfi–on

fsat

limited by fmax

CP/SP delay

CP/SP delay

(c)

(d) Df1 = Df1–on –Df1–off 1

1

0

fmax

fsat

Df1 = Df1–on –Df1–off Intensity

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0 Time (e)

Time (f)

9.31  Schematic pictures showing the condition for the constant phase-shift difference independent of carrier accumulation in QDs. (a)–(b) Relationship between the originally grown QD state and QDs pumped by the CP. (c)–(d) Accumulated phase-shift induced by the sequential CPs. (e)–(f) Sequential phase-shift difference between the two arms in the PC-SMZ.

A sample for experiment was a 400 µm long straight PC-WG with InAs-QDs embedded to characterize the sequential phase-shifts. Transmission spectra were measured by using a two-color pump/probe heterodyne method. A four-sequential CP train was generated with four beam-splitters and four retro-reflectors. Figure 9.32(a) indicates the transmittance of the CP and ∆φi of the SP as a function of the CP energy, while Fig. 9.32(b) indicates an example of the time decay of ∆φi

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Photonic crystal all-optical switches (a)

Absorption saturation

265

TPA 80

60

40 –25

Df of SP [deg.]

CP transmittance [dB]

fmax –20

20 –30 10

0

100 CP energy [fJ/pulse]

Phase shift [deg.]

(b)

40

t ~ 40 ps 20

0 0

100

200

300

CP/SP delay [ps]

9.32  (a) Dependences of CP transmittance (solid line) and phase-shift ∆φ of the SP (filled squares) on CP energy and (b) time decay of ∆φ pumped by the single CP with the energy of 10 fJ, showing the relaxation time of ~40 ps.

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pumped by a single CP with an energy of 10 fJ. The obtained phase-shift and decay constant are 30° and 40 ps, respectively. Next, the relationship between the accumulated phase-shift ∆φi pumped by the repetitive CP and the repetition frequency and energy of the CP was systematically obtained by using the PC-SMZ sample. A wide range of results are summarized in Fig. 9.33. For each CP energy (ECP ), the accumulated phase-shift ∆φi (upper part) and resultant phase-shift difference ∆φ (lower part) are shown. Repetition frequencies are set to 20, 40 and 80 GHz, while CP energies are set to 10 and 34 fJ/pulse. Dotted plots show measured accumulated phase-shifts, while solid-line curves show calculated ones. As discussed in Fig. 9.31(e), the successful sequential pulse operation of the PC-SMZ is achieved when a series of phase-shift difference ∆φi is time-invariant. Taking this into account, switching operation is achievable in two cases within the experiment, that is, 20 and 40 GHz repetition frequencies at the CP energy of 10 fJ, as surrounded by the bold solid line. 100% on–off switching ratio is achievable by slight change of the design for the current PC-SMZ structure.

Repetition frequency (GHz)

Phase shift [deg.]

34

80

40

20 Df1

100

fmax

50 0

0

25 50 75 CP/SP delay [ps]

0

50 100 150 CP/SP delay [ps]

0

100 200 300 CP/SP delay [ps]

50° Df(1)

Df

Ecp (fJ)

Phase shift [deg.]

80

10

fmax

60

Df1

40 20 0

0

25 50 75 CP/SP delay [ps]

0

50 100 150 CP/SP delay [ps]

0

100 200 300 CP/SP delay [ps]

30° Df

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Df(1)

9.33  Accumulated phase-shift ∆φi (upper part) and resultant phase-shift difference ∆φ (lower part) between the two ONL arms in the PC-SMZ as a function of the repetition frequency and the CP energy (Ecp).

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9.5

Photonic crystal all-optical switches

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Device structures and performances of photonic crystal all-optical switches

9.5.1 State-of-the-art performance of PC-SMZ In this section, state-of-the-art performances of the PC-SMZ are summarized. Figure 9.34(a) and (b) show a schematic diagram and an optical micrograph of the fabricated PC-SMZ, respectively. The ‘bar’ output port is a normally OFF port for the SP, while the ‘cross’ port is a normally ON port. Figure 9.34(c) and (d) show SEM photographs of the input DCs and output cleaved edge, respectively. Prior to switch operation of the PC-SMZ, influence of a low group velocity in the 2DPC waveguide on an enhancement effect in the ONL phase-shift has been investigated using a 500 µm long, QD-embedded straight 2DPC waveguide. Figure 9.35(a) shows a phase-shift in the ONL arm as a function of a wavelength of the single SP (Nakamura et al., 2004b). Desirable rapid increases for the phaseshift and group index (reciprocal of group velocity), as shown by a solid line curve, are due to a long light/matter interaction on account of the low group velocity for the SP. Figure 9.35(b) shows the ONL-induced phase-shift as a function of the CP energy when the SP wavelength is set to 1325 nm. The lower circles show the similar dependence for a 1 mm long non-PC ridge-waveguide with the similar QD. Importantly, the CP energy necessary for the π/2 phase-shift in the 2DPC waveguide is as low as 100 fJ and more than three orders of magnitude lower than that in the non-PC ridge waveguide. The result means that the PC/QD waveguide contributes to low-energy all-optical switches.

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9.34  (a) Schematic diagram of the PC-SMZ, (b) optical micrograph of the fabricated PC-SMZ, (c) magnified optical micrograph of the splitter and (d) magnified SEM photograph of the cleaved 2DPC waveguide cross-section.

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Optical switching responses of the PC-SMZ are shown in Fig. 9.36 (Nakamura et al., 2004b). Figure 9.36(a) shows decay characteristic of the output SP energy. The ON-CP and OFF-CP energies coupled into the waveguide were estimated as ~100 fJ. In case of ON-CP excitation only, the SP power slowly decays due to the slow carrier relaxation. The slow decay component is diminished when the OFF-CP is introduced in the CP-off port 27 ps after the ON-CP, as indicated by the ‘ON-CP/OFF-CP’. Rise and fall times were ~2 ps. Figure 9.36(b) shows the successful switching operation measured at the bar- and cross-ports. The current modulation is 50% due to the phase-shift ∆φ of π/2 and not π, partly because of a bad coupling efficiency between the input optical fiber and the 2DPC waveguide. This problem will be solved by improving the fiber/2DPC waveguide coupling. High repetition-rate operation of the PC-SMZ has also been demonstrated by using the four-pulse train with the period of 25 ps (Kitagawa et al., 2009), as shown in Fig. 9.37(a). As a result, sequential operations with repetition frequencies of 40 and 20 GHz were demonstrated, as shown in Fig. 9.37(b) and (c), respectively. The result suggests that the PC-SMZ is capable of switching speed of 20–40 GHz, though the switching ratio is as low as 17 %. The problem of the switching ratio is now under improvement.

9.5.2 Computational and experimental simulation of PC-FF The principle of the PC-FF discussed in section 9.2.4, has been verified by computer simulation, as shown in Fig. 9.38(a) and (b). For the calculation, period and width of set–reset pulses were 100–25 ps and 32–8 ps, respectively, while the repetition-rate of the clock pulse, delay time for the feedback loop and carrier relaxation time were 100–25 ps, 10 ps and 100–25 ps, respectively. For these parameters, the PC-FF was operated successfully at the speed of 10–40 Gb/s and eye patterns were clearly opened (S. Nakamura et al., 2008).

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Bi-stable operation of the PC-FF configuration, as shown in Fig. 9.39(a), has also been confirmed by using discrete optical components such as two hybridintegrated SMZ all-optical switches (S. Nakamura et al., 2008), optical fibers and semiconductor optical amplifiers (SOAs) as ONL phase shifters. Due to the feedback loop using fibers, the feedback delay time was about 37 ns. Consequently, durations of the set (S) and reset (R) pulses were set at 40 ns, as shown in Fig. 9.39(b). Without feedback, the output of the SMZ-1 showed switch-on only when either set or reset pulse was input as shown in Fig. 9.39(c). With feedback, a fraction of the output of SMZ-1 (A1) excited SMZ-2, where CW light (A2) at 1548 nm was modulated. The output of SMZ-2 (A3) was fed back into SMZ-1. As shown resultantly in Fig. 9.39(d), the output of SMZ-1 showed a flip-flop waveform with switching on after the input of a set pulse and switching off after the input of a reset pulse. Thus, the principle of the PC-FF has been verified preliminary, though the switching speed was on the ns order.

9.5.3 Challenges for PC-FF implementation TO design method for advanced 2DPC waveguides, SAG method for highdensity/uniformity QD with wavelength selectivity and experimental as well as computational simulations have been demonstrated so far. By using these technologies, design and fabrication of an actual PC-FF device is under implementation. Figure 9.40(a) shows an optical microscope photograph of two fabricated sets of PC-SMZ waveguides designed in the 300 × 500 µm chip. Two

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9.39  (a) Configuration of the PC-FF for demonstration of bi-stability using emulated setup, (b) set and reset pulses, (c) output showing no flip-flop operation without feedback and (d) output showing flip-flop operation with feedback.

shaded areas indicate the positions of the buried QD-1 with 1.29 µm wavelength and QD-2 with 1.31 µm wavelength. Measured properties of the QDs are shown in Fig. 9.40(b) and (c) (Ozaki et al., 2008). Areas of the QD-1 and QD-2 have been identified by the PL intensity mapping, as shown by the two separate zones in Fig. 9.40(b). Both QD-1 and QD-2 are located correctly on the 2DPC waveguides. In addition, control of different wavelengths for QD-1 and QD-2 have been confirmed to be designed 1290 and 1310 nm, respectively, as shown by the PL spectra in Fig. 9.40(c). Optical FF operation will be realized by solving some current problems such as improved coupling to optical fiber and easy packaging.

9.6

Conclusion

Ultra-small, ultra-low energy and ultra-fast SMZ-type all-optical analog switch, PC-SMZ, and digital optical flip-flop, PC-FF, have been proposed. They consist of GaAs-based 2DPC waveguides and InAs-based QDs. For implementation of

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these devices, two novel fundamental technologies have been developed. One is a TO design method for wide/flat-band 2DPC waveguides such as intersection, bend and Y-junction. It enabled quite the same excellent transmission properties as that of a straight waveguide. The other is SAG of high-density and highly uniform QDs using a metal-mask MBE method. It enabled QD ensemble with different wavelengths for different sites. By using these technologies, the ultralow energy (~100 fJ) PC-SMZ with 500 µm × 300 µm in size has been operated at ultra-fast (~ps) speed with as high repetition-rate as 40 Gb/s. On the other hand, computational simulation of the PC-FF verified successful optical bi-stability with a repetition-rate as high as 40 Gb/s. Experimental demonstration of the PC-FF is under way. This chapter has reviewed theoretical principles and experimental results of the TO method and SAG QD technology as well as PC-SMZ and PC-FF devices. The results will pave the way for the future ultrafast optical digital processing system.

9.7

Acknowledgments

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) within the framework of the Femtosecond Technology Association Project and NEDO grant. The authors would like to express their thanks to Drs Y. Tanaka, H. Nakamura, K. Kanamoto, Y. Nakamura,

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T. Yang, H. Ishikawa, K. Inoue and K. Ishida for technical suggestions and continuous cooperation on this work.

9.8

References

Analui, B., Guckenberger, D., Kucharski, D. and Narasimha, A. (2006), ‘A fully integrated 20 Gb/s optoelectronic transceiver implemented in a standard 0.13 µm CMOS SOI technology’, IEEE Journal of Solid-State Circuits, 41: 2645–955. Arakawa, Y. and Sakaki, H. (1982), ‘Multidimensional quantum well laser and temperature dependence of its threshold current’, Applied Physics Letters, 40: 939–41. Arentoft, J., Søndergaard, T., Kristensen, M., Boltasseva, A., Thorhauge, M. and Frandsen, L. (2002), ‘Low-loss silicon-on-insulator photonic crystal waveguides’, Electronics Letters, 38: 274–5. Asakawa, K., Sugimoto, Y., Watanabe, Y., Ozaki, N., Mizutani, A., Takata, Y., Kitagawa, Y., Ishikawa, H., Ikeda, N., Awazu, K., Wang, X., Watanabe, A., Nakamura, S., Ohkouchi, S., Inoue, K., Kristensen, M., Sigmund, O., Borel, P.I. and Baets, R. (2006), ‘Photonic crystal and quantum dot technologies for all-optical switch and logic device’, New Journal of Physics, 8: 208. Bendsøe, M.P. and Sigmund, O. (2003), Topology Optimization – Theory, Methods and Applications. Berlin: Springer. Borel, P.I., Frandsen, L.H., Harpøth, A., Kristensen, M., Jensen, J.S. and Sigmund, O. (2005), ‘Topology optimised broadband photonic crystal Y-splitter’, Electronics Letters, 41: 69–71. Ikeda, N., Sugimoto, Y., Watanabe, Y., Ozaki, N., Mizutani, A., Takata, Y., Jensen, J.S., Sigmund, O., Borel, P.I., Kristensen, M. and Asakawa, K. (2006), ‘Topology optimised photonic crystal waveguide intersections with high-transmittance and low crosstalk’, Electronics Letters, 42: 1031–3. Izhaky, N., Morse, M.T., Koehl, S., Cohen, O., Rubin, D., Barkai, A., Sarid, G., Cohen, R. and Paniccia, M.J. (2006), ‘Development of CMOS-compatible integrated silicon photonics devices’, IEEE Journal of Selected Topics in Quantum Electronics, 12: 1688–98. Jensen, J.S., Sigmund, O., Frandsen, L.H., Borel, P.I., Harpøth, A. and Kristensen, M. (2005), ‘Topology design and fabrication of an efficient double 90 degree photonic crystal waveguide bend’, IEEE Photonics Technology Letters, 17: 1202–4. Kanamoto, K., Nakamura, H., Nakamura, Y., Sugimoto, Y., Ikeda, N., Tanaka, Y., Ohkouchi, S., Ishikawa, H. and Asakawa, K. (2004), ‘Optical nonlinearity enhancement by the photonic-crystal waveguide with an InAs quantum dot core layer’, The 30th European Conference on Optical Communication 2004. Stockholm, Sweden. 84–5. Kitagawa, Y., Ozaki, N., Takata, Y., Ikeda, N., Watanabe, Y., Sugimoto, Y. and Asakawa, K. (2009), ‘Sequential operations of quantum dot/photonic crystal all-optical switch with high repetitive frequency pumping’, Journal of Lightwave Technology, 27: 1241–7. McNab, S.J., Moll, N. and Vlasov, Y.A. (2003), ‘Ultra-low loss photonic integrated circuit with membrane-type photonic crystal waveguides’, Optics Express, 11: 2927–39. Nakamura, H., Kanamoto, K., Nakamura, Y., Ohkouchi, S., Ikeda, N., Tanaka, Y., Sugimoto, Y., Ishikawa, H. and Asakawa, K. (2002), ‘Large enhancement of optical nonlinearity using quantum dots embedded in a photonic crystal structure for all-optical switch’, The 15th Annual Meeting of the IEEE Lasers and Electro-Optics Society 2002. Glasgow, Scotland. 764–5.

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Nakamura, H., Kanamoto, K., Nakamura, Y., Ohkouchi, S., Ishikawa, H. and Asakawa, K. (2004a), ‘Nonlinear optical phase shift in InAs quantum dots measured by a unique two-color pump/probe ellipsometric polarization analysis’, Journal of Applied Physics, 96: 1425–34. Nakamura, H., Nishikawa, S., Kohmoto, S., Kanamoto, K. and Asakawa, K. (2003), ‘Optical nonlinear properties of InAs quantum dots by means of transit absorption measurements’, Journal of Applied Physics, 94: 1184–9. Nakamura, H., Sugimoto, Y., Kanamoto, K., Ikeda, N., Tanaka, Y., Nakamura, Y., Ohkouchi, S., Watanabe, Y., Inoue, K., Ishikawa, H. and Asakawa, K. (2004b), ‘Ultra-fast photonic crystal/quantum dot all-optical switch for future photonic network’, Optics Express, 12: 6606–14. Nakamura, S., Watanabe, A., Wang, X., Ikeda, N., Sugimoto, Y., Ozaki, N., Watanabe, Y. and Asakawa, K. (2008), ‘Optical flip-flop based on coupled ultra-small Mach–Zehnder all-optical switches’, Proceedings of Optical Fiber Communication/National Fiber Optic Engineers Conference 2008, 1–3. Notomi, M., Shinya, A., Mitsui, S., Kuramochi, E. and Ryu, H.Y. (2004), ‘Waveguides, resonators and their coupled elements in photonic crystal slabs’, Optics Express, 12: 1551–65. Ohkouchi, S., Nakamura, Y., Ikeda, N., Sugimoto, Y. and Asakawa, K. (2007), ‘In situ mask designed for selective growth of InAs quantum dots in narrow regions developed for molecular beam epitaxy system’, Review of Scientific Instruments, 78: 073908. Ohkouchi, S., Nakamura, Y., Ikeda, N., Sugimoto, Y., Nakamura, H. and Asakawa, K. (2006), ‘Selective growth of high quality InAs quantum dots in narrow regions using in situ mask’, Journal of Crystal Growth, 293: 57–61. Ozaki, N., Takata, Y., Ohkouchi, S., Sugimoto, Y., Ikeda, N. and Asakawa, K. (2008), ‘Selective-area-growth of InAs-QDs with different absorption wavelengths via developed metal mask/MBE method for integrated optical devices’, Applied Surface Science, 254: 7968–71. Ozaki, N., Takata, Y., Ohkouchi, S., Sugimoto, Y., Nakamura, Y., Ikeda, N. and Asakawa, K. (2007), ‘Selective area growth of InAs quantum dots with a metal mask towards optical integrated circuit devices’, Journal of Crystal Growth, 301–2: 771–5. Sugimoto, Y., Ikeda, N., Carlsson, N., Asakawa, K., Kawai, N. and Inoue, K. (2002), ‘Fabrication and characterization of different types of two-dimensional AlGaAs photonic crystal slab’, Journal of Applied Physics, 91: 922–9. Sugimoto, Y., Tanaka, Y., Ikeda, N., Nakamura, Y., Asakawa, K. and Inoue, K. (2004), ‘Low propagation loss of 0.76 dB/mm in GaAs-based single-line-defect two-dimensional photonic crystal slab waveguides up to 1 cm in length’, Optics Express, 12: 1090–6. Sugimoto, Y., Tanaka, Y., Ikeda, N., Nakamura, H., Kanamoto, K., Ohkouchi, S., Watanabe, Y., Inoue, K. and Asakawa, K. (2005), ‘Fabrication and characterization of photonic crystal based symmetric Mach–Zehnder (PC-SMZ) structures based on GaAs membrane slab waveguides’, IEEE Journal on Selected Areas in Communications, 23: 1308–14. Tajima, K. (1993), ‘All-optical switch with switch-off time unrestricted by carrier lifetime’, Japanese Journal of Applied Physics, 32: L1746–8. Tanaka, Y., Nakamura, H., Sugimoto, Y., Ikeda, N., Asakawa, K. and Inoue, K. (2005a), ‘Coupling properties between 2D photonic crystal slab waveguides with a triangular lattice of air holes’, IEEE Journal of Quantum Electronics, 41: 76–84. Tanaka, Y., Sugimoto, Y., Ikeda, N., Nakamura, H., Kanamoto, K., Asakawa, K. and Inoue, K. (2005b), ‘Design, fabrication and characterization of a two-dimensional photonic-

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crystal symmetric Mach–Zehnder interferometer for optical integrated circuits’, Applied Physics Letters, 86: 141104. Têtu, A., Kristensen, M., Frandsen, L.H., Harpøth, A., Borel, P.I., Jensen, J.S. and Sigmund, O. (2005), ‘Broadband topology-optimized photonic crystal components for both TE and TM polarizations’, Optics Express, 13: 8606–11. Watanabe, Y., Ikeda, N., Takata, Y., Kitagawa, Y., Ozaki, N., Sugimoto, Y. and Asakawa, K. (2008), ‘Topology optimization of wavelength selective Y-junction for 2D photonic crystal waveguides’, Journal of Physics D, 41: 175109. Watanabe, Y., Sugimoto, Y., Ikeda, N., Ozaki, N., Mizutani, A., Takata, Y., Kitagawa, and Asakawa, K. (2006), ‘Broadband waveguide intersection with low-crosstalk in twodimensional photonic crystal circuits by using topology optimization’, Optics Express, 14: 9502–7. Watanabe, Y., Ikeda, N., Sugimoto, Y., Takata, Y., Kitagawa, Y., Mizutani, A., Osaki, N. and Asakawa, K. (2007), ‘Topology optimization of waveguide bends with wide, flat bandwidth in air-bridge-type photonic crystal slabs’, Journal of Applied Physics, 101: 113108. Yablonovitch, E. (1987), ‘Inhibited spontaneous emission in solid-state physics and electronics’, Physical Review Letters, 58: 2059–62.

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10 Fiber, holographic, quantum optical and other types of optical switches Y. ZHANG and B.J. LI, Sun Yat-Sen University, China Abstract: This chapter introduces some very different categories of optical switches as complementary contents for the knowledge gained from the previous chapters. This chapter first reviews the developments in designs and applications of fiber switches, holographic switches and quantum optical switches. Then the chapter provides some examples of specific optical switches. Key words: fiber switch, holographic switch, quantum-well-based switch, quantum-dot-based switch.

10.1 Introduction In chapters two to nine of this book, the main categories of optical switches were introduced. These switches exhibit some common features. First, they have attracted considerable attention from both the industrial and academic communities and are continuously being improved and reported in engineering and scientific literatures. The research and development carried out on these optical switches includes both theoretical studies and experimental demonstrations. Second, all of these optical switches are candidates for deployment in optical switching-/ demultiplexing-based systems, with some of them incorporated in commercial products. Third, all these switches have been designed by many research and development groups in the world. Most of them are not proprietary to any particular optical-component/device vendors, although certain switches have already been patented. The purpose of this chapter is to complement the knowledge gained about optical switches described in the previous nine chapters. So the discussion will not be as detailed as in previous chapters. However, our goal is to provide an overview with sufficient depth for the reader to appreciate as many of the existing approaches, especially very new approaches, to build optical switches. Some references provided at the end of this chapter will help the readers explore in more detail. In this chapter, four very different categories of optical switches will be covered. Section 10.2 discusses fiber switches, which may be considered in the category of MEMS-based optical switches due to the working principles. They can operate over a broad wavelength range. Section 10.3 discusses holographic switches, which can be regarded as a special group of electro-optical switches. They are more wavelength dependent than the ones discussed in section 10.2. Section 10.4 276 © Woodhead Publishing Limited, 2010



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discusses quantum optical switches including quantum-well-based and quantumdot-based types. They have emerged in recent years and have shown great potential in implementing high-speed all-optical switching. Section 10.5 introduces some specific switches.

10.2 Fiber switches Optical switches, whose operation is based on motion of fibers relative to fiber connectors, can be considered as fiber switches. These switches generally use mechanical positioning systems, where movement of optical fibers or other components relative to each other permits switching of light from input fibers to output fibers of the systeml–18. Although the movement can be realized manually, an actuation mechanism is more commonly employed, such as electrostatic actuation3,16, electromagnetic actuation6, thermal actuation7,10, comb-drive actuation5,17 and piezoelectric actuation18. Fiber switches can be classified into two main types19, namely, the fiber moving type and the optical-component moving type where the moving component can be prism, lens, mirror, collimator or other components. There are a number of different ways to construct fiber switches. Here we just introduce a few typical ones, especially introduced in recent years, as examples to give the readers some flavor of principles, configurations and characteristics of fiber switches.

10.2.1  Fiber moving type Fiber moving type switches can be simply built based on the principle schematically shown in Fig. 10.1. This is a l × N (N = 1, 2, 3 …) switch with one movable input fiber. For the special case (N = 1), the switch is the simplest ON/OFF switch. For N > 1, the switch is able to route an input optical signal to any one of the N output fibers. 1 × N fiber switches with relatively large N values can be realized by cascading multiple stages of 1 × 2 switches, as demonstrated in the work of Ford and DiGiovanni8. In other words, a 1 × 2 fiber switch can be considered as an elementary device to construct 1 × N fiber switches. Therefore, as examples, here Input fiber

Output fibers

10.1  Scheme of l × N fiber moving type switch.

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we mainly focus on several kinds of 1 × 2 moving-fiber type switches. An N × M (M = 1, 2, 3 …) switch with multiple movable input fibers and multiple output fibers may also be built based on the concept of l × N fiber switch. Fiber switch with electrostatic actuation As mentioned above, the actuation for the mechanical force on the movable input fiber can be generated in various ways. In principle, electromagnetic and piezoelectric forces do scale better, but are relatively difficult to fabricate. Electrostatic actuation is a simple way to move fibers. It can be generated by magnet acting on a magnetic pipe which circumvents the fiber16. Figure 10.2 shows schematically the structures of a typical 1 × 2 fiber switch of moving-fiber type with electrostatic actuation as reported in the work of Herding et al.16. To move the fibers directly, an electrostatic field is built up between a metalized flexible fiber and a fixed electrode which is also shown in Fig. 10.2.

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As shown in Fig. 10.2, the fiber switch is realized by a simple actuation chamber with suitable cavities for the fibers and appropriate electrodes for electrostatic actuation of the metalized fibers. Simultaneous application of the actuation voltage to the input fiber and the respective output fiber will pull the input fiber to the same wall of the actuation chamber as the output fiber, where they settle in a perfect optical alignment. The design uses planar electrodes integrated into the bottom surface of the actuation chamber. This concept is easy to realize and has the benefit of pulling the fiber not only to the side wall but also to the bottom of the actuation chamber. In addition, this design will provide a fiber alignment in two degrees of freedom and good optical coupling. To switch light between two optical fibers, it is necessary for the two fibers to overlap at least half of its diameter. For standard 125 µm fibers used here, the minimum travel range is 62.5 µm. In order to minimize the crosstalk between the two output fibers, a greater distance between the output fibers, i.e. a greater travel distance might be required. The main geometrical misalignments are summarized in Fig. 10.3. The extrinsic losses are related to a poor fiber-to-fiber alignment. The main sources of extrinsic losses are the longitudinal, lateral and angular misalignments. As the fibers are aligned by the specially formed electrodes to a common micromachined stop (see Fig. 10.2), the lateral and angular mismatch between the input and the output fibers are negligible. The optical coupling efficiency will be mainly determined by the longitudinal alignment. The specified maximal 2 dB insertion loss is achieved up to a longitudinal distance of 80 µm between the faces of the input and the output fibers for core guided light propagation and extended ray intensity profile. The crosstalk damping is mainly determined by the longitudinal distance between the input and the output fibers and by the lateral distance between the two output fibers. To ensure the specified coupling loss of less than 2 dB and to avoid optical degradation of the switch due to the intrinsic losses, the fibers used as input and output fiber pieces (pigtails) in a switch should be taken from the same piece of fiber, which will reduce the effect of different NA values of the coupled fibers. According to simulation results, the following design values for the fiber switch are used: actuation stroke of 65 µm (no additional separation of the two output fibers), length of movable fiber 30 mm and curved electrodes. As a result, the driving voltage as low as 40 V can be used. The specified crosstalk is achieved even with imperfect coupling into the input fiber as long as the longitudinal mismatch is less than 100 µm. The switching speed was measured to be 7 ms; the switching frequency is limited by the fiber mass to be less than 130 Hz. This fiber

x (a)

Q

y (b)

(c)

10.3  Different types of alignment errors: (a) longitudinal, (b) lateral and (c) angular misalignments.

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switch can be made out of a dry film photo resist TM100 in a simple one-step fabrication process. Fiber switch with comb-drive actuation 1 × 2 fiber switches can also be built by using various mechanical actuators. Among the reported types of designs, the one with comb-drive actuation shows its advantages in terms of endurance for long-term applications, switching time, requirement in structure length/thickness and convenience in fabrication process. Therefore, here we introduce a 1 × 2 fiber switch with comb-drive actuation reported in the work of Yang et al.17 for illustration. Figure 10.4 schematically depicts the structure of the 1 × 2 fiber switch. The device consists of a suspended fiber-holding table, folded flexures and two comb-drives. All the components can be monolithically fabricated using the dual-thickness SOI process. In order to achieve a displacement of about 125 µm, which is required for the fiber-switching design, the suspension beams are very flexible to produce a relatively large displacement under a reasonable driving voltage. Also, the inter-digital comb fingers should move only in the desired lateral direction with a minimal sideward movement. Doing so requires a large stiffness ratio between the sideward spring constant and the lateral spring constant. Also, stoppers are used for precise radial alignment to ensure a low butt-coupling loss between the input fiber and the output fiber. Based on the constraint of fabrication capacity for the large device thickness (around 100 µm) of suspended structures with a driving voltage of 100 V, the optimal device dimensions are listed in Table 10.1.

Folded-flexure

Hbeam

Output fibers Channel 1

Channel 2

Input fiber

Fiber holder

Stopper Lfiber Comb-drive actuator Fixed end of fiber

Fiber holding table

10.4  Schematic of the 1 × 2 fiber switch with comb-drive actuation17.

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Table 10.1  Optimal device dimensions of the 1 × 2 fiber switch17 Parameters

Values

Device thickness Gap between comb fingers Number of pairs of comb fingers Folded-beam length Folded-beam width Input fiber length

100 µm 5 µm 1000 2557 µm 15 µm 14.2 mm

Without the application of electrostatic voltage, the holding table and the input fiber are in the middle position (neutral position). When the left comb-drive actuator is actuated (100 V), the holding table is driven to the left and touches the left stopper; then the input fiber aligns with the output fiber (channel 1), as shown in Fig. 10.5(a). When the right comb-drive actuator is actuated with 100 V (and the left comb-drive is not actuated), the holding table is driven to the right and touches the right stopper; then the input fiber aligns with the output fiber (channel 2), as shown in Fig. 10.5(b). Note that the pictures in the right-bottom corner of Fig. 10.5(a) and (b) are the CCD photographs of the device in the corresponding status. Accordingly, 1 × 2 optical switching can be achieved by alternating the actuation of the comb-drive actuators. Since the thickness of the device is designed to be 100 µm and the depth of the fiber mounting groove is 65 µm, bulk micromachining technology must be utilized to fabricate the device. The gap between comb fingers is 5 µm and the maximum trench aspect ratio is 20. For fabricating suspended microstructures with high aspect ratios, the SOI process is one of the most popular approaches because of its simplicity. In order to reduce the complexity of fabrication and assembly, a dual-thickness SOI process is proposed to fabricate the suspended structure with layers of two different thicknesses. This process can monolithically create most of the parts of the device (folded-flexures, comb-drive actuators, stoppers, fiber holder and fiber-holding table) without the assembly process. The thicker layer serves as the structure layer and the thinner layer serves as the fiber mounting grooves. Figure 10.6(a) and (b) show the scanning electron micrograph (SEM) and the optical photograph of the device structures, respectively. Figure 10.7 shows the measured switching behaviors of the voltage output signals of channels 1 and 2. As shown in Fig. 10.7, the input fiber, which originally aligns with the output fiber of channel 1, switches to (align with) the output fiber of channel 2. The oscillation of the channel 2 signal arises from the bouncing effect when the fiber-holding table and the stopper collide with each other. This bouncing effect disappears within 2.5 ms. The total switching time (including the bouncing effect) is about 3.5 ms. The measured average insertion losses for channels 1 and 2 are about 0.92 dB and 0.89 dB, respectively. The deviations for the two channels are both 0.06 dB.

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10.5  Schematic views and CCD pictures of the switch at different switching positions: (a) the left comb-drive is actuated and (b) the right comb-drive is actuated17.

Fiber switch with piezoelectric actuation The most remarkable merit of 1 × 2 fiber switches with piezoelectric actuation is the very simple device structure18, which may greatly benefit mass production. The form of a 1 × 2 fiber switch with piezoelectric actuation is shown in Fig. 10.8. The input fiber is bonded inside the piezoelectric tube and the two output fibers are fixed closely together on the other side. The piezoelectric tube deflects when a step voltage is applied to the electrodes on the surface of the tube, as shown in Fig. 10.9. In this case, the deflection of the tube causes signals from the input fiber to be directed into the desired output fibers. The input fiber movement needs to be

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500 µm

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10.6  (a) SEM image and (b) optical photograph of the device structures17.

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–3

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1

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10.7  Measured switching behaviors. The transient curves are the measured photo-diode output voltages of channels 1 and 217.

Input fiber

Piezoelectric tube

Bending of the piezo tube

Output fibers

Wires for applying signals

10.8  Scheme of a 1 × 2 fiber switch with piezoelectric actuation18.

Input fiber movement

10.9  Deflection of the piezoelectric tube when a step voltage is applied18.

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Fiber, holographic, quantum optical and other switches Lp

Holder

Piezoelectric tube

Lg

Graphite rod

285 Lf

Input fiber

10.10  Construction of the input fiber18.

at least 62.5 µm (half the diameter of an optical fiber) in order to perform switching. The exact amount of deflection depends on the space between the output fibers. Figure 10.10 shows the construction of the input fiber. To achieve more flexible movement of the input fiber, the fiber protrudes beyond the piezoelectric tube. A thin graphite rod is bonded with part of the input fiber to control the bending of the fiber. In Fig. 10.10, Lp is the length of the piezoelectric tube, Lg is the length of the part of the input fiber, to which the graphite rod is bonded on its surface, and Lf is the length of free input optical fiber. When a voltage is applied on the piezoelectric tube, it deflects and actuates the input fiber. Once the input fiber hits the v-grooves at the output fibers (see Fig. 10.11), it stops and aligns with one of the output fibers. Accordingly, 1 × 2 optical switching can be realized by alternating the applied voltage on the piezoelectric tube. The piezoelectric tubes used in the switch were fabricated using the electrophoretic deposition process (EDP). EDP is a relatively low cost, fast and easy process to form ceramic tubes. Tubes of various dimensions can be fabricated using EDP. The cross-section of piezoelectric tube with two sectored outer electrodes for applying voltage is shown in Fig. 10.12.

Output fibers

0.16mm

v-grooves

10.11  Cross-section of the output fibers and v-grooves18.

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Outer electrode

Inner electrode

–

Piezoelectric tube

10.12  Cross-section of piezoelectric tube18.

The latest piezoelectric-based optical fiber switch prototype is shown in Fig. 10.13. The switch has a length of 110 mm. The fibers in the latest prototype are aligned using translation stages. Different components are then bonded together using low shrinkage adhesive. A compact electronic control unit is integrated into the switch. Fig. 10.14(a) and (b) show that the input fiber is being switched into different outputs. The space between the input and output fibers is 25 µm. Figure 10.15 is a graph showing the change in insertion loss of the switch by adjusting the applied voltage on the piezoelectric tube. The graph shows that as the voltage increases to around 65 V, the insertion loss drops to around 2 dB. This

10.13  Latest prototype of the 1 × 2 fiber switches with piezoelectric actuation18.

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(a)

(b)

10.14  Input fiber movement: (a) input aligned to the top fiber and (b) input aligned to the bottom fiber18.

is the voltage when the input fiber hits the v-grooves. Further increase of the voltage causes a decrease in the angular misalignment between the fibers and hence a drop in insertion loss. The insertion loss is the lowest (0.88 dB) when the applied voltage is 115 V. At this point, the angular misalignment between the

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20 18 16 Insertion loss (dB)

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14 12 10 8 6 4 2 0 0

20

40

60

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100

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140

Applied voltage (V)

10.15  Insertion loss of the optical switch against applied voltage on the piezoelectric tube18.

input and output fibers is minimized. Increasing the applied voltage further causes a rise in angular misalignment between the fibers. This can be seen in the graph as the applied voltage is increased from 115 V, the insertion loss rises from 1.05 to 2 dB. The average insertion loss of the switch is 1 dB and the average crosstalk is 245 dB. The switching time of the prototype can be controlled by adjusting the output capacitance of the power supply circuit. Lowering the output capacitance of the power supply increases the rise time of the step voltage and vice versa. When ceramic capacitors of 19.6 nF are connected to the output of the power supply, the step voltage from the power supply has a fast rise time of 0.8 ms. The input fiber is actuated too fast and causes vibration as it hits the v-grooves. Figure 10.16 is a graph showing the change in switching time of the optical switch versus the output capacitance. Based on this result, the output capacitance is adjusted to around 260 nF in order to achieve a switching time of less than 5 ms.

10.2.2  Component moving type Figure 10.17 schematically depicts a generic principle of a l × N fiber switch with component moving type. The movable component usually consists of a lens and a moving mirror. A gradient index lens can be used and the mirror can rotate to reflect light from the input onto one of the N outputs. In this section, two typical l × N fiber switches of the component moving type will be introduced for illustration. In the work of Peter et al.14, a l × N fiber switch using a conventional lens and an adaptive mirror to correct for the aberrations is proposed and studied. Figure 10.18 shows the switch composed of a fiber bundle, a lens and a mirror. The merit of this switch is its power coupling efficiency. The limitations, mainly due to aberrations and misalignments of the optical components, are overcome using a

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25

20 Switching time (ms)



15

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5

0 0

100

200

300

400

500

600

Output capacitance (nF)

10.16  Switching time of the switch against output capacitance of the power supply18.

Output fibers

Input fiber Movable component

10.17  Scheme of l × N fiber switch with component moving type.

Fiber bundle

Lens

Mirror

α

f

f

10.18  Scheme of l × N fiber switch composed of a fiber bundle, a lens and a mirror14.

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micromachined deformable mirror. The source fiber is imaged (4f system) onto one of the receiver fibers by moving the lens laterally. The switch serves two functions. First, it images a single-mode source fiber onto another single-mode receiver fiber (coupling function). Second, it deflects the beam to address one of the receiver fibers (switching function). In this fiber switch, an adaptive membrane mirror fabricated by bulk silicon micromachining is used as a deformable mirror, as shown in Fig. 10.19. Electrostatic deflection is generated by 37 electrodes arranged under the membrane in a hexagonal array. The maximum applied voltage is around 190 V. Since the membrane can only be deflected toward the electrodes, a bias voltage of about 141 V is applied to the membrane to achieve bidirectional movement. The deflection produced corresponds to a concave mirror with a focal length of about 2 m. The initial setup is made to compensate this slight defocusing. The optimization of the shape of the membrane for maximum power coupling efficiency of each connection is obtained with the help of an evolutionary algorithm. With good repeatability of the deformation of the membrane, the optimized deformation can be recalled for the specified connection to the receiver fiber. The measured crosstalk is less than 30 dB. The total loss due to the optical elements is estimated to be 22%, by considering two interfaces between the receiver fibers and air with 4% Fresnel losses each, 1% transmission loss inside the achromat and 0.3% of reflectance at both interfaces and a reflectivity of 87% for the mirror. Fluctuations have been measured which lead to deformation fluctuations of 0.1 µm at the center of the membrane at the maximum voltage of 190 V. In the work of Duparré et al.15, a l × 4 fiber switch for multimode fibers with 600 µm core diameters is proposed and studied. A microlens array (MLA) telescope allows for variable and fast beam deflection when the positions of the

Si chip

Al-coated SiN membrane

Control electrodes

V1

V2

V3

V4

Vb

10.19  Schematic for the micromachined deformable membrane mirror. The membrane has a diameter of 15 mm and a thickness of less than 1 µm. The surface of the membrane is coated with a 0.2-µm-thick reflective aluminum layer. Its active area has a diameter of 12 mm14.

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cylindrical MLAs relative to one another are altered by specially designed piezomechanical actuators. Standard achromats are used for collimation of light emitted by the input multimode fiber and for focusing of the deflected light onto a linear array of output multimode fibers. Figures 10.20 and 10.21 show the configurations of the decenterable cylindrical MLA telescopes for beam deflection and the l × 4 fiber switch, respectively. A multimode fiber with core radius Rin and numerical aperture NA 1 emits a light bundle. Bulk collimating and focusing optics with focal lengths FCOL and FFOC image the input fiber end face onto the output fiber end faces. After collimation of the light by the first collimating optics the bundle has a diameter of approximately A = 2NA 1/FCOL and a residual divergence (half-angle) of θmax = arctan (Rin/FCOL ). Beam deflection is achieved with two MLAs with pitches p1 and p2 (equal in value for this type of application) and focal lengths f1 and f2. The first MLA focuses the collimated beam into a large number of sub-foci and the second MLA re-collimates each beam. These microoptical elements permit fast beam deflection because even small displacements result in large deflection angles as a result of the short focal lengths of the microlenses. Deflection angle α, as a function of the focal lengths of the lens in the second MLA, f2, and the relative positions of the two MLAs, r0, is given by Collimator

Cylindrical lens arrays

Multimode fiber

Deflected beam

10.20  Scheme of a 2D micro-optical scanner based on MLA telescopes15. MLA

r0

p 2 Rin

Input fiber

A 2R

out

FCOL

f1 f2

Output fibers

FFOC

10.21  Scheme of the l × 4 fiber switch that uses a MLA telescope for beam deflection15.

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α = arctan(r0 /f2). The maximum angle of deflection is determined by αmax = arctan(p2/2f2) because r0max = p2/2. The displacement of the spot in the image plane deflected from the optical axis, v = FFOC tan(α), can then be given by v = FFOC r0/f2. The overall magnification of the imaged fiber end face is Mnet = f1/ f2(FFOC /FCOL ). For application of MLA telescopes in commercial devices such as multimode fiber switches, stability and repeatability are of major importance. Tolerance analysis shows that the decentration for beam deflection needs to be accurate on a submicrometer scale. Additionally, rotation of the two MLAs that build up a telescope about the optical axis led to a strong decrease in transfer efficiency. Therefore, the l × 4 fiber switch applies a newly developed piezo-based actuator for highly accurate and highly parallel decentration of the two MLAs for beam deflection, as shown in Fig. 10.22. Figure 10.23 shows the completely assembled prototype of the l × 4 fiber switch for multimode fibers with 600 µm core diameters without the control electronics. The overall system dimensions are 15 cm × 10 cm × 13 cm with an overall weight of the optomechanics of 3 kg. Switching among channels can be achieved by the variable beam deflector by digitally memorizing the values of voltage for the largest possible transfer efficiency for each channel and using a PC to assign a channel number to this voltage value. Figure 10.24 shows the measured switching behavior of the l × 4 fiber switch for switching among channels with different amounts of separation. The signal curves for the switching states channel Direction of translation

Frames holding the substrates of the cylindrical microlens arrays

Solid-state hinges

Space determined for implementation of piezostacks (2 × 2)

Two actuators acting in opposite directions

10.22  Mechanical setup for highly parallel decentration of cylindrical MLA telescopes15.

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10.23  Overall view of the prototype (without the driving electronics for the actuators)15.

0.4

Electronic control signal Optical output signal

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10.24  Activation and deactivation behavior of the four channels: (a) activation of channel 4 when one is switching from channel 1 (7 ms), (b) deactivation of channel 1 when one is switching to channel 4 (3 ms), (c) activation of channel 2 when one is switching from channel 3 (9 ms) and (d) deactivation of channel 2 when one is switching to channel 3 (9 ms)15.

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ON and channel OFF are shown. The ramps for switching between farther separated channels (Fig. 24(a) and (b)) are steeper than for channels that are closer to each other (Fig. 24(c) and (d)) because of the behavior of the control electronics. So switching off to farther separated channels is faster (3 ms) than switching to closer channels (9 ms). Switching on from farther separated channels takes place in 7 ms and from closer channels takes 9 ms. These switching times for the thick fibers are attractive for practical use. The measured insertion loss is 1.5 dB, and the experimental values of crosstalks are 234.2 to 244.7 dB.

10.3 Holographic switches Holography is a method to generate optical images. Instead of recording the image of the object as conventional photography does, holography provides the means to record the object wave itself. This wave is recorded in such a way that a subsequent illumination of the record serves to reconstruct the original object wave. The hologram consists of a complex distribution of clear and opaque areas corresponding to the recorded interference fringes. If the hologram is illuminated under certain conditions, a duplicate of the original reference wave can be obtained from the light transmitted through the hologram. Since a hologram can serve as a controllable, or programmable, diffraction grating for switching purposes, a number of approaches to holographic-optical switches have been reported20–25. Generally, holographic switches are capable of multi-dimensional steering of light using electrical control/activation of a holographic system. One of the advantages of holographic switches is that they involve no moving parts26. The functionalities of holographic switches may include space switching, wavelength selection, wavelength drift compensation, channel monitoring, variable optical attenuation, dynamic reconfiguration and parallelism (multiple interconnections may be handled simultaneously, which is also useful for multicasting). In the following paragraphs, we will introduce some typical approaches among the large number of reported approaches. l × N holographic switches can be formed based on the use of liquid-crystal spatial light modulators (LCSLMs) as the hologram recording media. Figure 10.25 shows a switching system where a LCSLM is used as a programmable Positive lens (achromat doublet) Programmable beam steerer

Single-mode fiber array

Nx

Ny f

f

10.25  Configuration of the holographic switch with a LCSLM as a programmable beam steerer25.

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beam steerer25. Here, many advantages are combined within the same device. First, the operation of diffractive liquid-crystal devices with no moving parts provides stable, accurate and reproducible switching maps, as well as a very simple addressing and supervision scheme. Then, it is shown both qualitatively and quantitatively that diffractive (or holographic) structures enable the extension of the device functionalities to multicast switching and variable-wavelength-band selection. Average insertion loss for the 14-channel device is 6 dB (in protection mode). Constant improvement on the LCSLM – in terms of mirror quality and liquid-crystal cell parameter control – as well as upgrades of the optomechanical alignment should rapidly lead to insertion loss values less than 4 dB. An N × N holographic switch can be built using a single spatial light modulator (SLM). However, the fan-out to the output fibers limits scalability and performance. A well-known solution to this problem is to build a holographic switch using two arrays of sub-holograms, as shown in Fig. 10.2623. This architecture ensures that the output beams are collinear with the axes of output fibers. Although the beamsteering angle has a wavelength dependence, in a double hologram system the ‘complementary’ nature of the holograms significantly reduces the overall wavelength dependence of the switch. This approach also has the added advantage of improving the crosstalk characteristics of the switch as the second hologram array tends to reject unwanted diffraction orders from the output channels. Finally, it should be noted that the insertion loss for such a switch architecture remains constant as the size of the system is scaled up. Thus, large switching fabrics scaling to many hundreds of channels are possible using this technology. Such a 3 × 3 switch was designed and constructed as part of the ROSES project23. The target performance figures for the switch are presented in Table 10.2. Table 10.2  Summary of 3 × 3 switch target performances23 Insertion loss Noise isolation (all paths) Back reflection Switching time Bandwidth

<20 dB >40 dB >40 dB 1 ms 30 nm

FLC-SLM #1

FLC-SLM #2 ∆h

φ h Input fiber array

z

10.26  Two hologram N × N holographic switch architecture23.

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Output fiber array

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(a)

Polarization selective A hologram

B Upper substrate

(b) (c)

Substrate mode hologram

Liquid crystal cell

Lower substrate Substrate mode hologram

10.27  Planar integrated polarization-insensitive 2 × 2 holographic switch: (a) fan-in stage, (b) switching and (c) fan-out stage21.

In the work of Moreau et al.21, a polarization-insensitive 2 × 2 holographic switch is realized by implementing multistage network in a planar configuration. The switch is a multilayered structure composed of a reflective polarizing hologram sandwiched between two planar substrates. Signals are coupled in and out of the switch by diffraction on substrate-mode holograms and their optical paths are controlled by total internal reflection in a liquid-crystal cell. Holographicoptical elements are recorded in high-index modulation photopolymer. Figure 10.27 schematically shows the polarization-insensitive 2 × 2 holographic switch proposed in the work of Moreau et al.21 Two or more parallel paths can be achieved in the same planar component by fixing the upper-substrate thickness to a fractional part of the lower-substrate thickness. In the present case, the uppersubstrate thickness is half of the lower one. When the spatial separation of A and B inputs is 2 mm, the signal polarization components are distributed in two parallel polarization-dependent switches. The first switch processes the TM component of signal A and the TE component of signal B, while the second one processes the TE component of signal A with the TM component of signal B. The switch symmetry ensures the correct fan-out of each signal. The measured crosstalk is 5.5% (in the worst case) and the switching time is 20 ms, which remains excessive for high-rate applications, but is sufficient to demonstrate the exchange operation and its applications in network protection and restoration services.

10.4 Quantum optical switches Quantum well (QW)-/quantum dot (QD)-based optical switches may be regarded as a special group relative to the nine main categories of optical switches. However, this group has been greatly expanded especially in recent years27–43. The most outstanding advantage of the QW-/QD-based switches is their ultra-high switching speed (response <10 ps). Moreover, most of the QW-/QD-based switches are capable of all-optical switching. Therefore, to some extent, QW-/QD-based switches can be considered as the most promising candidates for switches in ultrafast all-optical networks in the future. In this section, the QW-/QD-based switches will be introduced with several examples reported in the literature.

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10.4.1  Quantum-well-based switches QW-based switches are usually operated based on inter-subband transition (ISBT)27–35 or multiple QW intermixing (QWI)36–37. ISBT in semiconductor QWs possesses ultra-fast carrier relaxation (~1 ps) and high conduction-band offset at optical communication wavelengths, while QWI changes the effective transition energies of the electrons from the valence band to the conduction-band and shifts the position of the absorption edge. These are the optical switching principles of QW-based switches. QW-based switches based on ISBT ISBT in semiconductor QWs have been the subject of extensive research for the past two decades and are nowadays used in a variety of devices such as QW infrared photodetectors and quantum cascade lasers. These devices have been primarily developed with GaAs/AlGaAs or InGaAs/InAlAs QWs, whose relatively small conduction-band offsets limit the ISBT wavelength to the mid- and farinfrared spectral regions. More recently, however, near-infrared ISBTs have also been measured in wide-conduction-band-offset heterostructures such as GaN/ AlGaN and InGaAs/AlAsSb QWs, at wavelengths in the low-loss transmission window of optical fibers. This opens up the possibility of utilizing the unique features of ISBT for information processing applications in optical communications. In particular, due to their ultra-fast relaxation times and giant optical nonlinearities, ISBTs are ideally well suited to nonlinear optical switching at bit rates of several hundred Gb s21. Broadly speaking, an all-optical switch is a device that allows modulation of an optical signal by means of an optical control wave, e.g., through cross-absorption modulation as illustrated in Fig. 10.2831. In this example, strong control pulses of wavelength λc are used to temporarily saturate the absorption experienced in the waveguide by an input signal of wavelength λs, thereby P

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10.28  Schematic illustration of ISBT all-optical switch by crossabsorption modulation in a GaN-based waveguide31.

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increasing its transmission. These devices are expected to play an enabling role in the development of future ultra-broadband all-optical networks, in which information is not only transmitted but also processed in the optical domain, as required to meet future demands on network capacity and functionality. In the work of Li and Paiella31, an approach to the QW-based all-optical switch is proposed with GaN/AlGaN coupled QWs which allows for a favorable compromise between switching intensity and recovery time, based on Coulombinduced optical nonlinearities. In this approach, the control pulses were used to produce a large Stark shift of the ISBT spectrum through a redistribution of the electrons in the QWs. The optimized structure, as shown in Fig. 10.29(a), consists of 26 Å and 10 Å GaN wells coupled by a 17 Å Al0.4Ga0.6N center barrier and separated by 90 Å AlN outer barriers, with a Si doping density of 4 × 1019 cm23 in all the barriers. In addition, despite the well-known difficulties in doping bulk AlN, effective donor ionization is obtained in the barriers of GaN/AlN QWs. By design, in this structure the |1 –|3  transition energy hω31 (812 meV, or 1527 nm) lies within the wavelength range of optical fiber communications. Furthermore, the ISBT nonradiative scattering lifetimes satisfy the relation τ32 (0.11 ps) < τ31 (0.21 ps) << τ21 (1.82 ps). The notation τ32 is the relaxation time from subbands |3  to |2 . In equilibrium, all the electrons provided by the ionized Si donors reside in the ground-state subband |1 , localized in the well on the right-hand side. The resulting conduction-band diagram is shown in Fig. 10.29(a), together with the wave functions squared of the lowest three bound states referenced to their respective energy levels. In the presence of strong control pulses of photon energy near hω31, some of the electrons in |1  are optically excited into the upper subband |3 , from which they will then preferentially decay into the intermediate subband |2  given that τ32 < τ31. This temporary transfer of electrons from |1  to |2  is accompanied by an increase in the electronic potential energy in the well on the left (where subband |2  is primarily localized) relative to the well on the right, leading to a blue Stark shift of the |1 –|2  and |1 –|3  ISBT peaks. The absorption 6

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coefficient experienced by the signal is correspondingly modified, e.g., similar to the case of electro-absorption modulators. As an illustration, Fig. 10.29(b) shows the conduction-band diagram and bound states of the structure of Fig. 10.29(a) in the presence of a control wave of intensity 0.35 W µm22. A large blue shift of 71 meV is obtained in this case for the |1 –|3  transition energy. Figure 10.30 shows the time evolution of the signal absorption coefficient at 1584 nm (solid curve) in the coupled QW structure of Fig. 10.29, in the presence of the ultra-fast control pulse shown by the dotted curve. The solid curve is the signal absorption coefficient of the proposed structure versus time, computed at the optimal wavelength of 1584 nm for a peak control intensity of 130 mW µm22 (i.e. the saturation intensity at this wavelength). The 1/e absorption recovery time here is about 2.2 ps, primarily limited by the |2 –|1  relaxation time τ21 of 1.8 ps and fast enough for operation at a few hundred Gbs21. The same approach can also be used to demonstrate ultra-fast all-optical switching with inversion. This requires using a signal wavelength λs on the blue side of the |1 –|3  resonance. An illustration is shown in Fig. 10.31, where the signal absorption coefficient (solid curve) is temporarily doubled in the presence of a 1 ps Gaussian control pulse of peak intensity as small as 105 mW µm22 (dotted curve). For this, a variation of the coupled QWs was used, slightly modified so that in equilibrium the |1 –|3  transition energy (762 meV or 1627 nm) is on the red side of 1550 nm. In this new structure, the thicknesses of the left GaN well and of the intermediate Al0.4Ga0.6N barrier are 28 and 20 Å, respectively, while everything

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10.31  Time evolution of the signal absorption coefficient (solid curve) in the presence of the ultrafast control pulse shown by the dotted curve, in a coupled QW structure similar to that of Fig. 29. The signal wavelength in this case is on the blue side of the equilibrium |1 –|3  ISBT energy, leading to an increase in signal absorption induced by the control pulse31.

else is the same as shown in Fig. 10.29. The absorption coefficient plotted in Fig. 10.31 is computed at 837 meV (1482 nm), chosen for maximum absorption modulation. However, as shown in Fig. 10.30, the effective saturation intensity remains low over a wide wavelength range. The 1/e recovery time in this case is 1.5 ps, again sufficiently fast for bit rates of a few hundred Gbs21. QW-based switches based on QWI QWI, especially selective-area QWI, is also a very promising technique for the realization of monolithically integrated optical switches. To date, a number of QWI techniques have been reported, including impurity-induced disordering, impurity-free vacancy-induced disordering, ion implantation-induced interdiffusion and several laser-induced disordering processes. The frame of a QW-based optical switch based on QWI is usually a Mach– Zehnder interferometer (MZI)36–37, as shown in Fig. 10.3236. Here we consider the switch discussed in the work of Wong et al.36. It consists of two phase shifter elements placed along the interferometer arms, connected to the input–output waveguide ports through the MMI couplers and curved waveguides. The waveguides used throughout have a 2 µm wide ridge. The 3 dB MMI couplers have the dimensions of 6 µm × 220 µm. To achieve a compromise between bending losses and device compactness, the radius of the curved waveguides is chosen to be 500 µm. The total length of the device is 3.2 mm and the phase shifter

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10.32  Schematic layout of the MZI optical switch (left) and micrograph of the MMI coupler with the two output waveguides (right)36.

section is 400 µm long. The passive sections were blue-shifted using QWI to ensure low propagation loss, while the phase shifter sections were not intermixed to preserve the effectiveness of the electro-optic effects. The material structure used is a multiple quantum-well (MQW) heterostructure grown by MOCVD lattice matched to an n-doped InP wafer. The active region consists of six 7.0 nm thick InGaAs wells with 8.0 nm thick Al0.2Ga0.27In0.53As barriers. The MQWs are placed at the center of a 500 nm thick Al0.2Ga0.27In0.53As waveguide layer. A 1400 nm thick InP lower cladding, a 1400 nm thick InP upper cladding and a 100 nm thick In-GaAs contact layer complete the structure. Zn and Si were used as p-type and n-type dopants, respectively. The QWI technique used to bandgap shift the passive sections involves the generation of point defects during the deposition of a sputtered SiO2 film, followed by a high-temperature annealing stage to promote the diffusion. The entire device was intermixed, except for the active phase shifter sections, which were protected by a plasma-enhanced chemical vapor deposition (PECVD) SiO2 layer and thus, not intermixed. To induce refractive index changes in QW material, a reverse electric field perpendicular to the QWs layer is applied, i.e., to apply quantum confined Stark effect (QCSE). Figure 10.33 shows the switching characteristic of an MZI switch based on the QCSE with the input beam TE-polarized at a wavelength of 1580 nm. An extinction ratio of 20 dB is measured. The half-wavelength voltage (Vπ) is as low as 3.5 V, which corresponds to a phase shift efficiency of 130°/V/mm. By operating the system in the push–pull configuration with a dc pre-bias of 4 V, Vπ is found to reduce by nearly 50% to 1.9 V. At operating wavelengths below 1580 nm, extinction ratios <20 dB were recorded. Lower values of extinction ratio below 1580 nm were attributed to the lower values of chirp factor when the operating point is set close to the bandgap.

10.4.2  Quantum-dot-based switches Semiconductor QDs are expected to provide improved all-optical nonlinearities due to their delta-function-like density of states. QDs show sharp excitonic peaks

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with considerably larger peak absorption than in bulk material or QWs. An obvious enhancement of the electro-refraction as compared to QWs can be obtained in homogeneous QDs. Thus, it is expected to be a similar enhancement for the all-optical refractive index nonlinearity due to state filling in QDs. The refractive index nonlinearity in QDs is also enhanced, since a single electron-hole pair is able to induce transparency of the ground state transition, while two electron-hole pairs generate optical gain. In the work of Prasanth et al.38, an all-optical switch is proposed based on state filling of a single layer of InAs/InP QDs embedded in an InGaAsP/InP waveguide. The switch is schematically shown in Fig. 10.34. The pump beam is provided by a tunable optical parametric oscillator (OPO) which generates 200 fs pulses at 76 MHz repetition rate. The pump beam excites one of the two arms of the MZI from above, i.e., perpendicular to the substrate. The OPO (λ > 1350 nm) excites carriers directly into the QDs without exciting the bulk InGaAsP or InP. The resulting state filling in the InAs/InP QDs produces a refractive index variation leading to switching of the MZI. The switching of the MZI is probed with a CW tunable semiconductor laser, tuned into the 1530–70 nm wavelength window. The probe beam was coupled into the MZI by microscope objectives. The probe output was focused onto a slit to spatially separate the two outputs of the MZI. The all-optical switching signal was acquired by chopping (2 kHz) the pump beam and measuring the demodulated probe output with a lock-in amplifier. The pump laser excites a surface area of approximately 600 × 25 µm2 around the upper arm of the MZI, as schematically indicated in Fig. 10.34. The QD sample was grown by chemical beam epitaxy on a (100) oriented InP substrate. The QDs are prepared by Stranski–Krastanow growth by depositing 4.3 monolayers InAs at 500°C on top of a lattice matched GaxIn1–xAsyP1–y layer

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10.34  Scheme of the all-optical switch using a pump beam from the top to excite the QDs in the upper arm (in the shaded area) of the MZI. The QDs are contained in the core of waveguides from which the switch was made38.

(x = 0.25, y = 0.55). The QDs are subsequently capped with 0.62 nm InP and annealed for 5 min in PH3, thereby shifting the PL peak wavelength 100 nm down to 1500 nm. Atomic force microscopy of similar uncapped InAs/InP QDs shows a density of 1.4 × 1010/cm2. The single QD layer is embedded into a 370 nm thick InGaAsP waveguide core, which is covered with a 1.3 µm InP cladding. Subsequently, 2 × 2 MZI space switches were fabricated built on 3 dB MMI input and output couplers. The structures were defined in 100 nm SiNx. The ridge waveguides were shallow etched with a depth of 30 nm into the waveguide core using a CH4/H2 reactive ion etching process and an O2 descumming process. The upper and lower arms of the MZI have a width of 2.8 and 3.55 µm, respectively. The length of the phase shifting section is 605 µm, with 30 µm separation between the arms. Figure 10.35 shows the switching results for excitation of the QDs at 1450 nm and detection between 1530 and 1570 nm. The pump laser excitation density of 1 W/cm2 corresponds to a relative QD occupation of 1.4% at the highest power of 0.125 mW presented in Fig. 10.35. All-optical switching can be observed since the demodulated probe signals for the two outputs of the MZI are of similar magnitude but opposite in sign, as expected for an induced phase shift. A bleaching of the QD absorption would result in increased probe beam transmission for both MZI outputs. The observed refractive index nonlinearity at 1530–70 nm (probe wavelength) is related to a TM-polarized absorption bleaching at 1450 nm (pump wavelength) by the Kramers–Kronig relations. As deduced from the results presented in Fig. 10.35, 2.6 × 1024 rad phase shift at 0.125 mW pump power is observed. Since 10% of this power directly excites the waveguide and the temporal duty cycle is 0.5%, a phase shift of 4.2 rad/mW incident power

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is then obtained. The estimated index of refraction nonlinearity is 0.08/(µW absorbed power). In the work of Rodriguez et al.41, a SiO2/PbTe QD multilayer-based all-optical switch is reported, together with the fabrication process of SiO2/PbTe QD multilayers and the preliminary results of its characterization by grazing-incidence small-angle X-ray scattering. The samples were grown on Si(111) substrate by alternate use of pulsed laser deposition (PLD) and plasma enhanced chemical vapor deposition (PECVD) techniques. To improve the adherence of the sample to the substrate, the RF was turned on at full power (150 W) in argon only atmosphere for 5 min before the first layer deposition. After the initial treatment, twenty PbTe/SiO2 bi-layers were deposited on the substrate. For each sample, the number of bi-layers (20), the thickness of the SiO2 films (~20 nm) and the number of laser pulses (for PLD) were kept constant. Finally, a set of three samples was prepared each with different QD size (100, 150 and 200 laser pulses). A fourth sample (reference sample) was fabricated containing only a SiO2 film grown on the same substrate and with the same SiO2 thickness as in the multilayer structure (~400 nm). The top layer was always a SiO2 film for all samples. For the microcavities’ fabrication, five bi-layers were deposited in the form of Bragg reflectors using alternatively SiO2 (nL = 1.46) and TiO2 (nH = 2.35). Using those materials with high refractive index contrast (nH/nL = 1.6), only a few (5–10)

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10.36  SEM images of the fabricated device containing PbTe QDs multilayer inside a Fabry–Perot cavity. (a) Sample after preparation with the common procedure used to characterize by TEM and (b) image of the Bragg reflector41.

bi-layers are necessary in order to produce a significant increase in the electric field amplitude inside the nonlinear defect thus increasing the non-linear properties (102–104 times) of the device. To ensure the device can be operated in the infrared region, the thickness of each bi-layer should be kept at λ/4 and the thickness of the QDs layers inside the Fabry–Perot cavity must be kept at λ/2. Once the thickness of each dielectric layer was known, the following fabrication procedure was performed: First, a stack of Bragg reflectors was deposited on a glass substrate. Second, a PbTe/ SiO2 multilayer was fabricated on the Bragg reflector. The parameters for the multilayer fabrication were chosen to obtain PbTe QDs absorption in the infrared region (λ = 1400 nm). Finally, a second stack of Bragg reflectors was deposited on the multilayer to form a Fabry–Perot cavity. Figure 10.36 presents SEM images of the fabricated device. Parallel stripes are dielectric SiO2/TiO2 layers. Transmittance measurements were conducted using a spectrophotometer. The experimental results are presented in Fig. 10.37. The fabricated device exhibits a peak for maximum transmittance at 1450 nm. The device without nanoparticles inside the cavity shows approximately 100% transmittance at the resonance wavelength. The device containing nanoparticles exhibits an 80% lower transmittance due to the QDs absorption. By varying the intensity of the incident beam, one can saturate the absorption of the multilayer inside the etalon producing a bi-stability response of the device. Due to the fact that PbTe QDs exhibit absorption in the region of interest for optical communications, the device could operate as an optical switch for the infrared region.

10.5 Other switches So far, a fairly extensive coverage of special optical switches has been provided. However, the subject is diverse and there are still other approaches that we may have not covered. Some of them are variations of the switches that have been discussed. For example, we discussed nonlinear-effect-based optical switches in chapter seven. Readers may note that solitonic switches based on nonlinear-

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directional coupling44 and fractional Talbot effect45 were also explored. In section 10.2 of this chapter, fiber switches as a special group have been introduced. Nevertheless, the binary fiber optic switch demonstrated in the work of Xie and Taylor46 may be regarded as a little different from those introduced for its application in aircraft. However, some other optical switches may constitute very different technologies compared to those introduced. In the following paragraphs, we give a few more examples. In the work of Kang et al.47, an all-optical 1 × 2 Y-branch switch using photochromic dye doped polymer waveguides was presented, the schematic of which is shown in Fig. 10.38. The photochromic dye undergoes reversible structural change from the open form (low refractive index) to the closed form (high refractive index) by irradiation with ultraviolet (365 nm) light and in the reverse direction by irradiation with visible (514 nm) light. The 1 × 2 Y-branch switch exhibited a crosstalk of about 214 dB at the wavelength of 1.55 lm using the ultraviolet (365 nm) and visible (514 nm) lights. In the work of Liu et al.48, the authors demonstrated an all-optical switch using large volume 3D optical matter by optically trapping polystyrene spheres in a © Woodhead Publishing Limited, 2010



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capillary (see Fig. 10.39 for the photograph of the device). The formation of optical matter was confirmed by examining the diffraction pattern of the trap region. Optical switching with an extinction ratio as large as ~20 dB was realized. In the work of Bendix and Méndez-Bermúdez49, an optical switch by using chaotic 2D multiport waveguides was proposed. The 2D waveguide is formed by a cavity connected to two collinear semi-infinite ports of width d extended along the x axis. The prototype cavity has the geometry of the so-called cosine billiard: It has a flat wall at y = 0 and a deformed wall given by y(x) = d + a[1–cos(2πx/L)], where a is the amplitude of the deformation and L is the length of the cavity. Figure 10.40(a) shows the geometry of the waveguide. A prototype two-port waveguide is locally deformed to produce a ternary incomplete horseshoe characteristic of mixed phase space (chaotic regions surrounding islands of stability where motion is regular). Owing to tunneling to the phase-space-stability islands, quasi-bound states (QBS) appear. Then transversal ports were attached to the waveguide in the deformation region in positions where the phase-space structure is only slightly perturbed, as shown in Fig. 10.40(b), (c) and (d). The

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10.39  Photograph of the all-optical switch based on the generation of 3D optical matter 48.

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10.41  Optofluidic switch: (a) the functional area of the device (diffraction channel), consisting of a circular microchannel with a blazed grating imprinted onto its bottom, (b) schematic drawing of the blazed grating with the incident and transmitted laser beams, (c) layout of microchannels in the device: the flow layer (black and red) with four inlets (in0–in3), two vents (v1 and v2), and one outlet; the control layer (blue) with five inlets (c0–c4). The blazed grating is schematically shown as a patterned area and (d) a photograph of an actual microfluidic polydimethylsiloxane (PDMS) chip bonded to a cover glass50.

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results show that QBS can be guided out of the waveguide through the attached transversal ports, giving rise to frequency-selective switches. The last example is the optofluidic 1 × 4 switch demonstrated in the work of Groisman et al.50 The configuration of the switch is shown in Fig. 10.41. The switch is based on a blazed diffraction grating imprinted onto silicone elastomer at the bottom of a microfluidic channel that is filled with liquids of different refractive indices. When the condition of a maximum diffraction is met, the laser beam incident on the grating is deflected by an angle proportional to the refractive index mismatch between the elastomer and the liquid in the channel. The switch was tested using four different aqueous salt solutions generating the zeroth to third orders of diffraction. The insertion loss was <2.5 dB, the extinction ratio was >9.8 dB and the response time was 55 ms. The same basic design can be used to build optofluidic switches with more than four outputs. The switch does not consume electric power or liquid in its steady states, has a response time of 55 ms, insertion loss of ~2.5 dB, crosstalk of 29.8 dB and extinction ratio of 9.6 dB. The proposed 1 × 4 optofluidic switches have an advantage of potentially simple integration with other microfluidic elements for lab-on-a-chip applications. Furthermore, the crosstalk and the extinction ratio in the 1 × 4 switch are expected to be substantially improved by perfecting the shape of the diffraction grating and the response time of the switch can be reduced by modifying the microfluidic channels.

10.6 References 1 2 3

4 5

6

7 8 9

H. Yamamoto and H. Ogiwara, ‘Moving optical-fiber switch experiment,’ Applied Optics, 17, 3675 (1978). W. Tomlinson, R. Wagner, A. Strnad and F. Dunn, ‘Multiposition optical-fiber switch,’ Electronics Letters, 15, 192 (1979). K. Hogari and T. Matsumoto, ‘Electrostatically driven fiber-optic micromechanical on/off switch and its applications to subscriber transmission systems,’ Journal of Lightwave Technology, 8, 722 (1990). G. Pesavento, ‘Optomechanical M × N fiberoptic matrix switch,’ Proceedings of SPIE, 1474, 57 (1991). A. Müller, J. Göttert and J. Mohr, ‘LIGA microstructures on top of micromachined silicon wafers used to fabricate a micro-optical switch,’ Journal of Micromechanics and Microengineering, 3, 158 (1993). K. Sato, M. Horino, T. Akashi, N. Komatsu and D. Kobayashi, ‘Mechanical optical switch of a plane type with electromagnetic actuators,’ Proceedings of Pacific Rim Conference on Lasers and Electro-Optics 1997, 201–2 (1997). M. Hoffmann, P. Kopka, T. Gross and E. Voges, ‘All-silicon bistable micromechanical fiber switches,’ Electronics Letters, 34, 207 (1998). J.E. Ford and D.J. DiGiovanni, ‘l × N fiber bundle scanning switch,’ IEEE Photonics Technology Letters, 10, 967 (1998). S. Nagaoka, ‘Compact latching-type single-mode-fiber switches fabricated by a fibermicromachining technique and their practical applications,’ IEEE Journal on Selected Topics of Quantum Electronics, 5, 36 (1999).

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10 M. Hoffmann, P. Kopka and E. Voges, ‘All-silicon bistable micromechanical fiber switch based on advanced bulk micromachining,’ IEEE Journal on Selected Topics of Quantum Electronics, 5, 46 (1999). 11 M. Hoffmann, P. Kopka, T. Gross and E. Voges, ‘Optical fibre switches based on full wafer silicon micromachining,’ Journal of Micromechanics and Microengineering, 9, 151 (1999). 12 M. Hoffmann, D. Nüsse and E. Voges, ‘Electrostatic parallel-plate actuators with large deflections for use in optical moving-fibre switches,’ Journal of Micromechanics and Microengineering, 11, 323 (2001). 13 F. Gonté, Y.A. Peter, H. Herzig and R. Dändliker, ‘Massive free-space l × N fiber switch using an adaptive membrane mirror,’ Proceedings of SPIE, 4493, 64 (2002). 14 Y.A. Peter, F. Gonté, H.P. Herzig and R. Dändliker, ‘Micro-optical fiber switch for a large number of interconnects using a deformable mirror,’ IEEE Photonics Technology Letters, 14, 301 (2002). 15 J. Duparré, B. Götz and R. Göring, ‘Micro-optical 1 × 4 fiber switch for multimode fibers with 600 µm core diameters,’ Applied Optics, 42, 6889 (2003). 16 M. Herding, G. Somogyi, U. Mescheder and P. Woias, ‘A new micromachined optical fiber switch for instrumentation purposes,’ Proceedings of SPIE, 5455, 264 (2004). 17 Y.J. Yang, W.C. Kuo, K.C. Fan and W.L. Lin, ‘A 1 × 2 optical fiber switch using a dual-thickness SOI process,’ Journal of Micromechanics and Microengineering, 17, 1034 (2007). 18 M. Leung, J. Yue, K.A. Razak, E. Haemmerle, M. Hodgson and W. Gao, ‘Development of a 1 × 2 piezoelectric optical fiber switch,’ Proceedings of SPIE, 6836, 683603 (2007). 19 N. Kashima, Passive Optical Components for Optical Fiber Transmission (Artech House, Boston, 1995). 20 H. Yamazaki, T. Matsunaga, S. Fukushima and T. Kurokawa, ‘Large-scale holographic switch with a ferroelectric liquid-crystal spatial light modulator,’ Proceedings of the IEEE Lasers and Electro-Optics Society 10th Annual Meeting (LEOS’97), 1, 128 (1997). 21 V. Moreau, Y. Renotte and Y. Lion, ‘Planar integration of a polarization-insensitive optical switch with holographic elements,’ Materials Science in Semiconductor Processing, 3, 551 (2000). 22 B. Pesach, G. Bartal, E. Refaeli, A. Agranat, J. Krupnik and D. Sadot, ‘Free-space optical cross-connect switch by use of electroholography,’ Applied Optics, 39, 10 (2000). 23 W. Crossland, I. Manolis, M. Redmond, K. Tan, T. Wilkinson, M. Holmes, T. Parker, H. Chu, J. Croucher, V. Handerek, S. Warr, B. Robertson, I. Bonas, R. Franklin, C. Stace, H. White, R. Woolley and G. Henshall, ‘Holographic optical switching: the “ROSES” demonstrator,’ Journal of Lightwave Technology, 18, 1845 (2000). 24 A. d’Alessandro and R. Asquini, ‘Liquid crystal devices for photonic switching applications: state of the art and future developments,’ Molecular Crystals and Liquid Crystals, 398, 207 (2003). 25 B. Fracasso, J. de la Tocnaye, M. Razzak and C. Uche, ‘Design and performance of a versatile holographic liquid-crystal wavelength-selective optical switch,’ Journal of Lightwave Technology, 21, 2405 (2003). 26 T.S. El-Bawab, Optical Switching (Springer, New York, 2006). 27 T. Akiyama, N. Georgiev, T. Mozume, H. Yoshida, A.V. Gopal and O. Wada, ‘1.55-µm picosecond all-optical switching by using intersubband absorption in

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InGaAs–AlAs–AlAsSb coupled quantum wells,’ IEEE Photonics Technology Letters, 14, 495 (2002). T. Simoyama, H. Yoshida, J. Kasai, T. Mozume, A.V. Gopal and H. Ishikawa, ‘InGaAs–AlAs–AlAsSb coupled quantum well intersubband transition all-optical switch with low switching energy for OTDM systems,’ IEEE Photonics Technology Letters, 15, 1363 (2003). E.J. Gansen and A.L. Smirl, ‘Ultrafast polarization modulation induced by the “virtual excitation” of spin-polarized excitons in quantum wells: application to all-optical switching,’ Journal of Applied Physics, 95, 3907 (2004). K. Akita, R. Akimoto, T. Hasama, H. Ishikawa and Y. Takanashi, ‘Intersubband alloptical switching in submicron high-mesa SCH waveguide structure with wide-gap II-VI-based quantum wells,’ Electronics Letters, 42, 1352 (2006). Y. Li and R. Paiella, ‘Intersubband all-optical switching based on Coulomb-induced optical nonlinearities in GaN/AlGaN coupled quantum wells,’ Semiconductor Science and Technology, 21, 1105 (2006). N. Iizuka, K. Kaneko and N. Suzuki, ‘All-optical switch utilizing intersubband transition in GaN quantum wells,’ IEEE Journal of Quantum Electronics, 42, 765 (2006). R. Akimoto, B.S. Li, K. Akita and T. Hasama, ‘Ultrafast intersubband optical switching in II–VI-based quantum well for optical fiber communications,’ Physica Status Solidi (b), 243, 805 (2006). Y. Fedoryshyn, P. Strasser, P. Ma, F. Robin and H. Jäckel, ‘Optical waveguide structure for an all-optical switch based on intersubband transitions in InGaAs/ AlAsSb quantum wells,’ Optics Letters, 32, 2680 (2007). Y. Li, A. Bhattacharyya, C. Thomidis, T.D. Moustakas and R. Paiella, ‘Ultrafast alloptical switching with low saturation energy via intersubband transitions in GaN/AlN quantum-well waveguides,’ Optics Express, 15, 17922 (2007). H.Y. Wong, M. Sorel, A.C. Bryce, J.H. Marsh and J.M. Arnold, ‘Monolithically integrated InGaAs–AlGaInAs Mach–Zehnder interferometer optical switch using quantum-well intermixing,’ IEEE Photonics Technology Letters, 17, 783 (2005). J. Nah and P. Likamwa, ‘Monolithically integrated all-optical switch using quantum well intermixing,’ Optical and Quantum Electronics, 38, 567 (2006). R. Prasanth, J.E.M. Haverkort, A. Deepthy, E.W. Bogaart, J.J.G.M. van der Tol, E.A. Patent, G. Zhao, Q. Gong, P.J. van Veldhoven, R. Nötzel and J.H. Wolter, ‘All-optical switching due to state filling in quantum dots,’ Applied Physics Letters, 84, 4059 (2004). A.V. Uskov, E.P. O’Reilly, R.J. Manning, R.P. Webb, D. Cotter, M. Laemmlin, N.N. Ledentsov and D. Bimberg, ‘On ultrafast optical switching based on quantum-dot semiconductor optical amplifiers in nonlinear interferometers,’ IEEE Photonics Technology Letters, 16, 1265 (2004). J. Gea-Banacloche, M. Mumba and M. Xiao, ‘Optical switching in arrays of quantum dots with dipole-dipole interactions,’ Physics Reviews B, 74, 165330 (2006). E. Rodriguez, G. Kellermann, A.F. Craievich, E. Jimenez, C.L. César and L.C. Barbosa, ‘All-optical switching device for infrared based on PbTe quantum dots,’ Superlattices and Microstructures, 43, 626 (2008). N. Bickel and P. Likamwa, ‘Etched quantum dots for all-optical and electro-optical switches,’ Microelectronics Journal, 39, 362 (2008). G. Muñoz-Matutano, B. Alén, J. Martínez-Pastor, L. Seravalli, P. Frigeri and S. Franchi, ‘Optical switching of quantum states inside self-assembled quantum dots,’ Superlattices and Microstructures, 43, 494 (2008).

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44 A Zakery and M Hatami, ‘Design of an ultra-fast all-optical dark soliton switch in a nonlinear directional coupler (NLDC) made of chalcogenide glasses,’ Journal of Physics D: Applied Physics, 40, 1010 (2007). 45 S. Minardi, G. Arrighi, P. Di Trapani, A. Varanavicˇius and A. Piskarskas, ‘Solitonic all-optical switch based on the fractional Talbot effect,’ Optics Letters, 27, 2097 (2002). 46 Z. Xie and H.F. Taylor, ‘Fabry–Perot optical binary switch for aircraft applications,’ Optics Letters, 31, 2695 (2006). 47 J.W. Kang, E. Kim and J.J. Kim, ‘All-optical switch and modulator using photochromic dye doped polymer waveguides,’ Optical Materials, 21, 543 (2002). 48 J. Liu, Q.F. Dai, Z.M. Meng, X.G. Huang, L.J. Wu, Q. Guo, W. Hu, S. Lan, A.V. Gopal, and V.A. Trofimov, ‘All-optical switching using controlled formation of large volume three-dimensional optical matter,’ Applied Physics Letters, 92, 233108 (2008). 49 O. Bendix and J.A. Méndez-Bermúdez, ‘Design of switches and beam splitters by use of chaotic cavities,’ Optics Letters, 30, 1396 (2005). 50 A. Groisman, S. Zamek, K. Campbell, L. Pang, U. Levy and Y. Fainman, ‘Optofluidic 1 × 4 switch,’ Optics Express, 16, 13499 (2008).

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11 Summary: key trends in optical switches B.J. LI, Sun Yat-Sen University, China, and S.J. CHUA, National University of Singapore, Singapore

Optical switches are important devices for applications in optical communications, networks and systems. These can reduce the cost of network and increase fiber transmission capacity and, at the same time, distribute optical signals to different subscribers by electro-optic effect, acousto-optic effect, thermo-optic effect, magneto-optic effect, nonlinear optic effect, micro-electro-mechanical system (MEMS) principle, semiconductor optical amplifier (SOA) principle, liquid crystal principle, photonic crystal principle, holographic principle, quantum well/ dots, etc. Optical switches utilizing electro-optic effect (carrier-induced refractive-index change) have tremendous potential for application of optical processing because of their small size, single-mode operation and polarization independence. Electrooptical switches based on single-mode waveguide principle, multimode interference principle and plasma dispersion effect have been reported with lateral or vertical p-n junctions. The switch structures have been developed from one input/output port to four input/output ports. The future trends in electro-optical switches would be to reduce the injection current and injection current density by applying novel switch structures or methods. Optical switches utilizing thermo-optic effect were fabricated in polymer, silicon, silicon nanocrystals, III–V semiconductors, lithium niobate (LiNbO3), tantalum pentoxide (Ta2O5) and aluminum oxide (Al2O3). Among them, siliconon-insulator (SOI) technology is the most promising one for SOI-based optical switches, offering the potential for integration with SOI-based CMOS electronics towards achieving monolithically integrated optoelectronic systems-on-a-chip. In addition, as mentioned in Chapter 3, SOI is an attractive alternative materials system because it has a large thermo-optic coefficient, which, in combination with the small device dimension and the high thermal conductivity of silicon, allows low power and fast response time. Future trends of thermo-optical switches would be ultra-small structures using high index contrast of waveguide core, quick heating response, optimal uniformity of temperature distribution inside the crosssection of waveguide and minimum power requirements. Optical switches utilizing magneto-optic effect have been implemented in magneto-optical crystal fiber-type, polarization-preserving fiber-type, Faraday effect/ rotation, polarization rotation, high-speed magnetic field module, etc. But different switch implementation technologies with different performance characteristics in terms of power consumption, scalability, insertion loss, polarization-dependent loss, 313 © Woodhead Publishing Limited, 2010

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wavelength dependency and switching speed still should be focused. Besides, suitable materials for magneto-optic effect switches are also desired. Optical switches utilizing MEMS architectures appear very promising in optical cross-connect, wavelength selective cross-connect, optical add/drop multiplexer, etc. MEMS technology is recognized as one of the best candidates for optical switch application because of enabling an all-optical solution. Different architectures of MEMS-based switches including 2D and 3D structures have been reported. However, there are still important issues such as architectural design, fabrication and packaging that need to be addressed before the MEMS approach can become the technology of choice for the next-generation optical switches. In addition, reliability and scalability are the challenges surrounding MEMS-based optical switches. Optical switches utilizing SOA architectures have been used for space and wavelength switching. Mach–Zehnder and gain-transparent all-optical switches have also been reported. Polarization sensitivity and noise characteristics should be further improved to reduce their impact on the system. Optical switches utilizing nonlinear optic effect provide an ultra-fast option for processing signals due to their instantaneous nonlinear response. They also allow all-optical switching of signals that helps to realize future high-speed all-optical networks. Nonlinear-effect-based optical switches include nonlinear directional coupler switches, nonlinear Mach–Zehnder interferometer switches, nonlinear fiber switches, micro-ring/disk resonator type switches, quantum-dot switches, local nonlinearity, Kerr effect, phase modulation of spatial solitons, cross-phase modulation, four-wave mixing, frequency conversion, stimulated Raman scattering. Different types of nonlinear materials have different properties, which are important in various applications. Fok and Prucnal have mentioned in Chapter 7, that from the development of the nonlinear-effect-based switches, both size and switching speed are the trends that researchers are working on. Besides, other parameters such as the number of inputs and outputs, data format and the ability to cascade devices must also be considered when designing these devices. Optical switches utilizing liquid-crystal-based polarization management, refractive-index change, wavelength-selective, etc., have been described in Chapter 8. Vázquez et al. have mentioned that future trends would be liquidcrystal photonic crystal fibers, ring resonators with liquid crystal, high-capacity holographic liquid crystal switches. As mentioned in Chapter 8, to date, the OLED technology does not fulfill the telecom requirements concerning wavelengths, so liquid crystals still have a clean horizon in competing with MEMS. Optical switches in photonic crystal, such as symmetrical Mach–Zehnder-type all-optical digital switches, have been demonstrated in Chapter 9. For this implementation, wide/flat band 2D photonic crystal waveguides such as intersection, bend and Y-junction design are easy to design and fabricate. But to realize optical flip-flop operation for the future ultra-fast optical digital processing

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Summary: key trends in optical switches

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system, problems such as improved coupling to optical fiber and easy packaging need to be solved. Fiber, holographic, quantum-well and quantum-dot optical switches have also been designed as complementary contents of the optical switch family. But a smart drive design with the aim to emphasize switching performances and optically controlled all-optical switches would be key trends. Acousto-optical switch is another kind of switch. Optical switches utilizing acousto-optic effect, modulation of refractive index by acoustic waves, acoustooptical Bragg diffraction, etc., have already been designed. High sensitivity acousto-optical switches with less crosstalk, high optical response and improved pass band flatness are still desirable in applications. With rapid progress in nanoscience and nanotechnology, optical switches based on nanowires, nanoparticles, etc., will become more and more important for nanonetworks and nanochips. Therefore, ultra-compact, multifunctional and highly integrated nanophotonic switches will be another key trend for the optical switch family from materials to design. High confinement permits sharp waveguide bends and ultra-small device sizes, thus allowing the miniaturization of passive optical devices, such as nanowire-based waveguides, bends, splitters and interferometers and leading to high-density nanophotonic integrated circuits. By applying novel materials and using high-confinement nanowires, further research and work need to be done from materials to designs and fabrications. Through such efforts, many types of optically controlled nanophotonic switches will be invented and applied.

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Index

AC Kerr effect, 3 acousto-optical switch, 315 acrylic glass, 71 adiabatic mode coupler, 75–80 digital optical switch, 76 DOS and attenuators configuration, 79 vertical DOS configuration, 80 wide angle DOS with optical attenuators, 78 all-optical switches, 102–4, 297, 302, 306–7 based on 3D optical matter generation, 307 crossbar 2D MEMS switch, 102 diffraction grating creation, 103 digital thermo-optical waveguide switch, 104 EO modulator, 104 alumina, 75 amorphous materials, 72–5 amorphous silicon, 73 silica, 72 analogue bandwidth, 99 anisotropic etchings, 150 AO switches see acousto-optical asymmetric multiple quantum wells, 175 asymmetric nonlinear Mach-Zehnder interferometer, 201 asymmetric switch, 21 auger recombination, 167, 173 beam deflection-based switches, 224–7 effects of pixellated SLM, 226 two-stage N × N beam-steering switching architecture scheme, 225 beam propagation method, 19 Beneš network, 142 birefringence, 197 bismuth, 117 bismuth-substituted garnet Faraday rotators, 117 bismuth-substituted iron garnets, 116, 117 bit rate, 207 BPM see beam propagation method Bragg grating mirror, 81 broadcast and select switch architectures, 162 broadcast and select switching architecture, 215, 216 bulk micromachining, 150–1 carrier heating, 169 carrier injection, 166–7 region, 21, 24

carrier tunneling, 173 Cauchy law, 223 chalcogenide fibers, 188 Chappe optical telegraph network, 98 Clausius-Mossotti formula, 63, 64 Clos network, 142 closed-loop approach, 154 CMOS technology platform, 35 code division multiple access, 198 comb-drive actuation, 147–8, 280–1 communication network, 100 cross-absorption modulation, 184 schematic illustration, 200 cross-phase modulation, 183 crosstalk, 4, 75, 207 3D integrated optic switches, 79 2D photonic crystal waveguide, 243 design and fabrication for PC-SMZ, 251–7 fabrication processes, 244 phase-shift basic properties, 262–5 properties used in experiment, 244 DCs see directional couplers dense wavelength division multiplexing, 160 digital optical switch, 17, 75 digital photonic splitting, 14 directional couplers, 244–6 plan view SEM photograph, 245 DOS see digital optical switch double carrier injection, 30 2DPC see 2D photonic crystal waveguide DPS see digital photonic splitting eigenmodes, 108 electrical equivalent circuit, 211 electro-absorption modulator, 184, 186, 200 electro-optic effect, 313 electro-optical switches, 5–59, 103, 314 1 × 1 switch, 12–14 SOI 1 × 1 optical switch/modulator, 13 1 × 2 switch, 14–19 compact 1 × 2 waveguide DOS, 17 digital optical switch with epilayers, 15 double-etch and dual-taper digital optical switch, 16 near-field output intensity patterns, 20 optical waveguide switch, 16

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Index output optical power vs applied forwardbias, 20 symmetrical waveguide Y-branch DPS switch, 18 Y-branch digital optical switch, 15 1 × 4 switch, 51–2 experimental output near-field spots, 53 InP-based optical switch, 51 2 × 2 switch, 19–35 carrier injection TIR optical switch, 21 compact multi-mode coupler switch, 28 device optical response, 34 intersectional ridge optical waveguide switch, 24 modulation depth vs modulator voltage, 23 near-field output bright spots at 1.55 µm wavelength, 27 near-field output intensity patterns, 33 near-field output intensity spot at 1.3 µm wavelength, 25 normalized output optical power at port 4, 23 normalized output power at port 3, 26 optical power and modulation depth vs forward bias, 27 optical response, 28 output response at input modulation signal, 35 schematic view, 35 SOI zero-gap directional coupler switch, 29 TMI photonic switch with double carrier injections, 31 TMI photonic waveguide switch, 30 2 × 3 switch, 35–8 index-modulation switch, 36 2 × 4 switch, 52–8 optical signal A and B output states, 59 optical signal A output states, 57 optical signal B output states, 58 optical signal input from input A, 56 optical signal input from input B, 57 optical signal input from inputs A and B simultaneously, 58 proposed decoder switch, 54–5 3 × 2 switch, 38–46 C-band wavelengths theoretical results, 40–1 fabricated device array, 41 fabricated switch, 45 input waveguides, 42 multi-mode-based, 43–6 optical power splitter, 44 photonic switch, 39 single-mode-based, 38–43 wavelengths theoretical results, 44–5 3 × 3 switch, 46–50 logic switches, 48 NAND logic switch, 50 NOR logic switch, 50 NOT logic switch, 49 OR logic switch, 49 proposed all-optical logic switch, 47 SOI multimode interference optical switch, 50 waveguide cross-section, 47 device structures, 12–58

317

materials and fabrication, 10–12 fabrication procedure, 11 lateral p+-n junction switch, 12 optical switch waveguide cross-section, 11 vertical p+-n junction switch, 12 performance and challenges, 59 single-mode principle, 6–8 ridge waveguide cross-section, 6 SiGe ridge waveguide critical value, 9 SiGe/Si ridge waveguide cross-section, 7 SiGe/Si waveguide numerical aperture, 8 theory and principles, 6–10 multi-mode interference, 8–10 plasma dispersion effect, 10 electromagnetic actuation, 146 electron irradiation, 34 electrophoretic deposition process, 285 electrostatic actuation, 145, 278 electrostatic force, 145 EO switches see electro-optical etching process see bulk micromachining Fabry–Perot cavity, 82 Fabry–Perot filter, 73 Faraday effect, 2, 106–15 atomic model, 114–15 configuration index interactions, 115 ellipticity mechanism, 110 phenomenological model, 110–14 magnetic dipoles orientation, 110 magnetic field rotation, 112 spinning electron precession, 113 torque on single dipole moment, 111 polar geometry, 108 rotation mechanism, 108 rotation right-hand rule, 109 Faraday ellipticity, 109 Faraday rotation, 97, 98 characterization, 118–29 non-reciprocal transmission line, 118–19 wave formulation for non-reciprocal medium, 119–29 Faraday rotator, 117 fiber switches, 277–94 1 × 2 fiber switch comb-drive actuation, 280 optimal device dimensions, 281 piezoelectric actuation, 284 piezoelectric actuation prototype, 286 planar electrode design, 278 1 × 4 fiber switch prototype, 293 switch that uses MLA telescope, 291 1 × N fiber switch, 277 component moving type, 289 composed of fiber bundle, lens and mirror, 289 component moving type, 288–94 activation and deactivation behavior, 293 cylindrical MLA telescopes highly parallel decentration, 292 2D-micro-optical scanner, 291 micromachined deformable membrane mirror, 290 fiber moving type, 277–78 alignment errors, 279 comb-drive actuation, 280–1

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Index

device structures, 283 electrostatic actuation, 278–80 input fiber construction, 285 input fiber movement, 287 insertion loss against applied voltage, 288 measured switching behaviors, 284 output fibers and v-grooves, 285 piezoelectric actuation, 282–8 piezoelectric tube cross-section, 286 piezoelectric tube deflection, 284 switching positions, 282 switching time against output capacitance of power supply, 289 fluorine-containing polyimides, 71 forklift upgrade, 101 four-wave mixing, 183–4, 198–200 autocorrelation peak self-switching principle, 199 schematic illustration, 199 Fourier’s law, 67 Franz–Keldysh effect, 184, 186 free-ion theory, 114 gain dynamics effect of doping, 174 impact of strain, 174 gain media, 165–6 gain nonlinearity, 168–9 inter-band nonlinearities, 168–9 intra-band nonlinearities, 169 garnets, 116 Gaussian profile waves, 119, 120 geometrical fill factor, 213 Ghosh model, 66 Goos-Hahnchen shifts, 8 graded index separate confinement heterostructure, 172 grating equation, 226 GRINS CH see graded index separate confinement heterostructure halogenated amorphous silicon, 73 Hamiltonian, 114 heat conduction equation, 67 holographic switches, 294–6 3 × 3 switch target performances, 295 configuration with LCSLM, 294 high-capacity LC switches, 232–4 N × N holographic switch architecture, 295 planar integrated polarization-insensitive 2 × 2 switch, 296 impedance spectroscopy, 213 InAS QDs, 258 InGaAsP multiple-quantum wells, 16 ingress stage, 142 insertion loss, 4, 207 inter-band nonlinearities, 168–9 inter-band transition, 166–7 inter-subband transition, 297–300 interferometric configurations, 80–3 Michelson interferometer, 82 MZI with 3 dB splitter and with MMI coupler, 81 resonator configuration, 83 interpixel gap, 212 intra-band nonlinearities, 169

intra-band transition, 166–7 ISBT see inter-subband transition isentropic band gap, 66 isotropic etchings, 150 Jones calculus, 118, 213 Joule effect, 76 Kerr effect, 2, 107–8 Larmor precession frequency, 110, 111 LC see liquid crystal LCOS see liquid crystal on silicon LCP see left circularly polarized LCSLM see liquid-crystal spatial light modulator left circularly polarized, 105 light reflection see Kerr effect light transmission see Faraday effect liquid crystal, 206 liquid crystal devices, 235 liquid crystal on silicon, 213, 230 liquid crystal optical switches, 206–35 basic liquid crystal structures, 210–16 bi-stable bookshelf SSFLC cell, 213 spatial light modulators, 213–15 surface-stabilized ferroelectric liquid crystal cells, 212–13 twisted nematic cells, 210–12 future trends, 230–5 1 × N holographic switch architecture, 234 high-capacity holographic LC switches, 232–4 liquid crystal devices, 235 liquid crystal photonic crystal fibers, 231–2 ring resonators with LC, 232 vertically coupled micro-ring resonators, 233 liquid crystal theory and principles, 208–25 2D pixellated SLM geometry, 213 LCOS SLM structure, 214 liquid crystal cell schematic drawing, 211 liquid crystalline phases, 208–10 thermotropic liquids crystals mesophases, 209 optical switch schematic for parameters definition, 207 switches and applications, 215–30 1 × 2 liquid crystal optical switch, 218 beam deflection-based switches, 224–7 broadcast-and-select switching architecture, 216 cell contrast optimization, 222 compact and generic WWS structure, 229 materials for amplitude and phase modulation, 221–4 NLC blazed grafting with multi-electrode structure, 224 optical crystal switching architectures, 215 other structures for phase or amplitude switching, 223 polarization management-based switches, 216–21 PolRot state of the art and performance parameters, 219–20 space-routing switching architecture, 217 wavelength-selective switches, 227–30

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Index

liquid-crystal spatial light modulator, 294 liquid crystalline phases, 208–10 Lorentz-Lorenz formula, 63 Lucent LambdaRouter, 142 Lucent Microstar, 153 Mach–Zehnder interferometer, 2, 80, 194–6, 300 operation principle, 195 switch with asymmetric nonlinear media, 196 switch with two control signals configuration, 195 magneto-optic effect, 313–14 magneto-optical switches, 97–132 all-optical switches, 102–4 crossbar 2D MEMS switch, 102 contemporary networks, 100–1 global IP traffic forecasted growth, 100 wavelength division multiplexing, 101 Faraday effect, 106–15 Faraday rotation characterization, 118–29 non-reciprocal transmission line, 118–19 port-to-port phase delay calculations, 121 transmission line, 118 wave formulation for non-reciprocal medium, 121–29 material, 116–17 yttrium iron garnet unit cell, 116 optical communication history, 97–102 modern optical communications, 98–9 next-generation networks, 101–2 optical transmission windows, 99 polarization, 105–6 Lissajous figures, 106 scattering matrix, 129–31 models and variables, 129 theory and principles, 107–115 transmission matrix, 131–2 cascaded elements, 132 Maxwell equations, 63, 106, 107, 108, 120 MEMS see microelectromechanical systems MEMS-based optical switches, 136–55 actuating principles, 144–49 closed-loop vs open-loop control, 150 electromagnetic, 145–7 electromagnetic fiber optic bypass switch, 147 electrostatic, 144–7 stress-induced bending micromirrors, 146 challenges, 153–5 competing technologies, 154–5 manufacturability, 154 packaging, 154 reliability, 153–4 scalability, 154 2D optical switches, 138–43 Clos network implementation, 142 2D 2x 2 crossbar switch, 139 L-switching matrix, 141–2 multistage 2D switch, 142–3 N × N crossbar switch, 139 polygon OXC, 140 polygon switch, 139–40 re-arrangeable non-blocking switch, 140 3D optical switches, 143–4 3D micromirror, 144 Lucent Microstar, 143 microlens scanner, 143–4

319

fabrication techniques, 149–52 anisotropic vs isotropic etching, 150 bulk micromachining, 150–1 surface micromachining, 151–2 surface micromachining process, 151 wafer bonding, 152 list of abbreviations, 155 materials, 152–3 micromirror, 152–3 mirror spring/torsion beam, 153 substrate, 152 optical switch architectures, 138–44 optical systems, 136–8 optical add/drop multiplexer, 138 optical cross-connect, 137 wavelength selective cross-connect, 137 other actuating mechanisms, 147–49 Comb-drive, 147–8 micro-motor, 149 micromirror switching, 148 scratch drive actuator, 148 thermal, 148 metal-mask method, 258 selective area growth in InAS QDs, 259 selective area growth of QDs, 259 metal organic chemical vapor deposition, 51 micro-optical-electro-mechanical system, 83–6 micro-ring resonators, 232 microelectromechanical systems, 2, 84–8, 136, 137, 314 microlens array telescope, 290–1 microlens scanner, 143–4 micromirrors, 152–3 Microstar micromirror array, 143 middle stage, 143 mirror-OFF state, 39 mirror-ON state, 39 MLA telescope see microlens array telescope MMI see multi-mode interference modal evolution, 77 MOEMS see micro-optical-electro-mechanical system MQW see multiple quantum wells multi-mode interference, 8, 80 multi-mode scattering matrix, 125 multi-mode waveguide, 8 multiple access interference, 99 multiple quantum wells, 301 carrier uniformity, 173–4 gain/carrier recovery, 177 SOAs typical parameters, 177 MZI see Mach–Zehnder interferometer NAND logic switch, 48 output signals, 50 nanoparticles, 315 nanowires, 315 noise figure, 163 NOLM see nonlinear optical loop mirror non-blocking, 140 non-linear-effect-based optical switch, 202–3 non-linear optic effect, 314 nonlinear optical loop mirror, 189–90 basic configuration, 190 external optical control configuration, 191 in-loop optical control configuration, 192 modified configuration, 191

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Index

NOLM with external control signal, 191–4 nonlinear waveguides, 188–9 NOR logic switch, 48 output signals, 50 NOT logic switch, 48 output signals, 49 OLED, 314 on/off ratio, 4, 43 optical add/drop multiplexers, 206 basic principle, 138 optical constant, 63 optical cross-connect, 137, 206 optical de-multiplexer, 136 optical dispersion relations, 63 optical fibers, 99, 101, 186–8 step-index fibers, 187–8 chalcogenide fibers, 188 germania-doped highly nonlinear fibers, 187 highly nonlinear bismuth oxide fibers, 187–8 photonic crystal fibers, 188 optical Kerr effect, 3, 182 optical Kerr nonlinearity, 181 optical multiplexer, 136 optical nonlinearity, 182 optical switch architecture, 138–44 2D optical switches, 138–43 3D optical switches, 143–4 optical switches, 5, 59 fiber, holographic, quantum optical and other types, 276–309 fiber switches, 277–94 component moving type, 288–94 fiber moving type, 277–88 holographic switches, 294–6 implementation mechanisms, 1–4 key trends, 313–15 nonlinear devices, 185–89 optical fibers, 186–8 other types, 188–9 semiconductor-based devices, 185–6 nonlinear-effect-based optical switches structure, 189–202 1 × N switch based on spatial soliton interactions, 200–2 Mach-Zehnder interferometers, 194–6 NOLM with external control signal, 191–4 non-interferometric-based, 198–200 nonlinear optical loop mirror, 189–90 spatial soliton 1 × N switch schematic illustration, 201 ultrafast non-linear interferometers, 196–8 nonlinear effects, 182–5 cross-absorption modulation, 184 cross-phase modulation, 183 four-wave mixing, 183–4 self-phase modulation, 182–3 spatial soliton trapping and dragging, 184–5 other switches, 307–9 1 × 2 Y-branch all-optical switch, 307 all-optical switch based on 3D optical matter, 307 2D waveguide geometry, 308 optofluidic switch, 308

physical mechanisms, 2–4 quantum optical switches, 296–305 quantum-dot-based switches, 301–5 quantum-well-based switches, 297–301 semiconductor optical amplifiers, 158–78 design criteria, 171–7 structure, 165–71 switching strategy, 158–65 switching based on optical nonlinear effects, 181–203 ideal non-linear-effect-based, 202–3 optical transmission factor, 212 optical transmission plane, 159 optical well, 86 optofluidic switch, 308 OR logic switch, 48 output signals, 49 oscilloscope, 27, 34 OXC see optical cross-connect p-i-n Ge detector, 27 Pauli-Fermi principle, 114 PC-FF see photonic crystal flip-flop PC-SMZ see photonic crystal symmetrical Mach-Zender PCFF see photonic crystal flip-flop PDL see polarization-dependent loss PDLC see polymer-dispersed liquid crystal phosphosilicate glass, 153 photonic crystal, 314 photonic crystal all-optical switches, 241–72 advanced 2DPC waveguide design and fabrication for PC-SMZ, 251–7 air-hole patterns of TO-designed bend and Y-junction, 257 2DPC bend waveguides pattern, 255 2DPC intersection waveguides initial pattern, 253 fabricated bend waveguides, 256 PC-SMZ configuration with 2DPC intersection, 251 standard and TO designs, 254 Y-junction waveguide, 258 all-optical analog switches key roles, 243 device structures and performances, 265–71 accumulated phase shift and resultant phase shift difference, 267 control pulses transmittance dependence, 266 input four-pulse train, 269 optical nonlinearity-induced phase shifts and group index, 268 PC-SMZ schematic diagram, 267 PC-SMZ state-of-the-art performance, 265–8 photoluminescence intensity mapping, 272 time responses of SP in PC-SMZ, 269 key roles in nanophotonics, 242 optical QDs for PC-FF growth and characterization, 258–65 2DPC waveguide configuration, 261 high-density QDs and PL spectrum, 260 high density/uniformity in SAG-QDs, 259 metal-mask method for SAG of InAs QDs, 259 metal-mask method for selective area growth of QDs, 259

© Woodhead Publishing Limited, 2010



Index

molecular beam epitaxy grown sample, 260 PC-SMZ schematic, 263 phase-shift in QD-embedded 2DPC waveguide, 262–5 QD1 and QD2 photoluminescence intensity mapping, 262 rotatable MM, 261 sequential phase-shift excitation process in QD/PC waveguide, 264 wavelength-control technique in SAG‑QDs, 259–2 photonic crystal flip-flop computational and experimental simulation, 268–70 configuration, 271 implementation challenges, 270–1 theory and principles, 243–51 absorption and refractive index spectra, 247 calculated E-field distributions, 246 DCs in PC-SMZ waveguide pattern, 245 directional coupler plan-view, 245 2DPC waveguide, 243–6 optical bi-stability in PC-FF, 250 optical nonlinearity in quantum dots, 246–8 photonic crystal flip-flop, 249–1 photonic crystal symmetrical Mach-Zender, 248–51 transmittance changes in QD-containing waveguides, 247 topology and optimization design bend waveguide, 254–5 computational model, 253 intersection, 253–4 principles, 252–3 Y-junction, 255–7 Y-junction computational model, 256 Z-shaped 2DPC waveguide, 252 photonic crystal fibers, 188, 231–2 photonic crystal flip-flop, 242, 249–1 computational and experimental simulation, 268–70 computational simulation for repetition rates, 270 configuration for bi-stability demonstration, 271 implementation challenges, 270–1 principle of optical bi-stability, 250 photonic crystal symmetrical Mach-Zender advanced 2DPC waveguide design and fabrication, 251–7 DCs in waveguide pattern, 245 schematic diagram, 267 SMZ type all-optical switch principle, 248–9 analog switch, 248–9 diagram and waveguide configuration, 249 research items and numerical targets, 249 time-differential phase modulator principle, 248 state-of-the-art performance, 265–8 photonic crystals, 3 photonic liquid crystal fibers, 231 piezoelectric actuation, 282–8 piezoelectric tube, 286, 288 pixel pitch, 213 plasma dispersion effect, 10, 24

321

plasma-enhanced chemical vapor deposition, 32, 52, 73 polarization, 105–6 polarization beam splitter, 195 polarization-dependent loss, 75, 207 polarization insensitivity, 171 polarization management-based switches, 216–21 block diagram, 217 performance, 218–21 PolRot state of the art and performance parameters, 219–20 polarization mode dispersion, 207–8 polarization rotation, 216, 221 PolRot see polarization rotation polyimides, 71 polymer-dispersed liquid crystal, 210, 222 polymer materials, 69–72 benzocyclobutene, 70 bisphenol A-aldehyde, 70 ChemOptics Exguide resins, 72 fluorinated poly(arylene ether sulphide), 71 fluoroacrylate, 70 polyimide, 71 poly(methyl methacrylate), 71 polyurethane, 72 polymer waveguide technology, 69 polysilicon, 75, 153 power consumption, 207 power switching, 68 Poynting vector, 130 QDs see quantum dots quantum-dot-based switches, 301–5 quantum dot optical switches, 315 quantum dots absorption and refractive index spectra, 247 optical nonlinearity, 246–8 quantum mechanical approach, 86 quantum optical switches, 296–305 quantum-dot-based switches, 303–5 all-optical switch using pump beam, 305 demodulated probe transmission vs pump power, 304 fabricated device, 305 transmittance vs wavelength, 306 quantum-well-based switches, 297–301 based on inter-subband transition, 297–300 based on quantum-well intermixing, 300–1 conduction-band diagram and bound states, 298 ISBT all-optical switch, 297 MZI optical switch, 301 switching characteristics, 302 time evolution of signal absorption coefficient in coupled QW structure, 299 in presence of ultrafast control pulse, 300 quantum phenomena, 1 quantum-well-based switches, 297–301 quantum-well intermixing, 297, 300–1 quantum well optical switches, 315 QWI see quantum-well intermixing racetrack resonator, 82 RCP see right circularly polarized reactive ion etching technique, 32 reference planes, 131

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322 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 20 1 2 3 4 5 6 7 8 9 30 1 2 3 4 5 6 7 8 9 40 1 2 43X

Index

reference termination, 130 refractive index, 19 right circularly polarized, 105 Roman smoke signal telegraph, 98 Russel-Saunders coupling, 114 sacrificial layer, 151 SAG see selective area growth SAG-QDs high density/uniformity, 259 wavelength-control technique, 259–62 scalability, 207 scattering matrix, 119, 125, 129–1 models and variables, 129 scratch drive actuator, 148 selective area growth, 259, 272 self-imaging, 8 self-imaging theory, 8 self-phase modulation, 182–3 Sellmeier coefficients, 66 semiconductor optical amplifiers, 2, 158–78, 185–6, 192–4, 314 design criteria, 171–7 carrier distribution schematic, 173 carrier uniformity in MQW, 173–4 doping on gain dynamics, 174 gain/carrier recovery in MQW, 177 gain dynamic, 172–3 impact of strain on gain dynamics, 174 MQW SOAs typical parameters, 177 rate equations, 174–6 reducing threshold carrier density, 172 structure, 165–71 active region, 165–6 compressive and tensile quantum wells, 170 density of states, 166 gain nonlinearity, 168–9 generic structure, 166 inter-band and intra-band transition schematic, 167 inter-band vs intra-band transition, 166–7 noise, 171 polarization-insensitive SOA, 170–1 strained gain media, 169–70 transparency threshold, 168 switching strategy, 158–65, 159–5 broadcast and select switch architecture, 162 electrically vs optically controlled switch, 159–60 nonlinear effects, 160 reported SOA-based integrated switch matrices comparison, 163 switch fabric architectures, 160–5 wavelength routing and reconfigurable wavelength switch, 161 silica-based devices, 61 silicon, 151, 152, 153 silicon-on-insulator, 13, 82, 232, 281, 313 silicon wafers, 152 single crystal silicon, 153 single mode fiber, 187 single pitch, 142 SLM see spatial light modulators SMZ see symmetrical Mach Zender Snell-Descartes’s law, 224

SOA see semiconductor optical amplifiers SOI see silicon-on-insulator soliton switches, 200 space-routing switching architecture, 215, 217 space switch, 162–3 Spanke-Beneš network, 142 spatial light modulators, 213–15, 225, 295 spatial soliton dragging, 185, 201–2 switch, 202 spatial soliton trapping, 184–5, 200–1 spectral hole burning, 169 SSFLC see surface-stabilized ferroelectric liquid crystals stark effect, 3 step-index fibers, 187–8 strained gain media, 169–70 structural layers, 151 surface micromachining, 151–2 surface-stabilized ferroelectric liquid crystals, 210, 212–13 bi-stable bookshelf SSFLC cell, 213 switch, 1 see also specific type of switch switch fabric architectures, 16 1–6 broadcast and select, 162 optical switch matrix, 165 space switch, 162–3 wavelength/space switch architecture, 164 wavelength switch, 161–2 switching speed, 4 switching time, 207 symmetrical Mach Zender, 242 all-optical digital switch, 249–51 time-differential phase modulator, 248 synthetic garnets, 116 TEC see thermal-expansion coefficient temperature coefficient, 66 terahertz optical asymmetric demultiplexer, 181–2 configuration, 193 operation principle, 194 thermal-expansion coefficient, 64 thermal nonlinear optical effects, 68 thermo-optic coefficient, 62 thermo-optic effect, 28, 35, 62, 313 thermo-optical switches, 61–91 abbreviations, 89–90 adiabatic mode coupler, 75–80 digital optical switch, 76 DOS and attenuators configuration, 79 vertical DOS configuration, 80 wide angle DOS with optical attenuators, 78 amorphous materials, 72–3 amorphous silicon, 73 silica, 72–3 device structures, 75–86 interferometric configurations, 80–3 Michelson interferometer, 82 MZI with 3 dB splitter and with MMI coupler, 81 resonator configuration, 83 materials, 69–75 MOEMS configurations, 83–6 MEMS configurations by Zhong, 84–5

© Woodhead Publishing Limited, 2010



Index

MEMS configurations by Zhu, 87–8 polymer materials, 69–72 benzocyclobutene, 70 bisphenol A-aldehyde, 70 ChemOptics Exguide resins, 72 fluorinated poly(arylene ether sulphide), 71 fluoroacrylate, 70 polyimide, 70 poly(methyl methacrylate), 71 polyurethane, 72 recent thermo-optical switches, 89 semiconductor and crystalline materials, 73–5 aluminium oxide, 75 III-V semiconductors, 74 lithium niobate, 74 silicon, 73 silicon nanocrystals, 73–4 tantalum pentoxide, 74–5 symbols, 90–1 theory and principles, 62–9 thermotropic liquid crystals, 208 thin film transistor, 213 threshold carrier density, 172 time division multiplexing, 191 TIR see total internal reflection TO switches see thermo-optical TOAD see terahertz optical asymmetric demultiplexer TOC see thermo-optic coefficient TOE see thermo-optic effect Toshiyoshi design, 145 total internal reflection, 2, 19, 24 transmission matrix, 119, 131–2 cascaded elements, 132 transparent current density, 168

323

transparent polymers, 70 tunable wavelength converter, 162 twisted nematic cells, 210–12 impedance measurement, 211 operation, 211 two dimensional photonic crystal, 241–2 ultrafast non-linear interoferometer, 196–8 2 × 2 switch configuration, 198 configuration, 197 UV-imprinting technique, 72 variable optical attenuators, 218 wafer bonding technique, 152 waveguide vanishing, 13 wavelength-dependent loss, 75 wavelength division multiplexing, 101 wavelength-selective switches, 227–30 1 × 4 wavelength selective switch, 228 beam-shifting LC-WSS structure, 228 compact and generic structure using beam steering, 229 LC-WSS based on polarization switching, 227–9 LCOS beam-steering-based WSS, 229–30 wavelength switch, 161–2, 164 WDM see wavelength division multiplexing wet etching see anisotropic etchings WSS see wavelength-selective switches yttrium iron garnets, 117 Zeeman Hamiltonian, 115 Zeeman interaction, 114 zero water peak fibers, 99

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