Optoelectronics and Fiber Optic Technology

Optoelectronic and Fiber Optic

Technology

About the Author Ray Tricker (MSc, IEng, FIIE (elec), FinstM, FIQA, MIRSE) as well as being the Principal Consultant and Managing Director of Herne European Consultancy Ltd - a company specializing in ISO 9000 and ISO 14000 Management Systems - is also an established Butterworth-Heinemann author. He served with the Royal Corps of Signals (for a total of 37 years) during which time he held various managerial posts culminating in being appointed as the Chief Engineer of NATO ACE COMSEC. Most of Ray's work since joining Herne has centred on the European railways. He has held a number of posts with the Union International des Chemins de fer (UIC) (e.g. Quality Manager of the European Train Control System (ETCS), European Union (EU) T500 Review Team Leader, European Rail Traffic Management System (ERTMS) Users Group Project Co-ordinator, HEROE Project Co-ordinator) and currently (as well as writing books for Butterworth-Heinemann) he has prepared a complete Quality Management System for the European Rail Research Institute (ERRI) in Holland, and in assisting them in gaining ISO 9000 accreditation. He is also consultant to the Association of American Railroads (AAR) and Transportation Technology Centre Inc. (TTCI) advising them on ISO 9001:2000 compliance and has recently been appointed as a member of a Notified Body (for the harmonization of the trans-European, high speed, railway network).

To Claire

Optoelectronic and Fiber Optic Technology Ray Tricker

Newnes

OXFORD AMSTERDAM BOSTON LONDON NEW YORK PARIS SAN DIEGO SAN FF~NCISCO SINGAPORE SYDNEY TOKYO

Newnes An i m p r i n t of Elsevier Science Linacre House, Jordan Hill, Oxford OX2 8DP 225 W i l d w o o d Avenue, Woburn, MA 01801-2041 First p u b l i s h e d 2002 C o p y r i g h t 9 2002, Ray Tricker. All rights reserved. The right of Ray Tricker to be identified as the a u t h o r of this w o r k has been asserted in accordance with the Copyright, Designs and Patents Act 1988 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers

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Contents Foreword

xiii

Preface

xv

Acknowledgements 1

xvii

The history of fiber optics 1.1 Background 1.2 Basic principles of optical line transmission 1.3 Advantages of optical fibers and optoelectronic signalling 1.3.1 Immunity to electrical and magnetic fields 1.3.2 Low attenuation 1.3.3 Wide transmission bandwidth 1.3.4 Small physical size and weight 1.3.5 Increased flexibility 1.3.6 Electrical insulation 1.3.7 Immunity to electromagnetic interference and interception 1.3.8 Electrical protection 1.3.9 Analog and digital transmission 1.3.10 Receiver sensitivity 1.4 Disadvantages of optical fibers and optoelectronic signalling 1.4.1 Cost 1.4.2 Jointing and test procedures 1.4.3 Tensile stress 1.4.4 Short links 1.4.5 Fiber losses 1.4.6 Other losses 1.5 Practical applications of optoelectronics 1.5.1 Telephone networks 1.5.2 Urban broadband service networks 1.6 The future of optoelectronics 1.6.1 All-optical network 1.6.2 Computers 1.6.3 Transmission speed 1.6.4 Integration

1 19 23 24 25 25 25 26 27 27 28 28 28 29 29 29 30 30 30 31 32 32 32 33 33 34 34 34

vi

Contents

Theory 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

2.11

2.12 2.13 2.14

2.15 2.16

2.17

2.18

Reflection and refraction Refractive index Total internal reflection 2.3.1 Shell's law Angle of incidence Critical angle Acceptance cone Acceptance angle Numerical aperture Structure of a fiber Modes 2.10.1 Multimode step index 2.10.2 Multimode graded index 2.10.3 Single-mode step index Losses 2.11.1 Attenuation 2.11.2 Absorption 2.11.3 Scattering 2.11.4 Bending losses Optical power and power density Optical fiber input power Dispersion losses 2.14.1 Material dispersion 2.14.2 Waveguide dispersion 2.14.3 Chromatic dispersion Optical transitions in semiconductors 2.15.1 Semiconductor optical transitional mechanisms Usable b a n d w i d t h of optoelectronic transmission systems 2.16.1 Cut-off frequency of an optical fiber 2.16.2 Fiber bandwidth Modulation and demodulation of the subcarrier 2.17.1 Modulation techniques 2.17.2 Frequency modulation 2.17.3 Intensity (power) modulation Signal-to-noise ratio 2.18.1 Optical receiver thermal noise 2.18.2 Quantum noise (of light) 2.18.3 Dark current noise 2.18.4 Laser noise 2.18.5 Mode partition noise 2.18.6 Modal noise (in multimode fibers)

36 38 39 39 41 45 46 47 48 49 49 51 51 52 52 53 53 53 54 55 57 57 58 58 58 59 61 61 62 63 63 65 65 67 67 68 69 69 69 69 69 69

Contents

2.19 Types 2.19.1 2.19.2 2.19.3 2.20 Types 2.20.1 2.20.2 2.20.3 2.20.4 2.20.5 3

of mixers End fire mixers Transmissive mixer Biconical tapered transmissive mixer of multiplexing Fiber multiplexing Electrical multiplexing Wavelength division multiplexing Dense wavelength division multiplexing Time division multiplexing

Fibers and cables 3.1 3.2

3.3 3.4

3.5 3.6

3.7

3.8 3.9 3.10

3.11

Single-mode fiber Multimode fiber 3.2.1 Multimode step index fiber 3.2.2 Multimode graded index fiber Manufacturing processes and designs of optical waveguides Manufacturing techniques 3.4.1 Preparation of the glass preform 3.4.2 Forming the glass rod 3.4.3 Rare earth-doped fibers 3.4.4 Cap sealing Plastic fiber optic cables Cable characteristics and specifications 3.6.1 Tensile strength of the cable 3.6.2 Buffered fiber Types of cable 3.7.1 Tight buffered cable 3.7.2 Loose tube 3.7.3 Slotted core cable Fiber optic cable ducts 3.8.1 Tube identification Cable construction Types of ducted cable 3.10.1 External duct cable 3.10.2 Universal duct cable 3.10.3 Universal ruggedized cable 3.10.4 Water protection 3.10.5 Rodent protection 3.10.6 Metallic armouring 3.10.7 Hazardous environmental cable Installation possibilities 3.11.1 Training

vii

70 70 71 71 71 72 72 72 74 74 76 77 79 79 81 82 85 85 85 90 90 91 92 93 93 95 95 95 96 97 97 98 99 99 100 100 101 101 102 103 104 105

viii

4

5

Contents

T r a n s m i t t e r s - light emitting diodes and lasers

106

4.1 Light source 4.2 Conversion of electrical energy into light waves 4.3 Light emitting diodes 4.3.1 LED preparation 4.4 Types of LED 4.4.1 Edge emitting diodes 4.4.2 High radiance light emitting diodes 4.4.3 Diffused gallium arsenide LEDs emitting at 900 nm 4.4.4 Aluminium gallium arsenide/gallium arsenide (A1GaAs/GaAs) high radiance LEDs emitting a t 830 nm 4.4.5 InGaAsP/InP high radiance LEDs emitting at 1300nm 4.4.6 Indium phosphide 4.5 Laser diodes 4.5.1 Advantages 4.5.2 Disadvantages 4.5.3 Design of a laser diode 4.6 Basic differences between gain-guided (current-induced) and index-guided (built-in waveguide) laser diodes 4.6.1 Types of laser diodes 4.6.2 Microdisks 4.6.3 Multiple signalling

106 106 107 110 111 111 112 112

124 126 128 129

R e c e i v e r s - photodiodes

132

5.1 Conversion of light waves into electrical energy 5.2 Design characteristics of an optical receiver 5.2.1 The PIN photodiode 5.2.2 Avalanche photodiodes 5.2.3 Silicon photodiodes 5.2.4 Germanium photodiodes 5.2.5 Indium gallium arsenide photodiodes 5.3 Types of receiver 5.3.1 Analog fiber optic receiver 5.3.2 Digital fiber optic receiver 5.4 Fiber amplifiers 5.4.1 Laser amplifier repeater 5.4.2 Erbium-doped fiber amplifiers 5.4.3 Erbium-doped fluoride fiber amplifiers 5.4.4 Erbium-doped tellurite fiber amplifiers 5.4.5 Praseodymium-doped fluoride fiber amplifiers 5.4.6 Semiconductor optical amplifier

132 134 134 135 135 136 137 137 137 137 138 139 139 139 140 141 141

113 114 115 116 119 120 122

Contents

6

ix

Connectors and couplers

142

6.1 Connectors 6.1.1 Connector requirements 6.1.2 Quality of the connector 6.1.3 Connector attenuation 6.1.4 Types of connector 6.2 Couplers 6.2.1 End fire coupling 6.2.2 Lens coupling 6.2.3 Multiport couplers 6.2.4 Coupling losses 6.2.5 Attaching a fiber to a connector 6.2.6 Connecting an optical fiber cable to an LED 6.2.7 Connecting a fiber cable to an integrated circuit 6.3 Fiber jointing techniques 6.3.1 Epoxy splices 6.3.2 Heat-cured epoxy 6.3.3 Room temperature-cured epoxy 6.3.4 Pre-injected epoxy 6.3.5 UV adhesive 6.3.6 Cyanoacrylate adhesive 6.3.7 Anaerobic adhesive 6.3.8 Acrylic adhesive 6.3.9 Crimping 6.3.10 No epoxy and no crimping 6.3.11 Advantages and disadvantages of various types of termination methods 6.4 Splicing 6.4.1 Splice losses 6.4.2 Alignment errors during splicing 6.4.3 Mechanical splicing 6.4.4 Fusion splicing 6.4.5 Closures for fiber optic cables 6.4.6 Repeaters and regenerators

142 143 143 144 145 152 153 154 154 158 160 161 162 165 166 166 167 167 167 167 167 168 168 169

Communication systems

183

7.1 7.2 7.3 7.4

184 187 187 188 189 189 191

Local networks Long-distance networks Telephone networks Data networks 7.4.1 Local area network 7.4.2 Metropolitan area network 7.4.3 Wide area network

169 169 169 171 172 174 179 180

x

8

9

Contents

7.5 Designing an optical fiber cable system 7.5.1 Fiber versus cable 7.5.2 Digital or analog 7.5.3 ATM versus fast Ethernet 7.5.4 Multiplexing 7.5.5 Channel and cable capacity 7.5.6 Fiber loss 7.5.7 Cable routing 7.5.8 Distribution frames 7.6 Optical path loss budget 7.7 Installation techniques 7.7.1 Storage and handling 7.7.2 Cable laying 7.7.3 Installing fiber optic cables in buildings 7.7.4 Blown optical fiber

191 192 193 194 194 196 196 197 201 201 204 205 206 209 211

Optoelectronic test techniques

216

8.1 Testing fiber optics 8.1.1 Production testing 8.1.2 Installation testing 8.1.3 In-service field testing 8.2 Examples of test equipment 8.2.1 Production test kits 8.2.2 Laser light sources 8.2.3 Power meters 8.2.4 Fiber optic tracers 8.2.5 Power and attenuation measuring test sets 8.2.6 Other field testers 8.2.7 Optical time domain reflectometer 8.2.8 Optical fault locator 8.2.9 OFL versus OTDR 8.2.10 Bandwidth measuring test sets

216 217 218 219 219 22O 220 221 221 223 224 225 23O 231 231

Future developments

233

9.1 9.2 9.3

The global market UK market Development trends 9.3.1 Porous silicon 9.3.2 Ballistic transport 9.3.3 Fluoride glasses

233 235 236 237 237 237

Contents

9.4

9.5

9.6 9.7

9.8

9.9

9.10

9.11

9.3.4 Polarization 9.3.5 Synchronous optical network 9.3.6 Solitron pulses Lasers and amplifiers 9.4.1 Distributed feedback laser 9.4.2 Tuneable lasers 9.4.3 Rare earth-doped fiber lasers and amplifiers 9.4.4 Modular laser source 9.4.5 Erbium-doped fiber amplifier Fiber cables 9.5.1 Re-emergence of multimode fibers 9.5.2 Moulded-clad single-mode fibers 9.5.3 Multi-core fibers 9.5.4 Encapsulated fibers Moveable fibers 9.6.1 Optical bus Transmission systems 9.7.1 Development trends in higher-order optical fiber transmission systems 9.7.2 Transmitters and receivers 9.7.3 Fiberless transmission 9.7.4 Optical Ether 9.7.5 Optical radio Industry 9.8.1 Leak detection 9.8.2 Optical fiber drill bit temperature monitor 9.8.3 Optical fiber soldering iron 9.8.4 Optical computing 9.8.5 Optical multiplexers 9.8.60pto-isolators 9.8.70ptoelectronic switches 9.8.8 High speed optical switches Military 9.9.1 Laser dazzler 9.9.2 Plastic opto-chips 9.9.3 Fiber optic gyroscope Local government 9.10.1 Optical sensors 9.10.2 Office lighting 9.10.3 Decorations 9.10.4 Fiber optic road signs Medicine 9.11.1 Lasers - medical uses 9.11.2 Flexible fiber optic curvature sensors

xi

238 238 238 239 240 240 240 241 243 243 243 243 243 244 246 246 246 246 247 248 249 250 250 250 251 251 251 252 252 252 252 253 253 253 255 256 256 256 257 257 258 258 259

xii

Contents

9.12 Other possibilities 9.12.1 9.12.2 9.12.3 9.12.4

Internet Cars and planes Optical processing and computing Mining

9.13 Conclusion

259 259 260 260 261 261

Appendix A

Optoelectronic and fiber optic standards

263

Appendix B

Fiber optic chronology

276

Glossary

278

Acronyms and abbreviations

290

Books by the same author

300

Useful links

301

Index

305

Foreword During the last couple of decades, fiber optics has become increasingly popular in both commercial and military environments and although optoelectronics is based on a comparatively simple technology, it is nevertheless essential for today's engineers to be aware of the basic fundamentals and capabilities of this modern technique. In order to understand new electronic engineering principles, however, it is usually necessary for the reader to have a thorough technical foundation and for the author to derive and explain mathematical theories and progressively develop formulae, etc. to enable the reader to continue his studies with a complete understanding of the fundamentals surrounding this new technology. Although an understanding of the mathematical principles applicable to optoelectronics is of course necessary, it is the aim of this book to provide only a basic introduction to the basics of optoelectronics. A book that introduces the topic, and then leaves it up to others to provide a more in-depth explanation. A book that is easy to read, hard to put down(!) and (bearing in mind the saying that a picture is worth a thousand words) has lots of diagrams, pictures and graphics to aid understanding. As well as being a background reference manual for practising electronic and telecommunications engineers, technicians and students, it will also serve as a user-friendly overview of this increasingly popular subject, especially to those who would like to get to grips with this new area of communications technology.

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Preface For over a century, telegraph and telephone systems have been interconnected via copper cables and radio links and these have formed the basis of all suburban, urban and national telecommunication networks. But although the possibility of using a dielectric rod as a waveguide had been studied during the late 1800s and a paper had actually been written about it in the 1920s, it was not until 1966 that Charles Kao and George Hockham patented the principle of 'information transmission through a transparent dielectric medium' (i.e. glass fiber), that the use of glass fibers actually became a viable proposition. Since then, the development of optical fibers and their enormous bandwidth capability has meant that more and more communication systems are now using fibers to pass optical signals over long distances with very little l o s s - especially when compared with the losses experienced by electrical signals over copper lines. Without doubt this technology, following the development of laser diodes, has now become the preferred method for cable communication. However, in the very early days of fiber it was not evident just how dramatic optoelectronic and fiber optic technology would become, or to what extent it would impact on national and global economies. Indeed, a single glass fiber the thickness of a h u m a n hair can now carry the equivalent of 300 million simultaneous telephone c a l l s - roughly all the phone calls going on in America at any one time. But even though computer capacity is anticipated to increase by 1000-fold in the next decade (and 1 000 000-fold in the decade that follows) it is highly unlikely that an optoelectronic system would be incapable of satisfying these new requirements. In less than 20 years optical fiber has revolutionized the global telecommunications network. With the aim of providing a user-friendly background to this increasingly important technology, this book is structured as follows: Chapter 1 - a description of the history surrounding fiber optics and an introduction to some of the people who were involved in its discovery, use and development. Chapter 2 - an overview of the theory behind optoelectronics. Chapter 3 - a description of the different types of fiber that are available (e.g. single mode, multimode, step index, graded index, glass or plastic),

xvi

Preface

the manufacturing processes and techniques, cable characteristics and specifications and a look at some of the installation possibilities. Chapter 4 - a description of the optoelectronic transmitter (e.g. LED and laser) used to convert electrical signals into optical energy so that they can be propagated down an optical waveguide. Chapter 5 - a look at the other end of the fiber to see how light energy is received and converted back into an electrical signal by an optoelectronic receiver. Chapter 6 - a look at the method used to connect fibers to transceivers and how fibers can be joined a n d / o r spliced together. Chapter 7 - an overview of how fiber optics are utilized in local, long distance and global telecommunication networks. How an optical fiber cable system is designed and the installation techniques used. Chapter 8 - a look at installation and in-service testing techniques and some of the test equipment that is currently available for installers and system engineers. Chapter 9 - a look into the near and future possibilities associated with optoelectronic and fiber optic technology. New and perceived developments are described and an indication of how government, military and industry are increasingly making use of fiber optics is discussed. These chapters are supported by appendices that list some of the European and International standards currently available and concludes with a list of all the main acronyms and abbreviations associated with the optoelectronic and fiber optic technology, together with a detailed glossary of terms and a full index.

Acknowledgements Whilst every effort has been made to contact the copyright holders of the pictures, diagrams and drawings used to illustrate this book, it has not proved possible to make contact in all cases. If we have used your images in this book without permission, then we apologize and would request that you please contact us so that we can ensure that you are given due credit in the next reprint.

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The history of fiber optics

1.1 Background For over a century, telegraph and telephone systems have been interconnected via copper cables and radio links and these have formed the basis of all suburban, urban and national telecommunication networks. Although line transmission technology has improved over the years, it was not until the last few decades when revolutionary innovations caused by the development of semiconductors, digitalization and optoelectronics dramatically altered the method, technology and cost effectiveness of communication systems. Of course, the transmission of information by visual means is not an entirely new concept, as man has been sending messages optically for centuries and I suspect that the first form of optical communication was the caveman shaking his fist (an action which left no doubt as to its meaning!). This effective communication used the sun as a light source, the hand as the modulator and the eye as the detector. Optical communication then progressed to flags and smoke signals, which enabled messages to be passed over greater distances.

2 Optoelectronics and Fiber Optic Technology

From a military perspective, all sorts of signalling techniques have been used by armies in the field. The Greeks had the torch telegraph, the Phoenicians water telegraphs, and the Roman army used coloured smoke as a means of communication. The Greeks and Phoenicians also used reflected light from mirrors to communicate by code between watchtowers. Ship captains used both mirrors and lanterns to communicate with their fleets and army generals frequently used shields to signal troop movements. In the sixth century BC, legend has it that Aeschylus (Figure 1.1) (the 'Father of Tragedy', who apparently met his death when an eagle mistook his bald head for a rock and dropped a tortoise on it!) used a long chain of bonfires from Asia Minor to Argos to pass on the news of Troy's downfall.

Figure 1.1 Aeschylus (525-456 BC) In the second century BC, Polybius (a Greek historian, born in Megalopolis) developed a two digit, five level code by which the whole of the Greek alphabet could be transmitted. It is perhaps interesting to note that it is this last method that is generally recognized as being the first example of optoelectronic signalling. The Chinese, when threatened by the Tartars, lit signal fires along the boarders of their empire, increasing brightness by artificial means to penetrate fog and rain. Genghis Khan (1115-1227) (Figure 1.2) controlled and checked his horsemen by flag telegraphy. During the next few decades, a number of experiments were carried out utilizing various other visual methods. For example, in 1633 the Marquis of Worcester developed long distance communication systems using black and white signs, and in the papers of John Norris of Hughenden Manor in Buckinghamshire there is an enigmatic comment: '3 June 1778. Did this day heliograph intelligence from Dr Franklin in

The history of fiber optics 3

Figure 1.2 Genghis Khan (1167-1227) Paris to Wycombe.' In 1781, Norris also built a 30m tower on a hill at Camberley, Surrey, from the top of which he used to signal and place bets by heliograph with Lord le Despencer at West Wycombe. Turning the pages of history once again, it was in 1795 that the Reverend Lord George Murray (the fourth son of the Duke of Athol) was commissioned to supervise the installation of a chain of shutter stations stretching from Plymouth to London (Figure 1.3).

Figure 1.3 Admiralty shutter telegraph relay station sites ( i courtesy of the Royal Corps of Signals Museum, Blandford)

4 Optoelectronics and Fiber Optic Technology

Figure 1.4 The Admiralty shutter telegraph system (artist's impression of a relay station, J. courtesy of the Royal Corps of Signals Museum, Blandford)

These stations formed the Admiralty shutter telegraph system (Figure 1.4) and consisted of a wood frame 20 feet high by 12 feet wide, containing 3 foot square shutters arranged two by two vertically, which pivoted about their horizontal axis and were operated from the building beneath by ropes and pulleys. By careful positioning of the shutters, 63 changes were possible: sufficient to cater for the whole of the alphabet, the ten numerals, as well as a number of selected phrases - for example: 9 all shutters open - station not at work; 9 all shutters shut - station about to work; next station standby to answer. The stations were normally manned by three men (one to operate the shutters, the other two to act as telescope observers or 'glassmen', see Figure 1.5) and it is recorded that an average message passed from London

The history of fiber optics 5

Figure 1.5 Model of the inside of the Blandford Camp shutter telegraph station ( i courtesy of the Royal Corps of Signals, Blandford)

to Portsmouth in 15 minutes whereas a prearranged signal could be sent between London and Plymouth and acknowledged back in three minutes. Across the channel and a few years before the Admiralty scheme (i.e. 1792), Claude Chapp6 (a French engineer and cleric who converted an old idea into a reality by inventing the 'semaphore visual telegraph') had succeeded in passing a message between Paris and Strasburg (a distance of over 400 km) using movable signal elements. Chapp6's device (the T-type t e l e g r a p h - Figure 1.6) consisted of two side wings mounted at right angles to a main wing that was positioned at the top of a mast. Changing the position of the main wing with respect to the mast and varying the attitude of the side wings to the main wing by 45 degree steps could give 196 different and easily distinguishable signals. These signals were observed from nearby stations using telescopes and then passed on to the next relay station in a similar manner (Figure 1.7). It was far more efficient than hand-carried messages, but by the m i d nineteenth century it had been replaced by the electric telegraph and all that remains today are a few 'telegraph hills' scattered about. But Chapp6's semaphore system was not the only one in existence. Similar systems were being used in Denmark, Norway, Finland and Holland (Curacao) as shown in Figure 1.8.

6 Optoelectronicsand Fiber Optic Technology

Figure 1.6 Chappe's semaphore visual telegraph (courtesy of Telemuseum.se)

Figure 1.7 Telegraph post at Conde in November 1794 (courtesy of Telemuseum.se)

The history of fiber optics 7

One of the systems used in Denmark between 1794 and 1862.

The Norwegian signalling system used on the of the mountain tops Lyderhorn and Ilsfjellet.

Dutch optical telegraph system on the island of Curacao in the early/ mid-nineteenth century.

Figure 1.8 Examples of some of the other earlier opto-signalling devices (courtesy of Telemuseum.se)

Turn the pages of history once more and it is Napoleon (1760-1821) who, having quickly recognized the importance of optical signalling, established telegraph stations in the countries in which he was campaigning and equipped his armies with optical telegraph equipment (telegraph ambutantes), similar to Col. John Macdonald's anthropological telegraph illustrated in Figure 1.9. In Prussia, General Freiherr von Muffling carried out a number of trials during the period 1819 to 1830 utilizing optical telegraph systems. The success of these studies led to the construction of the first telegraph line between Berlin and Potsdam. During this time various heliograph

8 Optoelectronicsand Fiber Optic Technology

Figure 1.9 Napoleon I (Emperor of France 1769-1821) and CoL John Macdonald's anthropological telegraph ( ,~ courtesy of the Royal Corps of Signals Museum, Blandford)

Figure 1.10 Typical heliograph

The history of fiber optics 9

(Figure 1.10) and signal lamp systems were also developed enabling signalling over distances of around 100 kilometres. The heliograph was primarily used by armies for signalling to a distant point. It comprised a wooden tripod and a mirror assembly, the back of which was covered by a brass plate with a brass stump and pivot that connected to the Morse key via a brass tube. By adjusting the knurled tube and a Morse key the signaller could signal in Morse, by sunlight, to his distant observer. By the 1850s, heliograph systems were widely used by the military (Figure 1.11).

Figure 1.11 Heliograph operators in India (..,~..courtesy of the Royal Corps of Signals, Blandford) The size of the heliograph mirror depended on its intended use. The largest (which was primarily used by headquarters and garrisons) had a mirror 12 inches in diameter, but other standard sizes included the 5-inch mirror for infantry units and the lighter 3-inch instruments for cavalry units. The range of operation depended upon the strength of the sunlight and visibility. Under normal conditions, ranges of 20-30 miles with a 3-inch mirror, 30-40 miles with a 5-inch mirror and 80-90 miles with 12-inch mirror could be expected, but these ranges were frequently exceeded in practice.

10 Optoelectronics and Fiber Optic Technology

Heliographs had one unfortunate problem, however, they could only be used on cloudless days! This problem was largely overcome by inventors such as Henry Coxwell who developed balloon signalling. Figure 1.12 represents Coxwell's method of adapting semaphore arms of various shapes and symbols to convey, according to a preconcerted arrangement, any required information to those on the ground.

Figure 1.12 Henry Coxwell's balloon signalling (courtesy of London Illustrated News, 8 October 1879)

In the 1840s, a Swiss physicist (Daniel Collodon) and a French physicist (Jacques Babinet) showed that light could be guided along jets of water for fountain displays. Then in 1850, John Tyndall (Figure 1.13), a schoolteacher, noted that light could be guided inside a clear material, and by using water that flowed from one container to another and a beam of light, he demonstrated that light used internal reflection to follow a specific path. The principle was very simple, as the water flowed from the first container, Tyndall directed a beam of light at the path of water and the light followed a zigzag path inside the curved path of water. This simple experiment marked the first research into the guided transmission of light. But it was not until the 1880s that Alexander Graham Bell (in conjunction with Sununer Tainter) patented an optical telephone system, which he called the photophone, and successfully modulated a beam of sunlight using a diaphragm mirror. This is generally recognized as the

The history of fiber optics 11

Figure 1.13 John Tyndall, British scientist (1830-1893) first mobile phone! The action of the photophone is very simple (Figure 1.14a). Rays of sunlight (or a powerful light source) are reflected by a flat mirror (h) into a system of lenses (a and b) that form a beam of light on to a silver-coated glass plate (H). A person speaking into the mouthpiece of the transmitter (see Figure 1.14b) causes the silver-coated plate to vibrate which in turn causes the reflecting surface of the plate to change shape. This affects the strength of the light rays, which are compressed by a second set of lenses (c), and then reflected to the receiver. At the distinct end (see Figure 1.14c), the light rays are received by a silver-coated copper parabolic reflecting mirror (P). By using a sensitive selenium-photoresistor (S) (comprising a cylinder of copper plates, separated by strips of mica and filled with s e l e n i u m - whose resistance varies according to the quantity of light that strikes it), connected to a battery and a telephone instrument (t), the system is able to reproduce the speaker's voice. On a good day, when the weather was sunny, the photophone had an effective range of over 200 m, but it did not work at night or when the weather was cloudy. The transmission distance was increased to 2 km when Bell incorporated the use of an electric globe following Edison's invention of the light bulb, but the user still had to carry a fairly cumbersome piece of equipment around, with two such devices being needed for two-way communication. Alexander Graham Bell's earlier invention, the telephone, proved to be a far more practical proposition, therefore, and although he dreamed of sending signals through the air, he found that the atmosphere didn't transmit light nearly as reliably as wires could carry electricity.

12 Optoelectronics and Fiber Optic Technology

Key h

Flat mirror

Very thin mirror

b

Lens

Parabolic reflector

Allun cell lens to cut off heat rays

Selenium photoresistor

c

Compression lens

Figure 1.14

t

Alexander Graham Bell's photophone

Telephone earpiece

The history of fiber optics 13 Table 1.1

Patent number

Details of the photophone's US patent registration Type Inventor

235,199

U

235,496

U

Appfied

G r a n t e d Claims P a g e s

Bell

08/28/1880

12/07/1880

18

13

Bell

09/25/1880

12/14/1880

3

3

Alexander Graham

Alexander Graham and Tainter, Sununer

Description

Apparatus for Signalling and Communicating, Called 'Photophone' PhotophoneTransmitter

All of these methods were of course affected by the visual transparency of the medium through which they were being propagated (i.e. air), and fog, rain, storms, cloud and snow were some of the problems to be contended with. Another major problem that affected these early lightwave communication systems was the inability of lightwaves to go around corners, and avoid trees, buildings and mountains. The transmission path, therefore, had to be an unobstructed line of sight between transmitter and receiver. In the same period, William Wheeler invented a system of light pipes which were lined with a highly reflective coating, and by using light from an electric arc lamp placed in the cellar, he was able to illuminate almost the whole of the house by directing the light around the home via the pipes. Later on in 1888, the medical team of Roth and Reuss of Vienna used bent glass rods to illuminate body cavities and in 1895, the French engineer Henry Saint-Rene designed a system of bent glass rods for guiding light i m a g e s - in an attempt at early television. In 1898, American David Smith applied for a patent on a bent glass rod device to be used as a surgical lamp. By the turn of the century, however, several inventors had realized that bent quartz rods could carry light, and had patented them as dental illuminators, and many doctors were using illuminated plexiglass tongue depressors. But whilst all this was happening, a new technology was slowly developing that would eventually overcome the problem of optical transmission. This technology depended on a phenomenon called 'total internal reflection', which could confine light in a material that was

14 Optoelectronicsand Fiber Optic Technology

Figure 1.15 John Logie Baird (1888-1946)

surrounded by another material with a lower refractive index, for example glass in air. During the 1920s, the famous English inventor John Logie Baird (Figure 1.15) (remembered as being an inventor of mechanical television - an earlier version of today's television- and inventions related to radar and fiber optics) and an American (called Clarence Hansell) patented the idea of using bundles of hollow pipes or transparent rods to transmit images for television a n d / o r facsimile systems. In Munich during the 1930s a German medical student (Heinrich Lamm), whilst trying to find a way of looking at inaccessible parts of the human body, successfully demonstrated, with the aid of a 'light-stick', that by shining a light bulb filament on to an image, that image could be transmitted through a short bundle of optical fibers. However, the image transmitted by the unclad fibers was very poor and his effort to file a patent was denied because of Baird's previous British patent. Later on (i.e. in 1951), Holger Hansen also applied for a Danish patent for fiber optic imaging but was turned down by the Danish patent office because of existing Baird and Hansell patents. Hansen tried to interest some companies in his invention but without success. Historically, nothing else appears to have happened concerning the use of fiber bundles until 1954, when a Dutch scientist (Abraham van Heel) from the Technical University of Delft produced a paper concerning imaging bundles in the popular British journal, Nature. He had

The history of fiber optics

Cladding layer .... Core

15

~i

1

2 Figure

1.16

Basic fiber optic

proved that instead of using 'bare' fibers with total internal reflection at the glass-air interface, if the bare fiber (glass or plastic) was covered with a transparent cladding of lower refractive index, the reflective surface would be protected from contamination, and 'crosstalk' (i.e. the leakage of signal from one fiber to another) was greatly reduced between them. Figure 1.16 above shows the difference of a ray of light entering at a small angle (1) compared with another ray that enters at a different angle (2). The result is that different rays (or modes) of light will reach the end of the fiber at different times, even though the original light source was the same. Van Heel's experiments eventually led to the development of glass-clad fibers with an attenuation of less than one decibel per metre, which, whilst it was all right for medical imaging, was much too high for the telecommunications industry. Indeed, with an exponential growth rate in telephone traffic, television and the perceived use of videotelephony even in those days, telecommunications engineers were already asking for more transmission bandwidth. The use of higher optical frequencies seemed the best possible solution and with the invention of the laser (light amplification by stimulated emission~radiation) in 1960 came the announcement in the July 22 issue of Electronics that 'Usable communications channels in the electromagnetic spectrum may be extended by development of an experimental optical-frequency amplifier.' With the emergence of the continuous wave helium-neon laser this announcement quickly became a reality. By 1965, however, there were still technical barriers to the use of lasers, and optical waveguides were also proving to be a problem. This was mainly because, although optical fibers were very similar to the plastic dielectric waveguides that were used in certain microwave applications, the core of the drawn fibers was so small that they carried light in only one waveguide mode. Another problem was that most of the telecommunications industry had already discarded the use of fibers as being too lossy! So whilst attenuation of a decibel per metre was all

16 Optoelectronicsand Fiber Optic Technology

Figure 1.17

Charles Kao

right for medical purposes (e.g. for looking inside the body), telecommunications, which had a requirement for operating over long distances, could only accept a maximum loss of 10 or 20 decibels per kilometre. Help was around the corner, however, as a team from Standard Telecommunications Laboratories (STL) headed by a young Shanghai engineer called Charles K. Kao (Figure 1.17) and another young STL engineer who specialized in antenna theory, called George Hockham, were trying to find a way around fiber attenuation. With samples collected from a whole host of fiber manufacturers, they gradually built up a database of their (i.e. fiber's) properties, which enabled them to reach the conclusion that most of the high losses in early fibers were due to impurities and not to the silica glass itself! They were convinced that with careful manufacturing, these fiber losses could be reduced below 20 decibels per kilometre. Together Kao and Hockham presented a paper at a London meeting of the Institution of Electrical Engineers containing a proposal for longdistance communications over single-mode fibers. This was reported in the 1 April, 1966 issue of Laser Focus as follows: Dr C. K. Kao observed that short-distance runs have shown that the experimental optical waveguide developed by Standard Telecommunications Laboratories (STL) has an information-carrying capacity of one gigacycle, or equivalent to about 200 TV channels or more than 200 000 telephone channels. He described STUs device as

The history of fiber optics 17

consisting of a glass core about three or four microns in diameter, clad with a coaxial layer of another glass having a refractive index about one percent smaller than that of the core. Total diameter of the waveguide is between 300 and 400 microns. Surface optical waves are propagated along the interface between the two types of glass. According to Dr Kao, the fiber is relatively strong and can be easily supported. Also, the guidance surface is protected from external influences and the waveguide has a mechanical bending radius low enough to make the fiber almost completely flexible. Despite the fact that the best readily available low-loss material has a loss of about 1000 dB/km, STL believes that materials having losses of only tens of decibels per kilometre will eventually be developed. Based on Kao and Hockham's presentation, the British Post Office set up a new research fund of s million pounds to study ways to decrease fiber loss and in a similar manner, many institutes and laboratories from around the world were also actively investigating ways of trying to reduce fiber loss. Most of this research centred on an attempt to purify the glass used for optical fibers but at the Corning Glass Works, three researchers (Robert Maurer, Donald Keck and Peter Schultz, Figure 1.18) were experimenting

Figure 1.18 The original three Coming researchers with fused silica, a material that could be made extremely pure, but which had a high melting point and a low refractive index. By carefully adding controlled levels of dopants, the Corning researchers managed to increase the refractive index of the core slightly above that of the cladding, but without raising the attenuation dramatically. The Corning researchers' experiments came to fruition in September 1970 when they announced that they had made a single-mode fiber with

18 Optoelectronics and Fiber Optic Technology

an attenuation of below 20 d B / k m in a 633-nanometer h e l i u m - n e o n line. This breakthrough is generally recognized as the start of fiber-optic communications and opened the door to the commercialization of fiber optics, first for long-distance telephone service and later for computerrelated telecommunications (such as the Internet) and even medical devices (like the modern endoscope). This fiber optic wire or 'optical waveguide fiber' was patented by Corning (patent number 3,711,262) and was capable of carrying 65000 times more information than copper wire, via a pattern of light waves which could be decoded at a destination even a thousand miles away. The team had solved the problems presented by Dr Kao and it is now an accepted fact that fiber optic communications is one of the 10 most outstanding engineering achievements of the last 25 years. It is extremely doubtful (certainly not in my lifetime!) whether a superior method for long-distance information transfer will be discovered in the near future. In the same year that Corning patented the optical waveguide fibre (i.e. 1970), Bell Labs in America and a team from the Loffe Physical Institute in St Petersburg manufactured semiconductor diode lasers capable of emitting a continuous wave at room temperature. With the emergence of better manufacturing practices and the use of longer wavelengths, fibers were gradually being produced with even lower attenuation and in 1976 telephone field trials were conducted by AT&T using these types of fibers to transmit light (at 850 nanometers) with the aid of gallium aluminium arsenide laser diodes, for several k i l o m e t r e s - without repeaters. By then losses of less than 2 d B / k m were the accepted norm, but this was rapidly reduced to about 0 . 5 d B / k m when the new lnGaAsP lasers working in the 1300 nanometer range were introduced. So, although the possibility of using a dielectric rod as a waveguide had been studied during the late 1800s and a paper had actually been written about it in the 1920s, it was not until 1966 that Charles Kao and George Hockham patented the principle of 'information transmission through a transparent dielectric m e d i u m ' (i.e. glass fiber), that the use of glass fibers actually became a viable proposition. Since the development of optical fibers, more and more communication systems are now using fibers to pass optical signals over long distances with very little loss (compared to the losses experienced by electrical signals over copper lines). Following the development of laser diodes, the use of optoelectronics has now become the preferred method for cable communication. Indeed, a single glass fiber the thickness of a h u m a n hair (see Figure 1.19) can carry the equivalent of 300 million simultaneous telephone calls - roughly all the phone calls going on in America at any one time.

The history of fiber optics

Figure 1.19

19

Fiber size (courtesy of Corning Cable Systems GmbH)

1.2 Basic principles of optical line transmission Some crystals, for example gallium arsenide (GaAs), emit light at a wavelength very close to visible light when an electric current is passed through them. Figure 1.20 shows the frequency and wavelength of rays and light making up the electromagnetic spectrum. As can be seen from Figure 1.20, visible light (with wavelengths in the 400 nm to 750 nm range) is only a small part of the light spectrum. Above this is the near infrared region (i.e. 750 nm to 1600 nm) which fiber optic transmission u s e s - primarily because the fiber propagates the light much more efficiently at these wavelengths. This light emission is produced by the release of energy (photons) from the atoms of a material and is caused when the atoms are excited by heat, chemical reaction or some other means. Today's low-loss glass fiber optic cable can offer almost unlimited bandwidth and has many advantages over previous transmission media. In a fiber optic system, the sound waves are converted to electrical signals, which are coded as a series of pulses fed as a rapid burst of laser light through the fiber optic cable. At the other end they are received, decoded and converted back to sound waves. A lightwave system (Figure 1.21), therefore, is composed of four basic parts: a transmitter, fiber optic cable, regenerator and a receiver. The transmitter is the light source, which can be a light emitting diode (LED) or a laser. This source is modulated (i.e. turned on or off in a digital system) to represent the binary digits (ls and 0s) it receives from an electrical transmission system. The transmitter then converts the electrical analog or digital signal into a corresponding optical signal.

20

Optoelectronicsand Fiber Optic Technology

1 0 22

~- Cosmic rays

1021 - 102o - i0~9

~-- Gamma rays

- -

10~8

1017

~- X-rays

- -

-

Violet

(I THz)

~- Ultraviolet light

- -

1 0 TM -

-

1012 -

109

Yellow Orange

-

Red

101o -(I GHz)

Radar

10 7

(1 MHz) 106 10s

.._..

Television and FM radio

Shortwave radio '--~-- AM radio

Radio frequencies

550 580 620 750

Infrared

- -

10 8 - -

490

Green

<-- Infrared light

400 455

Blue

z.~._ Visible light

~3

1011

Ultraviolet

-

10~8

1015

10

Wavelength (nm)

800 850

10 4

(1 kHz) 103 -102 -I0 0

~- Sound

Wavelength =

Speed of light Frequency

~- Subsonic

Figure 1.20 Electromagnetic spectrum, showing wavelengths and frequencies of common signals (courtesy of Cable Talk Pty Ltd)

Signal input I

~'

Figure 1.21

Fiber I optic Transmitter t.~ (light source)

, Regenerator

Fiber = optic ~ , ~

I Receiver (photodetector)

Signal output

Basic tightwave system

Note: Impurity elements in the silica glass will produce high absorption at specific wavelengths, but windows of low attenuation are found around 1300 nm and 1550 nm and it is these that are being exploited by modern infrared optical fiber systems. Above 1700 nm glass starts to absorb light energy due to the molecular resonance of silicon oxide and so the most popular operational wavelengths for optical transmitters are 850, 1300 or 1550 nanometres.

The history of fiber optics 21

The next stage is the fiber optic cable, which acts as a waveguide for the optical signal. The cable can be a single strand of specially manufactured glass or multiple strands bundled together. Although each fiber optic strand will be no thicker than a human hair, the actual cable may measure as much as 189inches in diameter after the strands have been wrapped in protective coverings.

Note: In construction, although the fiber optic cable provides special protection for the optical fiber it contains, it is very similar in construction to an ordinary electrical cable. For systems requiring transmission distances of many kilometres, or where two or more fiber optic cables must be joined together, an optical splice is commonly used.

The third part of the lightwave system is the regenerator. As a photonic signal travels through a fiber optic strand, it attenuates and begins to lose its shape. If it is not regenerated periodically, the signal will not be recognizable at the receiving end. These regenerators can either be optical-electrical-optical devices, usually found in terrestrial systems, or all-optical systems found in undersea lightwave systems. Finally, there is the receiver, which is a photodetector that takes the optical signal from the fiber and converts it into a replica of the original electrical signal for transmission through the non-optical portions of a network. The photodetector is normally either a PIN (positive intrinsic negative) or avalanche-type photodiode. Utilizing this principle it is possible to communicate (for example, by telephone) over an optical link as shown in Figure 1.22. The microphone in the telephone set converts speech (i.e. sound waves) into electrical energy. A multiplexer is used to combine this with other similar information channels to form an information group. This is coded (if required) and amplified prior to being converted into light signals by an optoelectric crystal transducer (usually a light emitting diode or a laser diode) and transmitted over an optical fiber waveguide, which may be several kilometres in length. At the far end of the optical link the light signals are reconverted into electrical energy (by another type of optoelectronic signal transducer called a photodetector): amplified, decoded, split back into their individual speech channels and then converted into sound waves, which are audible to the subscriber, through the use of an electroacoustic transducer (i.e. telephone earpiece).

Figure 1.22 Basic optoelectronic communication system

The history of fiber optics 23

1.3 Advantages of optical fibers and optoelectronic signalling (Figure 1.23)

Figure 1.23 Advantages of fiber optics The main advantages of using optical fibers and optoelectronic signalling are: 9 9 9 9 9 9 9 9 9 9

immunity to electrical and magnetic fields; low a t t e n u a t i o n - longer distance between regenerators; wide transmission b a n d w i d t h - large number of channels; small physical size and weight; increased flexibility; electrical i n s u l a t i o n - removed earth PD (potential difference) problems; immunity to electromagnetic interference and interception- may be used in electrically noisy environments; electrical protection- security enhancement; analog and digital transmission; receiver sensitivity.

24 Optoelectronics and Fiber Optic Technology

1.3.1 Immunity to electrical and magnetic fields As photons have no electrical charge, they are unaffected by electrical storms or electric fields normally found in high voltage environments. They are also immune to endo- and exo-atmospheric pulse effects, and, in addition, high magnetic fields caused by transformers and motors etc. will have no effect on the fiber's optical transmission properties. Fiber optic cables are also virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup. Since the transmission medium used in fiber optics is glass, fiber optic cable can be run through harsh or noisy environments such as elevator shafts or subway tunnels, with no degradation or attenuation in signal quality (Figure 1.24).

Figure 1.24

Fiber optic cables attached to the walls of a railway tunnel (courtesy of QA Photos Ltd)

1.3.2 Low attenuation An electrical signal transmitted over a coaxial cable will, dependent upon its frequency, have lost almost half its power after only a few hundred metres; whereas an optical signal transmitted over an optical fiber (quite frequently referred to as an optical waveguide) will, dependent upon the

The history of fiber optics 25

transmitted wavelength (see Chapter 2), still be perfectly acceptable after 20 kilometres. A point to note w h e n describing attenuation is that it should always be m a d e clear whether one is referring to an electrical decibel m e a s u r e m e n t or an optical measurement. This is because an electrical m e a s u r e m e n t of I dB is equivalent to an optical m e a s u r e m e n t of 2 dB. This consideration is extremely important when comparing cable attenuations as: 20 dB km -1 electrical cable = 10 dB km -1 optical cable! The reason for this is that optical power is directly proportional to electrical current, whereas electrical power is proportional to the square of the current. Summarized: dB[optical] = 10 log(Pout/Pin) = 10 log(Iout/Ii~) whilst: dB[electrical] = 10 log(Pout/Pin) = 20 log(Iout/Iin) In addition: dBm[optical] = 10 log(Pout / 1 mW)

1.3.3 Wide transmission bandwidth The ability to carry much more information and deliver it with greater fidelity than either copper wire or coaxial cable is a very relevant consideration. For example, at the moment, m a x i m u m transmission rates of about 1 Gbit s-1 are possible using conventional coaxial cable, whereas transmission rates in excess of 10Gbits -1 are currently possible using optical waveguides and experiments are ongoing to increase this to 40 Gbit s-1 and perhaps even 80 Gbit s -1 in the near future.

1.3.4 Small physical size and weight Another very important advantage associated with optical fiber cables is their comparatively small physical size and weight compared to coaxial cables. Whereas the diameter of a typical m o d e r n coaxial cable will be approximately 10mm, a fiber rod is only 0.1 to 0 . 2 m m and even with a protective plastic coating around it, this would not be more than 0.25 to 0.5 mm. In addition, the weight of a coaxial cable can vary anything from 350 to 1100 kg km -1 depending upon the type used, but a single fiber, (unless armoured or having very heavy strengthening members), only

26

Optoelectronicsand Fiber Optic Technology

weighs about 1 2 k g k m -1. Previously the physical properties of copper cable have always severely restricted the size of cable that can be pulled through a duct and so these differences are very important w h e n installation techniques are being considered. Another factor worth considering is that typically, a 1000-pair armoured copper cable would normally be manufactured in 400 m lengths and this would necessitate frequent jointing. The use of the far lighter fiber optic cable means that not only can longer lengths (e.g. 2000m) be pulled through a duct without jointing, but transportation costs and problems associated with cable drums etc. are also considerably reduced. The small size and light weight of fiber optic cable allows installers to use existing ducts or cable runs without the need for new conduit.

1.3.5 Increased flexibility When designing telecommunications systems the increased flexibility of an optical fiber cable over a normal copper cable, together with its almost obsolescence-proof versatility, has to be considered. A fiber optic cable, even one that contains many fibers, is usually much smaller and weighs less than a copper wire or coaxial cable with similar information carrying capacity. It is easier to handle and install, and uses less duct space. Indeed, a fiber optic is frequently installed without ducts. In addition to being physically smaller, weighing less and possessing a relatively high tensile strength (approximately equal to twice the kilometric cable weight and comparable, when freshly drawn, to that of any high tensile material, including steel), optical fibers are far more mechanically flexible than normal copper coaxial cables. This is particularly advantageous with regard to transportation and in certain installations where space is at a premium (for example, in an aircraft). Optical fibers can also be incorporated into, or become part of, almost any existing network infrastructure with little or no difficulty.

1.3.6 Electrical insulation As an optical fiber is metal-free, there is no requirement to ground or earth the optical waveguide against, say, lightning strikes and, as no electrical signal is being transmitted, an optical waveguide is capable of bridging large differences in potential and could, for example, act as a control line for a high voltage switching station. As the basic fiber is made of glass, it will not corrode and is unaffected by most chemicals. It can be buried directly in most kinds of soil or

The history of fiber optics 27

exposed to most corrosive atmospheres in chemical plants without significant concern. It can also be installed in potentially hazardous areas such as those incorporating flammable explosive chemicals without the possibility of generating sparks.

1.3.7 Immunity to electromagnetic interference and interception As light waves carried inside a fiber do not set up any form of electromagnetic field, security of transmission is guaranteed and it is therefore virtually impossible to externally intercept a light signal. Tapping a fiber can only be achieved by isolating the bare fiber from the cable and then completing an optical tap. Isolating the fiber and inserting a tap would naturally reduce the power output of the optical waveguide which in turn could be easily detected with the use of an optical time domain reflectometer (OTDR) or similar instrument (see Chapter 8). The light wave is also unaffected by external electromagnetic interference and so the possibility of electromagnetic crosstalk is eliminated. If optical leakage does occur, then it is retained in the plastic coating or cladding that surrounds the fiber, thus ensuring that optical crosstalk between fibers cannot occur.

1.3.8 Electrical protection Because no electromagnetic field is present within a fiber rod, electrical protection (or shielding) is not required. The fiber is totally immune to virtually all kinds of interference, including lightning, and will not conduct electricity. It can, therefore, come in direct contact with high voltage electrical equipment and power lines. It will also not create ground loops of any kind. Fiber optic cables are virtually unaffected by outdoor atmospheric conditions, allowing them to be lashed directly to telephone poles or existing electrical cables without concern for extraneous signal pickup (Figure 1.25). On the other side of the coin, as the only carrier in the fiber is light, there is no possibility of a spark from a broken fiber. Even in the most explosive of atmospheres, there is no fire hazard, and no danger of electrical shock to personnel repairing broken fibers.

1.3.9 Analog and digital transmission Both analog transmission (the transmission of continuously varying signals either by direct modulation of the optical power or by using an electrical subcarrier to modulate the optical power) and digital transmis-

28

Optoelectronicsand Fiber Optic Technology

Figure 1.25

Fiber optic cables carried on overhead power lines

sion (the transmission of a discrete signal, obtained by the digitization of an analog signal using binary light pulses in a periodic time pattern) are possible with the use of optical fibers. As the receiver requires a high signal-to-noise ratio for analog transmission and because distortion of the optoelectronic system is non-linear, digital transmission is a far more viable proposition. Fiber optic cable can support much higher data rates, and at greater distances, than coaxial cable, making it ideal for transmission of serial digital data. As a consequence of these limitations, analog transmission is normally restricted to lower bandwidths and smaller distances.

1.3.10 Receiver sensitivity A further advantage of optoelectronic signalling is given by the inherent stabilization of optical transmitters and the simulation in single-mode systems of modulation and demodulation processes (i.e. heterodyning) whose effects are already well known from other electrical systems. Theoretical calculations have indicated that an increase of 10 to 15dB in receiver sensitivity can be expected from the use of optoelectronic systems, compared with more conventional systems.

1.4 Disadvantages of optical fibers and optoelectronic signalling (Figure 1.26) Unfortunately, there are also disadvantages associated with optical fibers and optoelectronic systems. These include:

The history of fiber optics 29

Figure 1.26

9 9 9 9

Fiber optic losses

cost; jointing and test procedures; short links; fiber losses.

1.4.1 Cost

Although the raw material used for producing optical waveguides (quartz sand) is inexpensive and available in unlimited quantities, the cost of obtaining a chemically pure glass rod (typically 1 foreign atom in 109 silicon atoms) is particularly high because of the sophisticated manufacturing equipment and processes required. 1.4.2 Jointing and test procedures

Owing to the small size of the fiber compound to other cables, microjoining procedures have had to be developed, together with their associated test practices (see following chapters). These are costly in manpower, training and equipment. 1.4.3 Tensile stress

Less well known is the fact that optical fibers are more susceptible to buckling, bending and tensile stress than copper cables. This can lead to

30

Optoelectronicsand Fiber Optic Technology

problems, particularly where the cables are retrofitted in existing buildings. It thus remains common practice to restrict optical fiber technology to premises and floor backbones with a few interfaces to the copper cabling. 1.4.4 Short links

Even supposing optical fiber cable was inexpensive; it would still not be cost effective to replace every small conventional connector (for example, between computers and peripherals) with an optical fiber link, as the price of optoelectronic transducers would be prohibitive. Currently, therefore, optical fiber links are used far more for long multi-transmission paths or high density local area networks. 1.4.5 Fiber losses

Attenuation coefficients are a function of the optical wavelength, and so the amount of optical power available to the photodetector at the end of a fiber length depends primarily upon the attenuation coefficient of the fiber. Most of the energy losses in optical fibers occur because of light scattering but this particle scattering (i.e. Rayleigh scattering, Figure 1.27) is very small compared with the wavelength of the radiation being considered and is due to microscopic imperfections in the glass.

Figure 1.27

The phenomena of Rayleigh scattering

A feature of Rayleigh scattering is that the scattered flux is inversely proportional to the fourth power of the wavelength. The degree of scattering increases as the light wavelength approaches the size of the i m p e r f e c t i o n s - thereby giving greater attenuation at shorter wavelengths.

The history of fiber optics 31

Note: The blue colour of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This Rayleigh scattering is more effective at short wavelengths. Therefore, the light scattered down to the earth at a large angle with respect to the

direction of the sun's light is predominantly in the blue end of the spectrum.

Dispersion is another cause of energy loss and is brought about by light rays with different critical angles causing rays to have different path lengths. This can be overcome by using a fiber cladding (see Chapter 2). Light propagation in a fiber is modal and similar to waveguide propagation. In practice, however, cable diameters are very much larger than the optical wavelength, and therefore produce multimode propagation, with each mode having its own velocity. This leads to a dispersion or spread of energy in time as the light travels down the fiber thus limiting the maximum signalling speed over a given length of cable. Because of fiber attenuation and material dispersion, optoelectronics (at the moment) is mainly concentrated in the near infrared or short wavelength (780 to 900nm: typically 850nm) and the long wavelength region (1200nm to 1600nm: typically 1300nm and 1550nm). Rayleigh scattering largely controls the short wavelength losses, while absorption of the glass materials controls long wavelength losses. Minimum loss is the region of about 1300 nm which is also the wavelength at which firstorder material dispersion goes through zero (see Chapter 2). Although designers would prefer to use the long wavelength region because their low fiber attenuation and comparatively low material dispersion permit higher bandwidths, the cost of doing so usually limits their choice.

1.4.6 Other losses Other than opaque obstructions on the surface of a fiber, there is always a loss due to reflection from the entrance and exit surface of any fiber. This loss is called the Fresnel loss and is equal to about 4 per cent for each transition between air and glass. There are special coupling gels that can be applied between glass surfaces to reduce this loss when necessary.

1.5 Practical applications of optoelectronics Practical applications of optoelectronics include telecommunications, data communications, video, control and protection switching, sensors

32 Optoelectronicsand Fiber Optic Technology and p o w e r applications (for example, traffic lights and pedestrian crossing lights).

Note: Using optoelectronics in traffic and pedestrian lights meant that the reflector behind the bulb could be dispensed with. This helped prevent a common cause of road accidents - the sun's reflection on the bulb giving the appearance that the bulb was lit when it was not.

Fiber optics can also be used in h a z a r d o u s environments such as oil refineries where, for example, fiber optics could control the p u m p s s u p p l y i n g oil to the tanker. Cameras positioned by a m o t o r w a y are not always intended to measure speed, some are to monitor traffic flow. The old system was to put wires across the road but n o w cameras are linked to a concentrator, with a fiber running from each camera. Approximately 30 cameras can feed into one single-mode fiber, allowing the personnel watching the monitors to choose which camera they want to look at. This system means that personnel can not only alert the emergency services to a road accident but they can also pass on vital information to the emergency services about the accident.

Figure 1.28 Fiber optic components in communication networks (courtesy of Corning Cable Systems GMBH)

The history of fiber optics 33

1.5.1 Telephone networks Because of the low attenuation and high transmission bandwidth of an optical waveguide compared with copper lines normally used in telephone networks, increasing numbers of long-haul coaxial trunk links between telephone exchanges are being replaced by optical fiber links (Figure 1.28). 1.5.2 Urban broadband service networks A normal coaxial cable television network using repeaters every 400 to 500 metres is only capable of transmitting 20 to 30 separate television channels within a total bandwidth of about 300 MHz. By using optical waveguides, not only can the bandwidth be increased, but also the number of repeaters required is considerably reduced. This is particularly important in modern suburban communications where many information systems (for example, videotext, videoconferencing, videotelephony, switched broadband communication networks (for extension of banking and shopping facilities as well as cable radio and television) can be supplied over a single fiber optic link.

1.6 The future of optoelectronics (Figure 1.29) Since the 1980s, the technology associated with optical line communication systems has advanced enormously. This is particularly relevant with regard to system networks and the integration of components. 1.6.1 All-optical network Without doubt, fiber optics is definitely the way of the future and the unprecedented growth in business and ('experts predict) consumer demand for high speed fiber optic links and voice-over-Internet protocol will continue. An 'all-optical network' is envisioned within the next few years and optical transmission lines and optical switches (switching operations triggered directly by light pulses) are now being used as replacements for electromagnetic and electronic switches. 1.6.2 Computers Fiber optics is increasingly becoming part of the computer age, is fast and economical and there is no reason to suppose that it will not eventually replace virtually all of the copper lines.

34

Optoelectronics and Fiber Optic Technology

Figure 1.29 Near future and future optoelectronic applications 1.6.3 Transmission speed A new fiber optic line is currently on the drawing board called 'fiber-2', which it is mooted will transmit far longer distances with even more speed.

1.6.4 Integration Although integration is mainly concentrated on monomode optical waveguides and the integration of a number of optical and electro-optical elements on to a simple substrate via a thin film planer, the advancement of this technology has provided an enormous reduction in the cost of producing optoelectronic converters and repeaters etc. As well as lower unit costs, however, there are many other advantages to integration: including higher packing density and increased reliability. But probably the greatest impact from integration has been with respect to modulation and switching devices, which are not only smaller, but also now have considerably reduced power consumption.

The history of fiber optics 35

Fiber optic cable is the solution to many of today's high speed, high bandwidth data communication problems and will continue to play a large role in future telecom and datacom networks.

2

Theory

It was around 300 BC that Euclid (a Greek mathematician who lived in Alexandria, Egypt) noticed that a pond always appeared shallower than it really was and he observed that light passing from one medium (such as air) into a denser medium (such as water or glass) always seemed to be bent (or refracted) by that denser medium.

Note: After receiving his education (probably at Plato's Academy in Athens) Euclid became a teacher and scholar at the school in Alexandria known as the Museum. While there, he wrote his most influential work, Elements. In the 13 volumes of this comprehensive treatise, Euclid compiled and systematically arranged mathematical results known in his day (for example, a list of definitions, postulates and axioms). He proved one proposition after another, basing each proof only on those results that had preceded it. This axiomatic method (as it is known today) served as the standard for scientific argumentation and is the core of today's mathematics.

In the second century Claudius Ptolemy, the Greek astronomer, measured the difference between the angle at which light was directed at a transparent medium (i.e. the angle of incidence) and the corresponding angle that the light was refracted by that medium (i.e. the angle of refraction), shown in Figure 2.2.

Theory 37 Note: Ptolemy was one of the most influential Greek astronomers, geographers and mathematicians of his time. From his studies and observations of the heavens, he formed his geocentric theory (i.e. Earthcentred) known as the Ptolemaic system. The theory assumed that the Earth is at the centre of the system followed (outward) by the moon, Mercury, Venus, Sun, Mars, Jupiter and Saturn. Everyone accepted this system until 1543 when Nicolaus Copernicus discovered the heliocentric (Sun-centred) system. Of Ptolemy's many written works, the Almagest (divided into 13 sections, each section dealing with a different astronomical concept) is the most famous and is still in use today.

Ptolemy later produced some tables of his findings and from these results he deduced that when a beam of light is directed at a transparent material such as glass, although some of the light is reflected back to the originating medium (for instance, air) a high proportion of that light would pass into that material. Ptolemy also termed reflectance as being the ratio of reflected power to incident power. Nowadays it has been recognized that as light passes from one medium to another (for example, air and a glass prism) it will change speed, and this will cause a deflection of light called refraction. In a vacuum, light will travel at approximately 300000 k m / s (but is slower in other media) and as shown in Figure 2.1, different wavelengths of light will travel at different speeds in the same medium, but be refracted at a different rate. Because each wavelength (or to put it another way, the different colours present in white light) will change speed at a different rate, each wavelength is refracted differently and produces a spectrum as shown in Figure 2.1.

Note: One everyday effect of refraction is that objects seen under water always appear to be at a shallower depth than they really are. This is because the observer sees an underwater object in a higher position, as the eye cannot tell that the light has been refracted on its path from the object.

38

Optoelectronics and Fiber Optic Technology

111111111111111111, :;;;2:Z:::Z

Red Orange Yellow Green Blue Violet

Figure 2.1

medium

The speed of different wavelengths of light through the same

2.1 Reflection and refraction When light travelling in a transparent material meets the surface of another transparent material, two things happen: 9 some of the light will be reflected; and 9 some of the light will be transmitted into the second transparent material.

Incident r a y ~

Angle of incidence "~

Reflected .... -- ~" ray ..

,...,..

s

s

n9 (air) n 2 (glass)

Angle of refraction

J Refracted ray

Figure 2.2

Showing how light is refracted when passing from one medium into a denser medium

Theory 39

The transmitted light usually changes direction w h e n it enters the second material. This bending of light is called refraction and d e p e n d s on the fact that whilst light travels at one speed in one material it will travel at a different speed in a different material. As a result, each material has its o w n refractive index and this is used to calculate the a m o u n t of bending that takes place.

2.2 Refractive index The optical density of a material is referred to as its refractive index and is a direct proportion between the velocity of light in a v a c u u m to the velocity of light in the material:

Refractive index -

Velocity of light in a v a c u u m Velocity of light in a m e d i u m

= n

As shown in Table 2.1, the refractive index of glass varies between the limits of pure crown glass and pure flint glass with a typical value of 1.6. Table 2.1

Refractive indices of various types of glass

Vacuum

1.00

Glass, flint, dense

1.66

Glass Glass, albite Glass, crown Glass, crown, zinc

1.51714 1.4890 1.520 1.517

Glass, Glass, Glass, Glass, Glass,

1.89 1.65548 1.80 1.58038 1.62725

flint, flint, flint, flint, flint,

heaviest heavy lanthanum light medium

The refractive index of a material affects light d e p e n d e n t on whether: 9 light goes from a material with a low refractive index to one with a high refractive index; or 9 light goes from a material with a high refractive index to one with a low refractive index. These two cases are s h o w n diagrammatically in Figure 2.3"

40

Optoelectronics and Fiber Optic Technology

/ / / / /

w*thalow \ e I e ,," refractiveIndex n, ~ //

/ //

//

/ /

Material with a high r e f r a c ~

Material with a high refractive index n=

Figure 2.3

Material witha low refractiveIndex nl ~

\

.~_~.~.~,, ~) I e "~x,\\ /

i

\ \\

~\

The effect on light through different materials

2.3 Total internal reflection Total internal reflection (TIR) (Figure 2.4) happens when light travelling in one material contacts a different material and the light is reflected back into the original material. In a glass tube, as the core and outer edges consist of different compositions of glass, light entering the core is confined to the boundaries of the core (because it reflects down the core), so long as the critical angle (see section 2.5) is not exceeded.

Figure 2.4

Total internal reflection

Theory 41

The two conditions necessary for TIR to occur are: 1 the refractive index of the first m e d i u m must be greater than the refractive index of the second one; 2 the angle of incidence must be greater than (or equal to) the critical angle.

2.3.1 Snell's law In 1621, Willebrord Snell (1591-1626) put forward a paper, which although never actually published concerned the relationship between the angle of incidence and the angle of refraction of a ray of light crossing the b o u n d a r y between two media. This paper postulated that the ratio of the sines of the angles of incidence and refraction of a ray shall always be constant. The refracted ray will always lie in the same plane as the incident ray and the sine of the angle of refraction (~) is dependent u p o n the sine of the angle of incidence (0) such that: sin 0

n2 m

sin 9

= a constant

nI

where n 1 and n 2 are the indices of refraction of the two materials through which the light is passing. There are two laws of refraction: 1 The incident ray, the refracted ray, and the normal all lie in the same plane. 2 For light rays passing from one transparent m e d i u m to another, the sine of the angle of incidence (0) and the sine of the angle of refraction (~) bear a constant ratio to one another. These laws are known as Snell's law.

Note: Willebrord van Roijen Snell (1591-1626) was a Dutch mathematician who developed the triangulation method of measuring distances using church spires and rooftops for reference points. He extended this technique to being able to estimate the radius of the Earth quite accurately. He is best remembered for Snell's law (1621) which is the basis of modern geometric optics.

42

Optoelectronicsand Fiber Optic Technology

A l t h o u g h the law of refraction is generally k n o w n as Snell's law, it was not until some years later, however, that a F r e n c h m a n called Ren6 Descartes was the first to really m a k e full use of the ratio b e t w e e n the sines of the two angles. Because of this, it is not u n c o m m o n (especially in France!) to hear Snell's law referred to as Descartes' law.

Note: Rene Descartes (1596-1650) was a famous French mathematician, scientist and philosopher. Born at La Haye (now called Descartes!), and educated at the Jesuit College of La Fleche between 1606 and 1614, Descartes later claimed that his 'education gave him little of substance and that only mathematics had given him certain knowledge'. He was arguably the first major philosopher in the modern era to make a serious effort to defeat scepticism. His views about knowledge and certainty, as well as his views about the relationship between mind and body, have been very influential over the last three centuries.

The laws of refraction can be used to explain how light waves will behave w h e n passing through more than two media with parallel boundaries. If the refractive index of the first and last m e d i u m is the same (for example, they are both air), but different to that of the intermediate m e d i u m (for example, glass), light will be refracted towards the normal on entering the glass, and on leaving will be refracted back a w a y from the normal to exactly the same extent. The effect of this is that the emergent ray is parallel to the incident ray, but is 'laterally displaced' from it. If, as in Figure 2.5, a ray of light is now directed at a block of glass, the angles ~1 and 0 2 must be equal because the sides of the glass are parallel. So: n 2 sin ~1 = n2 sin (}2 and, therefore: n I s i n (D 2 -- n I s i n

01

Thus the angles (I) 1 and 01 m u s t always be equal.

Theory 43

Incident ray~

Angle of incidence N.~

, n 1 (air) n2 (glass)

Angle of f refraction n2 (glass) i

~

air) ~

Refracted ray

'.

Figure 2.5 Showing the path of a ray of light through a block of glass with parallel sides If, however, the light source is n o w contained inside a glass tube (see Figure 2.6), then, by Snell's law, the light leaving the source strikes the surface of the glass at an angle of 0 and leaves an angle of ~, equivalent to: n 2 sin 0 = nl sin

n 1 (air) n2 (glass) Light source n2 (glass) n 1 (air)

Figure 2.6 Light source within a glass tube

44 Optoelectronicsand Fiber Optic Technology I I I I I I I I I I I I I

n I (air) n2 (glass)

Incident~ ra~ Light source

Figure 2.7

Reflected ~ray """""'~~

j ~

n2 (glass) n 1 (air)

Total Internal Reflection of a light ray within a glass tube

As n 2 is greater than n~ then 9 must be greater than f) and, if ~ is greater than 90 degrees, there will be TIR (i.e. no refraction) and all light will remain inside the glass tube (see Figure 2.7). Furthermore, as the sides of the glass tube are parallel, rays of light striking the surface will be internally reflected at an angle equal to the angle of incidence. Reflected rays will be reflected at exactly the same angle, as they are incident on the opposite side. This will continue until they reach the far end of the tube. The glass tube therefore acts as a guide for light and, as the light is trapped inside, bends in the glass tube are relatively insignificant. Scratches or surface contamination such as grease, however, will alter the Cladding Core

Light ray (travelling away) reflected through the fiber

Figure 2.8 End view of fiber illustrating propagation of light ray

Theory 45

effective angle of incidence at that particular interface. This is because grease has a refractive index almost the same as glass and to the approaching ray this will appear as an extension of the glass surface (Figure 2.8).

2.4 Angle of incidence Because light travelling from a denser to a less dense medium is refracted away from the normal, some rays striking the boundary between the two media at a large angle of incidence cannot pass through it, but are totally internally reflected. Typically, when a ray of light emerges from a denser to a rarer medium, the ray is deflected away from the normal, but in practice, there is also a weak reflected ray present. If the angle of incidence (Figure 2.9) increases, the refracted ray moves closer to the boundary between the two media because the angle of refraction has also increased. The refracted ray also becomes weaker, while the reflected ray within the glass becomes stronger. N o t e : TIR has many commercial uses. For example, a 90 degree prism, with light totally internally reflected off one face, can be used in prismatic binoculars.

Angle of refraction

Material with a low

refractive index Material with a high

refractive index ~ / I

~' " ,, , ~ ~L..._.~',

Angleof Incidence

Figure

2.9

Angle of incidence

~",,

,

Angle of reflection

, ",,

"\

\

\

\

46 Optoelectronics and Fiber Optic Technology

2.5 Critical angle If a situation is reached when the angle of refraction is 90 degrees, there is only a residual refracted ray grazing along the boundary between the two media, and most of the light is internally reflected. The angle of incidence for an angle of refraction of 90 degrees is called the critical angle. When the angle of incidence is greater than the critical angle, the ray is totally internally reflected, as it is clearly impossible for any light to escape from inside the glass. The critical angle for a ray of light emerging from glass into air is approximately 42 degrees. This is the principle on which all optical fibers work, since the light pulses passing along such a fiber have a high angle of incidence on the walls of the fiber, and so are unable to escape from it. Note: angle times come

The high refractive index of diamond gives it a very low critical (24 degrees). This means that light is reflected internally many before it can escape from a well-cut diamond, and can then out in any direction. This is why diamonds sparkle.

By encasing the glass tubes inside a cladding whose refractive index is only slightly lower than the refractive index of the glass (see Figure 2.10), the critical a n g l e - and hence any dispersion loss (see section 2.14) - is reduced. This, therefore, is the basic principle behind the use of optical fibers and optoelectronics, and is covered by the following three rules: 1 When light propagates, in a glass of refractive index n, it moves by a factor of n more slowly than in free space.

Figure 2.10 Critical angle for a tube encased in cladding with only a slightly lower refractive index than the critical angle

Theory

47

2 If light emerging from a glass with a high refractive index is incident on a material with a lower refractive index, it changes direction provided it strikes the interface at a steep angle. If the angle of incidence is shallower than the critical angle, TIR occurs. 3 When a light beam is passed from air to glass, because of the different refractive indices at the point of penetration, the light is propagated from within the glass core at various angles. A variable refraction is therefore present at the end f a c e - which is sometimes referred to as the aperture. The critical angle is defined as 'the angle of incidence, which provides an angle of refraction of 90 degrees'.

Note: The actual value of the critical angle is dependent upon the combination of materialspresenton each side of the boundary.For the water/air boundary, the critical angle is 48.6 degrees. For the crown glass/waterboundary,the critical angle is 61.0 degrees.

2.6 Acceptance cone Since fiber is circular, a cone (termed the acceptance cone, Figure 2.11) defines the angles of incident light that are totally, internally, reflected by the core. A large acceptance cone, although allowing a fiber to accept and propagate light from a larger field, coincides with greater dispersion. Conversely, a narrow acceptance cone accompanies low dispersion but requires a narrow, more precise source of light.

CC o , dc ,,"" ,,

Cladding

i

Figure 2.11

Acceptance cone

fiiiii C!ilor!e!iiiiiiiiiil l

48

Optoelectronics and Fiber Optic Technology

Acceptance

an~e

i

_....=,

i

__ ~.,,.. . . . .

n l (cladding)

/

/

n~ (cladding)

/

Figure 2.12 fiber

Acceptance angle of fight incident on the end face of an optical

2.7 Acceptance angle The acceptance angle is a function of the refractive indices of the core and cladding materials and is defined as 'the half angle of vertex of a cone within which rays are incident on the end face of the fiber and will be captured and propagated by that fiber' (see Figure 2.12). Optical fiber has a transparent inner core and a thin exterior cladding. The property of TIR causes light to travel through the core material as it is reflected at the core/cladding interface (Figure 2.13).

Cladding

/

I

i Fiber i I acceptance{. ' I angle!

Figure 2.13

~

\'

Ray entering fiber within acceptance cone angle

~ " ~ Core

Theory

49

The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in that material, and the index of refraction of vacuum is, therefore, equal to one. The ratio of the core index to the cladding index determines the acceptance angle. Much of the light that enters a fiber at an angle greater than that of the acceptance angle will be lost through the cladding because it is not internally reflected (Figure 2.14). /

/

/'- ~ -, "--

/

Cladding

" - --

/

I I Fiber ~ acceptance ~ angle

' " -- -Core

Loss

Loss

/-i~ ~ -z~ x

-

'~,

"~Loss

', \~

Figure 2.14

Light ray Ray entering fiber outside of the acceptance cone angle

Note: A n o t h e r word for a c c e p t a n c e angle is the numerical aperture.

2.8 Numerical aperture The sine of the acceptance angle is called the numerical aperture of the fiber and is more commonly quoted than the acceptance angle itself. Numerical aperture (NA) (Figure 2.15) is the light-gathering capability of a fiber. The calculated material NA is related to the refractive indices of the core and cladding. A fiber with a high NA gathers more light than a fiber with a low NA because the critical angle is greater, and the fiber accepts light injected from larger angles.

2.9 Structure of a fiber Optical fibers are very fine fibers of glass (Figure 2.16). They consist of a glass core, roughly 50 micrometres in diameter, surrounded by a glass

50 Optoelectronics and Fiber Optic Technology

Figure 2.15

Numerical aperture - principle of operation

optical cladding giving an outside diameter of about 120 micrometres. They make use of the phenomena of TIR (see section 2.3) to confine light within the core of the fiber. Fibers are identified by size with 8/125, 50/125 and 62.5/125 being the more common types. The two numbers indicate the dimensions of the core compared to the cladding and are measured in microns. The core has a higher refractive index than the cladding and although the cladding does not carry light, it is nevertheless an essential part of the fiber as it keeps the value of the critical angle constant throughout the whole length of the fiber. As the optical fiber is essentially an optical waveguide, this means that it doesn't matter if the fiber is bent a little (for example, within a building) and the light, which is confined to the core of the fiber, will follow the bends of the fibers. Optical fibers can, therefore, be used to make light literally bend round corners.

Cladding

Core

Figure 2.16

The structure of a fiber

Theory 51

2.10 Modes Although many different types of fibers are manufactured today (see Chapter 3), there are only two modes of propagation - single mode (often called fundamental or monomode and primarily used in telecommunications) and multimode (which is mainly used for data communications. The difference between the two types is their relative size and optical performance. As the name suggests, in a single-mode fiber, only one mode, the fundamental or monomode, can propagate. In multimode fibers, however, several hundred modes differing in field pattern and propagation velocity are possible. The upper limit to the modes is determined by the core diameter and numerical aperture of the waveguide. For example, a graded index fiber with a 501Jm core typically allows about 500 separate core modes, each having different propagation and attenuation characteristics.

2.10.1 Multimode step index Multimode step index fiber (Figure 2.17) is the simplest type of fiber and has a core diameter in the 501Jm to more than 10001Jm range. The large core of this fiber allows many light modes to propagate. Some rays will take longer paths than others with the lowest order mode, the axial ray, travelling down the centre of the fiber and arriving at the end of the fiber before the higher order m o d e s - leaving behind a narrow pulse of light as it travels through the fiber. In other words, the light passing down the fiber takes longer and shorter path lengths, and consequently the signal is dispersed in time (modal dispersion), which restricts the bandwidth of the fiber and the distance over which the fiber can operate satisfactorily. Stepped index multimode fiber was the first type of fiber available. It has a core with a uniform reflective index (RI) (i.e. the core is of equal

Figure 2.17 Multimode step index fiber

52

Optoelectronicsand Fiber Optic Technology

density throughout) and is large enough to support a large number of modes. It includes all plastic fibers- combination glass/plastic fibers and glass fiber 100 / 140 and larger. This type of fiber is generally used for short data links and control circuits but not usually for telecommunications.

2.10.2 Multimode graded index In an effort to reduce modal dispersion, multimode graded index fiber (Figure 2.18) was introduced. Its core is a series of concentric rings, each with a lower refractive index thus ensuring that as light travels faster in a lower index medium, the light farther away from the fiber axis will travel fasten

Figure 2.18 Mutimode graded index fiber

Because of its construction, the differing layers of the core will ensure that the rays of light are not reflected sharply by the core to cladding interface but are refracted successively. Since the high order modes have a faster average velocity than the low order modes they will arrive at any given point at nearly the same time. The path of travel, therefore, appears almost sinusoidal. Multimode graded index fiber has a bandwidth of up to 1600 M H z / k m and is generally used in data communication (e.g. LANs).

2.10.3 Single-mode step index Single-mode step index fiber (Figure 2.19) limits the amount of dispersion by having a core small enough to allow only one mode of light to travel through the fiber (e.g. about 10~). This fiber has extremely high bandwidths and is currently used in telecommunications, long-distance high capacity links, and in the future will probably also be used in data communications (e.g. FDDI).

Theory

53

Figure 2.19 Single-modestep index fiber

2.11 Losses Light is degraded during transmission due to attenuation, polarization and dispersion. 2.11.1 Attenuation

Attenuation results from the glass fiber absorbing the energy of the light. The rate at which light is absorbed is dependent on the wavelength of the light and the characteristics of the particular glass. Glass is a silicon compound (silicon dioxide, SiO2), and by adding different additional chemicals to the basic silicon dioxide the optical properties of the glass can be changed. By adding roughly 4 per cent germanium dioxide (GeO2), for example, a glass can be created with much less attenuation than silicon dioxide by itself. This process is called 'doping' the glass and the germanium dioxide is referred to as a 'dopant'. The dopant modifies the way the glass refracts light (the 'refractive index') and, hence, improves the light-carrying capabilities of the fiber. The absorption of light in the fiber results from minute variations (less than one-tenth of the wavelength) in the density or composition of the glass. This is called Rayleigh scatter' and it's the same optical property that makes the sky look blue. (See Section 1.4.5 and Figure 1.27.) The attenuation of fiber is governed by the materials from which it is fabricated, the manufacturing process and the refractive index profile chosen. Attenuation loss is measured in dB/km. 2.11.2 Absorption

Absorption is caused by impurities in the glass and refers to the conversion of optical energy into either optoelectronic activity or molecular vibrations. Although glass fibers are extremely pure (exceeding even that of semiconductors), some impurities will still remain as residues after purification. The amount of absorption by these impurities depends on their concentration and the light wavelength.

54 Optoelectronics and Fiber Optic Technology Absolutely pure glass has very little intrinsic absorption in the near infrared wavelength region (due to its base material structure). However, it does have two absorption mechanisms which exist in this region which give an attenuation minimum in the range of 800-1700 nm. One of these is referred to as the ultraviolet absorption edge (so called because it is formed by the tail end of a series of peaks centred in the ultraviolet region) where intrinsic absorption is caused by electronic transmissions stimulated by higher energy excitation. In the infrared and far infrared region, absorption bands also occur, the tails of which extend to the near infrared region. These absorption bands are caused by molecular vibrations within the glass.

2.11.3 Scattering Scattering (Figure 2.20) is a result of imperfections in fibers and the basic structure of the fiber. Rayleigh scattering derives from the atomic and molecular structure of the glass and from the density and composition variations that are natural (i.e. unintentional) by-products caused during fiber manufacture and cabling. The angle of incidence of rays striking such variations at the core to cladding interface will change sufficiently so as to cause some rays to be refracted on to new paths which are not subject to TIR.

Light is lost Cladding ................................... .

.

.

.

.

.

t .......

.

Figure 2.20 Scattering

In general, unless very high power optical sources are used, the attenuation caused by scattering is a linear process, i.e. the scattering process does not produce a frequency shift. There are two main types of scattering process, the most important of which is Rayleigh scattering, the other being Mie scattering.

Theory 55

2.11.3.1 Rayleigh scattering This is caused by inhomogeneities (regions of slightly different density or composition) within the glass, which cause fluctuations in the refractive index. These regions are frozen into the glass during the manufacturing process. The subsequent scattering caused by the density and index variations, which is omnidirectional, produces an attenuation increase, which is proportional to the reciprocal of the fourth power of the wavelength. The inhomogeneities are much smaller in size than the wavelength of light.

2.11.3.2 Mie scattering Linear scattering also occurs at inhomogenieties and these arise from imperfections in the fiber's geometry, irregularities in the refractive index and the presence of bubbles etc. caused during manufacture. Careful control of the manufacturing process, however, can reduce Mie scattering to insignificant levels.

2.11.4 Bending losses Losses due to the curvature and losses caused by an abrupt change in radius of curvature are referred to as 'bending losses'. Care must be taken when cabling and jointing fibers (otherwise the attenuation of the fiber can be significantly affected) and you will find that when jointing, fibers are normally curled up inside the joint housing (Figure 2.21).

Figure 2.21 Typicalfiber optic connection pane/(courtesy of BICC Brand-Rex)

56

Optoelectronicsand Fiber Optic Technology

Figure 2.22 Microbending For example, with single-mode fiber, a minimum bend radius of approximately 50 mm is usually specified to ensure that the performance at 1550 nm is not affected by macrobending loss.

2.11.4.1 Microbending Microbending (Figure 2.22) is a loss due to small-scale bending or distortions. The actual microbending may not be visibly apparent, and the losses may be temperature related, tensile related or crush related but they are reversible once the cause is removed. The effects of microbending on multimode fiber can result in anything from an increase in attenuation (which is independent of wavelength) to a series of periodic peaks and troughs on the spectral attenuation curve. Installers can minimize the effect of microbending during installation and testing.

2.11.4.2 Macrobending The change in spectral attenuation caused by macrobending (Figure 2.23) is slightly different to microbending. In general, there are no peaks and

Figure 2.23

Macrobending

Theory 57

troughs because in a macrobend no light is coupled back into the core from the cladding as can happen in the case of microbends. Macrobending losses are very wavelength d e p e n d e n t and are a loss that has been caused by large-scale bending of the fiber although the losses are reversible once the bends are straightened. Installers can minimize the effect during installation and testing by not exceeding the installation and long-term bend radii.

2.12 Optical power and power density A uniformly diffused light emitting surface is called a lamberton source and when a surface is radiating power, the power density at any point on that radiating surface can be expressed in terms of the power per unit area (normal unit = Wcm-2). This is called the irradiance of the source and its distribution across the surface gives the near field radiation pattern. Power density decreases with source distance until it eventually becomes almost insignificant. The density of this radiant power is expressed as the radiance of the source and the unit that describes the optoelectronic radiation characteristics is called the steradian. The p o w e r radiated in a given direction is referred to as the radiant intensity of the source and its distribution forms the far field radiation pattern.

2.13 Optical fiber input power If a large radiant source such as an LED is in close proximity to the surface area of a fiber end surface (see Figure 2.24), every single point on

;

\

.

.

.

.

.

.

.

.

: I

:

i

....................

"

Critical ray

Figure 2.24 Any point on a fiber's end surface accepts incident power within its acceptance cone

58

Optoelectronicsand Fiber Optic Technology

that fiber end surface accepts incident power within its acceptance cone, and the total of all such incremental powers gives the total power input.

2.14 Dispersion losses The dispersion or spreading of light pulses as they travel through a fiber results from two factors: the broad range of wavelengths emitted by the light source and the amount of wavelength dispersion inherent in the fiber. A short light pulse will gradually become longer and will eventually fall back far enough, so that the next pulse catches up with it. This has a limiting effect on the fiber's bandwidth and causes the pulse seen at the output to be wider than the input pulse (Figure 2.25).

Each pulse apparent

Pulses merged

Figure 2.25 Dispersion To overcome this problem, pulse rates must be slow enough so that dispersion does not cause adjacent pulses to overlap and the detector must be able to distinguish between each pulse. The most significant types of dispersion in fiber optic cable are called material, waveguide and chromatic.

2.14.1 Material dispersion As the refractive index of a glass prism varies with wavelength, different wavelengths will travel at different velocities along the fiber and arrive at different times. This causes an energy loss, which is called material dispersion. The amount of material dispersion depends upon the chemical composition of the glass.

2.14.2 Waveguide dispersion Waveguide dispersion can loosely be described as a way in which the refractive index profile affects the relationship between propagation constant and wavelength. As frequency is a function of wavelength, the

Theory

59

group velocity of the energy in a fiber varies with frequency. This produces an additional loss, which is called waveguide dispersion and arises from the distribution of light between the case and the cladding. Waveguide dispersion arises because the propagation constant varies with wavelength, the causes of which are independent of material dispersion.

2.14.3 Chromatic dispersion The combination of material dispersion and waveguide dispersion is called chromatic dispersion (sometimes referred to as wavelength dispersion just to make it a little more confusing!) and these losses primarily concern the spectral width of the transmitter and choice of the correct wavelength. As the light emitted by a typical red LED covers a waveband of approximately 40 nm between 780 nm and 820 nm (in contrast to modern laser diodes which have a spectral width of less than I nm) the choice of transmitter is one of the main methods of reducing spectral width. Choosing the correct wavelength is, however, equally important and Figure 2.26 illustrates the effects of material, chromatic and waveguide dispersion by showing a graph of effective refractive index against wavelength.

1.48

t._

Chromatic dispersion

~

1.47

._

m

~ 1.46-

Waveguide dispersion

~6 /

~ 1.45s

Material 1.44

.....................

dispersion

I

I

I

I

500

1000

1500

2000

Wavelength (nm)

P_--

Figure 2.26 Graph of an optical fiber's effective refractive index against

wavelength - showing effects of chromatic, waveguide and material dispersion

60

Optoelectronics and Fiber Optic Technology

Material dispersion and waveguide dispersion effects vary in opposite senses as the wavelength is increased, but there is an optimum wavelength around 1300nm where the two effects almost cancel each other out and chromatic dispersion is at a minimum. Attenuation is therefore also at a minimum and this makes 1300nm a highly attractive operating wavelength and a point at which there is a possibility of increasing propagation path lengths to hundreds of kilometres. By increasing the waveguide dispersion so as to cancel out material dispersion, it is also possible to shift the zero-dispersion wavelength to a 1550 nm loss minimum.

2.14.3.1 Dispersion-shifted fiber The first commercial dispersion-shifted fiber (Figure 2.27) was introduced in 1986 by Corning. This has a segmented core whereby a high index inner region is surrounded by a lower index segment, a segment with a higher refractive index and finally a low index cladding.

/••

High index region Lower index segment "J

High refractive index region / /

Figure 2.27

Region with low index cladding

Dispersion-shifted fiber

There are a number of other methods of achieving the same result and these include reducing the core diameter or making the core's refractive index have a triangular, trapezoidal or trenched-annular radial distribution. Although production runs of segmented-core dispersion-shifted fibers have shown average losses of 0.21 d B / k m -1 at 1550nm, initial costs for dispersion shifted fiber used higher than standard step index fiber, but this difference narrowed as production volumes increased.

Theory

61

2.15 Optical transitions in semiconductors The basic construction of semiconductor devices is such they have two electron energy bands (valance and conduction) separated from each other by a band gap, E. The interaction between photons (i.e. light particles) and valence/conduction band electrons is the basic principle behind all optoelectronic semiconductors.

2.15.1 Semiconductor optical transitional mechanisms There are three semiconductor optical transitional mechanisms: spontaneous emission, stimulated emission and absorption. These are illustrated in Figure 2.28.

o

o

o -"

O

(a) Spontaneous emission

Conduction band Band gap E

O

o

Valence band

(b) Stimulated emission

(c) Absorption

photon 9electron o hole

Figure 2.28 Semiconductor transitional mechanisms

When a conduction band has too many electrons, the electrons are passed to the free states (holes) in the valence band. This causes a radiative recombination of excess charge carriers and a photon (p) is spontaneously emitted whose energy is proportional to its frequency and wavelength and is thus dependent upon the size of the band gap E (Figure 2.28a). This is called spontaneous emission or luminescence. Each band occupies a range of energy values, which means that the wavelength can vary from photon to photon, and thus provides a bandwidth to the total emission. For germanium and silicon the band gap is such that emission is in the far infrared or heat band, but some compounds of band III-V elements can produce emissions of visible or near infrared wavelengths.

62

Optoelectronicsand Fiber Optic Technology

Because of photons stimulating excess charge carriers to radiative emission, additional photons are emitted. This is called stimulated emission (Figure 2.28b). The third optical mechanism that can occur is when an electron is being taken from the valence to the higher energy state or conduction band and the electron overcomes the photon. This is referred to as absorption (Figure 2.28c). Although absorption, spontaneous emission and stimulated emission always occur together, one type of emission will always be dominant. Forward biasing the semiconductor p-n junction will, similar to a transistor, cause charge carriers to move from one side of the junction to another where, as minority carriers, they will be recombined with the majority carriers. This phenomenon of emitting light after forward biasing is called injection luminance. The devices that provide this particular type of interaction are light emitting diodes (LEDs), laser diodes, and in the reverse mode, photodiodes.

2.16 Usable bandwidth of optoelectronic transmission systems Bandwidth is the information carrying capacity of the fiber and is measured in M H z / k m . The higher the value, the greater the data transmission. To transfer a digital waveform over a normal voice telephone system, the waveform must first be converted into signals that fall within the frequency of the speech channel, i.e. 300 to 3400 Hz. This can be carried out by means of a modem (modulator/demodulator), which is connected between the terminal and the telephone line. Because of the limitations of a normal telephone speech channel (e.g. line distortion, noise and the requirement to prevent interference to adjacent channels), the transmission of data at bit rates only up to 2.4 Kbit/s -1 is normal. Of course, if high quality private wires or 4-wire circuits are used, rates of up to 9.6 Kbit/s -1 and in exceptional cases 19.2 Kbit/s -1 are feasible; depending upon the bit error rate (i.e. the number of false bits relative to the total number of bits in a digital stream). Bit error rate is recognized as the quality measurement of a transmission system and typical requirements are 1 in 109 but could be as high as 1 in 1013. In digital communications, one speech channel utilizes a 64 Kbit/s -1 high speed data channel: to perfectly pulse code modulate a speech channel requires 64 Kbit/s -1 (delta modulation, incidentally, only requires 16 Kbit/s -1 and digital analysis only 2.4 Kbit/s -1). All of these limits are covered by recommendation CCITT Signalling System No. 7, which

Theory

63

stipulates that digital signalling shall be via a 2 M b i t / s -1 stream (actually 2048 Kbit/s), split into 32 x 64 K b i t / s -1 traffic channels. Depending upon the acceptable a m o u n t of voice degradation, this can be reduced to about 48 Kbit / s -1, which also enables 16 Kbit / s -1 of data to be transmitted at the same time. Research establishments are now investigating the possibility of utilizing 144Kbit/s -1 channels to allow a n u m b e r of speech and data channels to be transmitted simultaneously. This will provide: 9 2 x 64 K b i t / s - 1 speech channels; 9 1 x 8 Kbit/s -1 data channel; 9 1 x 8 Kbit s -1 high speed data channel. Currently a range of all digital services is being offered by PTTs worldwide. These are available at 6 4 K b i t / s -1 and 2.04Mbit/s -1 bandwidths and in certain circumstances (depending on the country) higher speeds are also available.

2.16.1 Cut-off frequency of an optical fiber The cut-off frequency of an optical fiber is determined not only by the fiber itself (mainly modal dispersion in the case of multimode fibers, and waveguide dispersion in the case of single-mode fibers) but also by the amount of material dispersion caused by the spectral width of the transmitter.

2.16.2 Fiber bandwidth In Figure 2.29 a ray of light that enters a fiber at a small angle (1) has a shorter path through the fiber than light which enters at an angle close to the m a x i m u m acceptance angle (2). As a result, different 'rays' (or modes) of light reach the end of the fiber at different times, even though the original source is the same LED or laser. This produces a certain amount of uncertainty as to where the start and end of a pulse occurs at the output end of the f i b e r - which in turn limits the m a x i m u m frequency that can be transmitted. In short, the fewer modes, the higher the b a n d w i d t h of the fiber. The way that the n u m b e r of modes is reduced is by making the core of the fiber as small as possible. Single-mode fiber, with a core measuring only 8-10 microns in diameter, has a much higher bandwidth because it allows only a few modes of light to propagate along its core. Fibers with a wider core diameter, such as 50 and 62.5 microns, allow m a n y more modes to propagate and are, therefore, referred to as 'multimode' fibers.

64 Optoelectronics and Fiber Optic Technology Cladding

layer -- ~......... Core . . . .

2

Figure 2.29

Different path lengths determine the bandwidth of a fiber

The frequency at which signal attenuation increases by 3 dB is called the fiber bandwidth. As the bandwidth of an optical waveguide is virtually reciprocal to its length, the bandwidth length product is frequently quoted as a fiber quality characteristic. Figure 2.30 shows frequency responses of I km lengths of a number of different types of fibers compared with I km lengths of balanced copper and coaxial cables. It should be noted, that the fairly constant response of an optical fiber obviates the need for equalization, which is normally required for coaxial systems. Multimode fibres Step index fibers

100

Grade index fibers

I

/

/ i

E /

rn

Singlemode

/

10

J

// 1

10

102

103 MHz

Figure 2.30

copper wires

=

Typical frequency responses for optical fibers compared with

Theory 65

Owing to material dispersion, for fibers greater than I km in length, the cut-off frequency is also dependent upon the actual material used and the number of splices.

2,17 Modulation and demodulation of the subcarrier The principle of signalling over a copper cable relies on an oscillator being modulated by the information signal in frequency, phase or amplitude. A detector at the distant end then demodulates the product.

2.17.1 Modulation techniques LEDs and lasers, as previously stated, are modulated in one of two ways: on and off, or linearly. 2.17.1.1 On/off modulation

As can be seen from Figure 2.31, a transistor is used to switch the LED or laser on and off in step with an input digital signal. This signal can be converted from almost any digital format by the appropriate circuitry, into the correct base drive for the transistor. Digital o n / o f f modulation of an LED or a LASER can take a number of forms. The simplest is light-on for a logic '1', and light-off for a logic '0'. Two other common forms are pulse width modulation and pulse rate modulation. In the former, a constant stream of pulses is produced with one width signifying a logic '1' and another width a logic '0'. In the latter, the pulses are all of the same width but the pulse rate changes to differentiate between logic '1' and logic '0'.

Figure 2.31

On~offmodulation of LEDs and lasers

66

Optoelectronicsand Fiber Optic Technology

I

Input

1

Figure 2.32 Linear modulation of an LED or laser

2.17.1.2 Linear m o d u l a t i o n

Linear modulation of an LED or a laser is accomplished by the operational amplifier circuit of Figure 2.32. The inverting input is used to supply the modulating drive to the LED or laser while the non-inverting input supplies a DC bias reference.

2.17.2 Types of modulation There are two main types of modulation as shown in Figure 2.33.

Figure 2.33

Types of modulation and demodulation

Theory 67

Note: With optoelectronics, the transmission methods used mainly depend upon whether the signal is analog or digital but, having said that, the principles are similar.

2.17.2.1 Frequency modulation If the optoelectronic transmission path bandwidth is much greater than the signal being transmitted, a suitable frequency modulated electrical subcarrier can be introduced. By modulating a subcarrier it is then possible to create an analog transmission channel which could be suitable for transmitting video and frequency division multiplex (FDM) signals as well as digital signals. Several analog signals (for instance, video channels) can be combined to form an FDM signal, which can then be used to frequency modulate the electrical subcarrier. This method would, however, reduce the dynamic range of the channels and a far better method would be to frequency modulate a number of carriers with video signals and then to multiplex these on a frequency division basis.

2.17.2.2 Intensity (power) modulation Because the spectral width of light emitting and (to a lesser extent) laser diodes is so high and the frequency stability so low, frequency and phase modulation of optical transmitters is almost impossible. Another disadvantage is associated with the fiber itself as, owing to the multipath propagation properties of step-index and graded-index multimode fibers, the signal is greatly distorted. Nowadays, therefore, the only practical type of modulation that can be used is intensity (power) modulation whose main advantage is that the optical source can be directly modulated by the drive current and the photodiode will convert the received optical power directly into an electrical photocurrent. There are two main types of intensity modulation: 9 direct intensity modulation; and 9 premodulation with subsequent intensity modulation.

Direct intensity modulation Direct intensity modulation is a very simple process, which relies upon the optical signal and the electrical signal being proportional. Provided that the transmitter and the receiver characteristics are exactly the same, a high linearity can be realized, but as this can only normally be achieved in small sections, its use is restricted mainly to analog transmission although it is sometimes used for transmitting signals over short distances.

68

Optoelectronicsand Fiber Optic Technology

Premodulation and subsequent intensity modulation With premodulation and subsequent intensity modulation, the advantage is that there is an improved signal-to-noise ratio and the non-linearity of the optoelectronic transducer no longer affects the signal. The design is again simple and this method is particularly suitable for transmitting video signals over short or medium distances. The usable bandwidth of an intensity (power) undulated optoelectronic transmission path is determined by: 9 the modulation bandwidth of the optical transmitter; 9 the dispersion effects in the fiber (e.g. modal, waveguide); 9 the modulation bandwidth of the optical receiver.

material

and

2.18 Signal-to-noise ratio The signal-to-noise ratio (SNR) at the output of optoelectronic transmission systems depends upon the elements shown in Figure 2.34.

Figure 2.34

Signal-to-noise ratio

Theory 69

Note: In any analog system, some of the voltage is what you want to measure (signal) and some of it is random fluctuations (noise). The ratio of the power of the two is called the 'signal-to-noise ratio' (SNR). SNR is a measure of the quality of a signal and is usually expressed in decibels (dB).

2.18.1 Optical receiver thermal noise This is noise due to the load resistor and the first amplifier. This noise can be minimized by utilizing a device such as a transimpedence amplifier to reduce the effective time constant and diode capacitance, which limits the receiver frequency response.

2.18.2 Quantum noise (of light) This is mainly due to the noise caused by avalanche diodes.

2.18.3 Dark current noise Even when the photodiode is at rest a reverse current flows and this produces a noise component. Although using an avalanche photodiode can reduce this effect, avalanche diodes themselves produce noise and so there is a limitation to their use. Dark current noise is normally reduced in optical receivers by using a low pass filter.

2.18.4 Laser noise The actual structure of the laser can affect the stability of the transmitter's output power.

2.18.5 Mode partition noise This is caused when individual laser modes arrive at the receiver separated in time and produce an increase in the noise.

2.18.6 Modal noise (in multimode fibers) When a graded index multimode fiber (see Chapter 3) is used together with a narrow spectral-width laser diode, the interference pattern produced is very unstable owing to fiber effects and fluctuations in the optical wavelength. This is particularly noticeable at junctions and splices.

70

Optoelectronics and Fiber Optic Technology

0

.m

i.._

G)

Section 1 receiver noise (i.e. thermal and quantum light

o

o

I

Section 2 transmitter noise (i.e. laser, laser modal and fibre model Power - ~

Figure 2.35 Graph of signal-to-noise ratio against power at the output of an optical receiver

|t has been ascertained, that at low powers, receiver noise (i.e. thermal and quantum light) predominates and the signal-to-noise ratio will improve with optical power until transmitter noise (i.e. LASER noise, LASER modal noise and fiber modal noise) becomes dominant and the signal-to-noise ratio then remains constant, as shown in Figure 2.35. Signal-to-noise ratios of around 40 dB can be expected with gainguided laser diodes (see Chapter 4).

2.19 Types of mixers There are three main types of mixers used with fiber optics" 9 end fire; 9 transmissive; 9 biconical tapered transmissive.

2.19.1 End fire mixers A thin platelet of quartz glass, embedded in a layer of low refractive index adhesive, whose thickness is equal to the core diameter of the fiber sections and whose width is the same as the total outer diameters of the fibers, ensures that incoming light is equally divided to outgoing fiber ports and therefore acts as an effective waveguide (Figure 2.36). A mirror reflects the light.

Theory 71

Figure 2.36

End fire mixer with p/ate/et and mirror

2.19.2 Transmissive mixer This is similar to the end fire mixer, except that the incoming and outgoing fibers are arranged opposite each other (Figure 2.37).

Figure 2.37

Transmissive mixer with mixer platelet

2.19.3 Biconical tapered transmissive mixer In the biconical tapered transmissive mixer, the power from any or all input fibers is spread among all of the output ports (Figure 2.38).

Figure 2.38 Biconical tapered transmissive mixer

2.20 Types of multiplexing Figure 2.39 Owing to the dispersion characteristics of optical waveguides and because of financial restraints, quite often it is better to use a high quality singlemode fiber (see Chapter 3) with a transmission bandwidth of 56.5 Mbit s-1, as opposed to multiplexing four 140 Mbit s-1 signals together.

72

Optoelectronicsand Fiber Optic Technology

Figure 2.39

Types of multiplexing

Currently there are four main methods of multiplexing several electrical signals together via an optical fiber link.

2.20.1 Fiber multiplexing This is where a separate fiber is used for each signal.

2.20.2 Electrical multiplexing Electrical multiplexing is where a number (for example, four) of input signals are first combined electrically and the resultant signal used to drive an optoelectronic transducer. Although electrical multiplexing is much more economic and considerably less complex than wavelength division multiplexing (see later) for normal telecommunications links, the use of a wavelength division multiplex for subscriber line networks is advisable due to the greater flexibility provided for the gradual expansion of installed networks. For instance, in an initial stage, narrow band sources such as telephone and data transmission can be transmitted over a single fiber to the subscriber using bi-directional wavelength division multiplex in the range 800-900nm. If, at a later date, visual communication is required, then signals can be transmitted in the 1300 nm region over the same fiber by the use of additional optoelectronic transducers.

2.20.3 Wavelength division multiplexing Wavelength division multiplexing (WDM) is the simultaneous transmission of several signals in an optical waveguide at different wavelengths.

Theory 73

Figure 2.40

Wavelength division multiplexing using a prism

Currently, most fiber optic links consist of a pair of f i b e r s - one used to transmit, one used for r e c e i v e - and that the highest single-channel standard data rates commonly being used are 2.5Gbit/s (about 30000 standard telephone channels), although 10Gbit/s systems are currently available and 40 and even 80 Gbit/s systems appear to be just around the corner. However, optical fibers have the ability to carry multiple channels using technology known as WDM. Put simply, this uses the same fiber to carry different channels using different wavelengths or 'colours' of light. Several techniques of achieving this exist, such as using a prism as a wavelength dispersion element (Figure 2.40) or using a multilayer dielectric filter as a wavelength selective beam splitter (Figure 2.41). The devices that perform wavelength division multiplexing and demultiplexing are commonly called couplers. In the example system shown in Figure 2.42, three ports on each coupler are used - and the coupler acts as either a wavelength division multiplexer or a demultiplexer if used in the reverse mode. This sort of coupler is called a muldex coupler: a contraction of the words multiplexer and demultiplexer. This technology enables signals that are independent of each other, at different carrier wavelengths and close channel spacing to be modulated in a different manner (i.e. digital or analog) and transmitted over the same fiber.

Dielectric bandpass filter Input signal (M and ~L2) 1 reflected signal

Figure 2.41 filter

;L2 transmitted signal

Wavelength division multiplexing using a multiplayer dielectric

74

Optoelectronicsand Fiber Optic Technology

Figure 2.42 An example of a wavelength division multiplexed optical fiber transmission system, with bi-directional muldex couplers

Insertion losses, however, are increased, which shortens the span length to about 5 km and this type of multiplexer is used mainly in local TV subscriber networks.

2.20.4 Dense wavelength division multiplexing Dense wavelength division multiplexing (DWDM) increases fiber capacity by transmitting data with multiple wavelengths, known as channels. This technology is ideal for high volume point-to-point or backbone links with minimal switching and routing requirements. With current technology, the signal must be converted at switching points along the route from optical to electrical, the header information analysed, then the signal must be returned to the optical domain for correct routing. One side effect of DWDM is that the overall efficiency of the link is reduced due to added noise and the bandwidth restrictions imposed by the electronics.

2.20.5 Time division multiplexing In a time division multiplexed (TDM) system, a number of sampled messages are used to modulate a pulsed carrier at different time intervals.

Theory

75

This ensures that only one carrier is actually t r a n s m i t t e d at a n y one instance.

3

Fibers and cables

As shown in the previous chapter, there are two types of raw fiber: multimode, which is used in data communications, and single mode, which is used in telecommunications. The difference between the two is their relative size and optical performance. In Chapter 3 their construction, use and technology are reviewed. Optical fiber communication systems were originally developed as a means of transmitting high bandwidth signals over long distances. One reason for this was the growing requirement for computers to manipulate and transfer data at ever-faster speeds.

Figure 3.1 Fiber optics can be quite 'hair raising'! (courtesy of AURIGA (Europe) PLC)

Fibers and cables 77

Figure 3.2

Make-up of an optical fiber

An optical fiber consists of a core region and a cladding layer surrounded by a protective primary coating (Figure 3.2). The core region's refractive index (or optical density) is greater than the cladding layer and the boundary between the layers is where the light reflects. The base material used in fibers for most long-distance communication is silica glass which, in its purest form, has the desired qualities of low attenuation, low cost and is readily availability. The high refractive index of the core can be achieved by adding dopants such as phosphorous, germanium and boron to the silica during manufacture. However, this does not mean that all fibers for all applications are made from silica glass.

3.1 Single-mode fiber Single-mode fiber (also known as fundamental or monomode fiber) will permit only one mode to propagate and, as such, cannot suffer mode delay differences. Single-mode fibers are capable of wide bandwidths (for example, up to 40 GHz), and are ideally suited for long haul, low cost and high capacity circuits for the transmission of telephone and cable television etc. Although they have primarily been developed for the 1300nm window, they can also be used effectively with time division multiplex (TDM) and wavelength division multiplex (WDM) systems operating in the 1550nm wavelength region. Figure 3.3 shows a single-mode fiber, together with its refractive index profile and cross-section. Although the outer cladding of an optical fiber cable is at least ten times the thickness of the core radius (so as to prevent microbending losses), the actual fiber core is only a few micrometres in diameter (typically, 2 to 8 ~m) and there is a refractive index difference of less than 0.3 per cent.

~

78 Optoelectronics and Fiber Optic Technology

|I ,

i

I i | i

i

h

Core Cladding Outer layer Buffer/protective coating

i Refractive index

Figure 3.3 Single-mode optical fiber, together with its refractive index profile and cross-section

By design, the core fiber of a single-mode fiber is very narrow compared to the wavelength of light being used. Instead of the 62.5 microns used in a multimode cable, single-mode cable can only be 8 microns in diameter with the result that only a single path exists through the cable core through which light can travel. Normally, as much as 20 per cent of the light in a single-mode cable actually travels down the cladding and the effective diameter of the cable is a blend of the single-mode core and the degree to which the cladding carries light. This is referred to as the 'mode field diameter', which can be larger than the physical diameter of the core depending on the refractive indices of the core and cladding. The unfortunate disadvantage of using a single-mode fiber is the difficulty experienced in trying to couple the extremely small crosssectional area of the fiber core to an optoelectronic detector or when trying to splice it to another length of fiber. If the fiber is subjected to mechanical stress, local discontinuities can be introduced. Curvatures of the fiber involving axial displacements of a few micrometres and spatial wavelengths of a few millimetres result in unwanted light radiance and extra attenuation losses. These are referred to as microbending losses or in the case of macroscopic axial deviation of the fiber from straight line, macrobending losses (see Chapter 2). To overcome these problems, high intensity laser diodes are frequently used. Another disadvantage of using single-mode fibers is that as the refractive index of glass decreases with optical wavelength, the light

Fibers and cables

79

velocity will also be wavelength dependent. Thus the light from an optical transmitter will have a definite spectral width (e.g. light emitting diodes: 40nm, multimode lasers: 3 nm, monomode lasers: 0.01 nm) and delay differences will occur. There is, therefore, an advantage to using a (limited) multimode fiber.

3.2 Multimode fiber In comparison to the single-mode fiber, multimode fiber has a relatively large core (typically 50-60 ]am) and a high numerical aperture (NA). As the name implies, multimode fibers are capable of propagating more than one mode at a time and they are ideally suited for high bandwidth (i.e. a few GHz), medium-haul applications. The outer cladding is normally ten wavelengths thick and approximately twice the size of the core diameter. As its name implies, there are multiple paths (modes) by which a light 'ray' may be propagated. For example, a fiber with a core diameter of 62.5 microns and a light with a wavelength of 1300nm will propagate about 228 modes. The problem with multimode cable, however, is that as some of the modes are longer than others, the pulse of light will be 'spread out' due to the modal dispersion. This causes an effect referred to as 'intersymbol interference', which restricts the distance that a pulse can be usefully sent over multimode fiber. There are two types of multimode fiber: 9 step index; 9 graded index. 3.2.1 Multimode step index fiber As described in Chapter 2, a glass fiber in air has a large critical angle of approximately 42 degrees. This enables propagation along paths that are much longer than the axial route but attenuation and dispersion losses are subsequently increased. By making the fiber core extremely narrow (CCITT Recommendation No. G651 indicates that 'the core diameter should be 50 ]am and the cladding 150 ]am') and enclosing it in a cladding material whose refractive index is only slightly lower than the fiber (but nevertheless capable of ensuring that the refractive index changes directly at the core-cladding interface) not only will the critical angle be reduced, but losses will also be minimized.

80 Optoelectronicsand Fiber Optic Technology

,, i

i

~ , ~

i

'

~ ',

!

~

Core Cladding Outer layer

i

', ',

i

i

efractive

index

Figure 3.4 Step index multimode fiber together with its refractive index and cross-section

A fiber optic constructed in this manner is referred to as a step-index fiber and they are ideally suited for short-haul and low-cost installations with typical bandwidths up to about 20 MHz. Figure 3.4 shows a step index multimode fiber, together with its refractive index profile and cross-section. In a simple step index fiber the refractive index of the cladding is typically 10 per cent lower than the refractive index of the core and, provided that the light coupled into the fiber is of a low acceptance angle, total reflection of light rays will occur. It should be noted, however, that although the light velocity will be constant within the core, a ray with a high angle will have a smaller velocity component than those with a smaller reflective angle. An angle of 12 degrees is usually considered as a typical value of acceptance angle. Light rays passing through the fiber are continuously reflected off the glass cladding towards the centre of the core at different angles and lengths, limiting the overall bandwidth of the cable. The main disadvantage of step index fibers is that the different optical lengths caused by the various angles at which light is propagated relative to the core, causes the transmission bandwidth to be fairly small. These effects can be avoided by using graded index fiber, and because of these limitations, multimode step index fiber is typically only used in applications requiring distances of less than I km. Despite the low bandwidth, high attenuation and relatively short distances associated with multimode step index fiber, its large diameter core (up to I mm in diameter) makes it easier to connect a n d / o r splice especially as core alignment is not so critical.

Fibers and cables

81

~Core Cladding

[

,'

/

i Refractive index

Figure 3.5 Multimode graded index fiber, together with refractive index profile and cross-section

3.2.2 Multimode graded index fiber One way to reduce the modal dispersion in multimode fiber is to modify the refractive index of the glass so that the light can be made to travel at different speeds. This can be achieved by carefully manufacturing the fiber so that the light entering the fiber with a narrow acceptance angle can be made to travel more slowly than the light that is being reflected by the modes near the outside of the core. Varying the path length speed (i.e. shorter paths are made to travel slower than the longer path lengths) ensures that the light pulse stays together and is not spread out as happens with non-graded index fiber. A graded index fiber typically transmits roughly 800 modes. Although previously only used for trunk (i.e. long-haul) networks, graded index fibers are nowadays frequently employed in local networks at the 850 and 1300 nm wavelengths. With multimode graded index fiber, the core's refractive index gradually decreases from the centre to the outer edge, resulting in a slow bending of the light rather than an abrupt bouncing or 'step' pattern and because of these variations in refractive index, the trajectory of the light is sinusoidal as opposed to zigzag and the light rays follow curved paths. Figure 3.5 shows graded-index fiber, together with its refractive index profile and cross-section. Figure 3.6 shows the light trajectory in detail. The reason for this phenomenon is that the core is, in effect, made up of a central membrane or fiber axis, surrounded by a succession of infinitely thin shells each consisting of a different, but constant, refractive index. From Figure 3.6 it can be seen that light rays running close to the

82

Optoelectronics and Fiber Optic Technology

n1+2 n1+3~,~ n 1+ 1 .... ~

~

n1 ~ ' ~ ' i i ~ i i n1+1 n1+2 ~ n 1+3

~ ....

Cladding -------------- -

i Cladding I

Figure 3.6 Light trajectories in a graded index fiber

fiber axis, although having a shorter path length, will have a lower velocity because they pass through a region with a high refractive index. Owing to the reduced refractive index at the core edges, the rays on more deviant paths will travel much faster than axial rays and cause the light components to take almost the same amount of time to travel the length of the fiber, thus minimizing dispersion losses. Light is, therefore, transmitted along a fiber in a multitude of different paths, varying from those parallel to the fiber axis to those approaching the critical angle. Each path at a different angle is termed a 'transmission mode', and the NA of graded index fiber is defined as the maximum value of acceptance angle at the fiber axis. Typical attenuation coefficients of graded-index fibers at 850 nm are 2.5 to 3 d B k m -1, while at 1300nm they are 1.0 to 1. 5 d B k m -i, with a 3dB bandwidth level of about 2000 MHz. The main advantage of using graded-index fiber is the reduced refractive index at the centre of the core. This is particularly advantageous when trying to couple the tiny emitting area and sharply focused beam of a small LED or Laser diode to a fiber. Another advantage of using graded index fiber is that it is comparatively cheap to produce. Table 3.1 compares the three main fiber types: single mode, step index and graded index.

3.3 Manufacturing processes and designs of optical waveguides The advantages associated with optical fibers have been known for a long time, but it wasn't until the early 1970s, when Corning Glass Works produced a usable fiber with an attenuation of about 1 6 d B k m -1, that optical fibers really became a viable possibility. Nowadays, of course, attenuations of less than I dB km -1, and in exceptional cases as low as 0.1 dB km -1, are to be found.

Table 3.1

Comparison of the three main optical fiber types

Fiber type

Bandwidth efficiency

Coupling Attenuation diameter factor (dB)

Step index

10-20 MHz km-’

70

10

Graded index

1 GHz km-’

40

Single mode

20-2000 GHz km-’

20

Core

Advantages

Disadvantages

Types

Use

100-200

Reduced refractive index at core.

Low transmission bandwidth.

Includes all plastic, PCS/ HCS and some glass fibers with large cores, 100 pm or larger.

Short-distance applications (e.g. less than 1 Km).

5

50

Comparatively cheap to produce. Modal Dispersion reduced.

3

10

Wide transmission bandwidth. Very low attenuation factor.

Typical sizes 50/125 pm, 623125 pm and 851125 pm.

Coupling efficiency.

Small core, typically 8/125pm.

Used for long-haul telecommunications (i.e. long transmission links with large information carrying capacity requiring few repeaters).

Table 3.2 Mode comparison table

Potential Bandwidth (MHz-km)

Core

Cladding diameter

Numerical aperture

8.0

125

0.110

Multirnode

50.0

125

0.200

Multirnode

62.5

125

Multirnode

85.0

125

Multirnode

100.0

140

0.290

Multirnode

200.0

Mode

Single-mode

diameter

850 nm

13 10 nm

1550 nm

Chromatic dispersion coefficient (ps/nm-km)

Attenuation Coefficient (dB/km)

1310 nm 1550 nm

850 nm 1310 nm 1550 nm

3.0

18.0

0.40

200 to 1000 400 to 1500

3.00

1.00

0.275

160 to 300

200 to 1000

3.75

1.75

0.260

100 to 200

200 to 400

4.00

2.00

100

100to200

5.00

2.00

10

>10.00

0.3

Fibers and cables 85

3.4 Manufacturing techniques How then is an optical fiber manufactured? Basically, there are three separate steps: preparation of the glass preform, forming the glass rod and fiber drawing.

3.4.1 Preparation of the glass preform Natural silicon dioxide (crystal quartz or silica sand) is manufactured into a solid, bubble-free, glass preform of pure silicon dioxide by removing all metallic chlorides (for example, iron) through fractional distillation.

3.4.2 Forming the glass rod (Figure 3.7) The preform is made into a glass tube of the desired core refractive index by doping the silicon dioxide with germanium and phosphorous to increase the refractive index, or with boron a n d / o r fluoride to reduce the refractive index. There are three main methods of forming the glass rod: internal vapour deposition, external vapour deposition and vapour axial deposition.

Figure 3.7 A glass fob falls free of the heated end of a silica band, thus beginning the fiber drawing process (courtesy of Corning Cable Systems

GmbH)

3.4.2.1 Internal vapour deposition Figure 3.8 shows the preparation of the preform by internal vapour deposition.

86

Optoelectronics and Fiber Optic Technology

Figure 3.8

Glass preform preparation by internal vapour deposition

Silicon dioxide p o w d e r (doped to provide the required refractive index) is deposited in layers on a rotating tube. The tube is then heated to 1600~ to form core and cladding layers. By further heating, this hollow rod is collapsed to form a solid cylindrical rod with an internal core and a cladding of exactly the right refractive index. An outer cladding region (tube) of commercial silica glass then surrounds the rod. This process is referred to as the deposition drying and sintering process.

3.4.2.2 External vapour deposition In the external vapour deposition process (Figure 3.9) layers of silicon dioxide are formed on to a ceramic substrate rod so as to make the required core and cladding properties. A steel mandrel is mounted in a lathe and rotated. A burner is then fed with chlorides of silica and the other required dopants and mode to traverse the mandrel. The dopants oxidize in the oxy-hydrogen flame and are deposited on the mandrel in the form of soot. After all the required layers have been deposited, the ceramic rod is then removed and the Silicon dioxide powder for core and claddino

Ceramic substrate rod i

0 Figure 3.9 External vapour deposition in which silicon dioxide layers are deposited on to a ceramic substrate rod

Fibers and cables 87

hollow tube heated so that it collapses to form a solid rod. This method was originally developed by Corning.

3.4.2.3 Vapour axial deposition A porous rod is heated and variously doped silicon dioxide powders are deposited on it (next to each other as opposed to after each o t h e r - as was the case in the other two methods). As it grows, the porous glass body is d r a w n off and heated to form a narrow solid rod. One advantage of this process is that the porous rod does not have to be removed. The area of the end face, however, limits the a m o u n t of silicon dioxide that can be deposited and therefore the growth rate. Figure 3.10 illustrates the process.

t ~---'r

~

Core lind

Sintering

Drying Silicon dioxide

"~"powder

#

Figure 3.10

Vapour axial deposition of glass rod preform

The glass rod obtained from one of the above methods is heated in a high temperature furnace to 2000~ and a thin fiber d r a w n off as shown in Figure 3.11. It is important that this is done at a steady rate to minimize the a m o u n t of diameter variations in the fiber. Care must also be taken to ensure that the surface of the glass is not contaminated by foreign particles, as this will cause microcracks in the fiber. Most manufacturers n o w a d a y s look for a chemically pure glass with less than one foreign atom in 109 silicon atoms.

88 Optoelectronicsand Fiber Optic Technology

f i

Preform (core and cladding glass feed)

i ,/

High temperature heatingelement Interaction zone

~

Plastic coating

~

Fiber with polymer film UV radiator - ~ Fiber with polymer coating

,

Figure 3.11

'

' ,

- - ~

Fiber take-off

An illustration of the fiber drawing process

A plastic cladding is now applied to the glass fiber. This makes fibers easier to handle, protects them from microbending losses and mechanical damage and is normally about 280pm in external diameter. In special cases (for example, light buffer cable), this outer coating could be as much as 500 ~rn in diameter. To improve strength, fatigue resistance, resistance to increased hydrogen-induced attenuation and to protect against chemical corrosives such as hydrofluoric acid, a hermetic coating can be applied between the glass and the coating. Other coatings, such as silicon and polytetrafluoroethylene (PTFE) (Teflon), can be applied to allow fibers to be used at high temperatures (for example, up to 200~ while metallic coatings can make fiber sensors respond more strongly to magnetic fields and pressures. One of the problems of making a fiber in this manner is an undesirable dip in refractive index produced within the central core region. This is the direct result of the intense heat required for the collapsing and fiber drawing stages because heat causes the germanium dioxide core dopant to be evaporated away from the tube's inner surface. In multimode fibers, this refractive index dip can cause reduced coupling efficiency, reduced

Fibers and cables 89

Figure 3.12

Full length outside deposition (FLOD) of glass rod preform

bandwidth and measurement errors. In single-mode fiber, the dip can result in increased microcurvature sensitivity and changes in the cut-off wavelength or the shortest wavelength at which only the fundamental mode of an optical waveguide is capable of propagation. In an attempt to overcome these problems, AEG Telefunken developed a far more efficient method of applying the dopant, called full length outside deposition (FLOD). This process is shown in Figure 3.12. In addition to maintaining a virtually constant refractive index throughout the fiber, the FLOD method has the advantage that it does not require such a high operating temperature. In the process, a tubular glass body with concentric regions containing a number of doped glass layers, each having a different refractive index, is produced first. The glass body is then heated to its softening point and drawn into a glass fiber. During drawing, a partial vacuum is maintained in the hollow centre of the tubular body. The size of this partial vacuum is sufficient to reduce the evaporation of the doping material from the tubular body interior, which effectively minimizes the dip in the index of refraction in the centre of the optical fiber. Initially, optical cladding, consisting primarily of fused silica, is deposited on the inside of the substrate tube. Core material, which may also contain germanium, is then deposited at a reduced temperature to form a diffuse and permeable layer known as a 'frit'. After deposition of the frit, this partially completed preform is sealed at one end, removed from the lathe and a solution of suitable salts of the desired rare earth dopant (e.g. neodymium, erbium, ytterbium etc.)

90

Optoelectronicsand Fiber Optic Technology

introduced. Over a fixed period of time, this solution is left to permeate the frit. After discarding excess solution, the preform is returned to the lathe to be dried and consolidated. During consolidation, the interstices within the frit collapse and encapsulate the rare earth. Finally, the preform is subjected to a controlled collapse at high temperature to form a solid rod of glass - with a rare earth incorporated into the core.

3.4.3 Rare earth-doped fibers (Figure 3.13) Rare earth-doped fibers are used primarily in fiber amplifiers and fiber lasers. To make a rare earth-doped fiber, you need to start with a rare earthdoped preform using the solution doping process. The actual manufacturing process is similar to the other methods previously described.

Figure 3.13 Rare earth-doped fibers 3.4.4 Cap sealing To ease the problem of pulling an optical fiber through a cable duct, fibers are normally provided with factory fitted caps at both ends. Where this is not available, or short cuts of cable are being used, an 'eye' can easily be locally manufactured, as shown in Figure 3.14. In small capacity optical fiber cables, the factory-fitted cap is often transparent (see Figure 3.15) so that the fibers inside can be illuminated to facilitate testing for damage after installation etc.

Fibers and cables 91

Figure 3.14 Optical fiber with hand formed pulling 'eye'

Figure 3.15 Transparent cap with pulling head

3.5 Plastic fiber optic cables Fibers can also be manufactured from transparent plastic and these offer the advantages of a larger diameter (e.g. I mm), increased flexibility and, because the ends can be cut using a hot razor blade, ease of termination. Unfortunately, however, owing to the material's high intrinsic loss, the use of plastic fibers is normally restricted to only a few metres and to environments protected from temperature extremes. Plastic optical fiber (POF), however, offers noise immunity and low cable weight and volume, and is very competitive with shielded copper wire, thus making it suitable for industrial applications, such as those found in factories etc.

Note: Typically, POF has a continuous operating temperature rating of-55~ (-167~ to 85~ (+185~ Most POFs can withstand up to 100~ (212~ for short periods of time (less than 1 minute). POF is a total reflective type which has a concentric double structure consisting of a core of transparent polymethylmethcrylate (PMMA) with

92 Optoelectronicsand Fiber Optic Technology Table 3.3

Advantages and disadvantages of different fiber claddings

Type of cladding

Advantages

Disadvantages

Glass/glass fiber

9Clean fracture surface. 9Retention of optical characteristics.

9Less flexible than plastic fiber. 9Prone to damage.

Transparent plastic/ plastic fiber

9Large diameter. 9Increased flexibility. 9Ease of termination.

9High intrinsic loss. 9Additional losses at the fracture zone. 9Limited distances of a few metres and to environments protected from temperature extremes.

a high refractive index, covered by a thin layer of special transparent cladding material of a lower refractive index. Silica (glass) optical fiber has better light transmission characteristics (less loss) than POF and silica fiber can tolerate higher temperatures than plastic fiber. On the other hand, POF is more flexible, less prone to breakage, easier to fabricate into special assemblies, and lower in cost than glass fibers. Another advantage of glass/glass fiber is that a very clean fracture surface can be obtained which ensures that the fiber cladding inside the connector retains its optical characteristics right up to the end face of the fiber. With glass/plastic fiber on the other hand, some additional losses are introduced due to the fracture zone of the plastic, which, even after grinding and polishing, will still have microscopic end face absorption areas (Table 3.3).

3.6 Cable characteristics and specifications Two of the most important characteristics associated with the installation of fiber optic cables are the bending and deflecting diameters. The bending diameter of an optical fiber cable is the minimum permissible diameter to which the cable can be bent without damaging the conductors. The deflecting diameter is the minimum permissible diameter over which the cable can be drawn or guided while being subjected to a high tensile force.

Fibers and cables 93

Figure 3.16 Construction of a multi-fiber ruggedized optical fiber cable, suitable for harsh (e.g. military) environments With an all-dielectric (i.e. non-metallic) construction, cables can be bent to a comparatively small diameter without damaging the fibers. For example, cables with an outside diameter of up to 30 m m can be bent to 400mm. Cables with metallic elements, however, are not so flexible as they use wire armouring or have an aluminium-laminated sheath with copper conductors.

3.6.1 Tensile strength of the cable Nowadays, most cables utilize high tensile, low elongation elements of aramid yarn, encapsulated in an outer jacket with additional strength members (Figure 3.16). To meet the overgrowing requirement for heavyduty cables, these ruggedized cables typically have a tensile strength in excess of 250 lb.

3.6.2 Buffered fiber The fiber is coated with a 6 0 ~ m thick protective sleeving such as polythene, ultraviolet cross-linked acrylate, or similar material and then

94

Optoelectronicsand Fiber Optic Technology

~_~,.......~ Polythenejacket //~'~~'~ /~ ~

~

Buffertube- - ~ ~ ' 7 ~ / - " ~ ~ " ~ Fiber ~.~~ ) ) ) Coating - ~ ~ , ~ ~ ~ /

"",,~h ixotr;picfi~Iing ~ " (a) Single Figure 3.17 Construction

(b) Multi-fiber of a buffered optical fiber cable

placed either by itself (in which case it would look and act almost like a coaxial c a b l e - Figure 3.17a or together with a n u m b e r of fibers (Figure 3.17b) inside a double-walled buffer tube. Normally, and to enable easy identification, each fiber is colour coded with a 2 p m thin film of coating material. This coating has no effect on the optical properties of the fiber.

Figure 3.18 Example of fiber optic cable construction

Fibers and cables

95

The fibers are then encased in a polyisobutylene-based filling compound to provide an additional buffer between the fibers and the buffer tube. These thixotropic filling compounds (gases, gels, pastes etc.) cater for a wide range of environmental conditions, with temperature ranges of t y p i c a l l y - 3 0 ~ to +70~ Because of this wide temperature range it is essential that the filling compound does not freeze, swell, or run. The flammability of the buffer tube must also be insignificant and the tube must be capable of being easily cleaned. The tube and filling compound enable the core and fibers to adjust to a wide range of tensions and they are also impact resistant and laterally strengthened. Past experience has shown that the wall thickness of the buffer tube should be about 15 per cent of the overall buffer diameter and single-fiber buffers combined in one cable form should ideally have an outside diameter of 1.4mm while a 10-fiber buffer should have an outside diameter of approximately 3 mm. The cable is often then encased in a rugged, flexible, age-resistant polyethylene (or similar) jacket which has a relatively low coefficient of expansion over a wide temperature range to allow the cable to be placed in ducts and pulled through in a similar manner to copper cables. A further Kevlar strength member may also be included as shown in Figure 3.18.

3.7 Types of cable There are three basic design types.

3.7.1 Tight buffered cable Tight buffered cables (or stranded tube) cables are cables with stranded, loose buffer tubes around a central metallic or non-metallic (dielectric) strength member. They are small, compact, relatively inexpensive and are by far the most common type on the data communications market.

3.7.2 Loose tube Loose tube (or central tube) cables (Figure 3.19) have a central loose buffer tube and strength elements partly embedded into the sheath. The fiber is loosely laid into a hollow tube which may be filled with a chemically neutral thixotropic gel to prevent ingress of water. (Many of these tubes may be laid up together and sheathed to make the final cable.) This type is primarily used for telecommunications applications, major links and inter-building cross-connections.

96 Optoelectronics and Fiber Optic Technology

Figure 3.19 Loose tube fiber (courtesy of Brand-Rex)

3.7.3 Slotted core cable

Slotted core cable (Figure 3.20) is a ruggedized cable provided with helical grooves in which several fibers can be accommodated. The trouble with this type of cable is the difficulty in providing sufficient protection at junction points.

Figure 3.20 Example of a slotted core, non-metallic, fiber ribbon technology (courtesy of Coming Cable Systems GmbH)

Fibers and cables 97

Table 3.4 Typical buffered fiber optical fiber colour coding Fiber number

1 2 3 4 5 6 7 8 9 10 11 12

Colour

Blue Orange Green Brown Grey White Red Black Yellow Violet Pink (or light blue) Turquoise (or neutral)

The cables have a central member with longitudinal slots which hold the fibers.

3.8 Fiber optic cable ducts Quite frequently, a n u m b e r of optical fibers are gathered together in a random arrangement for use as a single transmission element, these are called fiber bundles. Fibers can, of course, be grouped together in a very similar manner to multipair copper cables and are normally contained in plastic ducts. To ease identification, individual fibers are colour coded. Table 3.4 shows an example of the colour coding used by manufacturers. Note: If there are more than 12 fibers in a tube they are usually 'bundled' together in quantities of 12 and held together with a coloured binding yarn.

3.8.1 Tube identification

The normal system for tube identification is the 'pilot directional' one. The pilot tube is red, the directional one green and all the other tubes are natural coloured.

98 Optoelectronics and Fiber Optic Technology

3.9 Cable construction The layout and construction of fiber optic cables will, naturally, vary from manufacturer to manufacturer, but Figures 3.21 and 3.22 indicate the general layout of medium and large capacity cables.

Unit buffer tube Filling compound

Outer jacket

(a) Single

(b) Multi-fiber

Figure 3.21 Construction of a typical medium capacity optical fiber cable

/

~

Polythene jacket Barrier layer Strengthening member of aramid yarn

~~~---~ ~~_~~,,~

~~<~~~ "~

Core filling compound Fibers

Buffer tube ~ Buffer

All-dielectric support element

Figure 3.22 Construction of a typical large capacity optical fiber cable

Ruggedization of the cable is possible and this can be achieved by utilizing some form of strength member (e.g. Kevlar), which is either laid helically or braided around the fiber coating. This is then surrounded by a tough outer sheath to provide the required environmental and mechanical protection (Figure 3.23).

Fibers and cables 99

Figure 3.23 Brand-Rex)

Example of a high fiber count, loose tube fiber (courtesy of

3.10 Types of ducted cable There are m a n y different types of ducted cable on the market today, and here are a few examples.

3.10.1 External duct cable Features

Properties

Descriptions

Loose tube fibers Robust construction Non-metallic Water barrier Coloured fibers Polyethylene sheath

Fiber count: 2 to 36 Pulling tension: 1000 N Temp. range:-20 to 60~ Mind bend radius: 100 mm Waterblocking: IEC 708-1 Crush: 1000 N via 50 x 50 mm plates

Incorporating gel filled (nonpetroleum gel) loose tubes, this cable range is suitable for external use, typically in wet ducts. The nonmetallic construction eliminates problems caused by earth loops and induced voltages caused by lightning strikes.

Picture

100 Optoelectronics and Fiber Optic Technology

3.10.2 Universal duct cable Features

Properties

Descriptions

Loose tube fibers Robust construction Non-metallic Water barrier Coloured fibers Low smoke halogen-free (LSHF) sheath

Fiber count: 2 to 36 Pulling tension: 1000 N Temp. range:-20 to 60~ Mind bend radius: 100 mm Waterblocking: IEC 708-1 Crush: 1000 N via 50 x 50 mm plates

This cable range embodies all of the non-metallic and water barriered properties of the external duct cable, combined with a low smoke halogen free (LSHF) outer sheath which is designed to meet internal fire regulations, thus allowing wet duct or in-building deployment.

Picture

3.10.3 Universal ruggedized cable Features

Properties

Descriptions

Loose tube fibers Ruggedized dual sheath construction Non-metallic Water barrier Coloured fibers High tensile strength Low smoke halogen free (LSHF) sheath

Fiber count: 2 to 36 Pulling tension" 2000 N Temp range:-20 to 60~ Mind bend radius: 100 mm Waterblocking: IEC 708-1 Crush: 1000 N via 50 x 50 mm plates

Designed to meet the arduous requirements of harsh external conditions and internal safety, this cable features all of the environmental protection of the external ruggedized cable plus an LSHF sheath to meet internal fire regulations.

Picture

Fibers and cables 101

Note: The diameter of the fiber is an important consideration. Increasing the diameter of the fiber will reduce tolerance problems, but the difficulty of producing a clean fracture surface is increased, fiber flexibility is reduced and costs are increased.

3.10.4 Water protection Should water penetrate through a damaged point in the sheath, special fillings prevent the water from migrating along the cable. This is achieved by the use of a gel-type filling compound a n d / o r a dry swellable element. For additional protection against moisture cables are also available with a laminated aluminium sheath (Figure 3.24).

Figure 3.24 Example of a stranded tube design, non-metallic control member, dry core, moisture barrier (courtesy of Corning Cable Systems GmbH) 3.10.5 Rodent protection Damage due to rodents can be a problem with direct buried cables and when they are laid in shafts and duct systems. Protection is usually achieved by incorporating non-metallic armouring and protective poly-

102 Optoelectronics and Fiber Optic Technology

Figure 3.25 Example of a stranded tube design, non-metallic, rodent protection, oil resistant fiber (courtesy of Corning Cable Systems GmbH)

amide jackets (Figure 3.25). Non-metallic protection has advantages in that there are: 9no problems with grounding or potential equalization; 9 no lightning protection measures are required; 9 buildings can be electrically isolated.

Non-metallic armouring includes laminated glass-fiber yarns (which also serve as strength elements) and a polyamide (the hardest plastic used for sheathing cables) jacket. Metallic armouring, however, is still probably considered the most effective protection against rodent damage but it has cost limitations.

3.10.6 Metallic armouring Whenever a cable is directly buried (i.e. there are no ducts available or ducting is unsuitable), the fiber cable has to be given some type of additional strengthening. This is usually some form of metallic armouring such as corrugated steel tape under the outer jacket (Figure 3.26).

Fibers and cables 103

Figure 3.26

Example of a stranded tube, non-metallic central membrane, dry core, corrugated steel tape cable (courtesy of Corning Cable Systems GmbH)

A special coating is used to bond the outer jacket to the steel tape. Where the mechanical loads are exceptionally high, a further inner jacket can be inserted underneath the steel tape. 3.10.7 Hazardous environmental cable In areas such as airports or in the vicinity of high voltage power lines, some form of protective armouring is desirable but, because of the dangers of lightning strikes, a completely non-metallic construction is preferred (Figure 3.27). In this particular example, two individually coloured primary-coated optical fibers are housed within a gel-filled polyester tube. Up to 12 of these tubes can be helically stranded around a non-metallic central strength member. The spaces between the tubes are then filled with a non-halogenated water blocking compound. The inner sheath is made out of polyethylene whilst the outer sheath is made from a hard grade nylon. Normally the cables are weatherproof and resistant to attacks by rodents and insects.

104 Optoelectronicsand Fiber Optic Technology

Figure 3.27 Example of a stranded loose tube, non-metallic, armoured cable (courtesy of BICC Brand-Rex)

They are also ideal for chemical and petrochemical environments since the combination of nylon and polyethylene affords great protection against chemicals and solvents.

3.11 Installation possibilities Optical fiber cables, because of their mechanical strength and low weight, can be pulled into ducts, ploughed in, cleated to walls, installed in vertical runs of over 1000m in length (this is particularly important in multi-storey office blocks and mines etc.), placed on cable trays and planer shelves (or PVC conduit) installed aerially, attached to supporting wires (e.g. high voltage cables), or even immersed in water (oceanographic systems). The technique used, of course, depends not only upon the environment but on the length of the cable to be installed. Today, one of the most practised methods of installing short lengths into highly populated duct systems is to use compressed air to inject an auxiliary rope. This will then be connected to a winch rope, which in turn will be connected to the cable. Ideally, however, a system of ducts should be included in preliminary drawings of all new buildings. The price of the actual duct is very cheap and the building can be constructed so that it has a cost-effective futureproof wiring system capable of meeting changing communication requirements. Because optical fiber cables are very light, they can be incorporated into overhead power line routes without any additional stress being exerted on to the existing mast (Figure 3.28). In addition, as the optical fiber cable is completely non-metallic, any problems associated with inductive interference are eliminated. The process is very similar to installing normal copper aerial cables and the optical fiber is merely lashed to the phase conductors or ground wires of the overhead power line.

Fibers and cables 105

Figure 3.28 Scott)

Typical pylon incorporating fiber optic cables (courtesy of David

3.11.1 Training The methods previously used in copper cable installation are almost identical to those used for optical fibers and the use of optical fiber cables requires very little additional or specific training except with respect to field maintenance.

4 Transmitters - light emitting diodes a n d lasers

4.1 Light source At each end of the fiber optic link is a light source (called an electrooptical transducer), which changes electrical energy into optical energy and vice versa. The ideal requirements for this light source are: 9 9 9 9 9

small spot size; fast switching speed; high intensity; monochromatic light; operation at wavelengths between 800 and 1600nm.

4.2 Conversion of electrical energy into light waves The basic function of an optical transmitter is to convert the electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may be turned on and off or may have its intensity varied sinusoidally between two predetermined levels. Figure 4.1 shows a graphic representation of these two basic schemes. This conversion of electrical energy into optical energy is achieved by the use of an optoelectronic semiconductor device, such as a light emitting or laser diode. Although the radiation from a light emitting diode (LED) is quite safe to the naked eye, there is, of course, a potential hazard with the use of lasers and high radiance LEDs because of the damage that these high intensity point sources can cause through absorption of energy inside the eye's retina. In view of this potential health hazard, it is wise to supply operators, particularly those with constant access, with protective glasses

Transmitters - light emitting diodes and lasers

107

Intensity

On/off modulation

Figure

4.1

Linear modulation

Modulation methods

and insist, as a precaution, that no energized optical source or illuminated fiber is viewed through a microscope. The main requirements for lasers and LED devices is that they should: 9 be small; 9 be rugged; 9 possess a long operating life (at least comparable to other components within the system); 9 have an emission only in the low attenuation region of the optical waveguide; 9 have a high coupling efficiency between semiconductor and optical waveguide; 9 have a large radiating power (i.e. high electrical/optical conversion efficiency); 9 be capable of simple modulation from a transmission signal; 9 have an economical construction; 9 have low power consumption. See Table 4.1 for a comparison of LED and laser characteristics.

4.3 Light emitting diodes With certain reservations, silicon and germanium compound semiconductors meet these requirements, but as they both have a low breakdown voltage (caused by low electron mobility) and as they are very brittle, increasing use has been made of gallium arsenide. Although these are more expensive to manufacture, they have the advantage that they have a high breakdown voltage, high electron mobility, are a high speed device and so give a relatively greater bandwidth. The structure of

108 Optoe/ectronicsand Fiber Optic Technology

Table 4.1

Comparison of LED and laser characteristics

Characteristic

LED

Laser

Output power Speed Output potential Numerical aperture Spectral width Single-mode capability Throughput Distance Cost Ease of operation Frequency bandwidth Medium

Lower Lower Higher Larger Wider No Lower (MHz) Shorter Lower Easier 850 and 1310 nm Multimode

Higher Higher Lower Smaller Narrower Yes Higher (GHz) Longer Higher Harder 1310 and 1550 nm Single mode

a typical high radiance gallium arsenide (GaAs) LED is s h o w n in Figure 4.2. An LED is essentially a pn semiconductor that emits light w h e n a forward bias is applied (i.e. the negative battery terminal connects to the n-type material). The forward bias causes the electrons and holes to move towards one another and cross the depletion area of the junction. Electrons and holes combine and cause light to be emitted and as long as the voltage is applied, the electrons will keep on flowing through the diode and recombination will continue at the junction. An LED is, therefore, a very efficient semiconductor diode that converts electrical energy into light in a process k n o w n as spontaneous emission.

Figure 4.2

Small area, high radiance, gallium arsenide homojunction LED

Transmitters - light emitting diodes and lasers

109

Note: As the lightwaves are not in phase (as is the case of light emitted from a normal standard light bulb), there is no optical feedback and the light is said to be 'coherent light'.

In many semiconductors, the energy produced by recombination is released as heat; however, in materials used in the manufacture of LEDs, the recombination energy is released as a photon of light which can emerge from the semiconductor material. The materials used in the manufacture of LEDs (e.g. aluminium (A1), gallium (Ga), indium (In), phosphorus (P) and arsenic (As)), influence the wavelength emitted and the amount of energy released. GaAs LEDs will normally emit light at nearly 930 nm. Adding aluminium can increase the band gap and shift the emission to wavelengths of 750-900nm. Other semiconductor compounds (for instance, GaAsP) will produce wavelengths of 655nm (visible red) and InGaAsP the 1300 and 1550nm wavelengths. In practice LEDs are very rarely used in 1550 nm systems. There are various disadvantages associated with using an LED as a transmitter. For instance, electrons can recombine without emitting light, they lose energy because they collide with other electrons (the Auger effect) and the high refractive index of the semiconductor causes refraction and partial refraction of light at the semiconductor/air interface. Simple LEDs, such as the one shown in Figure 4.3, will emit light in all directions but are constructed so that light is emitted from the surface in a broad cone. More complex internal structures can concentrate the emission of surface emitting LEDs into a narrower angle (Burrus diodes) while other configurations can cause the light to be emitted from the edge of the LED in a more concentrated beam. Although this will produce more power, it requires more complicated circuitry to dissipate the extra heat. In general, the more complex the LED structure, the brighter and more collimated is the emitted light and the faster the LED can be modulated. As the average lifetime of a radiative recombination is only a few nanoseconds, the modulation bandwidth of an LED is limited to only a few hundred megahertz as opposed to laser diodes, which can be modulated to many gigahertz. Another disadvantage of LEDs is their low coupling efficiency and the large amount of inherent chromatic dispersion. On the other hand, the main advantage of using LEDs in optical fiber systems is that their small core diameter permits an enhanced coupling efficiency of the light into the optical waveguide. With a spectral width of about 40nm, an LED has a bandwidth of at least 100 MHz and about

110 Optoelectronics and Fiber Optic Technology

Figure 4.3 A selection of typical LEDs (courtesy of LaslRvis)

50~W of transmitted optical power launched into the fiber. Other advantages include: 9 9 9 9 9

simple design; ease of manufacture; simple system integration; low cost; and high reliability.

As the light and current characteristics of LEDs are only slightly dependent upon aging and temperature, the design of an LED optical transmitter is comparatively simple and virtually only consists of a driver to modulate the LED.

4.3.1 LED preparation For years, LEDs, like most transistors, were manufactured by the simple process of diffusing an element on to a crystal substrate. With the development of growth techniques, this practice is now almost nonexistent and the following are the preferred methods: 9 liquid phase epitaxi: where extreme heat (as much as 800~ in the case of gallium aluminium arsenide) is used to melt the source slice to provide a crystal deposit on the substrate;

Transmitters - light emitting diodes and lasers

111

9 vapour phase epitaxi: elements of the crystal are passed over a heated substrate and deposited epitaxially; 9 molecular beam epitaxi: the material is transferred directly as an ion beam from a heated source to a coated crystal substrate.

4.4 Types of LED 4.4.1 Edge emitting diodes The manufacturing technique of edge light emitting diodes (ELEDs) ensures that the inner p-n junction has a low energy band gap whereas the outer regions have a relatively wide band gap. This double heterostructure make-up restricts the production of photons to a narrow concentrated beam because, although they might have sufficient energy to drift across the inner forward biased junction, they cannot overcome the higher barrier potential associated with the outer regions. The structure of an ELED is shown in Figure 4.4.

p-contact Metal contact p-lnp SiO2 GalnAsP n-contact I

I//

n-lnp

Figure 4.4 Edge emitting LED structure

Lateral spread is restricted by having a narrow width electrode structure. In a similar manner to that of the fiber cladding, the outer layers also have a lower refractive index, which helps to confine the emitted photons by internal reflection. ELEDs are capable of delivering 30 ~W into a single-mode fiber which far exceeds the 5 ~W or so normally expected with most LEDS.

112 Optoelectronics and Fiber Optic Technology

Edge emitting diodes have a comparatively thick active region that produces low self-absorption and an edge radiance far greater than the radiance viewed perpendicular to the junction. By varying the mixture of the junction, different energy gaps can be produced which will provide different beam concentrations.

4.4.2 High radiance light emitting diodes Although LEDs operating in the visible 900 nm red region are suitable for simple applications, systems operating in the infrared spectral region can provide transmission rates varying from a few Mbit/s -1 to more than 100Mbit/s -1 over repeater distances of 15 to 20km or more. High radiance LEDS, emitting at 850 and 1300 nm, are often used as they have an intrinsically high quantum efficiency due to the advantages gained by the double heterostructure of the diode.

Note: Quantum efficiency- the average number of electrons for each absorbed quantum of light emitted.

By designing the diode so that the size of the emitting surface layer is very limited, the emission wavelength can be set to low fiber attenuation and dispersion. This method ensures that a low thermal resistance is maintained and a high radiance (therefore a high radiant power) is coupled to the fiber.

4.4.3 Diffused gallium arsenide LEDs emitting at 900 nm This particular type of diode utilizes planer technology and a typical example would be constructed as shown in Figure 4.5. The low dislocation density (less than 4000cm -2) of the n-doped substrate wafer minimizes losses due to non-radiative recombination at crystal defects. The p-region acts as a diffusion mask and the p-n junction is graded to reduce lattice strains and additional losses due to, for example, crystal defects. To improve the radiation output of the diode, the final p-surface is coated with an anti-reflecting layer of silicon nitrate. The emission wavelength of this particular type of diode would normally be about 900nm, the spectral bandwidth approximately 40nm, and the radiant power would be roughly proportional to the forward current (typically 120t2W at 100mA). The average lifetime (i.e. time until the radiant power is reduced by 50 per cent) of one of these types of LED is approximately 105 h and diffused

Transmitters - light emitting diodes and lasers

113

Anti-reflective nn~tinn

p-contact AI203 insulation )d-GaAs iffused) ,=,_

n-GaAs (substrate) n-contact

,,,

,,,

,

Figure 4.5

=

,

,

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Structure of a diffused gallium arsenide LED

GaAs LEDs are suitable for transmission rates up to about 5 M b i t / s -1. Doping the p-region to a higher level, although facilitating large modulation bandwidths, would reduce the amount of radiant power available.

4.4.4 Aluminium gallium arsenide/gallium arsenide (AIGaAs/GaAs) high-radiance LEDs emitting at 830 nm In this type of LED, shown structurally in Figure 4.6, three AIGaAs layers of varying thickness and doping levels (carefully selected to prevent any appreciable current spreading within the layer) are grown epitaxially on the GaAs substrate. To ensure that the A1GaAs layers have an extremely small transmitting area, the epitaxial layers point downwards. This is sometimes known as the 'upside-down configuration'. The diode is mounted on a thin gold heatsink and soldered to a silicon chip containing a conductor track and insulation layer. Thermal resistance is minimized by locating the p-contact and the p-n junction near the

Light emitting area

n """-'"*

n = AIGaAa

n-GaAs Sma p col Conducting lead ~ ~ S 1C2 insulation Silicon chip

Figure 4.6

......... ----

.........

Structure of an AIGa/GaAs high radiance LED

p-AIGaAs (captive layer) p-AIGaAs (active layer) ALO3 insulation Heatsink

114 Optoelectronics and Fiber Optic Technology

heatsink. The high aluminium content of the two AIGaAs layers ensures that a potential barrier is formed to reduce the a m o u n t of electrons and holes that would otherwise be capable of entering the active region. Q u a n t u m efficiency depends not only on the crystalline quality but also, to a significant degree, on the doping and composition of the active layer. O p t i m u m values are achieved with compounds for wavelengths between 800 and 890nm and doping levels around 2 x 101S/cm 3. Such a diode would exhibit a rise time of approximately 15 ns. By using A1GaAs layers the possible reduction of q u a n t u m efficiency due to non-radiative recombination is minimized. The diode is then encapsulated within a very thin protective film and produces a spectral b a n d w i d t h of about 45 nm. As the temperature rises, the emission peak is displaced to longer wavelengths at the rate of 0.3 to 0.4nm per Kelvin. Due to the small p-contact area, the diodes have a series resistance of 5 ohms. The voltage requirement for 100 mA is about 1.9 V and this produces a rise time of 15 ns. Shorter rise times (down to about 4 ns) are easily realized but only with a disproportionate decrease in optical power. It has been stated that if the temperature rises the powers decrease by 0.3 to 0.4 per cent per Kelvin. At 100mA (dependent upon temperature) an optical power of approximately 4 m W is produced which would launch approximately 60laW of optical power into a graded index fiber and 700 laW into fibers with a large core area of (for example) 100lam. Tile life expectancy of a typical A1GaAs/GaAs high radiance light emitting diode is ill excess of 10~ h.

Note: Laser diodes, manufactured using InGaAs quantum wells surrounded by AIGaAs guide layers, emit in the 900-960 nm range. Conventional AIGaAs lasers operating in the 800-870 nm range are subject to high internal absorption losses which not only reduce efficiency but which are also the main cause of catastrophic optical damage (COD) at the emitting facet of the device. The addition of in to the 'active region' shifts the wavelength outside the absorption band of GaAs. It is feasible to provide a laser diode with a simple 75 IJm stripe contact operating in excess of 1 watt without risk of COD.

4.4.5 InGaAsP/InP high radiance LEDs emitting at 1300 nm Figure 4.7 is a typical quartenary high radiance LED and has basically the same construction as those previously described. A p-doped layer of InGaAsP is lattice-matched to an InP layer. This acts as a barrier and

Transmitters - light emitting diodes and lasers

Figure 4.7

115

InGaAsP/InP high radiance LED structure

ensures a low resistant p-contact, but obviously even a slight lattice mismatch will reduce efficiency. To reduce reflection, the diode is given a thin coating of silicon nitrate. At 1300nm the substrate is transparent and so hole etching is not required. This type of LED would expect to have a rise time of approximately 8ns and a fall time of 18ns thus making it suitable for 3 4 M b i t / s -1 transmission rates. Rise and fall times can be reduced to 3 and 6 ns (for 140Mbit/s -1 transmission rates) by increasing the doping level, but of course there would be a corresponding reduction in radiated power. Although this particular type of LED reacts to temperature changes rather more than the others previously described (0.7 per cent per degree) they have a very low ageing rate (typically (10 6 h).

4.4.6 Indium phosphide Gallium arsenide (GaAs) and indium phosphide (InP) are Type III-V compounds, so called because they combine non-metallic elements from the third and fifth rows of the periodic table. Both offer considerable improvements over silicon in such key characteristics as electron mobility and resistance to radiation. Made of deadly poisons, these compounds are not only susceptible to defects during the crystal growing process, but also tend to result in very brittle devices and so are confined to market niches for the time being.

116 Optoelectronics and Fiber Optic Technology

Having said that, both GaAs and InP can emit light when a current is applied, something that silicon cannot do, and this property has led to the development of laser diodes for integrated circuits that combine optical and electronic functions on the same chip. The electron-to-electron capability has made Type III-V devices generally, and InP devices specifically, very attractive for tactical military applications as it allows them to be interfaced directly with the optical fibers used in guided missiles (such as fiber optic guided mortar rounds for the Army, air-to-air and submarine-launched missiles for the N a v y and air-to-ground bombs for the Air Force). GaAs emits light at 850nm, the wavelength of multimode optical fibers, but InP can emit light at the 1300 and 1550nm wavelengths of single-mode optical fibers. By operating at 1300 nm, (the region of least dispersion), or 1550nm (where the least light is absorbed), single-mode fibers reduce signal loss. Also, because the light no longer has to bounce around within the glass strands, the single-mode fibers can be made much smaller; typically less than 10 microns (i.e. millionths of a metre) in diameter instead of about 100 microns for multimode. By comparison, the average human hair is 100-150 microns in diameter.

Note: Corning Glass Works has developed what it calls its Corguide dispersion-flattened single-mode fiber that operates at both 1300 and 1550 nm.

4.5 Laser diodes In 1960 the laser was developed, and a milestone reached. The l a s e r - an acronym for light amplification by stimulated emission of r a d i a t i o n - is basically a source of light that is emitted when an electron in a semiconductor drops from a high energy level to a lower one and needs to get rid of its extra energy. In a laser, the photon of light produced stimulates other photons which have the same wavelength as the original photon in a process known as 'stimulated emission'. After a photon has been generated, one of two things can happen to it. It can either be radiated from the semiconductor or it can be absorbed in the creation of a hole-electron pair. If a photon should happen to be incident on a free electron in the conduction band it can stimulate the electron to prematurely recombine and in doing so emit a photon that is coherent (i.e. in step, in phase and monochromatic) with the incident photon.

Transmitters - light emitting diodes and lasers

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117

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Figure 4.8 Optical characteristics of an LED and laser compared

The power of two coherent photons is twice that of two non-coherent photons, so if the diode current is above a critical level then there is a chance that the two photons will stimulate further emission (before they are either radiated or absorbed) which causes a rapid increase in radiant power. This action is called 'lasing'. The laser beam is formed by a resonator, or optical cavity, which confines the light and causes it to pass again through the excited medium. The resonator can be a pair of mirrors, or facets at the end of a semiconductor crystal. The rear facet is coated with a reflective layer so that all light that strikes it is reflected. The front facet is left uncoated so that most light escapes, but in practice some light is reflected back into the material. The output of a laser depends on the drive current passing through it. At low drive currents the laser operates like an inefficient LED, but as drive current passes a threshold value, lasing begins. A laser diode is, therefore, essentially an LED with a wavelength selective element. Figure 4.8 shows a graph comparing optical powers of LED operation (due to spontaneous emission) and laser operation (due to stimulated emission), while Figure 4.9 shows spectral and spatial distribution differences between the two types of diode. At low current the laser diode acts in a similar manner to a normal LED while above the threshold current stimulated emission (i.e. the narrowing of the light ray to a few spectral lines instead of a broad spectral distribution) takes place. This not only enables the laser to be easily coupled to a single-mode fiber but also significantly reduces the a m o u n t of uncoupled light (i.e. spatial radiation distribution).

118 Optoelectronics and Fiber Optic Technology

Figure 4.9

and LEDs

Comparing (a) spectral and (b) spatial distribution of laser diodes

Because the size of the threshold current is dependent upon temperature and ageing, the drive current must be regulated to provide a constant power output. To achieve this, light from the laser's back mirror enables the output of a photodiode to regulate the drive current. Owing to their size, ruggedness and the need for only a very low supply voltage to produce high speed modulation, the laser diode is a most cost-effective and efficient light source for long-haul, high bit rate, communication systems. As a rule, at the lower end of tile light spectrum (820 to 880 nm) gallium aluminium arsenide oxide stripe lasers are used, while laser diodes with low fiber attenuation and a lower threshold

Figure 4.10 A typical laser (courtesy of LaslRvis)

Transmitters - light emitting diodes and lasers

Figure 4.11

S e m i c o n d u c t o r solid state laser

119

(courtesy of LaslRvis)

current (e.g. gallium indium arsenide potassium and indium potassium) are normally used at 1300 to 1600 nm. If in the double heterostructure ELED (see Secontion 4.4.1), the end faces of the emissive p-n region are cleaved and polished, sufficient photons would be reflected to maintain lasing action provided that the diode current is above the critical value. The region between two plane parallel, semitransparent crystal surfaces acts as a Fabrey-Perot cavity (i.e. it performs as a variable bandpass filter or spectrometer) and is similar in action to a waveguide resonant cavity in that it has very high Q, resulting in a narrowing of the spectral width of the laser emission. As previously mentioned, the principal characteristic of a laser diode is that the light rays are coherent: they are in phase, they travel in the same direction and they are virtually the same wavelength. This ensures that the laser light maintains a high energy density and that the ray does not diverge by any significant amount. Being coherent sources, laser diodes are virtually always used with m o n o m o d e fibers. Although gas solid state lasers (e.g. helium, neon, argon etc.) (Figure 4.11) have been used in the past, because of their size, cost and low efficiency, they are usually considered unsuitable for optical fiber applications and nowadays semiconductor injection lasers are used for most of the long-haul and high bandwidth applications.

4.5.1 Advantages Although the cost of a laser diode can be as much as ten times that of a simple LED, the advantages of using lasers instead of LEDs include:

120

9 9 9 9 9 9 9 9 9

Optoelectronics and Fiber Optic Technology

simple economical design; high optical power; production of a light that can be precisely controlled; the ability to work at increased temperatures; an improved modulation capability (bandwidths well into the GHz range); high coupling efficiency; low spectral width (3.5 nm); the ability to transmit optical output powers between 5 and 10 mW; the ability to maintain the intrinsic laser characteristics over long periods.

In addition, the linearity of an optoelectronic transmission system is mainly determined by the light current characteristics of the transmitter as opposed to the non-linear distortion caused by the receiver and transmission medium which, unless very high transmission powers are used, are almost negligible. Thus, although an LED exhibits signs of saturation at the higher power levels, the operational laser diode over a similar power range tends to improve as the power increases (see Figure 4.8).

4.5.2 Disadvantages It should be noted, however, that, depending upon their relative phases, two coherent light beams could add or subtract their electric fields. This causes a speckle pattern at the end of the fiber as illustrated in Figure 4.12. Because of fewer transmission modes, graded-index fiber will show a more pronounced speckle pattern than step index fiber. Although modulation rates are extended into the gigahertz range by the use of a DC bias, one of the main disadvantages of using a laser diode is that it is extremely sensitive to overload currents and at high transmission rates, when the laser is required to operate continuously, the use of a large drive current produces unfavourable thermal characteristics and necessitates the use of cooling and power stabilization.

Figure 4.12

Two coherent light beams causing a speckle pattern at the end of an optical fiber

Transmitters - light emitting diodes and lasers

Confining layer

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Advantages of a double heterostructure make-up

121

122 Optoelectronicsand Fiber Optic Technology

Owing to the reduced number of electron carriers (caused by diffusion and equalization of electrons and holes), stimulated emission from a single p-n junction (as used in earlier laser diodes) only occurs at high threshold currents. In later models this has been overcome in a double heterostructure make-up, by sandwiching the active semiconductor layer between two semiconductor layers with a wider band gap. This structure prevents electrons injected from the n-side and holes injected from the p-side spreading outside the active region. Carrier confinement has thus been achieved. Various characteristics of a double heterostructure laser are illustrated in Figure 4.13. As the wider band gap effectively has a smaller refractive index, sandwiching the active layer between two such layers produces a similar effect to that found in a glass fiber waveguide and the light wave is concentrated. The band gap (and hence the wavelength of the emitted light) can be varied by epitaxially depositing a new semiconductor on the substrate.

Note: Epitaxial l a y e r - a layer of semiconductor material having the same crystalline orientation as that of a substrate upon which it is grown.

4.5.3 D e s i g n of a laser d i o d e

Figure 4.14 shows the spatial confinement of a laser diode's active region. Longitudinal confinement of the electrical region around the p-n junction Lsteral

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Spat~a~ confinement of a laser diode's active region

Transmitters - light emitting diodes and lasers

123

and the optical cavity is achieved by cavity mirrors. Vertical confinement is achieved by the two mirror surfaces and lateral confinement by additional internal structures. Lateral confinement of the active region to a few microns is very important, as this will restrict the laser's oscillation to the fundamental transverse mode and thus prevent high order modes being propagated. There are obviously quite a number of different types of lasers on the market today, but the most commonly used are those that depend upon the type of lateral waveguide that is used. The two main types in this category are laser diodes with a current-induced waveguide and laser diodes with a built-in waveguide. 4.5.3.1 Laser d i o d e s with a current-induced w a v e g u i d e

Figure 4.15 shows a typical oxide stripe laser diode with a currentinduced waveguide. In this sort of laser, the current carriers are concentrated onto an active stripe-shaped window, which causes an imaginary refractive index to assume a lateral profile. Contact window

Metal contact Oxide mask (AIO) Y / / / / 2 / / / / / / Z / 2 / / ~ //,/Tf,~L,,~-- Capping layer

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Oxide stripe laser diode structure

As this lateral profile corresponds to an optical gain profile they are usually referred to as gain-guided laser diodes (GLDs). Other types of GLDs are proton-isolated lasers and v-groove lasers.

124

Optoelectronics and Fiber Optic Technology

4.5.3.2 laser diodes with a built-in waveguide Examples in this particular group are buried heterostructure (BH) lasers channelled substrate planer (CSP) lasers and metal clad ridge waveguide (MCRW) lasers. The MCRW laser structure is shown in Figure 4.16. Lateral current confinement and waveguiding are achieved simultaneously, owing to the blocking effect of the metal contact on the lightly doped capping layer outside the stripe. The increased reflection of the optical wave from the metallization on both sides of the mesa stripe causes an effective refractive index and hence an optical waveguide. As all of this particular family have a built-in refractive index profile, they are also known as index-guided laser diodes (ILDs).

,

-Capping layer

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Metal clad ridge waveguide (MCRW) laser diode structure

4.6 Basic differences between gain-guided current-induced) and index-guided (built-in waveguide) aser diodes (Figure 4.17)

t

There are a number of basic differences between gain-guided and indexguided laser diodes. These include: 9 threshold current: as the threshold current for an index-guided laser diode is approximately 20 per cent of that of a gain-guided laser diode, the amount of heat produced in an index-guided laser is much lower than a gain-guided laser; 9 spectral width: ILDs normally only emit one dominant spectral line at the peak of the gain curve. This characteristic is not maintained at high modulation rates, however, as the spectral position of maximum gain

Transmitters - light emitting diodes and lasers

Optical feedback sensitivity

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125

Spectral width

Basic differences

Spatial radiation distribution

Figure 4.17

Basic differences between gain-guided and index-guided laser

diodes

varies with modulation. The dynamic spectrum broadens and the number of oscillating modes rises with transmission rate; 9 temperature: temperature affects laser diodes enormously. In 850nm lasers, the differential gain is lowered by typically 0.8 per cent per degree and the threshold current (i.e. lasing point) rises by 1 per cent per degree. In 1300 nm lasers, on the other hand, threshold current rises 2 per cent per degree. Increases in temperature particularly affect ILD and GLD lasers, causing them to shift to a longer wavelength by as much as several tenths of a nanometer per degree. In GLDs this would normally only cause an envelope shift but in ILDs this could cause mode jumping; 9 spatial radiation distribution: the far field of a laser (i.e. the spatial radiation distribution measured some distance away from the laser mirror) is affected by the size of the cavity and is usually wider than

126 Optoelectronics and Fiber Optic Technology

conventional solid state lasers. Because of the differences in lateral and longitudinal dimensions the spatial radiation is normally asymmetrical; 9 optical feedback sensitivity: although maximum launching efficiency is desirable, the requirement to minimize the amount of reflected light feedback must also be considered. The optical feedback sensitivity varies widely between the different types of diode. ILDS, because of their high Q factor, have a high sensitivity while a GLD's feedback is intrinsically lower.

4.6.1 Types of laser diodes The main types of laser diodes are:

9 stripe geometry; 9 oxide stripe; and 9 vertical cavity surface emitting.

4.6.1.1 Stripe geometry lasers Stripe geometry lasers are gain-guided laser diodes. The principle behind a stripe geometry laser (Figure 4.18) is that the active width does not extend to the edge of the semiconductor chip and optical confinement is provided by the refractive index at the heterojunction interfaces.

Figure 4.18 Structure of a diffused planar striped geometry laser diode

4.6.1.2 Oxide stripe lasers An essential part of an oxide stripe laser's highly stable format is its ability to handle high pulse powers and hence have a reduced sensitivity to spikes from electronic supply and regulating circuits. Oxide stripe lasers (of GaA1As/GaAs construction) are used in optical multimode systems where the band gap of the GaAs determines the long wavelength limit and the AI content varies short wavelength limit. Oxide stripe lasers have a low sensitivity to temperature variations up to 100~

Transmitters - light emitting diodes and lasers

127

and do not require additional cooling. They are suitable for the transmission of large volumes of data over quite large distances.

4.6.1.3 Vertical cavity surface emitting laser In experiments conducted by the Institut for Festk6rperelektronik, Technische Universit~it Wein, two-dimensional arrays of vertical cavity surface emitting laser diodes (VCSELs) were made using an ion milling etching technique. This was a complex process requiring more sophisticated etching techniques than the simple wet-chemical technique. VCSELs are grown by molecular beam epitaxy (MBE) or by metal organic chemical vapour deposition (MOCVD). The geometry of a VCSEL structure is shown in Figure 4.19. First a Bragg mirror, which consists of 20-30 pairs of alternating GaAs/AI(Ga)As layers, is grown on the substrate. The thickness of these layers is typically between 60 and 80nm. Next is a cladding layer of A1GaAs, then a GaAs active region and finally a further cladding layer of A1GaAs. A second Bragg mirror on top, which consists of another 20-30 pairs alternating GaAs/ A1GaAs layers, completes the structure. The total thickness of the entire grown structure is 3-5 lJm.

l Figure 4.19 The geometry of a VCSEL structure with a Bragg mirror on both sides of the substrate side and on top Note: In addition to conventional VCSELs with Bragg mirrors on the substrate and topsides, frequency tunable VCSELs with integrated

modulators have also been processed. This type of VCSEL is very important for wavelength division multiplexing techniques in optical communication and they have an integrated modulator diode in the cavity, but no Bragg mirror on top.

128

Optoelectronicsand Fiber Optic Technology

VCSELs are an excellent type of surface-emitting laser due to their small size, planar geometry, integrability and potential for high power and high quality beams. VCSELs can be fabricated in two-dimensional arrays, because they have integrated resonator mirrors (Bragg mirrors) and require no cleavage. They are ideal candidates as light emitters in integrated optoelectronics devices. 4.6.1.4 laser diodes in the 1300 to 1 6 0 0 n m region

Because fiber's attenuation is lower in the 1300-1600nm region compared with that at 850nm, repeater spans are increased. Dispersion characteristics of the fiber are important and as a result, the previously mentioned dynamic spectrum broadening of lasers (due to the FabreyPerot cavity) becomes a limiting factor for transmission over long spans. However, a feature of the transmission band between 1300 and 1600 nm is that the overall dispersion of a single-mode fiber can be made to disappear almost completely at a particular wavelength thus negating the effect of dynamic broadening. Although most laser diodes used for the long wavelength range are fabricated predominately from GaInAsP/InP, one of the problems associated with this type of diode is the increased amount of nonradiative recombination caused by increasing wavelength that limits the length of the wavelength. As found in other communication methods, the phenomenon of phase interference must be considered. Phase interference is caused when one light wave is superimposed on another at a different phase causing attenuation when antiphase and reinforcement when in phase. In any particular fiber there will of course be hundreds of waveforms (called eigenwaves or fiber modes) that can be propagated without interfering with each other. The effect of delay differences between light components with different propagation paths is, however, the controlling factor with regard to the bandwidth of a fiber. 4.6.2 Microdisks

T&T Bell Laboratories recently claimed the record for the world's smallest laser by etching 'microdisks' from wavers of semiconductor laser material. The disks are 2-10~m in diameter, under 0.2 ~m thick and consist of either one or six 10 nm quantum wells of InGaAs sandwiched between 10nm barrier layers of InGaAsP, with 20nm end caps of InGasAsP. They are grown on an InP substrate, then etched to remove the InP which isolates them from the bulk semiconductor, except for a thin support column. A scanning electron microscope shows them as thumbtack-like structures.

Transmitters - light emitting diodes and lasers

129

Instead of oscillating between a pair of mirrors, the light circulates around the rim of the disk, confined by total internal reflection because of the large difference between the 3.5 refractive index of the semiconductor and the nominal 1 index of air. The developers say the structures also work if encapsulated in silica, because its refractive index is much lower than that of the semiconductor. Laser light emerges from the side and the top of the tiny disks.

4.6.3 Multiple signalling Scientists at GTE Laboratories in Waltham, Mass., have developed a diode laser array capable of sending telecommunications signals into four separate optical fibers simultaneously. The new device is expected to lower the cost of distributing voice, data and video signals over fiberbased networks dramatically. In the longer run, it might also be advantageous in helping high speed computers handle large data loads, as in parallel processing computers. As C.D. Decker, Vice President and Director of Research at GTE Laboratories, said: In essence, we have found a way to position four separate singlemode fibers so that they are automatically aligned to a single semiconductor chip containing four independently controllable lasers. Though multiple laser chips have been used experimentally for some time, until now no-one has been able to automate the process of fiber alignment or to control the laser signals being emitted into each fiber. According to Craig Armiento, research supervisor at GTE Laboratories, the method aligns the four fibers with the light emitting regions of each of the four lasers to within a tolerance of 1 ~m. Such high precision is achieved through the use of photolithographic and etching techniques similar to those used in the manufacture of integrated circuits. Pedestals and grooves are etched into a silicon 'waferboard' that serves as a platform for the fibers, lasers and associated electronic components. After the lasers are mounted and placed in contact with electrical leads, optical fibers are fitted into the precision-etched grooves and vacuum-drawn into alignment with the light emitting facet of each laser. 'This technology will permit us to make multiple-array transmitters employing 16 or more individual lasers', Armiento said. 'This is where size, assembly and cost advantages will be significant', he added. At the time of publication, no deployment timetable had yet been announced. Sanyo Semiconductor Corporation has also developed a four-beam individually addressable monolithic diode laser array on a single chip

130 Optoelectronics and Fiber Optic Technology

Figure 4.20

Schematic diagram of the structure of Sanyo's SDL-7952 four-beam diode laser (courtesy of Sanyo)

(see Figure 4.20), with each GaA1As laser capable of producing 100 mW of output power in single-mode operation. Each beam is individually controlled by a separate PIN photodiode backfacet monitor. By placing all four lasers on the same chip, alignment problems common to multichip implementations have been overcome; the beam pitch is highly controlled at 100 _+ 1 ~m. The position of the beam pitch enables optical systems to operate as if only a single beam were present. Another advantage is the ease of alignment. The operating temperature range for this device is -10 to +40~ threshold current is 80mA, operating current 190 mA, lasing wavelength 830 nm, and beam divergence 10 x 27 ~ Careful design of the Model SDL7952 four laser array, including a special heatsink structure, eliminated the thermal runway problems that plagued earlier 30 m W / b e a m versions of the device.

Transmitters - light emitting diodes and lasers

131

5

Receivers - photodiodes

A fiber optic detector is an optoelectronic transducer that converts optical energy to electrical energy. The most common is the photodiode which gets its name from its ability to detect light and produce current in response to incident light. The two types of photodetector used in fiber optics are the PIN diode and the avalanche photodiode (APD) (Figure 5.1).

Figure 5.1 PIN and avalanche photodiodes (courtesy of LaslRvis)

5.1 Conversion of light waves into electrical energy A photodiode is used to detect an optical light ray and convert its optical energy (photons) into electrical energy. Similar to LEDs and lasers, a photodiode's main requirement is that it should be small, rugged, have a

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I ! 'J

Basic photodiode operation

long operating life, be highly sensitive, possess a high response speed to the particular transmission rate (i.e. be designed to detect the lowest possible signal powers at a given bit rate), have an economical construction and have a low power consumption. Similar to LED and laser transmitters, a semiconductor p-n junction can be used for optodetection, the principle of which is illustrated in Figure 5.2. In the depletion region, the two charge carriers (holes and electrons) are separated by an electric field. The minority carriers diffuse to the p-n junction before they can reach the depletion region and be transported to the opposite neutral semiconductor region. This causes a photocurrent to flow in the external circuit and since the amount of light that exits a fiber is quite small, optical receivers usually employ high gain internal electrical amplifiers to improve input sensitivity especially at higher frequencies. Carriers generated outside the diffusion region are minimized because of recombination and do not contribute significantly to the photocurrent. Because of this conversion, overall efficiency is reduced. The actual frequency response of a photodiode detector depends very much upon the energy band gap material from which the detector is manufactured and the quantum efficiency of a photodiode is defined as 'the ratio of the number of generated electron-hole pairs to the number of incident electrons'. To produce a photocurrent, photons must have enough energy to raise an electron across the band gap from the valence band to the conduction band; this means that different materials (e.g. silicon, gallium arsenide, germanium and indium phosphide) are necessary for different wav-

134 Optoelectronics and Fiber Optic Technology elengths. Silicon, for example, is insensitive to wavelengths longer than 1000 nm whilst germanium, InGaAsP and OmGaAs can be used at longer wavelengths because their smaller band gap energies let them respond to less energetic photons.

Note: The composition of the detector determines the wavelength at which the detector is sensitive. Longer wavelength photons have less energy than shorter wavelength photons.

5.2 Design characteristics of an optical receiver Although the overall design of an optical receiver is fairly simple (consisting of just a photodiode to produce the current and an amplifier), the design criteria of the actual photodiode is far more involved. This is because it must take into consideration a number of factors such as the noise from the signal current, noise from the photodiode itself (e.g. avalanche diodes produce inherent noise dependent upon the multiplication factor), noise from the amplifier electronics, receiver thermal noise, dark current noise, as well as the signal-to-noise ratios required for the bit rate specified. 5.2.1 The PIN photodiode While the pn diode produces current from incident light, two characteristics make it unsuitable for use in fiber optic applications. First, the depletion region is relatively small and many of the absorbed photons do not produce an external current and so the received optical power must be fairly high to generate sufficient current. Second, the slow tail response makes the diode too slow and limits operations to the kilohertz range. The PIN photodiode is designed to overcome these problems. The depletion region is made as large as possible by the PIN structure. The lightly doped intrinsic layer between the heavily doped p-type and n-type materials means that most of the photons are absorbed within the depletion region which is a neutral layer between the n- and pregions. The inclusion of the intrinsic layer decreases the function capacity, increases the depletion region cross-sectional area, raises the maximum switching speed and the photon capture area. The result is more efficient production of external current and faster speed. A PIN photodiode's structure is shown in Figure 5.3.

Receivers - photodiodes

Figure 5.3

135

Structure of a silicon PIN p h o t o d i o d e

5.2.2 Avalanche photodiodes For a PIN photodiode, each absorbed photon ideally produces one electron hole pair which sets one electron flowing in the external circuit. In an APD a few incident photons result in many carriers being produced and an increased external current. To achieve this an APD relies on the phenomenon of avalanche multiplication whereby a strong electric field will accelerate current carriers so much that they can knock valence electrons out of the semiconductor lattice and with a high enough bias voltage an avalanche of carriers will result. The high operating voltage can make the APD faster than the PIN diode; however, the uneven nature of multiplication will introduce noise. A basic silicon photodiode will require several hundred volts to achieve the highfield strength gained from avalanche multiplication. By operating the photodiode in the APD or breakdown mode, the acceleration of carriers in the electric field reach such a high velocity that impact ionization occurs and further electron-hole pairs are generated which internally amplify the photocurrent produced. As the carrier transmit time increases, the gain-bandwidth product remains almost constant but large enough to convert optical powers in the nanowatt range at frequencies over a gigahertz. A guard ring is normally used to prevent edge breakdown and to limit possible inversion layers at the surface. Silicon is mainly used for the near infrared region while germanium is normally used in the long wavelength region. Avalanche photodiodes, although non-linear and fairly unstable, are very similar to normal silicon photodiodes except that they require a slightly lower operating voltage to achieve good multiplication.

5.2.3 Silicon photodiodes Basic silicon photodiodes (because of their small capacitance and low dark current noise) are very suitable as optical receivers. However, due to

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Optoelectronicsand Fiber Optic Technology

Figure 5.4

Silicon avalanche photodiode structure

their absorption rate being limited by the silicon band gap, silicon (and for that matter silicon avalanche) photodiodes are normally restricted to use in the 800 to 900 nm range. Figure 5.4 shows the structure of a silicon avalanche photodiode. Light passes through the p-layer (which is protected by a thin coating of silicon nitrate to prevent reflection losses) and separation of carrier pairs is achieved in the depletion layer (electrons to the n-side and holes to the p-side).

5.2.4 Germanium photodiodes Because of the silicon photodiode's low absorption rate above 1100nm (caused by the silicon band gap) germanium and alloys of various similar compound semiconductors are used in the 1300 to 1500nm range. In the structure of a germanium avalanche photodiode shown in Figure 5.5, the diode consists of a lightly doped p-type germanium substrate surrounded by an n-doped guard ring with p-n junction produced on the surface by diffusion or ion implantation. An antireflective coating of, for example, silicon oxide increases the quantum efficiency of the diode.

Figure 5.5

Germanium avalanche photodiode structure

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137

5.2.5 Indium gallium arsenide photodiodes Unfortunately, germanium photodiodes also have limitations in this spectral region such as high noise, temperature sensitivity, low responsiveness and dynamic impedance. Because of these problems, indium gallium arsenide photodiodes are quite frequently used. The main advantage of InGaAs photodiodes is that noise equivalent power is less than half that of the same size germanium diode. This enables the photodiode to detect at greater distances and with increased accuracy.

5.3 Types of receiver As in the case of transmitters, optical receivers are available in both analog and digital versions. Both types usually employ an analog preamplifier stage, followed by either an analog or digital output stage (depending on the type of receiver).

5.3.1 Analog fiber optic receiver The first stage of an analog fiber receiver is an operational amplifier connected as a current-to-voltage converter (Figure 5.6). This stage takes the tiny current from the photodiode and converts it into a voltage, usually in the millivolt range. The next stage is a simple operational voltage amplifier. Here the signal is raised to the desired output level. Current-to-voltage converter

Post-amplifier

Photodiode

Output

m

Figure 5.6

Basic analog (fiber optic) converter

I

5.3.2 Digital fiber optic receiver Figure 5.7 is a functional diagram of a simple digital optical receiver. As in the case of the analog receiver, the first stage is a current-to-voltage converter. The output of this stage, however, is fed to a voltage

138 Optoelectronics and Fiber Optic Technology Current-to-voltage converter

Comparator

Photodiode

vcc~ ~~ivge~er Figure 5.7

Output

Basic digital receiver

comparator, which produces a clean, fast, rise time digital output signal. Trigger level adjustment, when present, is used to 'touch up' the point on the analog signal where the comparator switches. This allows the symmetry of the recovered digital signal to be trimmed as accurately as desired.

Note: Additional stages are often added to both analog and digital receivers to provide drivers for coaxial cables, protocol converters or a host of other functions in efforts to reproduce the original signal as accurately as possible.

5.4 Fiber amplifiers Although low-loss optical fibers can enable signals and data to travel hundreds of kilometres, long-haul and submarine cables need regenerators a n d / o r repeaters to amplify the signal periodically (Figure 5.8). In a normal electronic repeater, the incoming light signal was converted (by the detector) into an electrical signal, amplified (by the regenerator) and then was retransmitted by another laser transmitter as an optical signal. These sort of repeaters, however, normally add quite a lot of noise to the signal and, being fairly complicated, are prone to failures. They have to be made for the specific bit rate of transmission and upgrading a n d / o r repair normally requires replacing all the r e p e a t e r s - which is rather difficult if it happens to be an undersea cable!

Receivers

Signal, n

Detector

Receiver regenerator

LASER

- photodiodes

139

Signal out

Figure 5.8 Basic electronic repeater

5.4.1 LASER amplifier repeater The prime function of a repeater or regenerator is to receive the optical signal, convert it into an electrical signal, amplify (or regenerate) the information and then convert it back into an optical signal. A repeater or regenerator is, therefore, required whenever the optical signal loss reaches a critical level. This depends upon a number of factors such as transmitter source type, transmitter wavelength, type of fiber, and transmission rate. Fluoride and tellurite-based fiber amplifiers can provide levels of amplification unobtainable with silica-based amplifiers. In the early 1990s, British Telecom successfully demonstrated an experimental laser amplifier repeater on a 120 km fiber link connecting two telephone exchanges in southeast England. The amplifier was based on a semiconductor lens that received the incoming pulse of laser light at one end and emitted an amplified pulse at the other. Effectively this was an optical equivalent of the travelling wave tube used to amplify microwave radio. This demonstration showed that the amplifier was capable of simultaneously amplifying several separate sets of pulses at different light frequencies using the technique of wavelength division multiplexing (WDM). The particular amplifier used by British Telecom was a double channel planar buried heterostructure laser, which was 5001Jm long and whose laser faces had been given an anti-reflective coating to reduce the reflectivity of the laser face over 500 times (i.e. from a typical value of 40 per cent to 0.08 per cent) and the amplifier gain was 17dB. The transmitter used was a distributed feedback laser emitting at 1.5 pm with a launched power o f - 2 dB. The receiver was a PIN-FET module.

5.4.2 Erbium-doped fiber amplifiers Over the years, manufacturers have experimented with fiber optic amplifiers and one of the most successful is the erbium-doped fiber amplifier (or EDFA), which operates in the 1550 nm range (Figure 5.9).

140

Signal in

Optoelectronicsand Fiber Optic Technology

..I vI

Coupler

Erbium-doped fiber

Filter

..,r- Signal out

Pump laser

Figure 5.9 Basic EDFA fiber ampfifier

A typical EDFA fiber amplifier consists of a length of fiber doped with erbium pumped (by a laser) at 980. The p u m p laser energizes the amplifier and the incoming signal stimulates emission as the pulse passes through the doped fiber. The stimulated emission in turn stimulates more emission and so there is a rapid, exponential growth of photons in the doped fiber. Bragg gratings (filters fabricated in fibers) are used to flatten the gain over the broadest wavelength range for use in WDM systems.

5.4.3 Erbium-doped fluoride fiber amplifiers Erbium-doped fluoride fiber amplifiers (EDFFA) exhibit a flat spectral gain without the need for an equalizer and offer advantages when used in WDM transmission systems. Gains greater than 40dB (i.e. x 10000) with power outputs (dBm) of 100 mW or more are capable with EDFAs and as Erbium ions (Er3+) can be stimulated to emit in the 1540 nm band (i.e. the band that has the least power loss in most silica-based fiber) they have the ability to amplify signals in a band where high quality amplifiers are most required. Another important property of erbium is that it is fairly soluble in silica, making it easy to dope into mixtures when making silica-based fiber.

5.4.4 Erbium-doped tellurite fiber amplifiers Erbium-doped tellurite fiber amplifiers (EDTFA) have an ultra-broad gain bandwidth of around 80nm and the potential for allowing the channel numbers of WDM systems to be increased.

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141

5.4.5 Praseodymium-doped fluoride fiber amplifiers Since most systems still work at 1310 nm, however, considerable research has been completed to find materials that will operate in a range that is effectively beyond the scope of practical optical amplifiers. Praseodymium-doped fluoride fiber amplifiers (PDFFAs) using fibers made from zirconium fluoride or hafnium fluoride are now available which have shown great promise.

5.4.6 Semiconductor optical amplifier A new amplifier (the semiconductor optical amplifier or SOA for short) is currently under development and this basically duplicates the function of a fiber optic amplifier (FOA) but in an integrated circuit fabricated like a diode laser.

6

Connectors and couplers

Optical connectors and couplers are the means by which a fiber optic cable is connected to peripheral e q u i p m e n t and to other fibers. Although these connectors are similar to their electrical counterparts in function and o u t w a r d appearance, they are actually high precision devices that have to centre the small fiber so that its light gathering core lies directly over and in line with the light source. Since the core size of c o m m o n 50 micron fiber is only 0.002 inches, this has to be achieved to tolerances of a few ten thousandths of an inch.

6.1 Connectors An optical fiber connector is defined as 'a device that makes a temporary connection between two fiber ends or between a fiber end and a transmitter or receiver'. Typically, the connector is m o u n t e d on the end of a cable or a bundle of fibers, and is designed for frequent connection and disconnection to matching connectors or e q u i p m e n t (Figure 6.1). Examples of where connectors m a y be used include: 9 9 9 9 9

connections with short intra-building data links; the point where a telecommunications system enters a building; interfaces between devices and local area networks (LANs); patch panels where signals are routed in a building; connections between terminal e q u i p m e n t and other c o m p u t e r devices; 9 t e m p o r a r y connections between remote mobile video cameras and portable recording e q u i p m e n t or t e m p o r a r y studies; 9 portable military systems.

Connectors and couplers 143

Figure 6.1 Examples of fiber optic connectors and cable assemblies (courtesy of Corning Cable Systems GmbH)

6.1.1 Connector requirements The basic requirement of a fiber optic connector is that it should be easy to assemble, have a rugged design, be low loss and have reproducible loss values even after repeated matings. It is also very important that fiber optic connectors are designed so that they do not damage the actual fiber end and introduce more losses. Ideally, a connector should include the following features: 9 low loss; 9 easy installation; 9 r e p e a t a b i l i t y - connection and reconnection many times without an

increase in loss; 9 consistency- no variation in loss whenever a connector is applied to a fiber; 9 e c o n o m y - the connector should be inexpensive, both in itself and in the tooling required for the fitting.

6.1.2 Quality of the connector The quality of the connector depends upon a number of things such as the type of fiber, fiber diameter tolerances, fiber asymmetry, concentricity of the fiber core, numerical aperture variations and the type of cladding used.

144 Optoelectronics and Fiber Optic Technology 6.1.3 Connector attenuation The key functional parameter of connectors is the a m o u n t of attenuation loss they exhibit. Loss in the best connectors is well below I dB, but for inexpensive connectors the loss m a y well be much higher. Loss in connectors refers to a mated pair, i.e. the loss in light passing from one fiber to another, or from one device to another.

Note: The loss in a single connector is not meaningful, as the light is not going anywhere.

The type of fiber used is also an important variable in connector performance because of the way light enters and leaves different fibers and connectors and most connectors are made for specific fibers. Connector attenuation is the sum of losses caused by several factors, which in themselves are the result of such things as the variation in the diameter of the core, concentricity of the core, eccentricity of the core and lateral alignment of the two fibers. The important factors that may be caused by intrinsic or extrinsic losses include: 9 9 9 9 9

overlap of fiber cores; alignment of fiber axes; fiber numerical aperture; fiber spacing; reflection at fiber ends. Intrinsic factors include:

9 c o n c e n t r i c i t y - cores not centralized within the cladding; 9 e l e p t i c i t y - cores oval shaped; 9 NA m i s m a t c h - this may be as a result of the transmitting fiber core being larger than the receiving fiber core (though within the manufacturing tolerances), or the cladding of the fibers being different sizes (which will prevent the cores from aligning correctly). Extrinsic factors include: 9 lateral a l i g n m e n t - connectors not aligned on the centre axis;

Connectors and couplers

145

9 end s e p a r a t i o n - fibers separated by a small gap suffer two losses. The first, Fresnel reflection, caused by the difference in the refractive index between the fiber and, usually, the air gap. The second is multimode loss caused when high order modes miss the acceptance cone and fail to enter the core of the other fiber. While the Fresnel loss m a y be reduced by the use of liquid matching fluid, its use is not recomm e n d e d in connectors as it can attract dust onto the connector face; 9 angular separation - ends should be perpendicular to the fiber ends; 9 surface f i n i s h - the fiber surface should be smooth and free from defects.

Note: The insertion loss of a connector is the loss that a connector introduces into a system, measured in dB.

6.1.4 Types of connector There are m a n y types of optical connectors in use today as shown in Table 6.1. 6.1.4.1 S M A c o n n e c t o r

The sub multiple assembly (SMA) connector, which was developed before the invention of single-mode fiber, was the most popular type of connector until recently. Figure 6.2 shows an exploded view of the parts of this connector.

Ali~]nment sueeve

High precision round centre pin

Hex-shaped threaded cap

Fiber cable crimp ring

Optical fiber access hole

Figure 6.2 SMA connector

il

l

Retaining 'C' ring

i

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Optoelectronicsand Fiber Optic Technology

Table 6.1

Types of connector Connector losses (for system planning)

Connector type

Multimode 62.5pm

Single mode

Connectors and couplers 147

6.1.4.2 ST c o n n e c t o r

The most popular type of multimode connector in use today, however, is the stick and turn (ST) connector (Figures 6.3 and 6.4). Initially developed by AT&T for telecommunications purposes, this connector uses a twist lock type of design. A typical mated pair of ST connectors will exhibit less than I dB (20 per cent) of loss and do not require alignment sleeves or other similar devices. The inclusion of an 'anti-rotation tab' assures that every time the connectors are mated, the fibers always return to the same rotational position assuring constant, uniform performance. ST connectors are available for both multimode and single-mode fibers, the primary difference being their overall tolerances.

Figure 6.3

ST connector

Figure 6.4

Typical ST connectors

148 Optoelectronicsand Fiber Optic Technology

Figure 6.5

Examples of PC board connectors

The SMA connector system, which is firmly mounted (see Figure 6.5), is mainly used in industrial applications, whereas the more common ST connectors, featuring bayonet locking, are mainly used in the field of computer data networks. 6.1.4.3 Single fiber connectors

Figure 6.6 shows an example of a graded-index fiber connector where the cable is secured to the plug by crimping. The cable can then be connected, or screwed, to (for example) an optoelectronic transducer using the coupling nut located on the plug.

Figure 6.6

Single fiber connector

Connectors and couplers 149

Many companies support their fiber ranges with such connectors. For example, in the early 1990s, STC produced a range of optical fiber cable assemblies designed specifically for harsh environments. These are composed of twin fibers, sheathed with high strength, low flammability, polyester, which have a breaking strain of 2000 N and are terminated with hermaphroditic expanded beam connectors. The cable and connectors have an overall loss per kilometre of less than 4 dB at 1300 nm. The weight of one kilometre of the cable is 31 kg so this particular cable is about a quarter the weight and half the size of the military coaxial cable it replaces. The connectors are shown in Figure 6.7.

Figure 6.7

Typical structure of an optical fiber assembly (courtesy of STC)

Each connector is provided with a scaled bayonet-locking cap attached by a stainless steel lanyard and each connector has a window to protect the lens and to facilitate easier cleaning. The windows and lenses are coated with an anti-reflective film to reduce losses and crosstalk. In the event of a window becoming permanently marked it can be replaced in seconds using a simple hand tool. Additionally, the connectors and cable materials are all proofed against petrol and diesel fuels, lubricants, hydraulic oils and alkaline wash solutions. The bulkhead connector is basically similar in construction to the free connector and the body can be moulded in carbon fiber-loaded polycarbonate to maintain electromagnetic shielding across the panel cut-out.

6.1.4.4 Connectors for fiber bundles

Connectors for fiber bundles (which can literally contain hundreds of single fibers) are readily available nowadays and although insertion losses can be a little higher, because of the high attenuation of fiber bundles, they are acceptable.

150 Optoelectronicsand Fiber Optic Technology

Figure 6.8 Connector with pre-assembled factory fitted fiber (courtesy of Corning Cable Systems GmbH)

In Figure 6.8, the connector incorporates a pre-assembled fiber end with a factory-made, high quality, connector. This system is ideal as a fast, cost-effective, field-installed connector. Figure 6.9 is an example of a connectorized riser cable suitable for indoor or outdoor applications. Up to 144 fibres can be broken out without any difficulty and individual fibres are normally colour coded to make installation easier.

Figure 6.9 GmbH)

Connectorized riser cable (courtesy of Corning Cable Systems

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151

6.1.4.5 U s i n g a fiber cable to c o n n e c t electric current as w e l l as l i g h t

In special cases, particularly when transmission lengths are longer than usual, both optical data and electric current can be transmitted, using one cable and one connector only. For this kind of application, male and female D-Sub connectors with mixed component parts have been developed for plastic fiber cables (Figure 6.10).

Figure 6.10 Electric current and fight in a fiber cable

Hybrid cables used for this purpose include both plastic fiber optic and copper cables. For high mechanical stress (for example, in heavy industry), designs with heavy-duty round and modular connectors are available and in most cases there is enough space to integrate the transducers in the male connector. The result is that the electrical contacts are plugged to the outside, whereas data is transmitted inside the cable by means of optical fibers. When the structure or function of an existing copper cable system needs to be extended, the possibilities are often limited by the amount of noise immunity and to the design of the electrical interface. These limitations can be overcome, particularly when the system length is to be increased, by using a plug-in optical fiber interface (Figure 6.11). For example, a system RS232 using copper cables of a few metres in length,

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Optoelectronicsand Fiber Optic Technology

Figure 6.11 Interfaces and buses

which would normally be very liable to interference if it were made any longer, can easily be extended (e.g. 3 km or more) using optical fiber interfaces. Multiplexing will also help to further reduce the costs. The same will apply to an RS485 system. Indeed, any extension is feasible, and the number of users is limited only by the number of logic addresses and not by the total impedance of the electrical interfaces.

6.2 Couplers When an optical fiber cable is damaged it can be repaired either by splicing a new piece of cable into the existing line or, if sufficient cable is available, by cutting out the damaged portion and splicing the two ends together. This process is referred to as concatenation. Quite frequently, especially when it is necessary to attach filters, optical couplers or transmit a n d / o r receive optoelectronic transducers etc. to the fiber, a low-loss, easily demountable coupler is used. There are two main types of optoelectronic demountable couplers: end fire couplers and lens couplers.

6.2.1 End fire coupling End fire coupling is where both optical faces (whether they are fiber-tofiber, fiber-to-photodiode, or LED/Laser-to-fiber etc.) are parallel and close to each other. Various coupling losses have to be considered, the most important being insertion loss.

Connectors and couplers 153

The three main situations encountered in end fire coupling are: 9 when the entrance and exit faces of the components are similar in size, have identical numerical apertures, symmetrical end fire coupling, and minimized losses (Figure 6.12);

r

Separation plate Figure 6.12

Identical entrance and exit face

9 when the face is larger than the entrance face: if the exit face is larger than the entrance face (e.g. LED-to-fiber) high losses will occur (Figure 6.13);

Separation plate Figure 6.13

Larger exit face

9 when the face is smaller than the entrance face: if a small exit face is coupled to a large entrance face (e.g. fiber-to-photodiode) the insertion loss is low (Figure 6.14).

Y

Separation plate Figure 6.14

Smaller exit face

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Optoelectronicsand Fiber Optic Technology

6.2.2 Lens coupling This particular method uses lenses to couple the face exit to the entrance face. Of the many examples, symmetrical lens coupling (Figure 6.15 showing a fiber-to-fiber coupling) and asymmetrical lens coupling (Figure 6.16 where the exit face is smaller than the entrance face as in a fiber-to-photodiode coupling) are the most widely used.

Figure 6.15 Symmetrical lens coupling

Figure 6.16 Asymmetrical lens coupling Although lens coupling permits greater mechanical tolerances at the junctions, the losses of a lens coupler are usually much higher than that of an end fire coupler, because of the additional losses caused by the imaging system and the different refractive indexes of the bonding layers. For this reason, end fire coupling is normally the preferred method.

6.2.3 Multiport couplers Optical couplers having more than one optical port to couple light in and out are called multiport devices. There are two main types of multiport couplers, those that provide three or four ports and those that can connect as many as 16 subscribers. These are called mixers and they depend upon the light being laterally transferred from one waveguide to another through the cladding. The amount of light being coupled depends upon the amount of overlap that exists at the fiber junction.

Connectors and couplers 155

Note: Extreme care has to be taken when measuring the output power of a multiport coupler so as to ensure that the termination of the individual ports is reflection free and that the emerging wave does not produce another wave, which re-enters the coupler.

Various examples of multiport couplers exist including: 9 symmetrical coupler, or switch (Figure 6.17): In this type of coupler, both sections are the same fiber type; 2 3

1 4

Figure 6.17

Symmetrical coupler or switch

9 symmetrical offset coupler (Figure 6.18): this coupler enables precise alignment and low transmission loss; 3 ~2

J

Figure 6.18

Symmetrical offset coupler

9 asymmetrical offset coupler (Figure 6.19): In this context, the word asymmetrical implies that at least one section of the coupling is of a different fiber type. Asymmetrical offset couplers can be manufactured with low insertion loss and are suited to step index fiber systems provided that the fiber cladding is thin. They are not suited to graded index fiber because of the possibility of a mismatch being caused by the offset positions;

156 Optoelectronicsand Fiber Optic Technology

4

Figure 6.19

Asymmetrical offset coupler Lapped core

2

<, 3 1

Figure 6.20 Asymmetrical three-port coupler with lapped core 9 Asymmetrical three-port coupler with lapped core (Figure 6.20): the lapped core of fiber 3 enables precise alignment in the axis of fibers 2 and 3, and hence low transmission loss for path 2-3 as well as 1-2; 9 four-port coupler with beam-splitting mirror (Figure 6.21): precise alignment of the fiber section is assured by use of the beam-splitting

2

1

Figure 6.21 Four-port with beam-splitting mirror

Connectors and couplers 157

111

12

4 v

Figure 6.22

Four-port coupler (bitaper)

mirror. This coupler has no mode dependence (apart from polarization effects) and can be used with step or graded-index fibers. Through a choice of mirrors, dielectric and Laser design, light division can be varied and the multilayer acts as a wavelength selective filter. four-port coupler (bitaper) (Figure 6.22): in the fused section, the fiber cores are separated from each other but the core modes are converted to cladding modes which provide optical power coupling from one fiber to another; couplers as switches (Figure 6.23); owing to their wavelength independence, couplers can also act as switches. These can range from a simple form (e.g. the symmetrical coupler of Figure 6.17), to an electromagnetically operated four-port switch, such as the one shown in Figure 6.21. The principle of operation is very simple. When the reed is energized, the four fibers are coupled together;

Figure 6.23

Electro magnetically operated four-port switch

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Optoelectronics and Fiber Optic Technology

Figure 6.24

Fused-fiber, transmissive-mode, directional star coupler

Figure 6.25

Reflective-mode, directional star coupler

9 transmissive and reflective star couplers (Figures 6.24 and 6.25): transmissive mixers are made by biconically tapering and fusing together a number of fibers. The resultant changes in the fiber cause the incoming light to strike the side of the fiber cladding at a steep angle which, as it exceeds the acceptance angle, transfers some of the energy into the cladding. At the point where the cross-sectional area widens, the light returns the core region of the other fibers. Fibers enter a reflective mixer at one end while light is reflected off a mirrored surface at the other end. In Figures 6.24 and 6.25, the power from all the input ports is spread amongst all the output ports to enable multiterminal links to be used over a shared fiber network; 9 couplers as signal dividers: couplers are also available that can divide a signal from one fiber into two fibers as shown in Figure 6.26. The design of the coupler ensures that the splitting ratio is 50/50 and independent of wavelength.

6.2.4 Coupling losses Connector insertion loss is not just a function of the tolerance of the connector but also the tolerance of the fiber itself and there are a n u m b e r

Connectors and couplers 159

Figure 6.26

Fiber optic signal divider

of factors that must be taken into consideration when designing optical fiber systems: 9 axial

fiber m i s a l i g n m e n t (Figure 6.27) is probably the most critical factor as it can introduce extremely high losses. For example, an axial (transverse) offset of around 30 per cent (equivalent to 30 pm for a fiber with a core diameter of 100 pro) is sufficient in itself to introduce a loss of approximately 2 dB;

Figure 6.27

Axial fiber misalignment

9 angular misalignment (Figure 6.28) is not so important in large fibers but the smaller the numerical aperture, the more significant the problem;

Figure 6.28 Angularmisa/ignment

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Optoelectronicsand Fiber Optic Technology

t

--J .....................

Figure 6.29

Separation between fibers

9 Fresnel losses are due to the reflection of the glass-air interface and can be as much as 0.35 dB at fiber-air-fiber combinations. The separation between fibers (Figure 6.29) is, therefore, an extremely important consideration but Fresnel losses can be reduced by using a dielectric coating a n d / o r an index matching liquid. The actual surface finish, or end face roughness, of an exit or entrance face also affects the amount of insertion loss that a junction exhibits as it can cause light scattering and absorption centres. For example, a fiber end face with a peak valley height of 10~lm introduces a loss of around 0.5 dB. Core diameter variations between the exit and entrance fiber core diameters will introduce additional insertion losses. For example, a core diameter variation (involving a transition to a smaller diameter fiber) of 5 per cent will introduce another loss of about 0.5 dB.

6.2.5 Attaching a fiber to a connector

This can be achieved by cleaving and either cleating the fiber to the ferrule so that the cleaved surface forms the fiber exit face, or using a low attenuation adhesive to secure the fiber to the ferrule. Cleaving can be achieved by use one of the m a n y pieces of equipment available today such as Fujikura's 'High Precision Fiber Cleavers', shown in Figure 6.30. Two models of the Fujikura cleaner are available. The conventional base (CT-04B) which is used for conventional fiber cladding material, and the enhanced base (CT-100B) which is suited for titanium (Ti) -doped silica as well as a conventional silica fiber. The cleaving method is diagrammatically illustrated in Figures 6.31 and 6.32. The primary coat will need to be stripped before being inserted into a connector and the instrument shown in Figure 6.33 can be used to do this. The strippers are designed to remove the primary coating from the optical fiber. These sorts of instruments are easy to operate and perform consistent quality stripping without damaging the fiber.

Connectors and couplers 161

Figure 6.30

Fiber cleavers (courtesy of Fujikura)

Figure 6.31

The conventional cleaver (courtesy of Fujikura)

3M's hot melt kit (Figure 6.34) is an example of one of the field termination kits that are currently available. Ferrules already containing 'hot melt' adhesive are used and once the ferrule has been heated (Figure 6.35a) the prepared fiber is inserted into the ferrule (Figure 6.35b). Cooling only takes a few minutes and it is then ready for use as a permanent, high quality, connector.

162 Optoelectronicsand Fiber Optic Technology

Figure 6.32 Enhanced cleaver (courtesy of Fujikura)

Figure 6.33

Primary coat stripper (courtesy of Fujikura)

Figure 6.34

Hot melt field termination kit (courtesy of AURIGA (Europe) PLC)

Connectors and couplers 163

Figure 6.35 Heating the ferrule in the oven and inserting the fiber (courtesy of AURIGA (Europe) PLC) 6.2.6 Connecting an optical fiber cable to an LED

The ability to consistently cleave to an accuracy of better than one degree every time is required for today's splicing and measurement applications and fortunately there are a number of cleavers and splicers on the market at the moment that are capable of doing this.

Figure 6.36 Butt coupling an optical fiber to an LED

Although fiber butt coupling (Figure 6.36) is possible, it is very inefficient owing to the relatively narrow acceptance angle of the fiber and so couplers using lenses to concentrate the beam are more widely used. From a purely thermodynamic point of view, it is impossible to increase the radiance of an LED by using a lens coupler. Lens coupling does, however, lightly magnify the active area and obviously increases coupling efficiency. 6.2.7 Connecting a fiber cable to an integrated circuit

There are two methods currently used to connect a fiber cable to an integrated circuit. They are end fire coupling and evanescent wave coupling.

164 Optoelectronicsand Fiber Optic Technology

6.2.7.1 End fire coupling

This is effectively a butt coupling of the integrated optical waveguide to the optical fiber which is then held together by an adhesive (Figure 6.37).

Fiber

Integral lensed

GaAIAs LED Figure 6.37

Lens coupling an optical fiber to an LED

6.2.7.2 Evanescent wave coupling

In this case (Figure 6.38), a sandwich ribbon fiber, having its core on the fiber surface, is pressed against the integrated optical waveguide. Coupling efficiencies of over 90 per cent have been achieved using this method.

Figure 6.38

Coupling configuration for a sandwich ribbon fiber with an integrated waveguide

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165

6.3 Fiber jointing techniques In the early 1980s, the basic method of joining fibers together was to attach a connector to a length of fiber optic cable and then use epoxy (applied with a cotton swab or toothpick) to fix it in place. The coated fiber was then inserted into the connector, the epoxy cured and the fiber then cleaved. Finally a few polishing steps were performed to get the fiber smooth and flat (relative to the end of the connector). The time to do all that was a very lengthy 15-20 minutes per connector. Similar methods are still being used today, but the epoxies have been refined, the method of applying the epoxy has been made simpler and the polishing films/tools optimized. There are now also a lot of different connector designs available and new termination methods have been developed that enable a connector to be installed onto a fiber cable in less than two minutes! While optical connectors can be used to connect fiber optic cables together, there are other methods that result in much lower loss splices. Two of the most common, and most popular, are the mechanical splice and the fusion splice. Joints, or splices, unlike connectors, are permanent connections between fibers. As a rule, splices are normally used in external, longdistance, high capacity optical fiber systems whilst connectors are used in internal short-distance applications. This is particularly significant as the loss of a connector is typically 0.7 dB whilst that of a splice is usually 0.1 to 0.5 dB depending upon the type of splice and fiber used. In a splice, the two fiber ends are bonded together either by melting (known as fusing) then gluing or mechanically holding them together in a tightly confined structure. This process normally takes between 5 and 10 minutes to complete but in general mechanical and epoxy splices lack the strength of fusion splices. Bonding the fiber ends together with no intervening air space reduces the Fresnel and transmission losses normally found at fiber/air interfaces but some interface and spreading losses can occur if a bonding agent such as an epoxy resin separates the fiber ends. These are nevertheless fairly minimal, if the refractive index of the material is close to that of glass and the layer is not too thick.

Note: Improper splicing can allow dirt into the junction and this will form a Iossy junction. For example, a solitary 101Jm dust particle would almost totally block light transmission through a single-mode fiber.

166 Optoelectronics and Fiber Optic Technology

6.3.1 Epoxy splices The basic steps for using epoxy as a termination m e t h o d (Figure 6.39) are as follows: 9 strip the cable d o w n to the bare fiber; 9 mix the two components of the epoxy (resin and h a r d e n e r - prem e a s u r e d packets are usually available); 9 load the mixed epoxy into a syringe (unless it's pre-loaded of course); 9 inject the epoxy into the back end of the connector ferrule; 9 insert the prepared cable into the connector; 9 if the connector is being installed on to jacketed fiber, crimp the connector on to the cable strength m e m b e r s and jacket; 9 install the connectorized cable into a curing holder; 9 cure the epoxy (by placing the connectorized cable and curing holder into an oven); 9 cleave the protruding fiber as close to the ferrule tip as possible (and remove all excess fiber); 9 dispose of all excess fiber; 9 polish to remove excess epoxy from the ferrule tip; 9 clean the ferrule tip/fiber (usually by using a lint-free 'wipe' d i p p e d in alcohol); 9 inspect; 9 test.

Figure 6.39

Typical ST connector using epoxy or adhesive (courtesy of Fotec)

6.3.2 Heat-cured epoxy As the term implies, exposure to an elevated t e m p e r a t u r e cures the epoxy. The choice of epoxy d e p e n d s on w h a t kind of reliability is desired together with the temperature or h u m i d i t y extremes anticipated. The main a d v a n t a g e to using heat-cured epoxy is that you can achieve the optimal resistance to extreme environments. The disadvantages are that the time to terminate is long (typically 15 minutes), the time to cure ! is even longer (typically ~ hour) and you need an oven to complete the curing.

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167

6.3.3 Room temperature-cured epoxy This method is essentially the same as the heat-cured epoxy method, except for the fact that it cures at room temperature. This makes it very useful as an in-service method as it can be completed without any need for power. The disadvantages associated with this method are that the pot life is very short and the curing time is a very lengthy (2-3 hours).

6.3.4 Pre-injected epoxy This method utilizes heat-cured epoxy but the connector manufacturer not the installer prefills the epoxy. The main advantage of this method is that you do not have to inject the epoxy, but to a degree this is counteracted by the fact that an oven is normally required to heat the epoxy and the connector housing can become hot to the touch. Another disadvantage is that the polishing generally takes longer.

6.3.5 UV adhesive This type of adhesive requires an ultraviolet (UV) light source for curing. It is a fast way to bond fiber to the connector ferrule and the polishing process is quite simple. On the downside, however, is the fact that as the UV light has to 'see' the adhesive, a solid ceramic, stainless steel, or polymer ferrule cannot be used. Another disadvantage is that its resistance to environmental extremes (heat, cold, humidity) is significantly less than epoxy. The main advantage is that the curing time is fast (e.g. 45 seconds) and no curing oven is required.

6.3.6 Cyanoacrylate adhesive The steps to use this adhesive system are as follows: inject the adhesive, insert the prepared cable and then spray the tip of the connector ferrule with an 'accelerator' as a means of escalating the curing process. The primary advantages are that this method is fast (e.g. 30 seconds to 1 minute) and there is no need for a curing oven. However, there are some disadvantages to the curing speed of this adhesive (for instance, the adhesive could harden before the fiber can be fully inserted into the ferrule) and, like UV adhesive, its resistance to environmental extremes is not as good.

6.3.7 Anaerobic adhesive This is a similar process to cyanoacrylate except, that instead of spraying an accelerator, you dip the fiber into a liquid 'primer'. The curing occurs

168 Optoelectronics and Fiber Optic Technology

as a result of no air being present within the ferrule hole (hence the term anaerobic, or 'absence of air'). Similar to cyanoacrylate, anaerobic adhesive's main advantages are that this method is very fast (i.e. 30-60 seconds) and a curing oven is not required. One of the disadvantages is that, as the primer is solvent based, it evaporates very quickly.

6.3.8 Acrylic adhesive This is fairly new on the market and its main advantage over other adhesive systems is that the time to cure is just as fast as an anaerobic but it does not begin to harden immediately (i.e. you have at least a full 30 seconds to get the fiber into the connector, so 'fiber-lock' will not occur). Once again, neither an oven nor (for that matter) a UV light source is required for curing and it offers resistance to environmental extremes. Another advantage is that as the activator is not solvent based it will not evaporate when exposed to a i r - it also has a long shelf life (e.g. 12 months). This adhesive system (compared to other adhesive and epoxy methods) probably offers the best combination of ease of use, termination speed and environmental resistance.

6.3.9 Crimping This kind of connector eliminates the use of e p o x y / a d h e s i v e altogether. Such connectors utilize a method to mechanically grip the fiber and other cable components (Figure 6.40).

Figure 6.40 Crimp terminations (courtesy of Fotec) The advantage of this method is that there is no e p o x y / a d h e s i v e to deal with and so termination is, therefore, easy and fast. One of the disadvantages, however, is that the long-term reliability is questionable and there are higher connection costs. Another disadvantage is that the crimping mechanism is not capable of retaining and aligning single-mode fibers well enough to provide adequate optical performance. Hence, this type is only suitable for multimode cable.

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169

6.3.10 No epoxy and no crimping These special connectors include a short length of fiber that has been preepoxied to the connector ferrule and pre-polished at the factory. A fiber 'stub' is left to protrude out the back of the ferrule which can be inserted into the splice mechanism that is pre-installed into the rear of the connector housing.

6.3.11 Advantages and disadvantages of various types of termination methods

Advantages: 9 the installer only has to prepare the cable, insert it into the connector and crimp the back end; 9 there is no need to deal with an epoxy/adhesive; 9 no polishing is required; 9 (hence) fewer consumables are required.

Disadvantages: 9 special tools (such as the cleaver) are required; 9 as there are effectively two fiber junctions (one at the ferrule tip and one within the splice mechanism) the number of fiber junctions increases to three (as opposed to only one for all other termination methods); 9 owing to the quality of the splice junction, long-term reliability can be less than epoxy or some adhesive systems and higher connector costs usually result.

6.4 Splicing For single- and multi-fiber splices a number of simple bonding and fusion-splicing techniques are currently available and ruggedized or permanent multi-fiber connectors are frequently used to connect the fiber optic cables to distribution frames and optoelectronic equipment.

6.4.1 Splice losses Splice losses can be due to variations in the outer diameter of the fiber core, differences in index profile, differences in the ellipticity of the core, misalignment of the fiber ends, poor quality of the refractive index match at fiber ends and waveguide imperfections etc. In practice, splice losses of about 0.5 dB are typical for fusion-spliced multimode fibers while single-

6.3.11 Advantages and disadvantages of various types of termination methods (Table 6.2) Table 6.2

Summary of the pros and cons of fiber connector termination methods

Termination method

Typical time to field terminate on to buffered fiber (minutes per connector)

Heat-cured epoxy Room temp.-cured epoxy Pre-injected epoxy UV adhesive Cyanoacrylate adhesive Anaerobic adhesive Acrylic adhesive Crimp No epoxylno polish Room temp.-cured epoxy Pre-injected epoxy

10 + 30 (cure time) 10 + 120 (cure time) 8 + 30 (cool down time) 5 3 3 2.5

UV adhesive Cyanoacrylate adhesive Anaerobic adhesive Acrylic adhesive Crimp No epoxyho polish

Medium Low Low Low High High

2 1.5 Low Medium

Time allowed to insert cable

2 hours 15 minutes 1 minute 30 seconds <5 seconds 5 seconds 30 seconds No limit No limit None Heating oven, cooling stand UV curing light None None None Special installation tool Special cleaver and crimp tool

Source of epoxyladhesive cure

Oven Air Oven UV light Air Activator Activator Not applicable Not applicable Good Good Fair Fair Fair Good Fair

Connectors and couplers 171

m o d e fibers (because of the narrowness of the core diameter) are not as m u c h (for example, 0.1 to 0.2 dB). Mechanical splicing, while still being more lossy than fusion splicing, is nevertheless appreciably less than using a connector. Although it is physically possible to splice non-similar fibers (e.g. single m o d e to multimode) sizeable losses can be expected if the light emerging from the o u t p u t fiber is not completely collected within the core of the input fiber. For example, connecting a 10 ]Jm core single-mode fiber to a 62.5/125 ~m m u l t i m o d e fiber w o u l d be minimal, for a light b e a m in the single-mode to m u l t i m o d e direction, but in the other direction it could be as m u c h as 20 dB. As the loss due to a splice can sometimes be as m u c h as the loss due to a kilometre of single-mode fiber it is important that the splicing and jointing techniques used are as accurate as possible!

Table 6.3

Alignment errors during splicing

Lateral misalignment

Angular misalignment

Axial misalignment

Poor end match

172 Optoelectronicsand Fiber Optic Technology

6.4.2 Alignment errors during splicing There four alignment errors in splicing optical fibers. These (see Table 6.3) are: 9 9 9 9

lateral; axial; angular; poor end finish.

6.4.3 Mechanical splicing In a mechanical splicer, after the two pieces of fiber have been cleaned and stripped, they are carefully butted together and aligned using a mechanical assembly such as a V-groove splicer. Either clamping them within a structure or gluing them together with some form of indexmatching adhesive then holds the two fibers together. Losses are normally much higher than those found in fusion splices but the main advantage of mechanical splicing is the simplicity of operation and the tools required. They are particularly suitable for test purposes which are moderately demanding and temporary.

6.4.3.1 V-groove splicer One of the simplest mechanical splicers is the V-groove splicer where the fibers are merely confined between two self-aligning V-groove plates. Some precision V-groove splicers employ hold-down springs made of beryllium copper and lever-activated rollers for dual-axis control. The principle of the V-groove splicer is illustrated in Figure 6.41.

Matching

top plate

Fiber

Plate

Figure 6.41

V-groove splicer for joining optical fibers

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173

A rapid curing index-matching adhesive is applied to the fiber ends to make a permanent fixture and the splice connector is then crimped on to the two buffer tubes. To ensure that attenuation caused by the joint does not increase due to seasonal temperature changes, it is essential that the adhesive used is age-resistant and an exact index-match of the fiber. Provided that extreme care is taken when completing the splice, an accurate cleave is achieved, and the right adhesive used etc., splice losses of less than 0.1 dB are possible.

6.4.3.2 Elastomeric splicer Another type of mechanical splicer is the elastomeric splicer, whose principle is illustrated in Figure 6.42. It is similar to the V-groove splicer, but the plates are made of a flexible plastic material. An index-matching adhesive is then used to fuse the two fiber ends together or, alternatively, the splice connector can be crimped to the optical fiber cable.

Figure 6.42

L .

.

.

.

.

.

!

Principle of an elastomeric spficer which aligns fibers in a hole in the flexible plate

The main disadvantage associated with mechanical splices is the possible loss introduced by an imperfect butt joint. These types of joints are therefore only suitable for short-term, short-distance, or temporary joints but, as these splicing methods are purely mechanical and there are no naked flames, these types of splices are particularly useful in hazardous areas.

6.4.3.3 Mechanical field splicing kits Low-loss mechanical splices are possible using one of the ultraviolet splicers that are available on the market today, the principle of which is illustrated in Figure 6.43. The procedure is very simple and, following fiber preparation (consisting of precision cleaving, removal of a small portion of the cable sheath or buffer tube and thoroughly cleaning the exposed fiber), one or two drops of ultraviolet-cured adhesive are applied to the end of one fiber before it is inserted into one end of the splicer.

174 Optoelectronics and Fiber Optic Technology Ultra violet light

",5,

Fiber

Ultra violet

transparent casnng Figure 6.43

Ultra-violet elastomeric splicing principle

The end of the other fiber is similarly treated, pushed into the other end of the splicer until it butts against the first fiber, and then cured by the application of an ultraviolet light source. Splice losses of less than 0.2 dB are not uncommon using this method. 6.4.4 Fusion splicing Fusion splicing (Figure 6.44) differs from mechanical splices and connectors in that it directly fuses optical fibers together in a high quality process using a fusion arc. The result is a physical low-loss bond between the fibers, free of air gaps and inclusions. In practical applications, fusion splicers are used for this purpose.

Figure 6.44

GmbH)

Principle of thermal splicing (courtesy of Corning Cable Systems

The splicing process typically involves six steps: 9 stripping the fiber ends with a stripping tool; 9 preparing the end faces with a fiber optic cleaver;

Connectors and couplers 175

9 inserting the fiber ends into the fusion splicer and alignment of the fibers by the fusion splicer; 9 fusing the fibers using an electric arc ignited between two electrodes; 9 analysing the finished splice; 9 protecting and storing the splice. Fusion splicing requires the alignment of the two ends with microscopic precision and the precisely controlled application of energy to fuse the two ends together. Through automation, manufacturers have been able to make the task simpler and there are many different types of fusion splicers available on the market today although, because of the accuracy required, they are still very expensive; sometimes costing thousands of pounds. Figure 6.45 is an example of a compact, lightweight, portable, fusion splicer. This splicer can be used for both multimode and single-mode fibers, with an average splice loss of 0.04dB for multimode (50/125) and 0.05 dB for single-mode.

Figure 6.45 A typical field splicing kit for splicing together optical fiber ends (courtesy of Brand-Rex)

For protection against damage and environmental effects, a heat-shrink splice protector similar to that in Figure 6.46 may be used to cover the splice with a protective coating.

176 Optoelectronicsand Fiber Optic Technology

Figure 6.46

Heat-shrink oven (courtesy of Corning Cable Systems GmbH)

Virtually all fusion splicers nowadays use electric arcs and have a projection microscope (generally a binocular model with a 50x magnification factor or more to enable the operator to observe the fibers during alignment). A method of comparing the amount of optical power transmitted through the fiber before and after splicing and a cleaver to cut the fiber to a precise perpendicular surface. Quite frequently, the equipment will also have a facility for coating the fiber ends after stripping (Figure 6.47).

Figure 6.47 A typical fiber fusion splicing unit (courtesy of Fujikura)

Connectors and couplers 177

Figure 6.48

Computer-controlled fiber splicer (courtesy of Brand-Rex)

Some manufacturers also use microcomputers to control the splicing process such as the one shown in Figure 6.48 which uses an integral local light injection and detection system (LLIDS) to accurately align the fibers.

6.4.4.1 Fusion splicing of graded index fibers This is the preferred method for long-term junctions as described in section 6.2.5. The fibers to be joined are first stripped of their protective plastic coating. The ends are then precisely cleaved to produce end faces that are within three degrees, perpendicular to the fiber axis. They are next placed in guides (where they are automatically aligned) and pushed together tightly enough so as to produce a visible compression when viewed through the microscope. The arc is then fired to weld the two fiber ends together. After the joint has cooled, it can be recoated with a plastic material to protect it against environmental degradation and make the outer diameter close to that of the original coated fiber. Further protection can be given by crimping a splice connector over the two buffer tubes and pouring an air-cured silicone rubber over the splice connector to recoat the joint.

178 Optoelectronics and Fiber Optic Technology

6.4.4.2 Fusion splicing of single-mode fibers Although similar splicing techniques can be used for single-mode fibers, the precision and accuracy required for cutting and splicing two such fibers together (perhaps with a core diameter of 1012m or less) is far greater than that previously used. Consequently, single-mode optical fiber fusion splices are not common.

6.4.4.3 Multi-fiber splicing During the installation process, considerable time is usually required for splicing individual fibers together. Following the introduction of multifiber splicing systems (similar to the one in Figure 6.49) this timeconsuming task has been dramatically reduced.

Figure 6.49 Example of a multi-fiber optical splice for 12 fibers (courtesy of AURIGA (Europe) PLC)

The example shown in Figure 6.49 is a Fiberlok 2612 by 3M which enables up to 12 single-mode fibers to be stripped, cleaved and spliced in under 5 minutes. The complete splice module contains a series of precision 'v' grooves that accurately locate the clad fibers into the correct position for splicing. It only measures 38 x 13 x 17 mm and is compatible with both discrete and ribbonized cable. Mean insertion loss is only 0.07 dB and reflectance <-60 dB with a 97.8 per cent yield. Splicing is a simple four-stage process. If individual fibers are to be spliced, they are first ribbonized using a special ribbonizing tool, after which the assembly is treated as if it were a ribbon cable. The primary coating is removed from all the fibers in one operation using a heated tool. The stripped fibers are installed into a ribbon holder and cleaved, with all the fibers being cleaved at the same time. A 25x viewer is used to

Connectors and couplers 179

check for cleaving defects such as shards or spikes. The cleaved fibers are then placed into the splice actuator tool which aligns the fibers and applies a pre-load to ensure end-face contact which completes the splice. Index-matching gel within the splice ensures low insertion loss, and moisture-resistant gel on the cable clamping mechanism prevents moisture entering into the splice.

6.4.5 Closures for fiber optic cables Copper cables use a variety of enclosures and containers (Figure 6.50) in which to protect their joints, terminations and other components at the joining or branching point. Typically, these may be in a manhole, on a utility pole, or in special boxes above or below ground level.

Figure 6.50

Examples of closures (courtesy of Corning)

One of the most important changes brought about by the use of optical fibers is that the size of the joint enclosure has had to be increased, to allow sufficient room to neatly arrange the splices and to provide enough space to ensure the bending diameter of the cable is within permissible limits. Fiber splice enclosures therefore have to be designed so as to hold cable strength members tightly together, to block entrance of water, to retain gas (if the cable is pressurized), to electrically bond and ground any metal elements in the cable (such as strength members' armour) and be capable of being re-entered, if required, to facilitate splice changes or repairs. Occasionally, extra space is also required for loading coils, jointing of cable shields, sheaths and strengthening elements.

180 Optoelectronicsand Fiber Optic Technology Fibers in multi-fiber optical cables are typically arranged in groups or units inside an enclosure. Normally each unit contains about 10 individual fiber splices and the required number of units are arranged inside an enclosure made for that particular cable. Currently, closures are readily available on the market to cater for all needs whether it be for 2 or 2000 fiber splices.

Note: Fiber splice enclosures should also be capable of allowing the organization of splices and fibers so they can be easily recognized, and providing room for initial splicing and future modifications.

6.4.6 Repeaters and regenerators The prime function of a repeater or regenerator is to receive the optical signal, convert it into an electrical signal, amplify (or regenerate) the information and then convert it back into an optical signal. A repeater or regenerator is, therefore, required whenever the optical signal loss reaches a critical level. This depends upon a number of factors such as transmitter source type, transmitter wavelength, type of fiber and transmission rate. One advantage of optical fiber systems is the long distances that can be maintained before signal repeaters are needed to regenerate signals. The use of optical fibers as opposed to copper cables, therefore, reduces the numbers of repeaters (in the case of analog signal transmission) and regenerators (in the case of digital transmission) that are required. Typically, a 100Mbits -1 copper coaxial link, because of the inherent attenuation introduced by the cable, requires regenerators every 2 km or so. Using Laser diodes and graded index fiber for the same circuits on the other hand, would mean that regeneration would only be required approximately every 20 km. Some modern single-mode fibers are now being produced with 200 GHz bandwidths which only need regenerating about every 100 km. From Table 6.4 the following conclusions about regenerator spacing may be drawn: 9 graded index fiber, for a wavelength of 850 nm, is well suited for 2, 8 and possibly 34Mbits -1 applications. If LEDs are used, regenerator spacing would have to be approximately every 8-13 km, depending upon the transmission rate. When laser diodes are used, attenuation as opposed to bandwidth tends to be the limiting factor and regenerator spacing is increased to 10-16 km, depending on transmission rate;

Table 6.4 Showing typical regenerator spacing required for graded-index multimode and single-mode fibers, at various transmission rates and wavelengths LED

Regenerator spacing limited by: Attenuation

Laser Bandwidth

Attenuation

Bandwidth

Graded index fiber for 850 nm (400 MHz km) Transmission rate

2 Mbit s-’ 8 Mbit s-‘ 34 Mbit s-’

8-1 2 krn 5krn

11 km 10-1 3 km

12-1 6 km 10-1 5 km

Graded index fiber for 1300 nm (1300 MHz km) Transmission rate

34 Mbit s-’ 140 Mbit s-’

Single-mode fiber for 1300nm (20 GHz km) Transmission rate 140 Mbit s-’ 565 Mbit s-’

12 km 7 km

17-30 km 20 km

20-40 km 16-30 km

182

Optoelectronicsand Fiber Optic Technology

9 graded index fiber is also suitable for a wavelength of 1300nm, together with 34 or 140Mbits -1 transmission rates. Use of LEDs is limited by the amount of material dispersion but distances can be increased by using Laser diodes. The regenerator spacing in 34Mbits -~ systems is dependent upon the link loss, while at 140Mbits -1 is limited by modal distortion. Any increase above 140Mbits -~ is not feasible due to the limited bandwidth; 9 single-mode fibers, at a wavelength of 1300 nm, are suitable for high bit rates and large section lengths, repeater spacing being limited almost entirely by link loss. Although current technology tends to limit the upper transmission rate to 564Mbits -1, with the use of very large scale integration (VLSI), transmission rates greater than 1 Gbit s -1 are becoming more common. Containers are required to house repeaters and regenerators and these are normally made of steel which have been hot-galvanized inside and out, and incorporate a plastic-based multilayer anti-corrosion coating. This coating enables the container to be buried in the soil where it will be subject to only slight external temperature variations and is extremely difficult to access by unauthorized personnel.

7

Communication systems

Although modem photonics began with the discovery of the laser in 1960, it wasn't until the 1970s that the telecommunications industry, with the perfection of low-loss optical fibers for undersea telephone cables, really made use of it. Since then, the driving force behind photonic development still remains the telecommunications industry, fuelled by the phenomenal growth in tile lnternet. Virtually any type of signal can travel via optical fiber. For example: analog video, NTSC or PAL, serial digital video (from 19.4Mb/s up to uncompressed 1.5Gb/s and beyond), analog audio, digital encryption, serial and parallel control data, intercom, triaxial RF camera links, Ethernet etc. The fiber is effectively just a large pipe the size of a human hair that has an (almost) unlimited bandwidth (Figure 7.1).

Figure 7.1 BrandRex)

The relative size of a fiber compared to a flea (courtesy of

184

Optoelectronicsand Fiber Optic Technology

7.1 Local networks It is interesting to note that there are seven colours in the rainbow, but until the 1960s the only colour available for telephones was black! Then the natural monopoly philosophy, which effectively dictated the type of service that could be made available to subscribers - as opposed to providing a service that fulfilled the customer's requirements - came to an end. Since that time we have moved from two phones on the desk (one for external, the other for internal calls) and gone through four generations of switching systems, from analog to digital, from copper to optical fiber networks and from the system is the solution to what the customer needs. Local cable systems (for example, cable television) are today experiencing extreme difficulty in selling services over existing copper cable networks and only a minority of homes having access to these facilities actually subscribe to them. Even such systems as the French Minitel, system whereby subscribers were provided with a terminal instead of a telephone book and had the facility of accessing weather reports, stock quotes and other specialized services over a copper network that joined together approximately 3.5 million users, were not fully utilized. Although attracting a great deal of interest it still lacked the ability to provide today's subscriber with exactly what he requires, i.e. an infinite amount of information, available at the flick of a switch or the press of a button. This situation was, of course, not overlooked by the cable network companies who were fully aware of the growing p h e n o m e n o n of telecommuters and the accepted fact that by the end of 2002, one in three workers will be operating out of his or her home. It is for precisely this reason that more and more interest is now being shown in the optical fiber market. In spite of the increased cost and difficult jointing procedures, the m a n y advantages associated with optical fibers over normal copper and coaxial cables ensure that fibers will be used more and more to provide switched, broadband communication services and networks for both business (videotelephony, videoconferencing and videotext) and private (multichannel radio/television programming, armchair shopping and videotelephony) subscribers. The huge increase in accessing the Internet has meant a phenomenal growth in the volume of traffic travelling through telecommunications networks. This traffic takes on a variety of forms, including Internet protocol, voice, data and video transmission. All of this d e m a n d s an increased bandwidth as their deployment becomes more widespread. Distribution of stereo sound programmes and television programmes are conventionally provided by sound and TV broadcasting services. In

Communication systems 185

an optical, broadband local area network, the users are arranged in a star configuration and the subscriber uses individual p r o g r a m m e selection channels to call up the desired p r o g r a m m e from the network control office. One of the advantages of this approach over the conventional distribution technique is that the n u m b e r of distributable p r o g r a m m e s is virtually unlimited. To ensure that several people can access each subscriber at a subscriber outlet, simultaneously, there must, of course, be sufficient transmission channels available for each service. When planning local area subscriber networks as well as providing for the current requirement, future expansion should also be anticipated. In copper cable systems this would have meant the inclusion of additional cable pairs. In optical fiber systems the availability of transmission modes virtually guarantees that sufficient channels will always be available. The layout of subscriber networks quite naturally varies from area to area and country to country, but typically it would be m a d e up of about 16 narrowband 16 Kbits -1 channels for telephone or data transmission, two 16 Kbits -1 channels available for subscriber video communication equipment, four stereo sound channels, four broadband channels for stereo TV and eight or so p r o g r a m m e selection and monitoring channels. At the subscriber premises the line will be terminated in a subscriber line terminating unit (LTU) that converts the optical signals into an electrical format and vice versa. The unit also performs some of the multiplexing and demultiplexing of the signals transmitted via the indoor cable between the line unit and associated equipment terminal. At the

Figure 7.2 Subscriber communication services system, using an optical fiber as a waveguide

186

Optoelectronics and Fiber Optic Technology

central switch, a line termination module acts as a muldex for the switched services being supplied to the subscriber and several signals, separated by time division multiplex (TDM), are incorporated with a binary line signal and digitally transmitted over the optical w a v e g u i d e to the subscriber. Such a system is illustrated in Figure 7.2. Mainly for economic and operational reasons, subscriber lines are, as a general rule, operated on an unrepeated basis. Although the cost of producing optical fiber cable is still fairly high, it is still more cost effective to install an optical system as opposed to a coaxial one, as can be seen from the graph in Figure 7.3.

I0 8

Coaxial line

7

Optical fiber

0

u 6 0 .>_. 5 I,, @

=:

4

~ Cost of installed cable plant I

!

10

20

_!

_

!

L ....

l

|_

_

30 40 50 60 70 Link length (km) "--------"

Figure 7.3 Cost comparison between coaxial and optical fiber systems for a transmission rate of 565 Mbit s -1

Previous experience with local networks is an essential consideration when planning new systems. For instance, some of the data obtained by engineers in the mid-1990s from a North American project (which used approximately 4000 km of fibers, installed in a cable route of about 70 km) produced some interesting results. Two of the main lessons learned were: 9 the tensile characteristics (the cables showed no significant increase in a t t e n u a t i o n - i.e. <0.1 dB km -1 at a wavelength of 850 n m - in response to a tensile force of 2500 N);

Communication systems 187

9 reaction to temperature changes (attenuation variations were always less than 0 . 8 d B k m -1 at a wavelength of 850nm in the r a n g e - 3 0 to +60~

7.2 Long-distance networks Until the wider use of optical fibers, both coaxial cables and balanced copper conductors were used in national long-distance networks. Depending on the method of transmission, these cables were designed either for frequency division multiplex (FDM) for analog, or pulse code modulation (PCM) for digital transmission. With the increased use of digital transmission techniques and the everincreasing amount of information services, expansion of long-haul networks has become very necessary and nowadays, because of the disadvantages (lack of bandwidth and size etc.) of copper conductors, optical fibers are becoming the preferred option. Obviously, it would not be cost effective to replace all of the existing copper cable networks, particularly if they were designed to cover a high density local environment where subscribers were dropped off in small numbers - but quite often. Instead, optical fibers are far more suited to carrying multigroup (e.g. 300) signal groups, which, in optoelectronic terminology, are referred to as 'highways'. Current technology suggests that the cost effectiveness of an optical fiber highway link decreases after about 10 km, and the use of some other method for very long distances has to be considered - for example, a microwave link.

7.3 Telephone networks Telephone networks are divided into two distinct levels, local and long distance. In local networks the subscriber is connected to the local or terminating exchange with local branch cables. Local exchanges are in turn interconnected with local inter-exchange trunks to form regional networks. The regional network is then extended to the main exchange via trunk lines (Figure 7.4). Typical cable capacities are: 9 trunk l i n e s - 2000 fibers or more; 9 branch l i n e s - 100-200 fibers; 9 subscriber l i n e s - 1-10 fibers.

188 Optoelectronics and Fiber Optic Technology

IIIIIII i~~! Illllll IIIIIII IIIIIII IIIIIII PBX Localbranch cables

T r u n k lines

N N oooN ooeN Telephone

IIIIIII I~o~1 IIii111 IIIIIII IIlllll IIIIIII

Local interexchange trunks

PBX Local branch cables

J

N

Telephone

N Telephone

,.,

Telephone

,_,

Publicswitch

I t~176!l

IIIIIII IIIIIII IIIIIII

J

III III IIInnll

T r u n k lines

IIIIIII I'o-o~'--~ IIIIIII IIIIIII IIIIIII IIIIIII

Local interexchange trunks

PBX Local branch cables

PBX Local branch cables

N

N Telephone

Figure 7.4

N Telephone

Example of a telephone network

7.4 Data networks There are three t y p e s of d a t a n e t w o r k : 9 local area n e t w o r k ; 9 m e t r o p o l i t a n area n e t w o r k ; 9 w i d e area n e t w o r k .

N Telephone

Telephone

Communication systems 189

7.4.1 Local area network A local area n e t w o r k (LAN) (Figure 7.5) consists of a n u m b e r of terminals, microprocessors and computers, located within adjacent buildings, connected via a central processor unit (CPU) or server which enables t h e m to share resources across a n e t w o r k (typically I k m radius) using Ethernet or similar protocol.

oo~176176176176176176176176176 loo~ /

o o o o o oo

Figure 7.5

oo

o o o o o

LAN

Note: Ethernet is a LAN protocol developed by Xerox Corporation in co-operation with DEC and Intel in 1976. It uses a bus topology and supports data transfer rates of 10 megabits (10 million bits) per second. Fast Ethernet (or 100BaseX) is an updated version of Ethernet which supports data transfer rates of 100 megabits per second.

7.4.2 Metropolitan area network A metropolitan area n e t w o r k (MAN) (Figure 7.6) is a data n e t w o r k intended to serve an area the size of a large city. It is effectively a large LAN m a d e u p of a collection of LANs connected to a router (a device that interconnects networks and decides the best path over which to send data b e t w e e n networks), operating over a metropolitan-sized area.

190 Optoelectronicsand Fiber Optic Technology

Local area network

Router

Local area network

Figure 7.6

MAN

Note- The term 'MAN' infers a network that is larger than most LANs, yet smaller geographically than a WAN.

Satellite

Router

Satellite dish

Figure 7.7

WAN

Communication systems 191

7.4.3 Wide area network

A wide area network (WAN) (Figure 7.7) is a network that spans a large geographical distance. It can consist of a number of interconnected LANs and MANs and many multinational companies operate WANs across continents, often utilizing public switched telephone networks (PSTN). Figure 7.7 illustrates how a collection of LANs might interconnect, through routers across a PSTN. The local networks can send data via satellite links to different continents, creating a WAN. A MAN could also be integrated into the WAN, again using a router.

7.5 Designing an optical fiber cable system The most important considerations when designing any optical fiber system are attenuation loss and usable bandwidth. Both of these are a function of the amount of input power that is required to produce a preset or desirable output power. The system loss is given as a function of power out against power in and is measured in decibels per kilometre of cable (e.g. 10 log[PoL,t/Pi,~] in dB km-1). Consideration must also be given to system expansion and this should not only include additions to the system and the necessity to splice new cables on to existing cables, but also excavation, cable routing etc. The design engineer of any local network, be it private or business, will also need to be aware of the following points: 9 the number of subscribers; 9 the application (i.e. indoor, ducted, buried, aerial or submarine); 9 the transmission properties (multichannel television, videoconferencing etc.); 9 the climatic and geographical environment (temperature, tension, pressure, ice or windloading); 9 the local environmental restraints (for instance, the requirement to utilize a special non-flammable or rodent-resistant, jacketed cable). Attenuation introduced by fiber splices (typically 0.05 to 0.02 dB), cable distribution cabinet jumpering, together with in-line or branch splices must also be kept to an absolute minimum. Instead of using main distribution frames (MDFs), this can be achieved by either laying separate cables to branch networks (as opposed to branch-splicing a cable off the trunk network) or distributing cables in the switch-room directly from the trunk cable (i.e. fanning out). Attenuation can naturally be minimized at spliced-cable sections if correctly dimensioned loose tube cables are used.

192 Optoelectronics and Fiber Optic Technology 7.5.1 Fiber versus cable

Basically, there are two choices for configuring hard-wired, high-speed system-to-system communications: fiberoptic or copper-based interconnects. Although the choice mainly depends on distance, bandwidth and cost, the points itemized in Table 7.1 also need to be considered. Figure 7.8 shows how existing copper local networks (a) can be replaced by optical fiber systems (b). In the Figure 7.8, a copper cable network requires more repeaters or generators, and so a large number of closures (10 in this example) are

Table 7.1

Considerations of fiber versus cable

Cost

Fiber costs less for long-distance applications because copper cables need many repeaters and/or regenerators over long hauls. For distances of only a few metres, however, copper systems are less expensive and will probably remain so for some considerable time.

Immunity to EMI, RFI, and lightning

Although it's a different story at the ends of the fiber cables, fiber optic transmission usually involves multiplexing signals into a serial data stream. Multiplexers and demultiplexers entail extremely high speed switching, giving rise to emissions that you must provide shielding against.

Weight and size

Optical fiber generally has an outside diameter ranging from 0.125 to 1 mm. The outer diameter of a communications cable pair or coaxial cable can range from 1 to 10 mm. Because the specific gravity of glass is approximately one-fourth that of copper and the dimensions of fiber cables are smaller, the cable weight of a fiber optic system is one-third to one-tenth that of a copper system.

Ease of installation

Although the pros and cons of the two technologies are highly dependent on the circumstances, in most cases, wiring a fiber optic system is more difficult and requires more specialized skills than connecting a copper-based system.

Ease of upgrading

Because current system applications use only a fraction of the available bandwidth, existing fiber optic systems are capable of covering any imaginable network upgrade.

Communication systems 193

Figure

7.8

Optical fiber local networks

required. On the other hand, optical fiber local networks require fewer generators, so fewer closures (only three in this example) are needed.

7.5.2 Digital or analog Should the system be digital or analog? Both of the following options have pros and cons.

7.5.2.1 Digital The only characteristic affecting the span length for digital transmission is the permissible attenuation between the optical transmitter and the optical receiver and the average received optical power necessary to achieve a certain bit error rate. Detailed consideration of the theoretical, achievable, receiver sensitivity shows that provided the system is not bandwidth-limited, binary optical transmission will give a receiver sensitivity several decibels higher than multilevel optical transmission. For systems up to 140 Mbit s-1 graded index fibers are used. Above 140 Mbit s-1, especially in long-haul communications, single-mode fibers are used as they provide the bandwidth required for regenerators. Because the spectral bandwidth must be low to minimize material dispersion at high bit rates, laser diodes must also be used.

7.5.2.2 Analog Apart from bandwidth and attenuation, linearity of the optoelectronic transmission path is also important for analog transmission. Although

194

Optoelectronicsand Fiber Optic Technology

linearity can be improved by reducing the power to the transmitter, a reduction in the modulation factor will reduce the already small systemto-noise ratio. This could be overcome by using: 9 negative feedback circuits (but these introduce excessive attenuation); 9 having non-linear pre-equalization (but diode losses are increased); 9 restricting the span length (but then the signal-to-noise ratio is reduced with power increases). An analog system is thus really only feasible if the quality of the system is modest. 7.5.3 ATM versus fast Ethernet

The networking market is traditionally split between asynchronous transfer mode (ATM) on the access side and Ethernet on the LAN side. However, Ethernet is moving to 10Gbit/s, so each technology is taking some of the other's market share. The main difference is in cost. LAN components tend to be cheaper than those for WANs, but WAN components have higher reliability. Many experts are now predicting that the future of networking to the desktop lies in ATM, which is said to be the technology best suited for handling multimedia. ATM is gaining acceptance from tile computer, communications and television industries, and is expected to play a major role in the delivery of full-motion video to the home and desktop in the future. But for the time being, it is predicted that Ethernet and fast Ethernet will be the most widely used network protocols for desktops, until demand for multimedia PCs increases in the home market and workstations in industrial applications need video. On the downside, whilst ATM has unlimited bandwidth, fast Ethernet is cheaper and works better with the cabling that's already installed. Indeed, dual-speed Fast Ethernet products connect computers at 10 times the speed of conventional Ethernet whilst still maintaining compatibility with existing 10 Mbit/s networks. 7.5.4 Multiplexing

With a simple low-cost fiber transmitter/receiver system, each video circuit, whether digital or analog, typically uses one fiber. Using a multiplexer, one fiber can support up to 64 channels of audio plus several data and intercom circuits. This type of multiplexer uses digital time division multiplexing (TDM) with a laser output. The fiber capacity could

Table 7.2

Optical fiber system types compared, showing typical regenerator spacings and system factors Transmit Receiver Connector Theoretical Maximum Fiber loss system power power section loss IOSS loss dBm dBm dB dB dB dBkm-’

Splice IOSS

Cable loss

dB km-‘

dB km-’

Regenerator Number spacing of channels km

8 Mbit s-’ 840/880nrn graded index LED 840/880 nrn graded index laser 1300 nm graded index LED 1300 nrn graded index laser

-16 -2 -1 7 -2

-56 -56 -49 -49

3 3 3 3

37 51 29 44

31 45 23 38

3 3 1 1

0.2 0.2 0.2 0.2

3.5 3.5 1.5 1.5

9 13 15 25

120 120 120 120

34 Mbit s-I 840/880 nrn graded index laser 1300 nm graded index LED 1300 nrn graded index laser 1300 nrn single-mode laser

-3 -1 9 -3 -4

-52 -47 -47 -47

3 3 3 4

46 25 41 39

40 19 35 33

3 0.8 0.8 0.5

0.2 0.1 0.1 0.1

3.5 0.9 0.9 0.6

11 21 39 53

480 480 480 480

140 Mbit s-’ 1300 nm graded index LED 1300nm graded index laser 1300 nrn single-mode laser

-20 -3 -4

-40 -40 -43

3 3 4

17 34 35

11 28 29

0.8 0.8 0.5

0.1 0.1 0.1

0.9 0.9 0.6

7 31 48

1920 1920 1920

-4

35

4

27

21

0.5

0.1

0.6

35

7680

565 Mbit s-’ Single-mode laser

196 Optoelectronics and Fiber Optic Technology

be further enhanced by optically multiplexing two or more light signals, say one HDTV laser transmitter operating at a wavelength of 1310nm and an audio multiplexer signal at 1550nm, on one fiber. This optical sharing of a fiber is referred to as wavelength division multiplexing, or WDM. Wavelength multiplexing is often used where the n u m b e r of fibers is limited, for instance when leasing an unused 'dark' fiber from a local carrier to transmit over longer distances, for studio-to-transmitter links or across towns. It is normally not necessary where fibers are plentiful.

7.5.5 Channel and cable capacity For applications having several communication links between the same points, it is economical to use some form of multiplexing to combine communication channels on to a single fiber-pair before transmission. TDM, whereby a number of sampled messages are used to modulate a pulsed carrier at different time intervals thereby ensuring that at any one instance only one carrier is being transmitted, is widely used. Typical regenerator spacings, wavelengths and transmitter types are compared in Table 7.2, with regard to other system factors.

7.5.6 Fiber loss (Table 7.3) Most general-purpose optical fibers exhibit losses of 4 to 6 dB per km (a 60 per cent to 75 per cent loss per km) at a wavelength of 850nm. When the wavelength is changed to 1300 nm, the loss drops to about 3 to 4 dB (50 per cent to 60 per cent) per km. At 1550 nm, it is even lower. P r e m i u m fibers are available with loss figures of 3 dB (50 per cent) per km at 850 nm and 1 dB (20 per cent) per km at 1300nm. Losses of 0.5 dB (10 per cent) per km at 1550nm are not uncommon. These losses are primarily the result of random scattering of light and absorption by actual impurities within the glass. Table 7.3

Fiber losses* Source

Fiber

850 nm

1300 nm

Multimode Single mode (premises) Single mode (outside plant)

3.5 dB

1.5 dB 1.0 dB 0.5 dB

*per km using EIA/TIA 568 maximum figures

Communication systems 197

Figure 7.9 Example of a trunk network (courtesy of Corning Cable Systems

GmbH)

All of the above attenuation factors result in a loss that is independent of bandwidth. In other words, a 3 dB loss means that 50 per cent of the light will be lost whether it is being modulated at 10 Hz or 100 MHz.

7.5.7 Cable routing Normally a communication network is divided into two major areas the trunk network and the access n e t w o r k - as shown in Figure 7.9 which depicts a typical trunk network connecting a n u m b e r of central offices and access networks together. Fiber to the curb (FTTC), fiber to the building (FTTB) and fiber to the home (FITH) are possible by using the trunk network to connect the central offices and the access networks with the homes, offices and customers (Figure 7.10).

7.5.7.1 Example 1: Campus cabling In this example, buildings A, B and C are to be interconnected using fiber optic cables. One building, in this case building 'A', is selected as the main building with a campus cross-connect and the network is configured in the form of a hierarchical star (Figure 7.11). A fiber optic outdoor cable is brought from the campus cross-connect out of building A and buried directly in the ground. Halfway towards buildings B and C, the cable is branched into two cables. This is

198 Optoelectronicsand Fiber Optic Technology

Figure 7.10 GmbH)

Fiber to the customer (courtesy of Corning Cable Systems

accomplished by fusion-splicing the individual fibers together, storing the resultant splices in splice trays and protecting them in a fiber optic closure. From this splice point, the two cables are routed directly to the intermediate cross-connects of buildings B and C. In this example, the cables are routed through concrete ducts.

Figure 7.11 Campus cabling (courtesy of Corning Cable Systems GmbH)

Communication systems 199

Figure 7.12 Riser area cabling (courtesy of Corning Cable Systems GmbH)

7.5.7.2 Example 2: Riser area cabling In this example, four floors of a building are interlinked by means of a fiber optic cable (Figure 7.12). One floor (in this case the ground floor) serves as the site for the intermediate cross-connect. From here, fiber optic cables are run in a star configuration to link all the other floors. They are terminated at horizontal cross-connects on the individual floors.

7.5.7.3 Example 3: Horizontal level (tertiary level) In this example, the cross-connect is to be connected to fiber optic wall outlet boxes at the workstations so that optical fibers can be used to connect terminal equipments. On the horizontal level, it is important to ensure flexibility and expendability and this calls for careful planning of the n u m b e r of wall outlet boxes and ensuring adequate reserve capacity. There are two methods of achieving this, conventional horizontal cabling and centralized horizontal cabling.

Conventional horizontal cabling In conventional cabling configurations (Figure 7.13), the horizontal crossconnects and wall outlets are linked in a star configuration with fiber optic cables converging on the horizontal cross-connect as the transition point to the riser area. Fiber optic cables with four fibers allow two terminals to be connected per wall outlet box. Duplex connecting cables are run from the wall outlet box to the terminals.

200 Optoelectronicsand Fiber Optic Technology

Figure 7.13 Conventional horizontal cabling (courtesy of Corning Cable Systems GmbH) Unlike copper cabling practice, the wall outlets and terminals can be located more than lOOm away from the horizontal cross-connect which greatly facilitates the cabling of large-area production sites and floors

Centralized horizontal cabling Since there are no strict rules for connecting fiber optic cables to the desk, it is possible to dispense completely with horizontal cross-connects and to route the cables from the intermediary cross connect via the riser directly to the desk (Figure 7.14). This eliminates a transition point, reduces the space required on the individual floors and simplifies cabling control, administration and maintenance.

Figure 7.14 Centralized horizontal cabling (courtesy of Corning Cable Systems GmbH)

Communication systems 201

Figure 7.15

Cab/e routing

7.5.8 Distribution frames Instead of running cables in a point-to-point manner and adding cables to ultimately form an unmanaged mesh, the objective should be to plan a fiber backbone. This will need to form a hierarchical topology composed of fiber optic cables and patch panels (often called distribution frames) similar in concept to the distributed star-hub configuration that airlines and data networks use. It should start with a main hub location, ideally one in a central location and in the main control area for convenient access. This is where simple fiber jumper patching allows you to reconfigure the system (alterations, improvements and changes, etc.) without having to recable. This 'hub location' is called the main distribution frame, or MDF (Figure 7.15). From this point cables radiate out to secondary locations where intermediate distribution frames, or IDFs, further distribute to local patch panels in telecommunication closets, equipment rooms, or user nodes. This topology allows the design engineer to flexibly reconfigure devices in one segment without disrupting the entire network. The star-hub is the most efficient, flexible topology for structured cabling networks. Figure 7.16 is an example of a 19" networking cabinet for distributing fiber optics. This particular example has 1440 ports and is suitable for short-haul and local networks. It can also be used as a fiber node and transition point in the LAN, MAN and WAN areas.

7.6 Optical path loss budget (Figure 7.17) When designing fiber optic systems, one must take into account the system loss budget (SLB), which describes the difference in power

202 Optoelectronicsand Fiber Optic Technology

Figure 7.16 GmbH)

Typical distribution cabinet (courtesy of Corning Cable Systems

between that emitted by the transmitting device and that received by the detecting device. The optical path loss is a way of determining whether the link, which has been installed, will work or not. As long as the m a x i m u m power loss does not exceed the budget, the link will work. All attenuation factors must be considered including the fiber attenuation per kilometre, connector loss, fiber to detector loss and fiber to transmitter loss.

Transmitted power out

Received power in

op,,ca,!/

1 !

transmitter

'

Connector losses

t

o,,ca,

receiver

t

Fiber losses

Fiber losses

Splice losses

Figure 7.17 Optical path loss budget

'

Connector losses

Communication systems 203

Note: Additional, spare power should also be provided to account for temperature variations, LED aging, mechanical stress loss on the fiber and bend loss on the fiber.

The m a i n i n f o r m a t i o n required before w o r k i n g out the loss b u d g e t is: 9 transmitter level; 9 receiver level; 9 length of link. O t h e r factors to take into consideration include: 9 9 9 9 9

type and length of fiber; w a v e l e n g t h of operation; n u m b e r of connectors; n u m b e r of splices; margin (allowance for future e x p a n s i o n or d e g r a d a t i o n of the l i n k hypothetical figure n o r m a l l y b e t w e e n 6 and 10 dB).

For example (using E I A / T I A 568 m a x i m u m values): 9 9 9 9 9

TX level = -13 dB; RX level = - 4 2 d B ; connectors = 2 @ 0.75 dB each; splices = 2 @ 0.3 dB each; m a r g i n = 8dB. Budget = (RX l e v e l - T X level) - (2 x Connectors) - 2 x splices) (margin) dB. Budget - (42 - 13) - (2 x 0.75) - (2 x 0.3) - 8 = 15.9 dB

As the total loss of the link is less than that of the b u d g e t , this link will w o r k - but for h o w far? To find out the m a x i m u m distance the link will w o r k to, the a t t e n u a t i o n coefficient of the fiber m u s t be k n o w n a n d d i v i d e d into the r e m a i n i n g figure of the budget. A s s u m i n g that the link consists of 1 k m of m u l t i m o d e 50/125 fiber using an 8 5 0 n m source w i t h a 3 dB per k m fiber loss, it will w o r k for a m a x i m u m distance of 5.3 k m (i.e. 15.9/3).

204

Optoelectronicsand Fiber Optic Technology

7.7 Installation techniques Cable types and sizes are chosen depending on the application and the overall cost of the system. As previously described, fiber optic cables are represented by a pair of numbers separated by a '/'. Two common fiber optic cable sizes are 50/125 and 62.5/125. In all cases, the first number represents the outer diameter size of the core and the second number represents the outer diameter size of the cladding, both are measured in microns. Selecting the right cable will provide optimum performance and leave room for system expansion. Typical uses of different types of fiber optic cable are:

9 single-mode cable for inter/intra-city trunks; 9 multimode 50/125 cable for intra-city and CATV trunks; 9 multimode 62.5/125 or 100/140 cable for wideband LANs.

Whenever possible, fiber optic cable should be run in a straight line, with as little bending as possible from end to end in order to keep fiber attenuation to a minimum. The cable can be terminated with a connector mechanically or spliced together using one of the previously described (see Chapter 6) chemical fusion techniques. In either case, it is important to remember that circuit attenuation increases with every splice or connector added to the system. Attenuation introduced by mechanical connectors (typically 1 dB) is mainly due to the misalignment between fiber cores. Therefore, a good fiber optic connector not only provides a means of connecting two fiber optic cables together, but also provides a method of aligning the cores and bringing the ends as close together as possible with the lowest attenuation possible. Unfortunately, mechanical connectors for fiber optic cable have not been standardized and are available from many sources which makes it increasingly difficult to interconnect equipment from different manufacturers. For this reason, several companies provide adaptors, which allow interconnection between different types of connectors. As with mechanical connectors, however, these adaptors also introduce additional attenuation in the order of approximately 1 dB. Fiber optic cables come in many different forms, for example unarmoured indoor/outdoor cable (breakout cable), armoured heavyduty external cable, tight jacket ruggedized internal cable and aerial cable (sky w r a p / s k y span). Each one of these cables will have their own installation specifications, which must be adhered to.

Communication systems 205

7.7.1 Storage and handling 7.7.1.1 Cable storage D r u m s should be stored (on a solid, even surface) in the open air or u n d e r cover, d e p e n d i n g on the location a n d / o r environment. They should be chocked to avoid i n v o l u n t a r y m o v e m e n t and should never be stored on their side or stacked u p o n one another. 7.7.1.2 Lifting Lags or battens are n o r m a l l y used to protect the outer layer of the cable, but as these are not load bearing, they m u s t not be used to lift the d r u m . There are two r e c o m m e n d e d m e t h o d s (Figures 7.18 and 7.19).

Figure 7.18 By crane

Figure 7.19

By forklift truck

206

Optoelectronics and Fiber Optic Technology

The spindle is placed through the centre of a d r u m and then suspended by rope with a spacer bar. The forks are placed under the bottom of the d r u m from the side.

7.7.1.3 Rolling The d r u m must only be rolled in the direction marked on the d r u m and should only be rolled on smooth or fairly even surfaces.

7.7.2 Cable laying By far the most expensive part of laying lengths of communication cable u n d e r g r o u n d is not the actual manufacturing costs but the excavation, trenching and refurbishment of cable ducts. Whenever a new underground cable is being installed, the possibility of including other cables routed in the same direction and capable of being laid in the same trench should be considered. Optical fibers, because of their size, lend themselves to sharing not only one trench but in some cases the same cable sheath as other fibers, coaxial, or copper quad cables. The cable (armoured), would normally be laid direct in the earth, which has been trenched to set specifications. The cable should be bedded in 75 m m of sand and should then be covered by a further 75 mm of sand, and, whenever possible, a plastic strip similar to those used when laying an electric cable, should be placed over the top of that. Earth would then be used to fill the trench and the cable will be marked for ease of identification (Figure 7.20). 1 A1

...................... :.......................

Earth

t:i:1:1:::::::::::::ii::i:i:i:i:::i:i::::::i::::i . . . . . . . . . . . . . . . . . . .

t ........................ Y V / ~ ' ~ ~

!

Tape sand Cab|e

Sand

Figure 7.20

Cable laid in a trench

Communication systems

207

If there is more than one cable to be installed, then they should not cross unless absolutely necessary and they should run parallel to the line of the trench. There are, of course, disadvantages to using optical fiber cables that have been trenched or ducted u n d e r g r o u n d - the most obvious being the damage that rodents can do to the cable. This problem can be overcome by using steel tape over the cable, but this is very expensive and is also against the basic principle of providing a completely non-metallic cable. Another method of overcoming the problems associated with rodents is to encase the cable in resin-impregnated glass or polyamides, but this too increases the cost of the cable.

7.7.2.1 Ducted Whenever possible a fiber optical cable should be installed in its own duct as opposed to sharing a duct with other cables. These small subducts can then share a large (e.g. 100 m m diameter) duct with other ducted cables (Figure 7.21).

Subducts

100 mm duct

Figure 7.21 Ducted cables

In the case of difficult pulls, it is advisable to pull in a cable containing extra fibers for future use to avoid returning to the same pull.

Note: As the cable will have a natural twist when it is removed from the cable drum, if it is to be installed in a duct with other cables, it will twist around these cables and tangle, resulting in certain damage to the outer sheath.

208

Optoelectronics and Fiber Optic Technology

7.7.2.2 Using pylons An alternative method of cable distribution across unpopulated areas (other than direct burying) is to utilize existing high voltage overhead power line pylons to support the cable, as illustrated in in Figure 7.22. In this method the non-metallic optical fiber cable is incorporated into the phase conductors or ground wires. Because of its low weight, the optical fiber cable subjects minimal additional stress on to the pylon.

Figure 7.22 Ground wire, incorporating optical fibers, mounted on the top of a pylon power line structure

7.7.2.3 Cable pathways and termination spaces The design of cable pathways (i.e. conduits, cable trays, riser shafts) and termination spaces (i.e. cross-connect telecom closets, work area telecom outlets) should adhere to the commercial building standard, EIA/TIA 569, for telecommunication pathways and spaces and the Fiber Industry Association (FIA) Code of Practice.

7.7.2.4 Maximum recommended pulling tension The maximum recommended pulling tension, specified by the manufacturer of the cable, must not be exceeded. The pulling tension may differ at different points in the route and care must be taken to adhere to the specifications.

Communication systems 209 Table 7.4

Typical bend radius specifications Minimum bend radius Unloaded

Loaded Application

Fiber count

cm

in

cm

in

Inter-building Backbone Intra-building Backbone

2-84 86-192 2-12 14-24 26-48 48-72 2 UTP + fiber STP + fiber

22.5 25.0 10.5 15.9 26.7 30.4 8.7 8.7 12.2

8.9 9.9 4.1 6.3 10.5 12.0 3.4 3.4 4.8

15.0 20.0 7.0 10.6 17.8 20.3 5.8 -

5.9 7.0 2.8 4.2 7.0 8.0 2.3

Horizontal Wiring

m

Note: When pulling cables around bends, (i.e. 45 ~ or more), a turning wheel should be used to avoid d a m a g e and stress to the cable. The manufacturer will normally specify the m i n i m u m bend radius Table 7.4).

7.7.2.5 Maximum vertical riser All fiber optic cables have a specified maximum vertical riser (Table 7.5) that is a function of the cable's weight and tensile strength. This is the maximum vertical distance that can be installed without intermediate support points. The following should be remembered: 9 all vertical cables must be secured at the top of the run; 9 the attachment point must adhere to the cable's minimum bend radius whilst supporting the cable securely; 9 long vertical runs should have an intermediate support grip when the maximum vertical rise has been reached.

7.7.3 Installing fiber optic cables in buildings The advantages of optical communications systems for building applications are well recognized: expanded data handling capacity, electrical

210

Optoelectronics and Fiber Optic Technology

Table 7.5

Typical vertical riser Maximum vertical rise

Application

Fiber count

M

Ft

Interbuilding Backbone Intrabuilding Backbone

2-84 86-192 2-12 14-24 26-48 48-72 2 UTP + fiber STP + fiber

184 69 500 500 500 500 500 40 40

640 336 1640 1640 1640 1640 1640 130 130

Horizontal Wiring

noise immunity, electrical isolation, and enhanced safety and data security. Many large building projects have already used fiber optic components, a trend certain to continue. Installing fiber optic systems is, in many ways, similar to installing twisted-pair or coaxial cable. Despite common misconceptions, fiber optic cables are quite easy to work with. They have excellent pull strength, even though they use glass fibers. Not only does their small size make them easier to pull than many copper cables, they have considerable resistance to crushing and cutting and their flexibility makes them easy to handle.

7.7.3.1 Patch panels Patch panels (Figure 7.23) are typically manufactured in multiples of 12 fibers but experience has shown that it is always better to overbuild and install at least twice as many fibers than are initially needed. They may not necessarily be required in the first instance, but it is much better to install plenty of excess fibers rather than having to recable later on. Whilst the cost of excess fibers is low, the cost of recabling, similar to coax, can be very high. For networks, since the most economical systems use one fiber per video circuit, it is essential to install far more fibers than the average LAN requires. It is not uncommon to see video-intensive campuses and large network facilities running 144-fiber cables between buildings, and perhaps 24 to 36 fibers to each floor.

Communication systems 211

Figure 7.23 GmbH)

Patch panel and splice tray (courtesy of Corning Cable Systems

7.7.4 Blown optical fiber Originally invented and pioneered by BT, the concept of blown fiber has since been further developed by some of the world's leading technological companies, including ICI and Optical Fibers (a partnership between Corning and BICC). The network infrastructure can be installed as part of a high performance optical communications system or as a microduct overlay to create a structured, 'futureproof' wiring system as a way of meeting today's ever-changing communications requirements. The basic microduct (Figure 7.24) is normally a 5 or 8 mm diameter tube with an inner layer of antistatic low-friction carbon-loaded polyethylene.

Figure 7.24 Microducts used for blown fiber (courtesy of Brand-Rex)

212 Optoelectronicsand Fiber Optic Technology

The optical fiber used is coated so as to give it a low coefficient of friction, low antistatic characteristics and a high aerodynamic drag factor. The outer layer will normally be a low flammability zero-halogen cablegrade material. The blown fiber principle involves installing small-bore plastic tubes into an existing building or including them as part of the plans for new buildings (Figure 7.25). Subsequently, the optical cabling can be added whenever required by using an installation equipment to blow specially designed optical fibers into selected ducts (Figure 7.26). These equipments generate compressed air, which is blown over the surface of the fiber to propel it through the ducts. Installed fibers are then terminated (using conventional on-site techniques or by splicing in a factory-made pigtail cable) and tested in the usual manner using power metres and light sources or optical time domain reflectometers (OTDRs). Blown fiber is suitable for LANS, where cable runs rarely exceed 1000m and for links between buildings.

Note: Blown fiber may be used in most circumstances where standard optical fiber would normally be used. Waterproof and UVproof microducts are also available.

7.7.4.1 Advantages of a b l o w n fiber system

The blown fiber system offers significant advantages over conventional networking systems, including: 9 low initial c o s t s - the only cost is in the installation of the empty microduct which can lie dormant until a customer or requirement is found. Blown fiber costs approximately the same as standard optical fiber cable; however, in terms of deferred cost, the year-one cost differential is huge; 9 f u t u r e - p r o o f i n g - once installed, the network can be switched to the latest fiber technology with the m i n i m u m of fuss and disruption; 9 ease of u p g r a d e - using the blown fiber method, fibers can be installed in heavily populated offices with the office users being completely unaware of the operation;

Communication systems 213

Figure 7.25 Brand-Rex)

Example of a complete system infrastructure (courtesy of

214

Optoelectronicsand Fiber Optic Technology

Figure 7.26 Blown fiber technique

9 f l e x i b i l i t y - networks can be configured quickly during initial installation and at any time thereafter to meet changing requirements; 9 excavation w o r k - need only take place once; 9 ease of r e p a i r - when a conventional optical cable is damaged, probably the most economical solution is to replace the whole cable length with all the attendant costs and disruption of service. By contrast, using the blown fiber method, damaged sections can be blown out from one end of the microduct and replaced by new sections which are then spliced in. The technique can also be used for costeffective network system upgrades.

7.7.4.2 Typical blown fiber equipment A number of specially developed portable blowing and data-logging equipments are currently available which ensure efficient installation with the minimum of inconvenience to customers. In the example shown in Figure 7.27, two low-noise units are used, one supplies air, the other for installing the fiber. A fiber blowing 'wand' connects the blowing unit to the network whilst the air supply module provides both compressed air and electrical power during installation. The blowing unit can be programmed manually or with a CAD package and its control computer automatically records data concerning the number and type of fibers installed, so as to provide a constantly updated picture of the completed installation. Its software also has built-in safeguards to prevent the insertion of incorrect fibers and an electronic record can be made available to customers on disk a n d / o r hard copy.

Communication systems 215

Figure 7.27

The 'Blolite' system (courtesy of Brand-Rex)

8

Optoelectronic test techniques

In this age of information technology and the requirement for high capacity wideband communication systems, it is of little wonder that industry is turning more and more to optical fibers, but with it has come a requirement for a completely new range of optical measuring instruments. Similar to other communication systems there are two distinctly separate areas that require production and field (i.e. in-service) testing and although the basic test requirements associated with optical fiber systems are well known, the range of instruments currently available is enormous. It is, therefore, essential for today's engineers to have a clear knowledge of exactly what he is testing, why he should test it, and how he should test it and this very much depends on whether he is a manufacturer, installer or end user.

8.1 Testing fiber optics During production, a manufacturer must constantly test the fibers to confirm that their characteristics meet the standard or client's specification. These tests will include: 9 9 9 9 9 9

core diameter; cladding diameter; numerical aperture; attenuation; refractive index profile; tensile strength.

Other tests will be required to ensure that the mechanical characteristics (e.g. impact resistance, tensile loading and crush resistance) confirm

Optoelectronic test techniques 217

the cable's ability to withstand physical and mechanical stress. Environmental tests will evaluate the changes in attenuation under extremes of temperature, temperature changes and humidity. Testing is also carried out during and after the installation of a link to ensure that it meets the performance requirements. Test results quantify system quality, identify system faults, ensure the longevity of the system, minimize maintenance and interruption to traffic and establish system accountability (particularly important when systems are being installed by outside contractors!). Testing the optical properties of the cable itself involves two measurable characteristics- attenuation and bandwidth. Attenuation (defined as optical power loss measured in decibels (dB)) is the primary limiting factor in most systems and consists of the loss introduced into the link by the physical properties of the passive components, connectors, couplers, splices, the fiber and switches. Additional losses may be introduced by bad installation practice (for example, tight bends and excess stress on the fiber during installation). Testing must also, therefore, be carried out after installation to ensure that no extra loss has been induced into the link and that the attenuation is within the limits set down by the end user. The equipment required for carrying out system testing includes power meters and optical time domain reflectometers (OTDR). Bandwidth is the maximum capacity of a line in an electronic communications network, measured in bits per second, bytes per second, or hertz (cycles per second). Bandwidth determines the amount of digital information that a network can carry and during periods of peak use, bandwidth may also determine the speed of transmission, particularly in the case of large data files such as those containing graphics, audio, or video. Bandwidth is also a measure of the information carrying capacity of the cable system. It is dependent on the fiber characteristics and varies inversely with system length (i.e. a long fiber will have less bandwidth available than a short one if they have the same modal bandwidth specification in MHz/km).

Note: Bandwidth testing requires complicated equipment and it is not common policy to carry out this type of testing in the field.

8.1.1 Production testing The whole purpose of production testing is to ensure that the user is provided with end-product consistency. Manufacturing optical fibers is a

218 Optoelectronicsand Fiber Optic Technology

precise science and in the same way as in any other volume manufacturing process, production testing is instrumental in ensuring a uniformity of manufacture and a means of verifying that the material specifications, within each phase of the manufacturing process, are to the required quality. Production testing usually involves the following: 9 measuring the refractive index profile: prior to coating the fiber with an acrylic material; 9 measuring the spectral response after coating: attenuation versus frequency; 9 checking fiber geometry; including concentricity of core and thickness of coating; 9 backscatter measurement: to detect breaks and other defects in the end product. In order to carry out such an exhaustive series of checks a variety of sophisticated measuring instruments, as well as highly trained staff, are required. Neither of these commodities comes cheaply and, as a general rule, manufacturers typically estimate to commit at least 30 per cent of their total manufacturing budget to production testing. Production testing therefore represents a significant proportion of the final product cost.

8.1.2 Installation testing The single most important test result is the end-to-end attenuation, which is a measure of the optical power loss between two cable termination points. Acceptable loss values are dependent upon the transmitter and receiver equipment and the system design (for instance, whether it is an analog, digital, communications LAN, or other system). In order for the system to operate correctly, the obtained loss figures should always be less than the optical path loss budget (see Chapter 7) calculated in the design.

Note: Measurements should be taken from patch panel to patch panel to ensure that all parts of the system are included.

Attenuation testing measures the overall power loss of the system and for short distances a simple optical power meter (OPM) can be used. For cable runs greater than 50 m manufacturers usually recommend the use of an OTDR.

Optoelectronic test techniques 219

8.1.3 In-service field testing When considering the purchase of an instrument, an engineer must be sure to take account of: 9 the permitted range of fiber connectors; are adaptors provided, are they easy to fit, will the equipment accept a bare fiber etc.; 9 the probable requirement to recalibrate the test equipment following the changing of connectors or adaptors, whether detectors are available for all nominal wavelengths (i.e. 850nm, 1300nm and 1550 nm); 9 whether the equipment is truly portable (i.e. battery powered). Other than system bandwidth, another important characteristic that has to be considered when engineering and maintaining optical fiber cable systems is the losses due to fiber splices, connectors, junctions and, of course, the actual fiber itself. In addition to the normal range of multimeters, oscilloscopes, power meters and other test equipment that a typical repair facility or field engineer would need, there is now the added requirement to carry an additional amount of test equipment specifically designed for light measurement.

8.2 Examples of test equipment As one may suspect there are a number of specific problems directly associated with f i b e r s - especially with respect to mechanical shock, excessive bending and inadequate i n s u l a t i o n - that are not normally found in conventional copper cables. It has, therefore, been necessary to develop a completely new range of laboratory and portable test instruments, which, due to the range and complexity of some of the measurements that have to be taken, often require a completely new form of measuring technique. Another factor that has to be considered is the importance of ensuring that the test equipment used with optical fiber systems is always of the highest accuracy. To provide this level of confidence it is essential that these instruments be regularly calibrated against a reference standard to detect, correlate, adjust and determine variations in their accuracy. This requirement is usually very expensive but hopefully will become cheaper as the technology progresses. The following are a selection of some of the equipments available for today's fiber optic engineers.

220 Optoelectronics and Fiber Optic Technology

Figure 8.1

The KDXIO00 (courtesy of KD-Optics)

8.2.1 Production test kits

As previously described, during production there is a need to have a facility for being able to make insertion loss and return loss measurements using LED or laser sources. The KD Optics KDX1000 modular test system shown in Figure 8.1 is a typical example of equipment that is suitable both as a laboratory instrument and as a production test equipment. The unit is of a modular construction and has a mainframe with power meter controller and the capacity for multimode, single-mode and visible light sources. The test software is fully automated providing the operator with a totally hands-free test environment. Test sequence and results are stored in an Access database. 8.2.2 Laser light sources

Most of the so-called portable laser light sources a n d / o r fault locators available prior to 1988 were ruggedized water-tight pieces of equipment, weighing about 5 kg or so and usually powered by rechargeable sealed lead-acid batteries capable of providing more than 20 hours' operating life. Industry quickly realized, however, that there was a requirement for a truly portable, drop-proof, multi-wavelength, reasonably priced piece of equipment and, thankfully, there are now a number of extremely efficient light sources available on the market at the time of publication, suitable for field maintenance and installation. A fault locator (Figure 8.2) is a simple, cheap method of tracing multimode or single-mode fibers and locating breaks or bad splices. They work on the principle of using a bright laser diode to shine a light down a fiber and seeing the glow (if a fault exists) through the buffer a n d / o r

Optoelectronic test techniques 221

Figure 8.2

Fotec $665 visual fault Iocator (courtesy of Fotec)

fiber jacket. Fault locators are a perfect companion to an OTDR and normally come with a wide range of connectors. Operating these equipments so that loss measurements can also be taken is usually an additional option and normally consists of connecting the light source to one end of the fiber link and measuring the amount of light intensity (and, therefore, the fiber system power loss) at the other end with the aid of some kind of power meter.

8.2.3 Power meters Power meters are used for measuring power, power loss and optical return loss during installation testing and field service of fiber optic networks. These fiber optic power meters (such as the Fotec DM3000 shown in Figure 8.3) quite frequently have dual silicon and g e r m a n i u m detectors that provide a spectral range of 400-1800 nm, a dynamic range of 90 dB and the facility of measuring two links at once (allowing one to be used as a reference measurement for the other). Features such as a menu-driven user interface, customizable displays, selectable calibration wavelengths, on-board storage of test results (for later readout in a PC) and data logging are usually available.

8.2.4 Fiber optic tracers Most fiber optic network problems are usually caused by improper connections but since the light is invisible to the eye, it is impossible to see if there is actually a signal in the fiber, and whether the fiber in the cable is transmitting light, how much power is coming out of the end of the cable

222 Optoelectronicsand Fiber Optic Technology

Figure 8.3 The Fotec DM3000 (courtesy of Fotec)

etc. Using a fiber optic tracer not only provides you with a visual check for fiber continuity and power m e a s u r e m e n t but is also very useful for: 9 9 9 9 9

checking for possible d a m a g e d u r i n g shipment; testing the fiber whilst it is still on the reel, before being pulled in; detecting fiber breaks up to a distance of more than 4 kilometres; identifying the next fiber to be tested for loss with the test kit; making sure the correct cables are being connected at patch panels etc.

Fiber optic tracers (Figure 8.4) are effectively little flashlights that have been modified to put light into a fiber and are invaluable for m a k i n g sure

Figure 8.4 Datacom)

Examples of fiber optic tracers (courtesy of Fotec, Noynes and

Optoelectronic test techniques 223

the fiber transmits light and for tracing the signal path throughout the network. Fiber optic tracers are usually housed in a pocket-sized, waterproof case that can be easily attached to all types of common connector without special adaptors. They are fairly cheap to purchase and are normally given to engineers as a general-purpose tool.

8.2.5 Power and attenuation measuring test sets Power and attenuation measuring test sets are as fundamental to testing optical fibers as the simple continuity tester is to testing conventional copper wires and coaxial cables! Indeed, one of the essential fiber optic measurements to be made during initial commissioning and in-service testing is the amount of optical power that is available at the end of a fiber and this forms the basis for determining a n d / o r confirming optical path loss. These measuring sets are designed to quickly test either multimode or single-mode fiber links. Results can be a simple PASS/FAIL display or the provision of a complete set of test results based on length, propagation delay, wavelength loss and user supplied data such as the number of splices and connections. Results can be downloaded to a PC and the documented results then used to generate network reports or certification reports based on the latest fiber standards. The equipments can also operate like a traditional optical power meter to measure optical power.

Figure 8.5

Turbotest 400 (courtesy of Noynes)

224 Optoelectronicsand Fiber Optic Technology

Most of these power and attenuation test sets are normally very simple (merely consisting of an optical transmitter with an associated receiver) and there are many hand-held as well as bench-mounted models currently available from manufacturers such as Philips, HDW Elektronik, Wandel & Golterman, Hewlett Packard, Photodyne, Solatron, Thorn EMI and Noynes etc. All of these manufacturers provide equipment with very similar facilities and, in most respects, their products often look similar to the example shown in Figure 8.5. While optical power meters are the primary measurement instrument, particularly for short cable runs, the use of an OTDR is preferred owing to the increased amount of system information that they provide. The downside to using OTDRs, however, is the actual cost of the instrument which, depending on the model chosen, can run into thousands of pounds. 8.2.6 Other field testers Datacom's FiberMeter T M is an example of a complete fiber certification system for field testing of multimode, premise-wired fiber links (Figure 8.6).

Figure 8.6

FiberMeter TM field certification system (courtesy of Datacom)

Optoelectronic test techniques 225

Figure 8.7 PM23 optical power meter and the LS2ODV LED Source (courtesy of KD-Optics)

The equipment provides automated measurement of optical loss, cable length, pass/fail certification against industry standards, detailed certification reports and a built-in talk set. Multimode equipment (see Figure 8.7) (suitable for the testing of LANs, FDDI) and single-mode equipments (suitable for testing SDH, CATV, Telecom) - whether inside or outside a b u i l d i n g - are also available. Although their main use is for cable attenuation testing, they can also be used for fiber continuity testing, connector testing and transmitter power testing etc. which make them ideal as field equipments

8.2.7 Optical time domain reflectometer Optical time domain reflectometry is a valuable and widely used measuring technique for optical fiber cable installations and was developed in the mid-1970s by Dr Stuart Persenick and Dr Michael Bainoski. The operation of the optical time domain reflectometer (OTDR) is based upon Rayleigh backscattering, whereby a small amount of light is scattered in all directions as it travels down the fiber. An OTDR makes use of this phenomenon by transmitting pulses of light into a fiber and then measuring the reflections that occur. The principle is illustrated in Figure 8.8. OTDRs are widely used for measuring the transmission performance and attenuation characteristics (i.e. loss versus distance) of an optical fiber, especially when fibers are first installed or after they have

226 Optoelectronicsand Fiber Optic Technology

Illustrating the principle of optical time domain ref/ectometry using Ray/eigh backscattering to test optical fiber installations

Figure 8.8

u n d e r g o n e repair. An OTDR can also be used to detect and locate faults, splices and other discontinuities within fibers that are not normally visible to the naked eye. They are used to test or characterize fiberoptic cables and, by extension, entire fiberoptic networks with a single-end test. They measure overall end-to-end loss, section loss and splice loss, as well as the reflectance of connectors and mechanical splices. They are used to locate breaks, defects and misalignments and any gradual or s u d d e n d e g r a d a t i o n of the fiber.

8.2.7.10TDR testing Attenuation testing measures the total a m o u n t of p o w e r loss between two points, but to discover the cause of the loss and the location it is necessary to use an OTDR. OTDRs provide a visual display of: 9 the location of individual system components, splices and connectors; 9 the loss caused by individual components; 9 faulty a r e a s - areas of high attenuation or breaks (discontinuities); 9 the general characteristics of the fiber; 9 the total length of the fiber; 9 the overall loss of the fiber; 9 a measure of the d B / k m .

Optoelectronic test techniques 227

Figure 8.9 A typical O TDR trace

The OTDR operates by sending a pulse of light through a fiber, then sampling the light that is reflected back to the source by Rayleigh scattering and Fresnel reflection. The sampled light is analysed and the time different a m o u n t of light can be used to measure distance and loss which can be displayed as a trace or, in some cases, stored on a c o m p u t e r disk. A typical OTDR trace is shown in Figure 8.9 where the y-axis describes the power of the signal while the x-axis represents the distance it travels in d B / k m . As a signal travels across the fiber, it continually loses power due to standard attenuation. An OTDR is a particularly useful piece of test e q u i p m e n t for cable acceptance testing and fault finding. If it is necessary to measure the output from a transmitter and the input to a receiver, then a power meter will also prove useful. 8 . 2 . 7 . 2 0 T D R characteristics

OTDRs are characterized by a n u m b e r of factors, these include: 9 Dead z o n e - this is an area immediately following the launch point, i.e. the point at which the light enters the fiber through the first connector. It is an effect caused by the large Fresnel reflection from the connector saturating the receiver and within this zone it is not possible to carry out measurements. The size of the dead zone is determined by the pulse length, i.e. the power o u t p u t from the laser in the OTDR. High resolution OTDRs are designed for the m e a s u r e m e n t of short lengths and therefore need short dead zones. It m a y be reduced by the use of

228 Optoelectronicsand Fiber Optic Technology

liquid matching fluid at the connector, though in many OTDRs this is not a recommended procedure. 9 Ghost reflections- these are reflections that appear on the trace twice and are caused by Fresnel reflections made from connectors, which can be relaunched and travel twice the distance through the fiber. The detector identifies the relaunch light as two events in the fiber and shows two traces which generally appear as equally spaced peaks. In a link containing a number of connectors a large number of ghost reflections may appear. 9 Dynamic r a n g e - the dynamic range of an OTDR is the difference between the output power from the laser and the loss at which the reflected signal is too weak to be identified from the noise. It is measured and specified in decibels (dB) and the range of an OTDR is determined by the dynamic range and the loss in the fiber. OTDRs used in production and laboratories are usually stand-alone equipments produced by specialist manufacturers. One example is the Nettest/PK Technologye Model 8000 OTDR (Figure 8.10) that supports up to eight wavelengths and multiple receivers. Optical unit configurations include standard and high performance single-mode, multimode and a combined s i n g l e - m o d e / m u l t i m o d e design. Test technicians can control the Model 8000 optical unit through a fully customizable, Visual Basic 'Front Panel' which runs under Windows NT on the 8000's Pentium PC controller.

Figure 8.10 Nettest/PK Technology Model 8000 OTDR (courtesy of Nettest)

Optoelectronic test techniques 229

There are many laboratory OTDRs on the market today but they are normally very expensive. Transportable and mobile OTDRs, whilst not being able to complete all of the tests available with a laboratory instrument, are a cheaper alternative option. GN Nettest's CMA4000 shown in Figure 8.11 is a typical field portable OTDR that offers all the features of a mainframe, laboratory-style OTDR whilst at the same time acts as a ruggedized, transportable equipment that is environmentally sealed and protected from dust, rain and impact.

Figure 8.11 The CMA4000 field portable OTDR (courtesy of Nettest) The CMA4000 optical test system is typical of equipment that provides a complete test and measurement solution for an optical network in a rugged, economical, field-portable unit. It can be customized as an OTDR with a visual fault locator, optical power meter, and light source, or as a high resolution optical spectrum analyser (OSA) for DWDM (dense wavelength division multiplexing) systems. It may be configured with a variety of optics to meet any application including single, dual, tri- or quad-wavelength OTDR modules. Whilst an OTDR is not a substitute for a test kit (e.g. source and meter) for testing loss, it can be an invaluable troubleshooting tool, especially in outside plant and campus networks. Fotec's PC-based Fiber U OTDR shown in Figure 8.12, is a portable OTDR module that converts any Windows PC to a full function OTDR.

230 Optoelectronicsand Fiber Optic Technology

Figure 8.12

Fiber U OTDR portable module (courtesy of Fotec)

The OTDR consists of an optical module for 850/1300 multimode or 1310/1550 single-mode testing. A mainframe controls the optical module and processes the optical signals and a user-supplied PC that processes and displays the traces. The module can be operated off an AC a d a p t e r / recharger or a built-in battery. This is perhaps the easiest OTDR to use, since the operation is based on normal Windows operations such as pointing with a mouse to choose functions, using buttons on the screen or 'hot keys,' displaying traces in windows, and standard Windows file management.

8.2.8 Optical fault Iocator Although an OTDR is an important piece of equipment that can provide an enormous amount of data about a fiber optic network (e.g. signal loss, magnitude of reflections and reflective and non-reflective faults), and are also good for certifying and documenting new networks, there are times when their use is limited and many test procedures don't require the expense and complexity of an OTDR. The number and nature of the tests an OTDR is capable of performing also require some fairly extensive operator training and the complexity of

Optoelectronic test techniques 231

an OTDR requires a significant set-up, self-diagnostic and calibration time before each use. The answer is to use some form of optical fault locator (OFL), which is capable of performing the basic procedures that make up the majority of tests run on fiber optic networks. Fault locators are ideal for checking the length and integrity of new cable on the reel before it is pulled through a duct. They can also be used for checking the cable (once it is installed) and the length and integrity of the installation, as well as the viability of splices and connectors. After the network is up and running, an OFL can also be used to locate a break or defect in the cable and check for the presence of u n w a n t e d reflections. 8 . 2 . 9 0 F L versus OTDR

Despite its relatively limited function, an OFL has some important advantages over an O T D R - not just limited to the cost of equipment alone. Every telephone or cable company usually deploys only one or two OTDRs to a service, installation, or repair depot to be shared among the crew as required. This would mean that either the entire crew would have to be trained in OTDR operation, or at least several employees would need to become specialists who would then be brought to the site to operate the OTDR. All this adds to the cost of operation. However, a simple OFL requires no training and no special personnel, enabling virtually any employee to operate one. An OFL can also be used to measure loss (in conjunction with an optical source and a power meter or an optical loss test kit). They also offer the advantages of speed and minimal dead zone (i.e. the distance of the front end of the cable that is difficult to analyse because the reflection time is too short for accurate readings). 8.2.10 Bandwidth measuring test sets

One of the limiting factors governing the transmission of information in an optical fiber is the operational b a n d w i d t h of the fiber itself. Although details of the operational bandwidth of the original fiber are always available from the manufacturer, in practice, when one or more fibers are spliced together in the field (owing to the possibility of less than perfect splices, junctions and mismatches etc.) there is no real means of accurately predicting the resultant bandwidth. In wideband applications this is, however, an exceedingly important aspect as it is essential for the installer, user and maintainer to be aware of the precise b a n d w i d t h of the system after installation. Although fairly limited, there are a few of these specialist equipments available to assist the engineer. One of these is Noyes's portable

232 Optoelectronicsand Fiber Optic Technology

Figure 8.13 Noynes)

WSP 100 Multi-Wavelength Selective Power Meter (courtesy of

Multi-Wavelength Selective Power Meter (Figure 8.13) that is designed for installation, maintenance and troubleshooting of long-haul and Metro loop applications. The equipment automatically analyses each channel and measures the centre wavelength/frequency, power level, and optical signal-to-noise ratio (OSNR). The test results are displayed in either tabular or graph format and it can operate in either local or remote mode.

9

Future developments

The use of optical fibers in the commercial world is split into three separate classes: telecommunications (mainly concentrating on high performance trunk lines and area networks); military and industrial (where the emphasis is centred on digital communications and safety/ security); and local area networks (LANS) or Integrated Services Digital Networks (ISDNs)- which are mainly used in the business world. More than 50 per cent of today's optical fiber applications are in the telecommunications field and according to the UK Advisory Council on Science and Technology (ACOST) in a recent report on optoelectronics, this field is of strategic importance and it is estimated that the world market would exceed s billion annually. ITT Corning of America, one of the first companies to become involved in optoelectronics, has estimated that the US market alone will require an astonishing 60 million fiber km in 2002. This confirms a recent report by Kessler Marketing Intelligence that the American optical fiber market alone is now expected to increase by an average annual growth rate of 15 per cent. On the other hand, the Department of Trade and Industry (DTI) estimates that total UK production of photonic systems and components for 2002 will be around s billion, accounting for more than half of the total EU production. Interest in applications such as government systems, oceanographic projects and industrial processor-controlled networks is rapidly growing and the use of LANs and associated optoelectronics in customer premises is expected to increase by an annual average growth of 70 per cent.

9.1 The global market Potentially, the European Community is the largest and most lucrative single telecommunications market in the world. Currently it has a population of 320 million, more than double Japan's, and 50 per cent

234 Optoelectronics and Fiber Optic Technology more than in the US. Including members of the European Free Trade Association (EFTA), this total rises to 360 m i l l i o n - 500 million all the Eastern European countries are considered! The global market for optical communications equipment is already a s billion business and is forecast to grow rapidly, perhaps reaching s billion by 2010. Britain has the largest semiconductor design industry in Europe, the world's fifth largest electronics industry and is a world leader in photonics. The predicted demand, in fiber km, for optical fiber systems is illustrated in Figure 9.1.

I

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Figure 9.1 Predicted demand, in fiber km, for optical fiber systems as cumulative curves

In Europe, the bulk of the optical fiber market is in France, West Germany and the United Kingdom: traditionally strong in telecommunication technology, became involved in optical fibers at a comparatively early s t a g e - the Biarritz installation was the first in the world to bring services to the subscriber via optical fibers. France, however, has still to adopt optical fibers for all of its main trunk networks. 9 Germany: has also been fairly conservative in adopting optical fibers and although experimenting with an integrated glass fiber network in

9 France:

Future developments 235

a number of cities and committed to completing all of its future trunk installations with single-mode fibers, the initial impetus has waned somewhat. United Kingdom: extensive networks of single-mode trunking have already been installed for telecommunications and data and much more is planned for the future together with a strong commitment to LANs, MANs and WANs. The British Government has set up a standalone optical fiber network between its establishments, offering better security than existing data networks and providing many more services including telex, facsimile, data transfer, high fidelity sound, television library and information. Other European markets: these include: Switzerland (single-mode trunking between cities as part of their ISDN network); Denmark (replacing existing inter-island trunks with high capacity optical fiber cables); Sweden (which leads the world in telephone density number of phones per capita - already uses single-mode fibers throughout and have already installed a complete nationwide optical fiber long-haul network in the 1990s). With such a worldwide interest and dependence in optoelectronics, it is not surprising that the advance of this particular technology is progressing far more quickly than any other, and many new techniques are being developed.

9.2 UK market Since 1999, several international companies have announced major UK expansion plans: 9 Nortel, the Canadian communications equipment maker, now employs over 5000 people in optical fiber components production at its Paignton, Harlow and Belfast p l a n t s - twice as many as in 1998. The Paignton site is one of Europe's largest manufacturing centres of excellence for optical amplifiers and wavelength division multiplexers (WDM). 9 Agilent, part of Hewlett Packard in the US, has doubled its Ipswich plant and now employs 1000 people making optical components for communications networks and 'pump lasers' for undersea telecomms cables. 9 JDS-Uniphase of Canada has taken over Sifam Fiber Optics and announced the creation of over 1000 jobs at its Torquay and Plymouth sites.

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Optoelectronics and Fiber Optic Technology

9 Agere, formerly the Microelectronics Group of US-based Lucent Technologies, opened its European optical amplifier design at Ascot in late 1998 / early 1999. Within the UK, several companies are also expanding rapidly: 9 Britain's own telecoms group Marconi, formerly GEC, announced in early 2001 plans to form an optical components company to pull together its photonics R&D activities in a new state-of-the-art manufacturing facility. The new company will create another 400 jobs. 9 Bookham Technologies in Abingdon was floated as a public company in 2000. Founded in 1988 it has developed a world beating technology called ASOC (Application Specific Optical Component) to make optical fiber components. Using a combination of automated assembly and standard microelectronic silicon chip fabrication techniques, Bookham has been able to reduce the cost, time and complexity of making optical components. It has since opened a second manufacturing facility at Swindon, doubling capacity. 9 In June 2000, a new start-up company, Southampton Photonics, raised s million in one of the biggest private investments seen in the UK in recent years. It is a spin-off from the Optoelectronics Research Centre at Southampton University, which specializes in making conventional optical fiber components from single pieces of fiber, instead of silicon. The university played a key role in the development of the erbiumdoped fiber amplifier (EDFA), one of the biggest breakthroughs in optical technology in recent years. EDFA uses a laser to pump light into the coating of the fiber, which has been specially treated, in order to stimulate the release of extra energy into the light signals passing through the core, thus effectively amplifying them. The new company will create 200 skilled jobs. 9 There are several other start-up companies in the south-east region, mostly developing a variety of new display technologies. The UK is also an important world source of optical grade glass, particularly for North American companies.

9.3 Development trends The introduction of optoelectronics in the 1980s is now viewed as the most important discovery for communications systems and the search for improvement continues unabated. Even non-gravitational environments such as space stations are now being used to experiment with molecular beam epitaxi (sometimes referred to a 'super lattice'), thin-film technology and advanced semiconductor manufacturing techniques such as the high electron mobility ionizer which uses an InGaAs layer on a GaAs

Future developments 237

strip capable of producing bandwidths up to 250 GHz. Meanwhile back on earth, development trends also continue and include the following: 9.3.1 Porous silicon Porous silicon has attracted quite a lot of scientific attention since a British team discovered that laser light shone on a solid-state porous silicon device was re-emitted at a different wavelength. These results suggested that porous silicon might be used in photonic switching and could be a cheaper option than using the more expensive gallium arsenide. Meanwhile, American researchers have achieved visible electroluminescence in solid-state porous silicon devices during research for the US Army Research Office. The devices were made by depositing a transparent n-type indium tin oxide conducting layer on the surface of porous Si samples produced by anodic etching of p-type crystalline Si wavers, in hydrofluoric acid solutions. Applying a forward bias made the devices emit a red-to-orange light, depending upon the etching time. 9.3.2 Ballistic transport Ballistic electron injection or 'ballistics', involves sending electrons across the discontinuous band gap created by layers of A1GaAs and GaAs. As the A1GaAs layer possesses a larger band gap, the resulting discontinuity provides a step potential that is picked up by the A1GaAs electrons as kinetic energy and electrons travel at speeds up to six times faster than they can in conventional structures. Use of microthin layers of A1GaAs and GaAs stabilizes the path and thereby increases the chance of an electron hitting an atom (or positive ion) in the lattice and interacting with it. 9.3.3 Fluoride glasses In a joint research agreement with Nippon Telegraph and Telephone Company, Corning Cable Systems conducted some basic research in fluoride glasses as a first step towards bulk-melt fluoride glasses. The team reported that oxygen is instrumental in stabilizing the inherently unstable fluoride glass and that fluoride glasses were believed to hold much promise as materials for future optical fibers. Theoretically, losses in fluoride glasses are two orders of magnitude less than other glasses, making the compositions desirable for low-loss, longer-distance transmissions. The team also determined that vapour phase deposition of fluoride glass can give high purities and produce preforms (from which low-loss silica fibers can be drawn) and that vapour deposited beryllium fluoride glass showed very low scattering losses.

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Optoelectronics and Fiber Optic Technology

It is anticipated that halide glasses will be an ideal material for future optical fibers and these are expected to be an enormous improvement over silica glass - but quite a lot of work still remains to be completed in this area.

9.3.4 Polarization Polarization refers to a very specific engineering property of light and its ability to react as an electromagnetic wave that is vibrating in a single plane. When a particular plane of vibration is introduced to one end of a fiber optic cable (such as a light beam being shone into the cable) the orientation of the plane of the light from one end of the cable to the other varies. Although fiber optic communications in the 1990s have not made use of light polarization in the transmission of data, it is anticipated that future engineers may rely on the polarization angle of a transmitted light plane to encode some types of signal.

9.3.5 Synchronous optical network In the 1990s, most fiber networks used single wavelengths contained in individual fibers, arranged in SONET (synchronous optical network) rings, normally consisting of four fibers, one to transmit, one to receive and two for future expansion. The main problem with SONET rings, however, is that they are speed specific and if a network operator wanted to increase the bandwidth of (for example) a 2.5 Gbits SONET ring to 10 Gbits, then they had the choice of either digging up the ground and laying more fiber, or replacing all of the associated multiplexing equipment with faster equivalents. By using dense wave division multiplexing (DWDM), however, the network operator can now multiplex almost as many wavelengths as he likes down a single fiber. Increased use of DWDM and alternative amplification methods such as the erbium-doped fiber amplifier (EDFA) have greatly reduced the cost of networking and improved performance.

9.3.6 Solitron pulses A new generation of transmission systems that exploit rather than fight against the non-linear properties of fiber are now at an advanced R&D phase. The key to these systems is the solitron which is a particular pulse shape that, when correctly launched into the fiber, will maintain its shape as it propagates by the reaction of linear distortion (pulse spreading through dispersion) cancelling, exactly, the non-linear portion (pulse

Future developments 239

narrowing). After a certain distance solitrons will, of course, eventually succumb to attenuation and dispersion and so break down. However, by using optical amplifiers at critical intervals along the cable (or better still by using distributed fiber amplification), total pulse breakdown is avoided. Figure 9.2 summarizes the key bit rate/distance achievements in laboratory trials to date and multi-Gbits transmission over international distances using dispersion-shifted fiber is clearly possible. Peak Power Peak Power

Peak Po

Pulse

Iw,0,h

f

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Figure 9.2 A solitron pulse travelling down a dispersion shifted fiber

The theory behind a solitron pulse is that as the light pulse's wavelength gradually gets shorter, this will cause the leading pulse edge to be overtaken by the faster trailing edge (i.e. the leading edge of the light will undergo a Doppler red-shift whilst the trailing edge is experiencing a Doppler blue-shift). The result of this effect is a shortening of the pulse duration, with a corresponding increasing peak power. Although the pulse loses energy over distance, it increases in peak power - thus insuring that it stays above the peak threshold of the dispersionshifted fiber.

9.4 Lasers and amplifiers The use of lasers in optoelectronics is on the increase. Hospitals rely on photonics to improve medical imaging and lasers are routinely used for a wide range of diagnostic and surgical procedures. Other industries use lasers to measure components to high accuracy and for communication

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Optoelectronicsand Fiber Optic Technology

purposes (they can even be used to cut cloth for the latest fashions, and weld metals!). Since its discovery, the technology behind lasering has been constantly improved and the following are a few examples. 9.4.1 Distributed f e e d b a c k laser

With the introduction of the distributed feedback (DFB) laser even more advances are being made in laser technology and PIN-FET receivers capable of receiving an increasingly high data rate are also available. DFB devices will increase the amount of individual communication channels that can be sent over an optical fiber as well as reduce the number of regenerators that are required. Commercial development of all-optical amplifiers is another promise and will represent considerable s a v i n g s especially in the cost of optical communication links.

9.4.2 Tunable lasers Currently, a network using DWDM with 64 different wavelengths needs 64 separate lasers, each emitting a separate wavelength on to which the digital data stream is modulated. The disadvantage with this system is that if a laser goes faulty, then the network loses a compete channel and as each laser operates at a different wavelength, the maintenance of this network requires an enormous amount of s p a r e s - half of which will probably never be used! One solution is to make use of a tunable laser, but while this might overcome the problem of inventory control and spares holding, they increase network costs, require more spectrum, more output power and are prone to unreliability. One solution (being developed by Southampton Photonics) is to use an erbium- and ytterbium-doped fiber core with a Bragg grating pattern written into it that defines each wavelength. Lasing will take place within the fiber itself and this solution has the advantage of lower noise, higher reliability and increased power output when compared with similar devices. More importantly, a relatively low grade semiconductor p u m p laser can drive it.

9.4.3 Rare earth-doped fiber lasers and amplifiers Doping single-mode optical fibers with rare earth dopant has given rise to a new set of active and passive fiber devices which include in-line optical amplifiers, absorption filters, distributed sensors and, although at the time of writing still very much at the research stage, an all-laser amplifier.

Future developments

241

Rare earth-doped single-mode fiber lasers produced so far have s h o w n to be a more efficient, compact and versatile source of laser radiation and it is predicted that these devices will be used in a wide variety of system applications in the near future. 9.4.4 M o d u l a r laser s o u r c e

Unlike the LED, at low current, the laser diode produces little or no optical power but when the m i n i m u m current threshold is exceeded, then optical power increases d r a m a t i c a l l y - as shown in Figure 9.3.

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Figure 9.3

Although an LED loses very little optical power as temperature increases (approximately 0.7 per cent per degree), a laser diode is very susceptible to temperature changes and an increase of only a few degrees can cause the laser to cease its lasing operation. Because of this, heat extraction, together with temperature stabilization, is very important. Another point to be remembered is the established fact that an LED is capable of producing a far greater optical power density, (e.g. 4 Wcm -2) than a laser, (typically 1 Wcm -1). To ensure that the m a x i m u m amount of power is transferred to the fiber, therefore, it is essential that the laser diodes are protected against environmental aging effects such as dust or dirt, organic vapours and humidity etc. In recent years, encapsulating the laser diode into a hermetically sealed, temperature-controlled, loss-free enclosure or module that is environmentally matched to the fiber has effectively overcome these problems and it has also produced an excellent method for ensuring that

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Optoelectronics and Fiber Optic Technology

the junction losses between the laser source and optical fiber are minimized. A typical example of such an encapsulated laser diode is shown as a block diagram in Figure 9.4, and as a cutaway in Figure 9.5. The internal temperature of the module is measured by a thermistor (attached to the laser's heatsink) which produces the control for a Peltier cooler which ensures that the laser diode is held to an almost constant temperature.

Figure 9.4 Block diagram of a typical encapsulated laser diode

Figure 9.5 Cutaway, showing monitor diode, laser diode, thermistor, as well as Peltier cooler (in background)

To minimize matching losses, the junction at the interface of the laser diode and the optical fiber is usually completed inside the module and an external pigtail is made available for connection to the optical waveguide.

Future developments 243

A photocurrent is produced by a monitor diode using the light from the rear mirror and this is fed to an electrical control and drive circuit to provide an external, variable, optical power control.

9.4.5 Erbium-doped fiber amplifier Southampton Photonics have now produced what it says is the next generation of erbium-doped fiber amplifier (EDFA). With this technology, eight amplifiers have been integrated into a single package, each sharing common pump lasers. This multiport amplifier provides gain for eight independent signals, for example eight DWDM channels.

9.5 Fiber cables Just as no single electrical cable can handle all electrical transmission needs, no single optical fiber is optimized for all optical transmission requirements. The current trend towards specialized fibers reflects growing user sophistication and the following are some of the latest developments.

9.5.1 Re-emergence of multimode fibers Manufacturers and users alike are now predicting the re-emergence of multimode fibers particularly in respect of military applications.

9.5.2 Moulded-clad single-mode fibers In the early 1990s, Corning Glass Works produced a new single-mode fiber that had a matched-clad, step index design, offered greater resistance to microbending losses and gave improved performance. Attenuation losses were less than 0.5dB/km -1 at both 1300nm and 1550 nm. The manufacturing process was radically improved during year 2000 and this single-mode optical fiber is now recognized as one of the most cost-effective mediums for transmitting bit rates greater than 6 Mbit/s -1 and at distances greater than 3 km.

9.5.3 Multi-core fibers Compared to conventional copper cabling, fiber optics offer an increased signal carrying capability but, although they can be bent a little, they do have the disadvantages of being extremely fragile and are not particularly flexible.

244 Optoelectronicsand Fiber Optic Technology Glass is an inherently weak substance, particularly in the amounts present in fiber optics, and for this reason stress relief and large loops have to be built into cable runs to prevent fractures. Because of its fragility, the cables have to be protected to prevent damage from impact or snagging. Designers need to be aware of these issues when envisaging systems incorporating fiber optics and frequently resort to expensive solutions, such as heavily armoured cable, or in extreme cases may even resort to copper conductors. Keyence UK believes it has found the solution. A standard fiber typically has only one core. Their new fiber unit has 217 core fibers, each with a diameter of only 66 ~m (Figure 9.6).

Figure 9.6 Multi-corded fiber providing additional strength (courtesy of Keyence Corporation Japan)

This multi-core idea allows the fiber to bend to a much greater extent, down to a radius of 2 m m (Figure 9.7). It also enables it to be more resistant to physical hazards.

9.5.4 Encapsulated fibers The LIGHTRAY OFX TM optical fiber circuit from Tyco Electronics (Figure 9.8) consists of a number of optical fibers that are encapsulated in thin protective film substrates.

Future developments 245

Figure 9.7 Typical bending radius of a multi-cored fiber (courtesy of Keyance Corporation, Japan) As well as saving valuable space in equipment and equipment racks, LIGHTRAY OFX T M also provides optimum optical performance. Operating between-25 and +80~ this fiber optic equivalent of a copper circuit allows fiber crossovers without performance penalty and very low insertion loss (maximum insertion loss is lower than 0.1 d13, 0.05 dB being typical).

Figure 9.8 Optical fiber circuit

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Optoelectronicsand Fiber Optic Technology

9.6 Movable fibers As well as the normal connectors and couplers described in Chapter 6, movable fibers can now be used as the basis of connectors, switches, demultiplexers and interferometers. The technology, instead of having to physically move the fibers back and forth, consists of using piezoelectric actuators to squeeze the sides of the connectors together. By moving one fiber very slightly in relation to another, the gap between them will form a Fabry-Perot cavity and light will be passed if the gap corresponds to one (or one and a half etc.) wavelengths. Present day wavelength division multiplexing (WDM) allows 1.55 jam band channels to be 0.8nm apart (although the bandwidth of single channels is only 0.1 to 0.2nm) opening up the possibility of denser packing in future. Varying the gap can also be used to separate out individual data channels. It is anticipated that using these movable Fabry-Perot cavities as plug and socket connectors etc. will prove to be more reliable than their predecessors and should provide many potential LAN applications, plus, in view of their low weight and being less of a fire hazard than electrical systems, other possibilities such as fiber optic buses in cars etc.

9.6.1 Optical bus The concept of distributed amplification with erbium doping can also be applied to the data bus. For example, the optical equivalent of an optical bus has been demonstrated using D-fiber couplers in a network of weakly amplifying fiber. Although noise accumulation will eventually dominate, it is nevertheless feasible that such an optical bus could grow to accommodate >1000 taps. This, coupled with the ability of using fiber amplifiers to counteract splitter losses, opens up the concept of a virtually lossless network (VLON) at all l e v e l s - metropolitan, national and international.

9.7 Transmission systems 9.7.1 Development trends in higher-order optical fiber transmission systems In the 1970s, the possibility of increasing the transmission bandwidth by operating a system at a rate of 34 Mbit s-1 was actively researched and in 1979, the Deutsche Bundespost successfully opened a 15 km link between Frankfurt-Ginnheim and Oberursel. In modern system designs a transmission rate of 8 Mbits -1 is normally used for the first optical transmission window (840 to 880nm) and, occasionally, at the second

Future developments 247

optical wavelength (1300 nm). In order to achieve optimum conditions, a laser diode is normally used as the transmitter for an 8 Mbit s -1 system while the receiver is usually an avalanche photodiode. These 8 Mbit s -1 systems would normally be used for such applications as branch carriers for digital exchanges, video telephony, data etc., and (as high transmission bandwidths are required) graded index or single-mode fibers are invariably used. The larger the bandwidth, the lower the losses a fiber may have and, while graded index fibers are perfectly acceptable for 8, 34 and possibly 140 Mbits -1 transmission rates, when this is increased to 560 Mbits -1, single-mode fibers (operating at wavelengths of 1300 nm or so) may well have to be used.

9.7.2 Transmitters and receivers

Studies are now progressing to develop systems capable of operating in the Gbit s -1 range and at wavelengths in excess of 1500 nm. During the past few years there has also been a number of radical changes to the transmission system and these have resulted in vast improvements to both transmitters and receivers. For example, the 8 Mbit s -1 system block diagram shown in Figure 9.9 uses the multimode emission spectrum of a GaA1As oxide stripe laser diode. To ensure stable laser operation, a Peltier cooler and temperature sensor are incorporated into the hermetically sealed laser module. NTC Photodiode thermistor 1 ~ Peltier Laser elements diode r . ~

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248

Optoelectronicsand Fiber Optic Technology

Also contained in the laser module is a drive circuit, which is used to regenerate the amplitude of the signal and supply the modulation current for the laser diode. Biasing, to permit the light pulse rise-time being delayed with respect to the drive current pulse is also normal. Regulation of the modulation depth and optical power is achieved with control loops and an alarm circuit is incorporated to ensure the laser diode is not overloaded. The receiver, on the other hand, normally uses an avalanche photodiode to convert the received optical signal into an electrical signal. The current pulses of the photodiode are converted into voltage pulses by a gain-controlled preamplifier called a transimpedance amplifier. This is followed by a dual-gated field effect transistor (FET) amplifier. A lowpass filter provides band limiting. Such an optical receiver is shown in block diagram form in Figure 9.10.

Figure 9.10

Functional diagram of an optical receiver

It is not uncommon to find PIN photodiode, FET and bipolar transistors integrated on a ceramic substrate. Normally, germanium avalanche photodiodes are used for wavelengths between 840 to 880 nm, while PIN photodiode with GaAs FETs are usually used for 1300 nm. 9.7.3 Fiberless transmission

Using fiber optics for communications purposes has many advantages particularly with regard to their enormous data carrying capacity. LANs can be made virtually futureproof by maximizing available bandwidth whilst at ttie same time leaving the majority of the existing infrastructure unchanged. This is all right, of course, provided that you already have fiber optic cable running throughout your building. Having to install an optical network based on an anticipated growth rate could well prove too expensive for many businesses.

Future developments 249

During 2000, Bell Lab demonstrated an optical system called OpticAir that uses similar technology to that found in a normal fiber network except that it transmits the photons through the open air. According to Kathy Zheleg, Vice President of Marketing for Lucent's optical networking group, only 5 per cent of the buildings in the USA have a fiber network installed. Just as the main m e m o r y requirements of a PC are forever increasing, the amount of data passed around the network is unlikely to drop and the need for additional b a n d w i d t h will always be there. OpticAir employs the same lasers as used in a fiber link, based around 1550nm, but uses special optics to reduce the power density. Lucent Technologies claims this gives a line of sight, point-to-point communication link that can reach twice visibility. 'If you can see for a mile, you should get a reliable link over two miles.' Their experts tell us the system uses state-of-the-art lasers, amplifiers and receivers that can be placed on rooftops or in office windows to transmit voice, data or video traffic from point to point through the air. It makes use of DWDM technology to increase network capacity in metropolitan areas and campus environments where cost, geography or other constraints may make fiber connections impractical. While there are no requirements for a licence to operate an optical communications link in the open air, there are rules against firing a laser across a city. To comply with UK and US laws, the power density is downgraded to less than 100 m W / c m 2 using a diffusion lens on the output of the transmitter, to split the beam into two as it leaves. This is in contrast with the traditional fiber beam former, which concentrates the laser w i n d o w energy to maximize the distance between repeaters. The divergent beam and lower power means if someone inadvertently peers into the transmitter they should not be blinded. Because this system is designed for short-range transmission, the reduced power density does not present a problem. Potential applications for the OpticAir system include transmitting data between high-rise office buildings, enabling naval ships to share huge amounts of information while in port and establishing temporary, high capacity data links for special events.

9.7.4 Optical Ether The concept of an 'Optical Ether' that exploits the b a n d w i d t h of fiber in a similar manner to the radio spectrum has been demonstrated in the laboratory with a number of terminals capable of switching 100 G b i t / s capacities. Related studies have shown that this approach could be extended to the replacement of the central office for up to 200 000 lines with a 100 per cent non-blocking capability and with a potential for

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Optoelectronicsand Fiber Optic Technology

improving them to four million lines. At today's service levels and with a similar level of call/service blocking, up to 20 million lines could then be accommodated, with a potential for significantly more. Beyond this level the need for conventional switching of some kind becomes necessary again. The key problems and limitations now hinge on the availability of sufficiently miniature and reliable electro-optics, memory capacity, control systems and wavelength standards. Commercialization is not anticipated before 2010.

9.7.5 Optical radio Another interesting prospect for transparent networks concerns customer mobility. As it is possible to transport raw microwave signals over fiber with only a single stage of frequency translation at each end, a radio cell can therefore be replicated over several remote and diverse locations to give the illusion of a single location. Effectively six dispersed company locations would, therefore, appear as one to the individual user. The certainty of this prospect increases with the realization that optical wireless is also being developed for in-building and street use for both telephony and broadband services.

9.8 Industry The increased use of microelectronics in industrial controls has resulted in extremely high signalling rates being present in the system. The problem, of course, with high signalling rates is the requirement for low signalling voltages and this is one of the main reasons why industry is turning to optical fibers and a few of these applications are described below.

9.8.1 Leak detection Whiteman Controls Corporation of America has designed and installed a leak detection method for double-walled piping systems containing hazardous fluids. The theory behind this is very simple but extremely effective and consists of a number of minute pressure sensors, which are placed at regular intervals along the pipe. The sensors are joined to an electro-optical connector and the light is passed via the optical fiber cable to an optoelectronic interface located some 2 to 3 km away from the hazardous area.

Future developments 251

9.8.2 Optical fiber drill bit temperature monitor An optical fiber drill bit temperature monitor has been designed by Vanzetti Systems for use with printed circuit board laminate drill bits. An optical fiber infrared detector head, connected to an optical fiber cable looks directly at, and measures the temperature of, the drill bit. The infrared head is capable of detecting overheating of drill bits (due to dulling or overuse), and details are then passed, as a signal, to an alarm circuit via the fiber cable.

9.8.3 Optical fiber soldering iron Although still very much at the drawing board stage, the development of an optical fiber soldering iron that can be used for field maintenance as well as in the laboratory is well advanced.

9.8.4 Optical computing The fastest computer in the world used to be (probably still is?) the BBC Andromeda, but even this has to rely on step processing or sequential functions which severely restricts its capability. Light can emit hundreds of modes in parallel without interference and, in the late 1990s, American SDI research investigated the possibility of an optical switch. This was effectively the first step towards optical computing which can be achieved by optically multiplexing many signals together and then switching one or any number of information channels in parallel. As described by David Larner in New Electronics, August 1999, mainstream optical components generally fall into two groups, those that emit light and those that detect light. There is a third group of optical components, h o w e v e r - those that direct light. Since the discovery of nonlinear crystals, which have the ability to change their refractive index when an electric field is applied, scientists have dreamed of building the optical computer. An advanced research initiative has set a 10-year design window for achieving a petaflops supercomputer, a system 1000 times faster than today's leading edge, teraflops supercomputing technology based on hybrid technology multi-threading. The central processor will be built with low temperature superconducting circuits. High speed RAMs will use CMOS chips cooled by liquid nitrogen. System interconnect will be optical, rather than electronic, three-dimensional holographic crystals will enable fast, high volume storage. In addition, the heterogeneous hardware will be tied together with an innovative software system blending such concepts as threaded execution, data flow processing and object oriented programming.

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For the time being, optical computers will have to remain a fascination because the functional elements, such as flip-flops, gates and latches take up far more space than the equivalent electronic devices. Second, because the means of switching optical signals around often relies on micro-machined mirrors, they have a mass which limits the switching speed.

9.8.5 Optical multiplexers It is strange, but true, that the material used to make an optical fiber is silica (Si) whilst the material used as the insulator in the manufacture of integrated circuits is silicon dioxide (SiO2)! This fact has not gone unnoticed by researchers around the world and the possibility of building optical multiplexers using semiconductor production techniques is currently being investigated.

9.8.60pto-isolators Opto-isolators provide a low-cost, space-efficient, easy-to-use solution to high voltage isolation requirements. With careful printed circuit board (PCB) design, the input circuit can be electrically isolated from the output stage for up to 7500 volts peak differential.

9.8.70ptoelectronic switches Optoelectronic switches operate on the principle that objects opaque to infrared will interrupt the transmission of light between an infrared emitting diode and a photosensor. Similarly, photoreflective switches operate by detecting light transmitted from an infrared emitting diode with a photosensor positioned to sense a reflective object positioned at a point of optimum response. Optical switches provide non-contact sensing of linear or rotary motion, and have many different individual applications such as in printers, photocopiers, weighing, measuring, automotive and entertainment equipment. Unlike conventional mechanical solutions, optoelectronic switches are not subject to wear or corrosion and they can interface directly to, and with, electronic control circuitry.

9.8.8 High speed optical switches Researchers at Glasgow University have demonstrated a high speed optical switch based on a semiconductor waveguide. The Scottish researchers claim the aluminium gallium arsenide waveguide can carry light pulses of less than ten picoseconds in duration and could lead the

Future developments 253

way to optical telephone systems capable of handling 1.2 million simultaneous telephone calls. On the other side of 'The Pond', scientists at Bell Labs have built a microscopic optical switch that works like a child's seesaw and demonstrates the first practical optical switching technology using micro electromechanical systems (MEMS). The seesaw in the experimental switch is a tiny, pivoting bar with a gold-plated mirror on one end. The mirrored end fits in a space about one-tenth as wide as a human hair, between two optical fibers. In the rest position (i.e. when the switch is off) the mirror rests below the cores of the two fibers, allowing light to travel from one to the other. To turn the switch on, a voltage is applied at the other end of the pivoting bar which generates an electrostatic force sufficient to lift the mirror, reflecting the light away from the fiber cores. MEMS devices are built in much the same way as integrated circuits with various films, such as polysilicon, silicon nitride, silicon dioxide and gold, deposited and patterned to produce multiplayer, three-dimensional structures. The major difference is a release step at the end of fabrication.

9.9 Military In the early 1960s, optical fiber technology, along with semiconductor and laser technology, became a particularly attractive possibility for strategic and tactical military applications. The military has always been concerned with radiation of energy from the transmission media and the susceptibility of communication systems to high energy electromagnetic radiation especially from nuclear weapons. Unfortunately, cost and doubtful performance factors have limited large-scale military implementation. With the emergence of more cost-effective manufacturing techniques, together with increased reliability, the military is now actively upgrading existing copper systems with optical fiber cables and extending digital ISDNs and LANs for fixed installations. In keeping with this general trend, the American forces are now replacing a microwave system with a single-mode optical fiber backbone transmission cable in the Republic of Korea. 9.9.1 Laser dazzler

Jane's Defence Weekly reported

in their May 1998 edition that the US Defence Advanced Research Projects Agency (DARPA) and the National Institute of Justice (NIJ) had developed advanced technology (which they are now seeking to commercialize) into the 'laser dazzler', a device that could be widely used by the military enforcement communities as a nonlethal weapon.

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The technology is being incorporated into a 250 milliwatt 532nm greenlaser hand-held flashlight. The laser dazzler device includes a miniature laser and power supply and has output optics that can temporarily expand the eye-safe laser into a blinding light. It can also penetrate smoke and fog at more than twice the distance of white light. DARPA and the NIJ believe the market for the technology will include military security personnel, law enforcement, border control, crowd control and underwater surveillance.

9.9.2 Plastic opto-chips Currently, most fiber optic networks employ WDM technology. Each system accommodates large numbers of optical wavelengths, each of which is modulated and carrying data. To place even more optical data modulated wavelengths that are on a fiber will require devices such as an opto-chip to allow modulation of each wavelength component at a much wider bandwidth and with lower signal power. As reported by AFCEA (signal Jul 2000), a new polymer-based electrooptic modulator (that serves as the interface for placing electronic data on to an optical carrier for transmission across a fiber optic network) could also be used to increase the bandwidth of fiber optic networks and clear the way for applications ranging from broadband |nternet access to fullscale holographic projection currently found in science fiction television programmes! Developed in a joint research effort by scientists at the University of Southern California, Los Angeles, and the University of Washington, Settle, this new technology also uses less power than present-generation modulators and features low noise disturbance. The universities have now succeeded in integrating polymeric materials with semiconductors. This has been achieved in both horizontal and vertical integration, where polymeric electro-optic circuits have been produced on top of very-large-scale chips. The fabrication process involves placing a layer of planarizing polymer with an optical-quality surface on top of a chip. Reactive ion etching techniques handle deep interconnections. The same etching technology can be teamed with shadow ion etching or grey scale etching to build three-dimensional, vertically integrated passive/active optical circuits which will allow an optical waveguide to be constructed that vertically links different chip layers. The competing technology for these polymer devices currently is lithium niobate, a crystalline technology that has been used for several decades. Lithium niobate modulators are limited to a speed of between 10 and 20 gigabits per second, however, and each requires an operating voltage of between 4 and 5 volts. The current polymer modulators have a drive requirement of less than 1 volt and can operate from 40 to 80

Future developments 255

gigabits per second. This lower power consumption also results in reduced heat. Whilst it is not anticipated that these polymer devices will replace all lithium niobate systems - especially those in standard, single modulator device a p p l i c a t i o n s - preliminary tests have indicated that a single modulator measuring about 1 micron can provide more than 300 gigahertz of bandwidth, enough to carry all of a major military unit's telecommunications, computer, television and satellite traffic. The new modulator technology is not bound to its current polymer make-up, however. Chemists are actively pursuing development of even better materials. From a military perspective, near-term benefits could be realized in systems such as large communications networks, radar and electronic countermeasures, electrically-steered antennas and radar transmitters as well as optical gyroscopes in airborne guidance systems. In the commercial sector, both satellite and fiber optic telecommunications could be prime beneficiaries and the first commercial customers of these modulators would likely be firms that operate fiber optic systems, such as the regional and international telecommunications companies. The long-term view is that the technology has the potential to open up many new applications, similar to those that emerged from the development of silicon semiconductors some 30 years ago. The next step for the researchers is to move these modulators from the laboratory into the commercial marketplace. Commercializing the technology will require significant long-term testing, for temperature and photostability and funding (!) - but this work is underway.

9.9.3 Fiber optic gyroscope The fiber optic gyroscope (Figure 9.11) consists of a long length of fiber w o u n d into a coil. Laser light is sent into both ends of the fiber using a beam splitter, which reflects 50 per cent of the light and transmits the remaining 50 cent. Light travelling clockwise round the coil emerges from the end of the fiber with the same phase as the light travelling in the

Figure 9.11 Fiber optic gyroscope

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anticlockwise direction. This is because both have travelled exactly the same distance. If the gyroscope is now rotated, say, in the clockwise direction, then the light travelling round the fiber coil in the clockwise direction will take longer to reach the end of the fiber because the end is always moving away from the light. In a similar manner, light travelling in the anticlockwise direction will take less time to reach the other end because that end is moving towards the light. This introduces a phase difference between the two emerging beams of light, which is proportional to the rate of rotation of the gyroscope.

9.10 Local government Within local government, fiber optic cables and optoelectronic systems are now becoming commonplace, as shown in the following examples.

9.10.1 Optical sensors Military researchers have long dominated 'smart structure' work, with most of their efforts devoted to embedding fiber sensors in composite materials used in aircraft. Now the car industry is looking at optical sensors to monitor and control tomorrow's family car. At Liverpool University, a group of scientists is currently testing a new approach to sensing temperature, pressure, linear and rotational movement, voltage and current. The project concerns fiber sensors that measure the change in colour of light (i.e. chromatic modulation) as it passes through, or is reflected by, a suitable modulator such as glass. Conventional electronic sensors can provide some data, but they make only point measurements. Fiber sensors, including multimode sensors (based on mode-distribution statistics and single-mode polarization sensors), on the other hand, can evaluate stresses throughout structural elements. In construction projects, optical sensors are now being used to measure stress during construction. This is extremely important, because buildings typically experience their heaviest structural loading prior to completion. Goals include evaluating stresses during construction, monitoring vibrations in the building once it is completed and observing how well the fiber sensors survive construction.

9.10.2 Office lighting As well as being used for communications systems and data transfer, fiber optics have also been used for decoration and in mainstream lighting. Indeed, it is anticipated that increased energy costs will drive the need for more energy efficient fiber optic systems.

Future developments 257

For example, in Australia in a temperate climate, air conditioning is calculated on the basis that 70 per cent of total building energy for air conditioning, and 50 per cent of the air conditioning load is used to combat the lighting system! As fiber optics do not generate any heat into the air-conditioned load area, this provides a potential energy saving of 35 per cent of the total building energy load. The office of the future will be dramatically different. Once all offices were illuminated with levels of illumination around 400-800 lux depending on the country, and this illumination was on the total horizontal and vertical surfaces. This was driven by the types of tasks one carried out in the office, wide desk and table top areas for the pyramids of paper spread all around. But the new computerized office is radically different and the screen and the keyboard dominate office tasks. As the screen is already illuminated only the keyboard and local task areas in a booth will require localized light, light that has no electricity, no radiation and no h e a t - fiber optic light. The surrounding areas to contrast correctly will have low levels of 100-150 lux for simple movement within the area.

9.10.3 Decorations Instead of using hundreds of tiny bulbs to simulate the firework display over the Euro Disney castle during the millennium celebrations in London, the effect was achieved from 400 fibers transmitting light from one 150 W metal halide light source. A multi-colour wheel in the optics created a kaleidoscope of intense moving colours, bringing the tableau dramatically to life. The entire display consisted of ten 60 ft wide, 22 ft high double-sided garlands, suspended across Regent Street. All were additionally highlighted with 500 W tungsten halogen haline floodlights to uplight the characters from their mounting positions at the base of each tableau display.

9.10.4 Fiber optic road signs A range of designs such as speed-activated 'slow down' signs, 'overheight' warnings for installation at low bridges and messages such as 'queue ahead', 'flooding' and 'overweight vehicle' are increasingly used on today's highways. The fiber optic sign houses tungstenhalogen lamps that transmit light through glass filaments, the legend being formed by their intensely illuminated terminals. Normally blank, the 'secret' sign is either switched on manually or automatically activated by a triggering mechanism, such as a speed or infrared detector.

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Optoelectronics and Fiber Optic Technology

The latest Department of Transport (DOT) specification requires that the illuminated message shall be continuously dimmed over the full range of ambient light, by means of an electronic 'contrast control' device, rather than a single light/dark setting previously allowed. This is normally achieved by incorporating two photocells in the sign head, which automatically adjust the luminous intensity across the complete brightness scale, reacting from brilliant sunlight (40000 lux) down to virtual total darkness (40 lux). The new specification also stipulates that the design should provide for full remote monitoring of all sign functions, safeguarding against lamp failure a n d / o r displaying the message at an inappropriate time. Equipment must also incorporate provision for manual switching and electrical interlocking to prohibit the simultaneous display of two legends, as well as a thermostat-controlled anti-condensation heater, to prevent the clear polycarbonate face misting up in low ambient temperatures.

9.11 Medicine In medicine, recent growth in the use of optics technology for biomedical research and health care has been explosive. Using fiber optics in medical procedures allows things like magnetic resonance imaging (MRI) and position emission tomography (PET) images to be transmitted to anyone, anywhere. Using fiber optic equipment during surgery also means using less invasive procedures, which in turn reduces recovery time, is less traumatic on the body and costs less for the procedure. 9.11.1

Lasers

- medical

uses

Lasers have been expanding into the medical arena and improving clinical results since the mid-1960s. These intense beams of light are used to photocoagulate, cut a n d / o r vaporize tissue and, in many types of surgery, have replaced the scalpel. It is estimated that approximately 5 per cent of the half-million doctors who are candidates to use the laser, employ it actively and this number is expected to grow to 25 per cent over the next five years. From brain surgery to kidney stones, eye surgery, gall bladder operations, oral surgery and irradiating cancer cells, the laser is redefining surgical procedures. It has become a tool the surgeon must master and use, not just to improve his own techniques but because his patients are increasingly asking for it. Medical lasers deliver either pulsed or continuous wave beams to tissue. When the beam is absorbed by tissue, localized heating occurs, which produces the desired surgical effect. As tissue temperatures rise,

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the tissue blanches white as it coagulates, shrivels as it desiccates and turns to steam and vapour as it vaporizes. The wavelength of the laser and the spectral absorption characteristics of the tissue determine how effectively the heat of the beam is transferred to the tissue. The three primary surgical LASERs used today are the carbon dioxide laser, emitting at 10.6 ]am, the Nd:YAG laser, emitting at 1.06 ]Jm and the misnamed argon/KTP laser (actually lasers), emitting in the green at 488, 514 and 532 nm. Recently, interest in the excimer lasers has been renewed for ophthalmology and two new solid-state l a s e r s - Ho:YAG and er:YAG - are being made available for specialized surgery. 9.11.2 Flexible fiber optic curvature sensors

Measurement sensors mounted on tape or attached to small subassemblies are frequently used to measure angular positions or movements on humans or machines. They are capable of detecting every movement and produce an output voltage that is proportional to the curvature or displacement of the sensor. A typical measurement sensor will respond to any curvature as a form of interference with the linear, bipolar modulation of light that is being transmitted through specially treated fiber optic loops and they will produce a linear, stable, high level output voltage in response to the bending or displacement of the sensor and where the output can be used to form real time computer images and data collection corresponding to complex shapes. Measurement sensors can be used to measure the angle of any small movement but they need to be robust enough to withstand millions of bends and at the same time possess attributes such as low cost, inherent safety, high speed and great flexibility. Applications include biometrics, CAD input, virtual reality (VR), crash testing, medical visualization and deformation studies.

9.12 Other possibilities The opportunities of photonics worldwide are immense and the global photonics industry is expected to be worth around s billion by 2010. Some 6000 new photonics R&D and manufacturing jobs have recently been announced in the UK, including major UK expansion plans by a number of multinationals. One immediate application for lasers in bringing optical communications to the desktop will be in distributing data around office buildings. lasers will be used in the risers, which take data around the buildings, but the drop to the desk will be handled by LEDs over multimode fiber.

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Bringing fiber to the home is a more difficult task than bringing fiber to the office, however. The problem is more economical than technical and as soon as homes start to be wired for this new technique, the market will open up. If users want broadcast quality video, there will be the need for more sophisticated technology. As far as fiber to the home is concerned, cost is seen as one of the main barriers. The major cost is installing the fiber itself and this dominates the cost of components.

9.12.1 Internet It is a recognized fact that the speed of components used by telecommunications networks doubles roughly every 18 months, and the cost per bit of data they transmit halves. Compared to telecommunications, the use of the Internet doubles every 100 days! The use of optoelectronics and fiber cables to provide a backbone for this ever-increasing need for speed and connectivity is now widely accepted and with its virtually limitless bandwidth, single fibers can carry thousands upon thousands of simultaneous transmissions. Indeed, Time Warner Telecom in the US has recently asked industry to provide a North American fiber optic telecommunications network that will be fast enough to allow everyone in the US and Canada to simultaneously send a one page e-mail across the n e t w o r k - assuming, that is, that they ever wanted to!

9.12.2 Cars and planes Today's cars have bulky lighting systems. With fiber optic cable being small enough to manoeuvre around, car designers can be given more freedom to design their vehicles. Optical systems used in cars can also control things like stereo, heat and airconditioners. In airlines, fiber optic cable is light, so there is no need for heavier cables to be used on board and this enables load savings. With the use of fiber optics, airplanes can have internal communication systems which are not exposed to outside elements and pilots will be able to see damage to certain parts of the plane without having to wait to land.

9.12.3 Optical processing and computing The use of optical technology in storage and signal processing is n o w becoming commonplace in certain application areas and the increased availability of optical hard disks for personal computers and domestic CD players is phenomenal. The impact of optical processing on telecommunications has not been so dramatic but it is anticipated that optical processing will provide a major input to neural network

Future developments 261

development. Many research-level neural networks are being produced with optical components. The main reason for this is the ability of optics to offer massive parallelism and interconnections. Since these parameters are fundamental to the principles of neural computing, optics is clearly suitable, so much so that the technologies are likely to develop very closely.

9.12.4 Mining Modern underground or open-cast mines use fiber optic transmission systems to monitor and control (from the surface) safety systems, fire and gas alarms, water, compressed air and electrical networks. Fiber optics can also be used in support of underground and surface services such as voice, data, LAN and video.

9.13 Conclusion Often optoelectronics is depicted, especially by those who are afraid of it as a competing technology, as a science in its infancy, with inevitable teething problems. As the costs of cables and connectors are reduced, however, the non-electrical nature of fibers will result in optoelectronics becoming more and more popular. In less than 20 years, optical fiber has revolutionized the global telecommunications network. During the next 20 years optical fiber technology can be expected to impact even more prominently by realizing new and revolutionary networks, services and applications. The provision of network facilities and capacities on a scale that is hardly imaginable today will be essential to meet the demand for both fixed and mobile telecommunications. It is anticipated, for example, that the computer will increase by 1000-fold in the next decade, and 1 000 000 fold in the decade that follows. At the same time, clock rates will increase by 10- and 100-fold respectively. But distributed computing is only effective with the availability of wide bandwidth channels between computers. Channels that are capable of data transfer rates on a par with the speed of the computers they serve. The future of fiber optics is the future of communications. With fibers, you have high bandwidth and almost unlimited potential for upgrading. Applications like multimedia and videoconferencing make high bandwidth very essential. Over wide area networks, the installed fiber optic infrastructure can be expanded to accommodate almost unlimited traffic. CATV (community antenna television or 'cable TV') operators are installing fibers very fast, since advanced digital TV will thrive in a fiberbased environment. Even wireless communications need fiber, connecting local low power cellular or PCS transceivers to the switching matrix.

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During the 1990s, the use of optical fibers has increased considerably in both commercial and military environments. Without doubt, optoelectronics will become the dominating technology of the next decade.

Appendix A Optoelectronic and fiber optic standards The following are summarized details of some of the most important standards available today. These details have been extracted from ILI's (Infonorme London Information) Standards Infobase with their kind permission. For further details of these and other standards, the reader is recommended to visit their website at www.ili.co.uk.

Standard No.

Title

Date

Summary

EN 181000

Harmonized system of quality assessment for electronic components Generic specification fiber optic branching devices

1994

Gives a generic specification that classifies fiber optic branching devices by wavelength dependence into two sub families: wavelength selective devices and non wavelength selective devices. Coverage includes: climatic category, materials, packaging, ordering information, package marking, symbols, component marking, crush resistance, mould growth, salt mist, dry heat, cleaning of optical services, solar radiation, quality assessment procedures, and change of temperature. Also gives detailed definitions and annexes.

EN 186000 PT1

Generic specification: Connector sets for optical fibers and cables Requirements, test methods and qualification approval procedures

1993

Applies to fiber optic connector sets for optical fibers and cables. Coverage includes definitions, environmental category, gauges, corrosion resistance, component marking, package marking, spectral loss, cable torsion, cable pulling, static load, nuclear radiation, solar radiation, spectral loss, axial compression and industrial atmosphere.

EN 187102

Optical aerial telecommunication cables

1995

Covers optical aerial telecommunication cables.

EN 187200

Sectional specification Optical cables to be used along electrical power lines (OCEPL)

2001

Describes the requirements of single-mode and graded index optical fiber cables for OCEPL.

-

264

Optoelectronics and Fiber Optic Technology

Standard No.

Title

Date

Summary

EN 50173

Information Technology Generic cabling systems

1995

Specifies generic cabling for use within commercial premises that may comprise single or multiple buildings on a campus. Covers balances, copper cabling and optical fiber cabling. Specifies the structure and minimum configuration for generic cabling; implementation requirements; conformance requirements and verification procedures; performance requirements for individual cabling links.

EN 50174-1

Information Technology - Cabling installation - Part 1: Specification and quality assurance

2000

EN 50174-2

Information Technology - cabling installation installation planning and practices inside buildings

2000

This standard describes the basic requirements for the planning and operation of information technology cabling using balanced copper and optical fiber cabling. This standard is applicable to cabling designed to support particular analogue and digital telecommunications services including voice services and general cabling systems designed and intended to support a wide range of telecommunications services.

EN 50174-2

Information Technology - Cabling installation Installation planning and practices inside buildings

2000

This standard describes the basic requirements for the planning and operation of information technology cabling using balanced copper and optical fiber cabling. This standard is applicable to cabling designed to support particular analogue and digital telecommunications services including voice services and general cabling systems designed and intended to support a wide range of telecommunications services.

EN 60794 PT3

Optical fiber c a b l e s Duct, buried and aerial cablesSectional specification

1998

Gives requirements of single-mode optical fiber cables intended for use mainly in public telecommunications networks, and considers other types of applications requiring similar cable types. Particularly applies to cables for use in ducts or for directly buried application and aerial cables. Excludes submarine cables.

Appendix A - Optoelectronic and fiber optic standards

265

Standard No.

Title

Date

Summary

EN 60794-1-1

Optical fiber c a b l e s Part 1-1 - Generic specification General

1999

Applicable to optical fiber cables used with telecommunications equipment and devices which employ similar techniques. Also applies to cables consisting of electrical conductors and optical fibers. Defines uniform generic requirements for the transmission, geometric, mechanical, material, ageing (environmental exposure) and climatic properties of optical fiber cables, and electrical requirements where appropriate.

EN 60794-1-2

Optical fiber c a b l e s Part 1-2 - Generic specification - Basic optical cable test procedures

1999

Deals with optical fiber cables to be used with telecommunication equipment and devices using similar techniques, and to cables with a combination of both electrical conductors and optical fibers. Specifies methods to establish requirements for the material, transmission, geometrical, mechanical, ageing and climatic properties of cables, and electrical requirements.

EN 60825 PT2

Safety of laser products - Part 2: Safety of optical fiber communication systems

2000

Provides requirements and specific guidance for the safe use of optical fiber and/or control communication systems where optical power may be accessible at great distance from the optical source. Coverage includes: cable design, cable connectors, automatic power reduction and labelling. Also gives detailed annexes.

EN 60869-1

Fiber optic attenuators Part 1 - Generic specification

2000

Applicable to fiber optic attenuators, which are passive because they do not contain any opto-electronic or other transducing elements, have two ports for transmitting optical power and attenuate the transmitted power in a variable or fixed fashion, and that the ports are optical fibers or optical fiber connectors. Determines uniform requirements for attenuator requirements and quality assessment methods. Does not include test and measurement methods which are covered in IEC 61300-1.

266

Optoelectronics and Fiber Optic Technology

Standard No.

Title

Date

Summary

EN 60874-1

Connectors for optical fibers and cables- Part 1 Generic specification

1999

Deals with fiber optic connectors sets and single components, eg. plugs, sockets and adaptors, for all kinds, sizes and structures of cables and fibers. Covers connector set criteria and quality assessment procedures. Test and measuring methods are not included.

EN 60875-1

Fiber optic branching devices - Generic specification

2001

Applicable to fiber optic branching devices, which are passive, have three or more ports for entry/exit of optical power and share optical power in a predetermined fashion, and where the ports are optical fibers or optical fiber connectors.

EN 61202-1

Fiber optic isolatorsGeneric specification

2000

Applicable to isolators used in the field of fiber optics which have the following features: passive components which contain no opto-electronic or other transducing elements; non-reciprocal optical devices where each port is an optical fiber or optical fiber connector, or they have two optical ports for directionally transmitting optical power. Establishes uniform requirements for fiber optic isolator requirements and quality assessment procedures.

EN 61269 PT1

Fiber optic terminus sets - Generic specification

1997

Applicable to fiber optic terminus sets for all types, sizes and structures of fibers and cables, including terminus set requirements and quality assessment procedures.

EN 61274 PT1

Fiber optic adaptors - Generic specification

1997

Applies to requirements and quality assessment procedures for individual fiber optic adaptors of various types.

EN 61280-4-2

Fiber optic communication subsystem basic test procedures - Fiber optic cable plantsingle-mode fiber optic cable plant attenuation

1999

Covers methods for measurement of the optical attenuation (loss) performance in installed single-mode fiber optic cable plants. Not meant for component testing and does not specify the installation elements requiring measurement.

Appendix A - Optoelectronic and fiber optic standards

267

Standard No.

Title

Date

Summary

EN 61281-1

Fiber optic communication subsystems Generic specifications

1999

Defines a generic specification for fiber optic communication subsystems (FOCSs), and is structured according to the IEC Quality Assessment System (IECQ). Subsystems are classified by having a common sectional specification. Each are supplemented by blank detail specifications, and those appropriate to the specific individual type or types of subsystems. These parameters form a minimum set of specifications that are common to all fiber optic subsystems. Additional parameters may be needed depending on the particular application and technology. These will be in the relevant sectional specification and/or detail specification, as appropriate. Each parameter may be measured using one of these test procedures.

EN 61754 PT1

Fiber optic connector interfaces - General and guidance

1997

Describes general information on the subject of fiber optic connector interfaces, including references, definitions and rules for creating and interpreting of the standard drawings.

EN 61757-1

Fiber optic sensorsGeneric specification

1999

Defines a generic specification covering optical fibers, sub-assemblies and components as they pertain specifically to sensing application, in those aspects not already addressed by previous or concurrent standardization efforts. A sensor contains an optical or optically powered sensing element where the information is created by reaction of light to a measurand. The object is to classify, define and provide the framework for specifying fiber optic sensors, and their specific components and subassemblies, fiber optic sensors are devices for extracting information from the environment using fiber optic technology.

EN 62077

Fiber optic circulators - Generic specification

2001

268

Optoelectronics and Fiber Optic Technology

Standard No.

Title

Date

Summary

IEC 60331-25

Tests for electric cables under fire conditions - Circuit integrity Procedures and requirements Optical fiber cables

1999

Gives test procedure and the performance requirement, including a recommended flame application time, for optical fiber cables that are needed to maintain circuit integrity when subjected to fire under specified conditions. Describes continuity checking arrangements, optical testing procedure, sample preparation, the method of burning the cables and gives requirements for evaluating test results.

IEC 60747 PT5

Semiconductor devices. Discrete devices Optoelectronic devices

1995

Gives standards for the following categories or sub-categories of devices: - semiconductor photoemitters; semiconductor photosensitive devices; photocouplers, optocouplers.

IEC 60793 PT1-3

Optical fibersPart 1-3: Generic specification Measuring methods for mechanical characteristics

2000

Applicable to the tests of mechanical strength, ease of handling or the recognition of physical defects or primary coated or primary buffered optical glass fibers. The methods are to be used for inspecting fibers for commercial purposes. The aim of this part is to establish uniform requirements for mechanical characteristics of optical fibers.

IEC 60794 PT1

Optical fiber c a b l e s Generic specification

1996

Applies to optical fiber cables for use with telecommunication equipment and devices employing similar techniques and to cables having a combination of both optical fibers and electrical conductors. Presents requirements for the geometrical, transmission, mechanical and climatic characteristics of optical fiber cables, and electrical requirements. This entirely revised edition contains new measuring methods which were previously under consideration, such as torsion, snatch, kink, cable bend, temperature cycling, sheath integrity and water penetration. Temperature cycling measuring method (IEC 794-1-F1) is substituted. A guide for optical cables for short distance links is added in an annex.

Appendix A - Optoelectronic and fiber optic standards

Standard No.

Title

Date

Summary

269

IEC

60794

Optical fiber cablesProduct specifications (indoor cable)

1998

Provides product specifications for single-fiber and dual-fiber cables. Describes single-fiber optical cables for indoor use with applications such as transmission equipment, telephone equipment, data processing equipment and communication and transmission networks.

IEC

60794

Optical fiber cablesDuct, buried and aerial cablesSectional specification

1998

Specifies the requirements of single-mode optical fiber cables which are intended to be used primarily in public telecommunications networks. Other types of applications requiring similar types of cables can be considered. In particular, requirements for cables to be used in ducts or for directly buried application and aerial cables are included in this standard. Underwater cables for lakes and river crossings and cable for indoor applications will be incorporated in this standard at a later stage. For aerial applications, this standard does not cover all functional aspects of cables installed in the vicinity of overhead power lines. In the case of such applications additional requirements and test methods may be necessary.

Optical fiber cablesPart 1-1: Generic specification General

2000

Applicable to optical fiber cables for use with telecommunication equipment and devices that employ similar techniques, and to cables that are combined with both optical fibers and electrical conductors. The aim of this is to establish uniform generic requirements for the geometrical, transmission, material, mechanical, ageing (environmental exposure) and climatic properties where appropriate, of optical fiber cables and electrical requirements. Annex A - information pertinent to short-distance links for many of the cables of this specification set. Annex B - guidance for users of this specification set in procuring optical cables compliant with these specifications. Annex C - guide to the installation of optical fiber cables.

PT2

PT3

IEC 60794-1-1

270

Optoelectronics and Fiber Optic Technology

Standard No.

Title

Date

Summary

IEC 60794-1-2

Optical fiber c a b l e s Generic specification Basic optical fiber test procedures

1999

Deals with optical fiber cables to be used with telecommunication equipment and devices using similar techniques, and to cables with a combination of both electrical conductors and optical fibers. Specifies methods to establish requirements for the material, transmission, geometrical, mechanical, ageing and climatic properties of cables, and electrical requirements.

IEC 60794-4-1

Optical fiber cablesAerial optical cables for high-voltage power lines

1999

IEC 60825-2

Safety of laser products Part 2: Safety of optical fiber communication systems

2000

Provides requirements and specific guidance for the safe use of optical fiber and/or control communication systems where optical power may be accessible at great distance from the optical source. Helps to protect people from optical radiation and helps to ensure adequate warning of hazards associated with optical fiber communication systems through signs, labels and instructions. Lays down requirements for manufacturers and operating organizations. Helps to reduce the possibility of injury by minimizing unnecessary accessible radiation.

IEC 60874 PT1

Connectors for optical fibers and cables - Generic specification

1999

Deals with fiber optic connectors and single components, e.g. sockets, plugs and adaptors, for all kinds, sizes and structures of cables and fibers. Covers connector set criteria and quality assessment procedures. Test and measuring methods are not included.

IEC 60875 PT1

Non-wavelengthselective fiber optic branching devicesPart 1: Generic specification

2000

Applicable to non-wavelength-selective fiber optic branching devices. It establishes uniform requirements for branching devices requirements and quality assessment procedures.

IEC 61073 PT1

Mechanical splices and fusion splice protectors for optical fibers and cablesPart 1: Generic specification

1999

Deals with fiber optic splice hardware (protection parts, alignment parts, etc) for optical fibers and cables and covers fiber optic splice hardware criteria and quality assessment methods. Test and measuring methods are detailed in IEC 61300-1, IEC 61300-2 and IEC 61300-3.

-

Appendix A - Optoelectronic and fiber optic standards

271

Standard No.

Title

Date

Summary

IEC 61073 PT2

Splices for optical fibers and cablesSectional specification- Splice organizers and closures for optical fibers and cables

1993

Covers the general requirements and the minimum quality assessment procedure for splice organizers and closures. All dimensional, mechanical and environmental requirements are to be defined in the detail specification.

IEC 61202 PT1

Fiber optic isolatorsPart 1: Generic specification

2000

Also bears the number QC 830000 which is the specification number in the IEC Quality Assessment System for Electronic Components (IECQ). Applies to the family of isolators used in the field of fiber optics, which have all of the following general features: they are non reciprocal optical devices; they are passive components in that they contain no optoelectronic or other transducing elements; they have two optical ports for directional transmission of optical power; usually they are optical fiber connectors or optical windows; they are wavelength sensitive. The object of this publication is to establish uniform requirements for the following: optical, mechanical and environmental properties of performance parameters; classification; quality assessment procedures; test and measuring methods.

IEC 61274 PT1

Fiber optic adaptors - Generic specification

1994

Relates to individual fiber optic adaptors such as: adaptors for connecting a plug to an identical type of plug adaptor; adaptors for connecting one type of plug to a different type of plug connector; adaptors for connecting a fiber optic connector to their optical devices such as LEDs, switches etc. The specification includes: adaptor requirements; quality assessment procedures.

IEC 61281-1

Fiber optic communication subsystems Part 1: Generic specification

1999

Covers a generic specification for fiber optic communication subsystems (FOCSs) which is structured according to the (IECQ), IEC Quality Assessment System. These are classified in families having a common sectional specification, each supplemented by blank detail and detailed specifications appropriate to the specific individual type or types of subsystems.

272

Optoelectronics and Fiber Optic Technology

Standard No.

Title

Date

Summary

IEC 61291-1

Optical fiber amplifiers- Part 1 Generic specification

1998

Applicable to optical fiber amplifiers (OFAs) and optically amplified, elementary subsystems using active fibers and containing rare-earth dopants. Sets out uniform requirements for transmission, operation, reliability and environmental properties of OFAs and supplies assistance to the purchaser in the selection of consistently high-quality OFA products for his especial applications.

IEC 61315

Calibration of fiber optic power meters

1995

IEC 61663-1

Lightning protectionTelecommunication lines - fiber optic installations

1999

This part is concerned with the lightning protection of telecommunication lines in fiber optic installations. Its aim is to limit the number of possible primary failures which occur in the optical fiber cable in a specified installation within values which are lower than or equal to the limit value and defined as the tolerable frequency of primary failures.

IEC 61753-1-1

Fiber optic interconnecting devices and passive components performance standard Part 1-1: General and guidanceInterconnecting devices (connectors)

2000

Gives general information on fiber optic connector performance standards. It includes references, definitions and rules for creating a performance standard. It also includes additional information pertinent to the subject.

IEC 61757-1

Fiber optic sensorsGeneric specification

1998

Gives a generic specification that covers optical fibers, components and sub-assemblies as they pertain specifically to sensing applications, in those aspects that are not already addressed by previous or concurrent standardization efforts.

IEC 61935-1

Generic cabling systems Specification for the testing of balanced communication cabling in accordance with ISO/IEC 11801 Part 1: Installed cabling

2000

Gives reference measurement methods for cabling parameters and the requirements for field tester accuracy to measure cabling parameters identified in ISO/IEC 11801. Applies when the cable assemblies are constructed of cables conforming to IEC 61156-1, IEC 61156-2, IEC 61156-3 or IEC 61156-4, and of connecting hardware as defined in IEC 60603-7 or IEC 60807-8.

Appendix A - Optoelectronic and fiber optic standards

273

Standard No.

Title

Date

Summary

IEC 61978-1

Fiber optic passive dispersion compensators Part 1: Generic specification

2000

Defines fiber optic passive dispersion compensators that are wavelength sensitive and may be polarization sensitive. Establishes quality assessment and uniform requirement procedures.

IEC 62005-1

Reliability of fiber optic interconnecting devices and passive components Part 1: Introductory guide and definitions

2001

Handbook for assessing the reliability interconnecting devices and passive components. Applicable to all types of fiber optic interconnecting devices and passive optical components.

IEC 62077

Fiber optic circulators - Generic specification

2001

Applicable to circulators used in the field of fiber optics bearing following features: - non-reciprocal optical devices, in which each port is either an optical fiber or optical fiber connector;passive components containing no opto-electronic or other transducing elements; - three or more ports for directionally transmitting optical power.

IEC 62099

Fiber optic wavelength switches - Generic specification

2001

Applicable to fiber optic wavelength switches. It covers devices and assemblies with following attributes:While the switch actuation means is by necessity active, the optical paths through the switch are passive.- The switch function is restricted to the routing of light rather than intentional power division plus routing.- They have two or more ports for the transmission of optical power and have two or more states in which power may be routed or blocked between these ports. - The ports are optical fibers or optical fiber connectors.

ISO 11149

Optics and optical instruments -lasers and laser-related equipment Fiber optic connectors for nontelecommunication laser applications

1997

Defines information for provision by a supplier of connectors between laser devices and optical fiber and optical cable assemblies.

274

Optoelectronics and Fiber Optic Technology

Standard No.

Title

Date

Summary

ISO TR 11802/4

Information Technology Telecommunications and information exchange between systems - Local and metropolitan area networks- technical reports and guidelines

1994

This publication provides the functional, electrical, optical and mechanical characteristics of a fiber optic interface for connecting a 4- or 16-Mbit/s token ring station to the trunk coupling unit (TCU) of a token ring.

ISO TR 9578

Information Technology Communication interface connectors used in local area networks

1990

Specifies the physical layer connection device for local area networks, sometimes referred to as the medium interface connector and is used between the terminal equipment and the trunk coupling unit for the trunk cable. The connectors described in this document were proposed by ISO/IEC JTC1 SC 6, SC 13, SC 83, IEC TC 46, IEC TC 48 and IEC TC 86. This document brings together the work of these groups into one collection which describes the physical devices which are currently available in the standards world.

ISO/IEC 11801

Information Technology Generic cabling for customer premises

2000

Specifies generic cabling for use within commercial premises having a geographical span of up to 3000m, with up to 1 000000 m squared of office space and a population between 50 and 50000 persons. It covers the structure and minimum configuration for generic cabling, implementation requirements, performance requirements for individual cabling links, and conformance requirements and verification procedures.

ISO/IEC 14763-1

Information Technology Implementation and operation of customer premises cabling Part 1: Administration

2001

Appendix A - Optoelectronic and fiber optic standards

275

Standard No.

Title

Date

Summary

ISO/IEC TR 14763-2

Information Technology Implementation and operation of customer premises cabling Part 2: Planning and installation

2000

Requirements are defined and provide general considerations for the planning, specification, quality assurance and installation of new cabling in accordance with ISO/IEC 11801.

ISO/IEC TR 14763-3

Information Technology Implementation and operation of customer premises cabling Part 3: Testing of optical fiber cabling

2000

The test procedures used to ensure that optical fiber cabling, designed in accordance with ISO/IEC 11801 and installed according to the recommendation of ISO/IEC 14763-2 is outlined in this report and is capable of delivering the level of transmission performance specified in ISO/IEC 11801.

Appendix B Fiber optic chronology 2500 BC: Earliest known glass. 800 BC:

Glass is drawn into fibers.

1790s:

Claude Chapp6 invents 'optical telegraph' in France.

1854:

John Tyndall demonstrates light guiding in water jets.

1880:

Alexander Graham Bell invents Photophone, Washington.

1880:

William Wheeler invents system of light pipes to illuminate homes from an electric arc lamp in basement, Concord, Mass.

1926:

John Logie Baird applies for British patent on an array of parallel glass rods or hollow tubes to carry image in a mechanical television. He later built an array of hollow tubes.

1930:

Heinrich Lamm, a medical student, assembles first bundle of transparent fibers to carry an image (of an electric lamp filament) in Munich. His effort to file a patent is denied because of Hansell's British patent.

1952:

Harold Horace Hopkins applies for a grant from the Royal Society to develop bundles of glass fibers for use as an endoscope at Imperial College of Science and Technology. Hires Narinder S. Kapany as an assistant when he receives grant.

1962:

Alec Reeves at Standard Telecommunications Laboratories in Harlow, UK, commissions a group to study optical waveguide communications under Antoni E. Karbowiak. One system they study is optical fiber.

1964:

Charles K. Kao takes over STL optical communication programme when Karbowiak leaves to become chair of electrical engineering at the University of New South Wales. Kao and George Hockham soon abandon Karbowiak's thin-film waveguide in favour of single-mode optical fiber.

1966:

Kao tells Institution of Electrical Engineers in London that fiber loss could be reduced below 20 decibels per kilometre for interoffice communications.

1966:

Kao and Hockham publish paper outlining their proposal in the

Proceedings of the Institution of Electrical Engineers.

Appendix B - Fiber optic chronology 277

1967:

British Post Office allocates an extra s some goes to fiber optics.

million to research;

1972:

Maurer, Keck and Schultz make multimode germania-doped fiber with 4 decibel per kilometre loss and much greater strength than titania-doped fiber.

1975:

First non-experimental fiber optic link installed by Dorset (UK) police after lightning knocks out their communication system.

1977:

AT&T and other telephone companies settle on 850 nanometre gallium arsenide light sources and graded index fibers for commercial systems operating at 45 million bits per second.

1978:

AT&T, British Post Office and STL commit to developing a single-mode transatlantic fiber cable, using the new 1.3 micrometre window, to be operational by 1988. By the end of the year, Bell Labs abandons development of new coaxial cables for submarine systems.

1982:

MCI leases right of way to install single-mode fiber from New York to Washington. The system will operate at 400 million bits per second at 1.3 micrometres. This starts the shift to singlemode fiber in America.

1984:

British Telecom lays first submarine fiber to carry regular traffic to the Isle of Wight.

1986:

First fiber optic cable across the English Channel begins service.

1986:

AT&T sends 1.7 billion bits per second through single-mode fibers originally installed to carry 400 million bits per second.

1988:

TAT-8 begins service of first transatlantic fiber optic cable, using 1.3 micrometre lasers and single-mode fiber.

1996:

Fujitsu, NTT Labs, and Bell Labs all report sending one trillion bits per second through single optical fibers in separate experiments using different techniques.

Glossary Because of their 'international acceptability' many of these definitions and terms have specific meanings and applications as opposed to generic definitions that are normally to be found in dictionaries.

Absorption: Loss of power in a fiber optic cable resulting from conversion of optical power into heat. Principally caused by impurities, such as transition metals and hydroxyl ions, it can also be caused by exposure to nuclear radiation. Acceptance angle: The half angle of the cone which incident light is totally, internally, reflected by the fiber core. It is the angle over which the core of an optical fiber accepts incoming light, usually measured from the fiber axis. It is equal to the arcsine (NA) where NA is the numerical aperture. Acceptance cone: A cone describing the complete envelope of light rays that will be propagated along an optical fiber. Active area: The area of a detector with greatest response. Amplitude modulation: A transmission technique in which the amplitude of a carrier is varied in sympathy with the information being communicated. Analog: A format that uses continuous physical parameters (such as voltage amplitude and carrier frequency) to transmit information. Angle of deviation: The angle between the original incident ray and the emergent ray. Angle of incidence: The angle between an incident ray and the normal to a reflecting or refracting surface. Angular misalignment loss: The optical power loss caused by angular deviation from the optimum alignment of source to optical fiber. Angular tilt: The angle formed by the axes of two fibers to be joined. Angular tilt causes an extrinsic loss that depends upon the joining hardware and method. Aramid yarn: Strength element used to provide support and additional protection of the fiber optic cable bundles. Kevlar is a particular brand of Aramid yarn. Armour: Extra cable protection to improve resistance to crushing, cutting and shearing forces. The usual form is a braided steel outer jacket but tough plastic with steel or plastic strengtheners is also used in many modem cable designs.

Glossary

279

ASCII: American Standard Code for Information Interchange. An 8-bit code in which letters, numbers and symbols are represented by 7-bit binary characters. The 8th bit is often used as a parity check. Note: The decimal numbers 0 to 47 code are used for symbols and computer commands. Decimal numbers 48 to 57 code for the numerals (0-9), decimal numbers 65 to 90 code for capital letters (A-Z), and decimal numbers 97 to 122 code for lower case letters (a-z). For example, in a computer using the ASCII code, 'A' is represented by the decimal number 65. The computer 'reads' this as the binary n u m b e r 01000001 and encodes the letter A. Most computers use an 8-bit code (extended ASCII) which produces 256 different combinations to represent symbols. In addition to the regular character set represented by ASCII (in the decimal range from 0 to 127), extended ASCII has an additional 128 codes that can be used to represent additional symbols (e.g. non-English characters or graphical symbols). Asynchronous transmission: A free-running transmission mode in which the time intervals between characters may be unequal. Attenuation: The reduction in optical power as it passes along a fiber, usually expressed in decibels (dB). See Optical loss. Attenuator: A device used to increase the attenuation of an optical fiber link, generally used to ensure that the signal at the receive end is not too strong. Avalanche photodiode (APD): A photodiode that exhibits internal amplification of photocurrent accomplished by avalanche multiplication of carriers in the junction region. As the reverse-bias voltage approaches the breakdown voltage, electron-hole pairs created by absorbed photons acquire sufficient energy to create additional electron-hole pairs when they collide with ions. A multiplication or signal gain is thereby achieved. Average power: The average level of power in a signal that varies with time. Axial ray: A light ray that travels along the axis of a fiber optic cable. Back reflection: Light reflected from the cleaved or polished end of a fiber caused by the difference of refractive indices of air and glass. Note: Typically 4 per cent of the incident light. Expressed in dB relative to incident power. Backscattering: The return of a portion of scattered light to the input end of a fiber optic cable. It is the scattering of light in the direction opposite to its original direction of propagation. Backscattering: The scattering of light in a fiber back toward the source, used to make OTDR measurements. Bandpass: A range of wavelengths over which a component will meet specifications.

280

Optoelectronicsand Fiber Optic Technology

Bandwidth: 1 The information capacity of a fiber optic cable. 2 The range of signal frequencies or bit rate within which a fiber optic component, link or network will operate. 3 The difference between the lower and upper frequencies which can be sent along a communication channel. Note: When used in radio engineering, bandwidth is quoted in cycles per second (hertz, or Hz) but the word 'bandwidth' is now also used to specify digital data transmission channels and Hz then refers to bits per second. The bandwidth of Ethernet, for example, is 10 MHz (megahertz). Bandwidth-limited operation: The condition in a fiber optic-cable based communications link when bandwidth, rather than received signal power, limits performance. This condition is reached when the signal becomes distorted (principally by dispersion) beyond specified limits. Baseband: A method of communication in which a signal is transmitted at its original frequency rather than being impressed upon a carrier frequency. Baud: A unit of data transmission signalling speed equal to the number of signal symbols per second. Note: With binary modulation systems this is the same as the data transmission rate in bits per second. However, it is different with nonbinary modulation systems. Beam splitter: An optical device, such as a partially reflecting mirror, for dividing an optical beam in two or more separate beams. It can be used in a fiber optic cable data link as a directional coupler. Bend loss: Attenuation to an optical signal that occurs when an optical fiber is bent round a tight radius. Light effectively 'spills out' from the core and is lost in the cladding. Bend radius: The minimum radius around which a fiber may be curved without risk of permanent damage resulting in excessive attenuation, or even breakage. Biconic: A connector type which has a taper sleeve which would be fixed to the fiber optic cable. This was one of the earliest connectors used in fiber optic systems but is in little use at present. Binary code: A digital coding system that uses a sequence of only two types of symbols (e.g. 0 and 1) to represent data. The two symbols are called bits (an abbreviation for binary digits). Bit" A binary digit which is generally either '0' or '1.' It is the smallest representation of information in a communications a n d / o r computing system. Bit error rate (BER): 1 The percentage of bits that are incorrectly received. 2 The fraction of data bits transmitted that are received in error.

Glossary

281

Bit rate: The number of bits of data transmitted per second over a communications link. Bit transfer rate: The bit transfer rate is expressed as bits per second or bps. Break out cable: Same as a fan out cable. This is a multiple fiber optic cable constructed in the tight buffered design and designed for ease of connecting rugged applications a n d / o r intra-building and interbuilding requirements. Broadband: The transmission of digital data streams over a wide b a n d w i d t h channel using modulated analog signals. Several data streams can be carried simultaneously by using different carrier frequencies. Broadcast: A communication strategy in which a single communication channel is used by all the nodes in a network. Messages from any source node are received by all other nodes, but will be ignored by nodes to which they are not specifically addressed. Buffer" A protective layer over the fiber optic cable, such as a coating, an inner jacket, or a hard tube. Buffer coating: A protective layer, such as an acrylic polymer, applied over the fiber optic cable cladding. Buffer tube: A hard plastic tube, having an inside diameter several times that of a fiber optic cable, that holds one or more fiber optic cables. Buffered fiber: Fiber optic cable protected with an additional material, usually hytrel or nylon, to provide ease in handling, connecting and increased tensile strength. Bundle- Many individual fiber optic cables within a single jacket or buffer tube a n d / o r a group of buffered fiber optic cables distinguished in some fashion from another group in the same cable core. Bus" C o m m o n l y called data bus. The term is used to describe the physical linkage between stations on a network sharing a common communication. Bus network: A network topology in which all of the terminals are attached to a transmission m e d i u m serving as a bus in which all other terminals receive all signals transmitted from a terminal connected to that bus. Byte" A unit of 8 bits. Cable: One or more fibers enclosed in protective coverings and strength members. Cable assembly" Fiber optic cable that has connectors installed on one or both ends, used to interconnect multimode and single-mode fiber optic cable systems and optoelectronic equipment. Note: If connectors are attached to only one end of the cable, it is known as a pigtail. If connectors are attached to both ends, it is known as a jumper.

282

Optoelectronics and Fiber Optic Technology

Cable bend radius: During installation this infers that the cable is experiencing a tensile load. Carrier sense multiple access (CSMA): A network access procedure whereby before transmitting to the network a station checks for the presence of a carrier signal showing that the network is already being used. Transmission only starts if no other carrier is detected. Carrier sense multiple access with collision detection (CSMA/CD): A technique employed in Ethernet-based LANs to control the transmission channel. It assures that there is no conflict between terminals that wish to transmit. Carrier sense multiple access with collision detection (CSMA/CD): A type of network (e.g. Ethernet) in which all connected devices monitor continuously. If a transmitted message overwrites one already on the network, a collision is detected and the message is retransmitted. Centre wavelength: The wavelength of an optical source that might be considered its middle. Central member: The centre component of a fiber optic cable that serves as an anti-buckling element to resist temperature-induced ( a n d / o r a strength element). The central member is composed of steel, fibreglass or glass-reinforced plastic. Central office: The place where communications common carriers terminate customer lines and locate switching equipment that interconnects those lines. Centro-symmetrical reflective optics: An optical technique in which a concave mirror is used to control coupling of light from one fiber optic cable to another. Channel: 1 A communications path derived from a specific transmission medium, for example fiber optic cables. The channel supports the endto-end communications of an information source and destination. Note: Besides the transmission m e d i u m a channel needs to have a transmitter/receiver (transceiver) and a m o d u l a t o r / d e m o d u l a t o r (modem). By multiplexing, several channels can share the same specific transmission medium. Channel is s y n o n y m o u s with link. The term channel is usually employed within the context of m u l t i p l e x i n g but not always. 2 A communication path for transmission of data between two points. Chromatic bandwidth: The inverse of the chromatic dispersion. Chromatic dispersion: The temporal spreading of a pulse in an optical waveguide caused by the wavelength dependence of the velocities of light. Cladding: A low refractive index glass or plastic that surrounds the core of the fiber optic cable. Cladding mode: A mode that is confined to the cladding.

Glossary

283

Cleaving: The controlled breaking of a fiber so that its end surface is smooth. Coating: A material put on a fiber optic cable during the drawing process to protect if from the environment. Coherent light or light waves: Light where all parameters are predictable and correlated at any point into time or space, particularly over an area perpendicular to the direction of propagation or over time at a particular point in space. Concentrator: A multi-port repeater. Conduit: Pipe or tubing through which cables can be pulled or housed. Connector: Termination component installed on ends of fiber optic cables. It provides physical attachment and optical coupling to further cables, transmitters, receivers and other devices. Core: The centre of the optical fiber through which light is transmitted. Core eccentricity: A measure of the displacement of the centre of the core relative to the cladding centre. Coupler: An optical device that splits or combines light from more than one fiber. Coupling efficiency: The efficiency of optical power transfer between two components. Coupling losses: The power loss suffered when coupling light from one optical device to another. Coupling ratio: The percentage of light transferred to a receiving output port with respect to the total power of all output ports. Critical angle: The limiting angle at which a light ray will be totally internally reflected. Crosstalk: The pickup in one particular fiber optic cable of unwanted light from another fiber optic cable. Cut-off wavelength: The wavelength beyond which single-mode fiber only supports one mode of propagation. Dark current: The noise current generated by a photodiode when it is not exposed to any light. Data link: A transmitter, optical fiber cable and receiver that transmits digital data between two points. Data rate: The number of bits of information sent per second in a data communications transmission system. dB: Decibel, a measure of loss or equivalently attenuation. In fiber optics the ratio is power and defined by: dB = 10 lOgl0(P1/P2). Demodulation: Re-creation of an original data stream from a modulated signal. Demultiplex: Splitting up a multiplexed signal into its component parts. Detector: A device (photodiodes, photodarlingtons and phototransistors) that generates an electrical signal when illuminated by light.

284

Optoelectronics and Fiber Optic Technology

Diameter-mismatch loss: The loss of power at a joint that occurs when the transmitting half has a diameter greater than the diameter of the receiving half. Diamond connector: A type of connector. Dichroic filter: An optical filter that transmits light selectively according to wavelength. Diffraction grating: An array of fine, parallel, equally spaced reflecting or transmitting lines that concentrate the diffracted light in a few directions, determined by the spacing of the lines and by the wavelength of the light. Digital: A data format that uses a discrete, countable and finite number of levels to transmit information. Binary is a special case of this corresponding to two levels. Digital signal: A signal encoded in discrete levels, typically binary '1' and binary '0'. Diode: An electronic device that lets current flow in one direction only. Diodes used in fiber optics include light emitters (LEDs and laser diodes) and detectors (photodiodes). Directional coupler: An optical fiber coupler where light at the input ports is only transferred to one or more defined output ports. Directivity: The amount of power observed at a given input port with respect to an initial input power. Dispersion: A general term for those phenomena that cause a broadening or spreading of light as it propagates down a fiber optic cable. Distributed system: A system containing two or more intelligent stations linked by a communication network. Dopant: A material such as germanium or boron oxide that is added to silica to change the refractive index. Drawing: The manufacturing process by which fiber optic cable is pulled from preforms. Duplex cable: A two-fiber cable suitable for duplex (two-way) transmission. Duplex transmission: Simultaneous independent transmission in both directions over a communication link. Edge-emitting diode (ELED): An LED that emits from the edge of the semiconductor chip, producing higher power and narrower spectral width. Electromagnetic radiation: Oscillating electrical and magnetic fields, perpendicular to each other, travelling at the speed of light. The spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet light and X-rays. Emitter: Term used for a light source. Encoding: A scheme to represent digital ones and zeros through combining high and low voltage states.

Glossary

285

End separation: The distance between the ends of two joined fiber optic cables. End-to-end loss: The optical loss due to the fiber optic cable, splices and connectors on an installed fiber optic cable data link path. Endoscope: A device for transmitting an optical image along a rigid or flexible tube, used for mechanical inspection and medical instruments. Equilibrium mode distribution: The steady modal state of a multimode fiber optic cable in which the relative power distribution among the modes is independent of the fiber optic cable length. Erbium-doped fiber amplifier: A type of fiber optic cable that amplifies 1550 nm optical signals when pumped with a 980-1480 nm light source. Ethernet: A network using CSMA/CD that is widely used for LANs. Evanescent wave: Light guided along the inner part of the cladding of an optical fiber. Expanded beam connector: A connector in which the diameter of a beam of light emerging from a fiber is expanded, then focused on to the core of another fiber. Extrinsic losses: Signal loss in transmission down fiber optic cable caused by imperfect alignment of fiber optic cables joined by a connector or splice. Ferrul: A component of a connector that holds a fiber optic cable in place and aids in its alignment. Fiber: An optical waveguide consisting of thin filaments of glass with a core and a cladding which is capable of carrying information in the form of light. Fiber amplifier: An all optical amplifier using erbium or other doped fibers and pump lasers to increase signal output power without electronic conversion. Fiber bandwidth: The lowest frequency at which the magnitude of the fiber transfer function decreases to a specified fraction of the zero frequency value. Fiber bundle: An assembly of unbuffered fiber optic cables usually employed as a single transmission channel. Fiber distributed data interface (FDDI): A very high speed local area networking architecture based upon fiber optic cable as the transmission medium. Fiber distributed data interface (FDDI) network: A token passing ring network designed specifically for fiber optic cable and featuring dual counter-rotating rings and 100 Mbps operation. Fiber loss: The attenuation (deterioration) of the light signal in transmission through a fiber optic cable. Fiber optic inter-repeater link: Standard defining a fiber optic cable link between two repeaters in an IEEE 802.3 network.

286

Optoelectronicsand Fiber Optic Technology

Fiber optic link: Any transmission channel using a fiber optic cable as the transmission medium to connect two end terminals or to be connected in series with other channels. Fiber optics: The branch of optical, communication and electronic technologies concerned with the transmission of electromagnetic radiation through thin fibers made of transparent material such as glass, fused silica and plastic. Fiber optic waveguide: A relatively long strand of transparent substance (usually glass) capable of conducting an electromagnetic wave of optical wavelength (visible or near-visible region of the frequency spectrum) with some ability to confine longitudinally directed, or near longitudinally directed, lightwaves to its interior by means of internal reflection. Frequency division multiplexing (FDM): Multiplexing of signals by assigning a different carrier frequency to each and then combining in a single signal. Frequency modulation: A transmission technique in which the frequency of a carrier is varied in sympathy with the information being communicated. Fresnel reflection loss: Loss of optical power due to Fresnel reflections. Fundamental mode: The lowest order propagation mode of a waveguide. Fused coupler: A method of making a multimode or single-mode coupler by wrapping fiber optic cables together, heating them and pulling them to form a central unified mass. By doing this, light on any input fiber optic cable is coupled to all output fiber optic cables. Fusion splice: A joining of two fiber optic cables by physically fusing through heat the two fiber optic cable ends. Fusion splicer: An instrument that splices fibers by fusing or welding them, typically by electrical arc. Fusion splicing: A permanent joint accomplished by the application of localized heat. Graded index fiber: A fiber optic cable where the core is composed of concentric rings of glass where the refractive indices decrease from the centre axis whose purpose is to reduce modal dispersion. Graded index profile: Any refractive index profile that varies with radius in the core. Hard-clad silica: A fiber optic cable with a hard plastic cladding surrounding a silica glass core. Hermaphrodite connector: A connector pair in which each part includes both pins (male) and sockets (female). Hybrid cable: A composite cable composed of both a fiber optic cable and electrical conductors. Hydrogen losses: Increases in fiber attenuation caused by diffusion of hydrogen.

Glossary

287

Incident angle: The angle between an incident ray and a line perpendicular to an optical surface.

Index matching material: A material used at an optical interconnection with a refractive index close to that of the fiber optic cable core.

Infrared: The designation for electromagnetic waves at wavelengths between the visible part of the spectrum (approximately 750 nm) and the microwave band (approximately 30 mm). Injection laser: Semiconductor or diode laser. Insertion loss: The loss in the power of a signal that results from inserting a passive component (such as in-line star couplers and splices) into a previously continuous path. Integrated optics: Design and application of optical devices that perform several functions on a single substrate. Integrated optoelectronics: Integration of optical and electronic devices on the same chip. International Standards Organization (ISO): An independent international body formed to define standards for multi-vendor network communications. Intrinsic losses: Loss caused by fiber optic cable parameter mismatches when two non-identical cables are joined. Isolator: A two-port component having greater attenuation in one direction than the other often used to prevent return reflections in a transmission path. Jacket: A layer of material surrounding an optical fiber but not bonded to it; part of the cable, not of the fiber.

Jumper cable: 1 Single fiber optics cable with connectors on both ends. 2 A short single fiber cable with connectors on both ends used for interconnecting other cables or testing. Junction laser: A semiconductor laser diode. Kevlar: A strong synthetic material used widely as a strength member and as a jacket in optical fiber cables. Xhe name is a trademark of Dupont. Large core fiber: A fiber optic cable with a core of 200 mm or more. Laser: A device which artificially generates coherent light within a narrow range of wavelengths. Lateral displacement loss: The loss of power that results from lateral displacement from optimum alignment between two fiber optic cables or between a fiber optic cable and an active device. Light: That portion of the electromagnetic spectrum that can be handled by the basic optical techniques used for the visible spectrum extending from the near ultraviolet region of approximately 0.3 mm through the visible region into the mid-infrared region of approximately 30 mm.

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Light emitting diode (LED): A semiconductor diode that spontaneously emits light from the pn junction when forward current is applied.

Light source: Source of light, typically an LED or laser, which is usually modulated and terminated over a fiber optic cable.

Lightguide: A fiber optic cable or fiber bundle. Light waves: 1 Electromagnetic waves in the region of optical frequencies. 2 The technology of optical communication, including fiber optics. Link: A fiber optic cable with connectors attached to a transmitter (source) and receiver (detector). Local area network (LAN): A data communications network serving a limited area such as a building or factory site. Local loop: The subscriber link in a telephone network. Long-haul network: A network that carries data between towns and cities, involving distances. Long wavelength: A commonly used term for light in the 1300 and 1550 nm ranges. Loose tube: A protective tube loosely surrounding a fiber optic cable often filled with a water blocking gel. Loss: Attenuation of optical signal. It is usually measured in dB. Loss budget: An account showing the overall loss in an optical fiber link, compared with the minimum received power specified at the receiver and the power available from the transmitter. Macrobend: A large fiber bend that can be seen with the unaided eye. Macrobending: Macroscopic axial deviations of a fiber optic cable from a straight line. Margin: The additional amount of loss that can be tolerated in a link. Material dispersion: Light pulse broadening caused by various wavelengths of light travelling at different velocities d o w n a fiber optic cable. Material scattering: That part of the total scattering attributable to the properties of the materials used for waveguide fabrication. Mechanical splice: A semi-permanent connection between two fibers made with an alignment device and index matching fluid or adhesive. Microbend loss: The loss attributed to microscopic bends in fiber optic cable. Microbending: Curvatures of the fiber optic cable which involves axial displacements of a few micrometres and spatial wavelengths of a few millimetres. Misalignment loss: The loss of power resulting from angular misalignment, lateral displacement and end separation. Modal bandwidth: A bandwidth limiting mechanism in multimode fiber optic cables.

Glossary

289

Modal dispersion: Dispersion caused by the difference in time taken by different modes to travel along a multimode fiber.

Modal noise: The fluctuation in optical power due to the interaction of the power travelling in more than one mode.

Mode: A single electromagnetic field pattern that travels in fiber. Mode coupling: The transfer of energy between modes. Mode field diameter: The diameter of the one mode of light that is being propagated in a single-mode fiber.

Mode stripper: A device that removes high order modes in a multimode fiber, to standardize conditions for measurement.

Modulation: The process by which the characteristic of one wave (the carrier) is modified by another wave (the information signal).

Multimode fiber optic cable: Type of fiber optic cable that support more than one propagation mode.

Multiplexing: Combining several data channels so that a composite signal can be transmitted over a single communications link.

Nanometre (nm): A unit of measure, 10-9m, used to measure the wavelength of light.

Network: A system of cables, hardware and equipment used for communications.

Numerical aperture: An imaginary cone which defines the acceptance area for the fiber optic cable core to accept light rays.

Optical amplifier: A device that amplifies light without converting it to an electrical signal.

Optical cable: An assembly of fiber optic cables and other material providing mechanical and environmental protection.

Optical fiber: Synonym for fiber optic cable. Optical fiber coupler: A device whose purpose is to couple power between a fiber optic cable and a source or detector.

Optical link: Any optical transmission channel designed to connect two end terminals.

Optical switch: A device that routes an optical signal from one or more input ports to one or more output ports.

Optical time domain reflectometer (OTDR): An instrument that measures transmission characteristics of an optical fiber by sending a short pulse and measuring the return signals resulting from backscatter and reflections. Optical time domain reflectometry: A method of evaluating fiber optic cables based upon detecting backscattered (reflected) light.

Optical waveguide: 1 Synonym for fiber optic cable. 2 Any structure that can guide light along a preset path. Optical window: Wavelength range of a fiber optic cable with a very low attenuation. Fiber optic data links using LED sources work in the first

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window at 850nm or in the second window at 1300nm. Fiber optic data links using LASER sources work in the second window at 1310 nm or in the third window at 1550 nm. Optoelectrical converter: Converts an optical signal into an electrical signal. Optoelectronics- The range of materials and devices that generate light (lasers and light emitting devices), amplify light (optical amplifiers), detect light (photodiodes) and control light (electro-optic circuits). Passive coupler: A coupler that divides a light input between output ports without adding any light power. Passive star coupler: Uses passive optical components to couple one or more input optical signals coming from fiber optic cables to one or more output fiber optic cables acting as receivers. Patch panel: Distribution area to rearrange fiber optic cable connections and circuits. Patchcord" A length of optical fiber or cable with optical connectors on both ends. Peak wavelength: The wavelength at which the optical power of a source is at a maximum. Phase modulation: Modulation of the phase angle of a signal in accordance with an input signal. Photocurrent" The electrical current that flows through a photosensitive device, such as a photodiode as a result of exposure to radiant power. Photodetector" An optoelectronic transducer, such as a PIN photodiode or avalanche photodiode that converts a light input into an electrical output signal. Photodiode- A semiconductor that converts light to an electrical signal, used in fiber optic receivers. Photon: A quantum of electromagnetic energy. Photonics: The technology of transmission of information using light. Pigtail: A short length of fiber optic cable, permanently fixed to a component, used to couple power between the component and the fiber optic cable used for transmission. PIN photodiode: A fast linear photodetector, widely used in fiber optic receivers. Planar waveguide" A step index multimode fiber in which a silica core is surrounded by a plastic cladding with lower refractive index. Plastic-clad silica fiber optic cable: A fiber optic cable having a glass core and a plastic cladding. Plastic fiber optic cable: Fiber optic cables having a plastic core and plastic cladding. Plastic-clad silica (PCS) fiber: A fiber made with a glass core and plastic cladding.

Glossary

291

Plenum: The air handling space between walls, under structural floors and above drop ceilings which can be used to route intra-building cabling. Plenum cable: Fiber optic cables whose flammability and smoke characteristic allow it to be routed in a plenum area without being enclosed in a conduit. Point-to-point: A fixed link secured between two distinct nodes or stations in a network. Polarization stability: The variation in insertion loss as the polarization state of the input light is varied. Polarized: Light for which the electrical vector of the electromagnetic field has been oriented, instead of being at random. Polishing: Preparing the end of a fiber optic cable by moving the end over an abrasive material. Power budget: The difference (in dB) between the transmitted optical power (in dBm) and the receiver sensitivity (in dBm). Power meter: Device used to measure attenuation of a plastic fiber optic cable. Preform: A solid rod of plastic material from which a plastic fiber optic cable is drawn or a glass structure from which glass fiber optic cable is drawn. Prefusing: Fusing with low current to clean the fiber optic cable end. Precedes fusion splicing. Primary coating: The plastic coating applied directly to the cladding surface of the fiber optic cable during manufacture to preserve the integrity of the surface. Pulse coded modulation (PCM): A technique in which an analog signal is converted to a digital signal. Pulse dispersion: The lengthening of a pulse as it travels along an optical fiber. Pulse spreading: The dispersion of an optical signal with time as it propagates through a fiber optic cable. Quantum efficiency: The proportion of received photons that produce useful output current in a photodetector. Quaternary: Made from four different elements. Radiance: Optical power per unit solid angle per unit area from a source.

Radiation pattern: The angular flux distribution radiating out of a fiber.

Rayleigh scattering: The scattering of light that results from small inhomogeneities in material density or composition which causes losses in optical power varying with the fourth power of the wavelength. Receiver: A device that detects an optical signal and converts it into an electrical signal.

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Reflectance: Light that is reflected back along the path of transmission, from either the coupling region, the connector or the terminated fiber optic cable. Reflection: The abrupt change in direction of light as it travels from one material to a dissimilar material. Refraction: The bending of light as it passes between materials of different refractive index. Refractive index: Ratio of the speed of light in a vacuum to the speed of light in a material. Regenerative repeater: A repeater designed for digital transmission that both amplifies and reshapes the signal. Sometimes called a regenerator. Regenerator: A receiver-transmitter unit that detects a weak optical signal, cleans and amplifies it, then sends the regenerated signal on through a further length of optical fiber. Repeater: One or more regenerators. Ribbon cable: A cable in which many optical fibers are embedded in parallel, in a plastic sheath, to form a ribbon-like structure. Electrical conductors may also be included. Ring network: A network topology in which terminals are connected in a point-to-point serial fashion in an unbroken circular configuration. Rise time: The time required for the leading edge of a pulse to rise from 10 per cent to 90 per cent of its amplitude. Riser: Application for indoor cables that pass between floors, normally in a vertical shaft or space. Scattering: The change of direction of light after striking small particles that causes loss in optical fibers. Scattering losses: Losses in optical fibers caused by the material of the fiber or by imperfections. Semiconductor laser: Same as a laser diode. Sensitivity: The minimum optical power required to achieve a specified level of performance. Serial transmission: A method of transmission in which each bit of information is sent sequentially on a single channel. Sheath: The outer protective layer of an optical fiber cable. Short wavelength: A commonly used term for light in the 665, 790 and 850 nm ranges. Shot noise: Noise caused by random current fluctuations arising from the discrete nature of electrons. Signal-to-Noise Ratio (SNR): The ratio of signal power to noise power measured in dB. Silica glass: Glass made mostly of silicon dioxide, used in optical fibers. Simplex: Transmission in only one direction.

Glossary

293

Simplex cable: A term sometimes used for a single-fiber cable. Simplex transmission: Operation of a communication channel in one direction only.

Single mode: A small core, fiber optic cable that supports only one mode of light propagation above the cut-off wavelength.

Single-mode fiber: A fiber with a small core, only a few times the wavelength of light transmitted, that only allows one mode of light to propagate. Solitron: An optical pulse that does not suffer dispersion as it propagates over a distance. Source: A LASER diode or LED used to inject an optical signal into fiber. Spectral attenuation: Measure for the attenuation in dependence on wavelength. Spectral bandwidth: The wavelength interval in which a radiated spectral quantity is not less than half its m a x i m u m value. Spectral width: The measure of the wavelength extent of a spectrum. Speed of light: Approximately 2.998 x 108 metres/second (in vacuum). Splice: An interconnection method for joining the ends of two fiber optic cables in a permanent or semi-permanent fashion. Splice box: Housing for one or more splice organizers. Splice closure: A container used to organize and protect splice trays. Splicing: The permanent joining of fiber optic cable ends to identical or similar fiber optic cables without using a connector. Star coupler: A coupler for a fiber optic cable in which power at any input port is distributed to all output ports. Star network: A network in which all terminals are connected through a single point, such as a star coupler. Station: A single addressable unit on a network, representing one or more devices that receive and transmit data. Steady state: Equilibrium mode distribution. Step index fiber: A fiber optic cable, either multimode or single-mode, in which the core refractive index is uniform throughout so that a sharp step in refractive index occurs at the core-to-cladding interface. Step index profile: A refractive index profile in which the refractive index changes abruptly from the value nl to n 2 at the core/cladding interface. Strain member: The part of an optical fiber cable which ensures that no strain is imposed on the fibers. Materials used include steel and synthetic yarns. Strength member: That part of a fiber optic cable composed of Kevlar aramid yarn, steel strands or fibreglass filaments that increase the tensile strength of the cable. Stripping: Removing the coating from a fiber optic cable.

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Subscriber loop: The link to individual subscribers in a telephone network.

Surface emitting diode: An LED which emits light from its surface rather than from its edge.

Synchronous transmission: A transmission method in which the synchronizing of characters is controlled by timing or clock signals. Transmitters and receivers operate continuously at the same frequency. Tap loss: The ratio of power at the tap port to the power at the input port. Tap port: The splitting ratio between output ports is not equal; it is the output port containing the lesser power. Tee coupler: A three-port optical coupler. Telecommunications: The communication of information over a distance by means of radio waves, optical signals or along a transmission line. Ternary: Made from three different elements. Test kit: A kit of fiber optic instruments, typically including a power meter, source and test accessories used for measuring loss and power. Thermal noise: Noise resulting from thermally induced random fluctuation in current in the receiver's load resistance. Thermal stability: A measure of the insertion loss variation as the device undergoes various environmental changes. Threshold current: The minimum current needed to sustain laser action in a laser diode. Throughput loss: The ratio of power at the throughput port to power at the input port.

Tight buffer: 1 Type of cable construction whereby each glass fiber optic cable is tightly buffered by a protective thermoplastic coating to a diameter of 900 microns. 2 A material tightly surrounding a fiber in a cable, holding it rigidly in place. Time division multiplex (TDM): Digital multiplexing by taking one pulse at a time from separate signals and combining them into a signal bit stream. Time division multiplexing (TDM): A transmission technique whereby several low speed channels share a given transmission medium, for example a fiber optic cable. Token: A unique bit sequence which permits a station to transmit to a token ring network. Token ring network: A network that can only be accessed by a station that is allocated the token, which is passed around the network in a predetermined sequence. Total bandwidth: The combined modal and chromatic bandwidth. Total internal reflection: Confinement of light into the core of a fiber by the reflection off the core/cladding boundary.

Glossary

295

Transceiver: A combination of transmitter and receiver providing both output and input interfaces with a device.

Transduce: A device for converting energy from one form to another, such as optical energy to electrical energy.

Transmission loss: Total loss encountered in transmission through a system.

Transmission media: The physical media through which communication signals are transmitted, for example optical fiber, coaxial cable, twisted pairs.

Transmitter: 1 An electrical package which converts an electrical signal to an optical signal. 2 A device which includes an LED or laser source and signal conditioning electronics that is used to inject a signal into fiber. Transparency: A communication system which is transparent and imposes no restrictions on the code or bit patterns in the information being transmitted. Tree coupler: A coupler which distributes light signals to several output ports. Ultraviolet: Electromagnetic waves with wavelengths around 100 to 400 nm. Uniformity: The maximum insertion loss difference between ports of a coupler. Videophone: A telephone service that offers transmission of pictures as well as sound. Virtual reality: A computer simulation that closely resembles reality. Visible light: Electromagnetic radiation that is visible to the h u m a n eye, at wavelengths from 400 to 700 nm. Waveguide: A two-dimensional substrate which carries light in channels inscribed in the material. Wavelength: Distance an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Wavelength dependence: The variation in an optical parameter caused by a change in the operating wavelength. Wavelength division multiplexer: A passive fiber optical device used to separate optical signals of different wavelengths carried on one fiber optic cable. Wavelength division multiplexing (WDM): Simultaneous transmission of several optical signals of different wavelengths on the same fiber optic cable. Wide area network (WAN): Communication network covering long distances, often using some public network facilities. Working margin: The difference (in dB) between the power budget and the loss budget (i.e. the excess power margin).

Acronyms and abbreviations ACOST: UK Advisory Council on Science and Technology ACR: Attenuation to Crosstalk Ratio Al: Aluminium AM: Amplitude Modulation APD: Avalanche PhotoDiode APF: All Plastic Fiber As: Arsenide ATM: Asynchronous Transfer Mode BBC: British Broadcasting Company BER: Bit Error Rate bit: Binary digit CAD: Computer Aided Design CATV: Community Antenna Television (or cable TV) CPU: Central Processor Unit CSMA: Carrier Sense Multiple Access CSMA/CD: Carrier Sense Multiple Access with Collision Detection CSP: Channelled Substrate Planer (LASER) DARPA: US Defense Advanced Research Projects Agency dB: Decibel dB(mu): Optical power referenced to 1 microwatt dB/km: Decibels per kilometre dBm: Optical power referenced to 1 milliwatt dBr: Decibels relative to a known power level DC: Direct Current DFB: Distributed FeedBack DOT: Department of Transport DSB: Digital Signal Processing DSF: Dispersion Shifted Fiber DTI: Department of Trade and Industry D W D M : Dense Wavelength Division Multiplex(ing) EDFA: Erbium-Doped Fiber Amplifier EDTFA: Erbium-Doped Tellurite Fiber Amplifier EFTA: European Free Trade Association EIA: Environmental Impact Assessment EIA: Electronic Industries Association ELED: Edge Light Emitting Diode ELFEXT: Equal Level Far End Cross Talk EMC: ElectroMagnetic Compatibility

Acronyms and abbreviations 297

EMD: Equilibrium Mode Distribution EMI: Electromagnetic Interference EU: European Union FDDI: Fiber Distributed Data Interface FDM: Frequency Division Multiplex(ing)

FEXT: Far End Cross Talk FIA: Fiber Industry Association FITL: Fiber In The Loop FLOD: Full Length Outside Deposition FM: Frequency Modulation FO: Fiber Optic FOA: Fiber Optic Amplifier FOTP: Fiber Optic Test Procedure FOTS: Fiber Optic Transmission System FPLD: Fabry-Perot Laser Diode FTTB: Fiber To The Building FTTC: Fiber To The Curb FTTH: Fiber To The Home FWHM: Full Width Half Maximum FXC: Fiber switch Cross Connect Ga: Gallium Gbit: Gigabit GI: Graded Index GIPFOC: Graded Index Plastic Fiber Optic Cable GOF: Glass Optical Fiber IDF: Intermediate Distribution Frame IDP: Integrated Detector/Amplifier ILD: Index guided Laser Diode ILD: Injection Laser Diode In: Indium IR: Infrared ISDN: Integrated Services Digital Network ISO: International Standards Organization kg: Kilogram km: Kilometre km/s: Kilometres per second LAN: Local Area Network LASER: Light Amplification by Stimulated Emission/Radiation lb: Pound (weight, UK) LD: Laser Diode LED: Light Emitting Diode LLDPE: Linear Low Density PolyEthylene jacketing LLIDS: Local Light Injection and Detection System LPE: Liquid Phase Epitaxy

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LSFH: Low Smoke Halogen Free m: Metre mA: MilliAmpere M A N : Metropolitan Area Network MBE: Molecular Beam Epitaxy M B P S : (see Fiber distributed data interface network in the Glossary) MCRW: Metal-Clad Ridge Waveguide (laser) MDF: Main Distribution Frame M E M S : Micro ElectroMechanical Systems M H z : MegaHertz mm: Millimetre MMF: Multimode Fiber optic cable

MOCVD: Metal Organic Chemical Vapour Deposition M o d e m : Modulator/demodulator

MOVPE: Metal Organic Vapour Phase Epitaxy MRI: Magnetic Resonance Imaging mW: MilliWatt NA: Numerical Aperture NEXT: Near End Crosstalk NIR: Near Infrared NIU: Network Interface Unit NLO: Non-Linear Optics nm: Nanometre NTSC: National Television System Committee OSI: Open Standards Interconnect OTDR: Optical Time Domain Reflectometer P: Phosphorous PAL: Phase Alternation Line (European TV format) PCB: Printed Circuit Board PCM: Pulse Code Modulation PCS: Plastic Clad Silica PD: Potential Difference PD: Photodiode PDFFA: Praseodymium-Doped Fluoride Fiber Amplifier PE: Polyethylene. This is a type of plastic material used to make cable jacketing PET: Position Emission Tomography PF: Perfluorinated PIN: Positive Intrinsic Negative photodiode PIN-FET: PIN-Field Effect Transistor PIN-PD: PIN-Photodiode P M M A : Polymethylmethacrylate POF: Plastic Optical Fiber POFA: Plastic Optical Fiber Amplifier

Acronyms and abbreviations 299

PSTN: Public Switched Telephone Networks PTFE: Polytetrafluoroethylene (Teflon) PTT: Postal Telegraph and Telephone PUR: Polyurethane PVC: Polyvinyl Chloride R&D: Research and Development RF: Radio Frequency RI: Reflective Index RX: Receiver SC: A connector type, primarily used with single-mode fiber optic cables ScTP: Screened Twisted Pair SDM: Space Domain Multiplexing Si: Silicon SIPFOC: Step Index Plastic Fiber Optic Cable SLB: System Loss Budget SMA: Sub Multiple Assembly SMA: A connector type that was the predecessor of the ST connector SNR, S/N: Signal to Noise Ratio. Usually expressed in dB SOA: Semiconductor Optical Amplifier SONET: Synchronous Optical Network ST: A keyed bayonet connector type similar to a BNC connector STL: Standard Telecommunications Laboratories TDM: Time Division Multiplex(ing) TIA: TransImpedence Amplifier TIR: Total Internal Reflection TX: Transmitter UK: United Kingdom US: United States UTP: Unshielded Twisted Pair UV: Ultraviolet VCSEL: Vertical Cavity Surface Emitting Laser VCSL: Vertical Cavity Semiconductor Laser VLON: Virtually Lossless Network VLSI: Very Large Scale Integration W: Watt WAN: Wide Area Network WDM: Wavelength Division Multiplex(ing) WIC: Wavelength Independent Coupler

Books by the same author

7~t/e

Details

Publisher

ISO 9001:2000 for Small Businesses (Second Edition)

Fully revised and updated, ISO 9001:2000 for Small Businesses explains the new requirements of ISO 9001:2000 and helps businesses draw up a quality plan to allow them to meet the challenges of the marketplace.

Butterworth-Heinemann ISBN: 0 7506 4882 1

ISO 9001:2000 Audit Procedures

A complete set of audit check sheets and explanations to assist auditors in completing internal, external and third part audits of newly implemented ISO 9001:2000 Quality Management Systems, existing ISO 9001:1994, ISO 9002:1994 and ISO 9003:1994 compliant QMSs and transitional QMSs

Butterworth-Heinemann ISBN: 0 7506 5436 8

ISO 9001:2000 in Brief

A 'hands-on' book providing practical information on how to cost effectively set up an ISO 9001:2000 Quality Management System.

Butterworth-Heinemann ISBN: 0 7506 4814 7

CE Conformity Marking

Essential information for any manufacturer or distributor wishing to trade in the European Union. Practical and easy to understand.

Butterworth-Heinemann ISBN: 0 7506 4813 9

Environmental Requirements for Electromechanical and Electronic Equipment

Definitive reference containing all the background guidance, ranges, test specifications, case studies and regulations worldwide.

Butterworth-Heinemann ISBN: 0 7506 3902 4

MDD Compliance using Quality Management Techniques

Easy-to-follow guide to MDD, enabling the purchaser to customize the Quality Management System to suit his own business.

Butterworth-Heinemann ISBN: 0 7506 4441 9

Quality and Standards in Electronics

Ensures that manufacturers are aware of the all the UK, European and international necessities, knows the current status of these regulations and standards, and where to obtain them.

Butterworth-Heinemann ISBN: 0 7506 2531 7

Use ful lin ks www.ailu.org.uk: AILU is an independent non-profit-making organization serving the industrial laser industry to help make the most of laser technology. Primarily a source of technical, product and service data on industrial lasers and their applications. Includes industry directory, online catalogue and events notice board. Also offers courses. www.alertt.org.uk: ALERTT is a DTI funded project run by the UK Consortium for Photonics and Optics to provide a national database on activities in photonics. www.auriga-europe.co.uk: AURIGA is a specialist optoelectronics and fiber optic distributor who offers a combination of copper, fiber optic and wireless products to suit all networking needs. They started trading in 1989 and have negotiated partnerships with blue chip manufacturers such as 3M, AMP, BICC Brand-Rex and Diamond. AURIGA. www.brand-rex.com: Part of the newly formed Intelligent Building Systems sector of Novar Plc, Brand-Rex is a major global supplier of structured cabling systems for data networks and a world leader in cabling solutions for the military, naval, aerospace, communication, automotive and mass transit industries. www.corning.com: Corning Cable Systems, wholly owned by Corning Incorporated (NYSE: GLW), provides system solutions and services for the various communication networks of all applications ranging from trade and industry through to universities and research institutes. The product range comprises cabling systems with fiber optic or shielded and unshielded copper cables with the appropriate hardware for campus, building and home cabling. www.fibreoptic.org.uk: Formed in February 1991, the aims and objectives of the Fibre Optic Industry Association Ltd (FIA) are to promote high standards of service within the fiber optic industry and to represent its members at a national and international level. FIA is committed to assisting the creation of standards, practices and guidelines for manufacturers, installers and users of fibre optics. www.fotec.com: Fotec has twenty years of experience in fiber optics, building test equipment and training. It provides technical support, online courses, and links to an online store where you can purchase training programs and cabling products. This website has lots of useful information whether you are just getting started or are trying to locate some references.

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www.herne.org.uk: Herne European Consultancy Ltd specialize in the development of Quality Management Systems that meet the recommendations and the specifications contained in the ISO 9000 (quality system and quality assurance) standards and the ISO 14000 (environmental management system) standards. They also specialize in designing and implementing internal and external audit procedures and are available to complete impartial, third party audits to suit the customer concerned. www.iee.org.uk: The IEE is involved in all aspects of photonics, including optical communications, optoelectronics devices and optical fiber sensors, thermal imaging and lasers. Has Optical Technology and Applications special interest group. www.ili.co.uk: Founded in 1949, ILI are specialists in worldwide hardcopy Standards and Specifications (DIN, ISO, BSI, ASTM, NFPA, API, IEC, ASME, etc.) and hold huge stocks for sale from distribution centers in Europe and the US. ILI also publish a range of engineering, technical and regulatory databases. www.iop.org/www.olemag.com: Runs a number of special interest groups including the Quantum Electronics Group and the Optical Group. Also publishes monthly magazine. www.kdoptics.com: KD Optics designs and manufactures optical test equipment for use in the fibre optic and datacomms and telecomms industry. In addition to a range of hand held equipment, there is a complete range of bench style fibre test equipment for use in manufacturing and test environments. www.lasirvis.com: LasIRvis are specialist distributors of fiber optic components, driver boards, collimators, modules and special assemblies such as Thermo Electric Coolers (peltier effect). OptoGuard HSR (for high precision applications) and encoder products. LaslRvis also produce fiber to component assemblies using PMMA, (plastic/acrylic multi mode fiber) for fiber pigtailed lasers, detectors and special emitters. www.mma.org.uk: Run by the Institution of Mechanical Engineers with the support of the DTI. MMA encourages the commercial exploitation of microsystems technology and nanotechnology. www.nettest.com: NetTest (formerly GN Nettest) is a leading worldwide provider of testing, monitoring and management systems across both the optical and network layers of communications networks. NetTest provides network operators, network equipment manufacturers, component manufacturers and enterprise service providers with the network testing solutions they need. www.noyes-fiber.com: Noyes is an industry leader in the design and manufacturing of fiber optic test equipment for measuring, maintaining and documenting the performance of fiber optic networks. Product line consists of DWDM, optical power meters, light sources, loss/

Useful links

303

return test sets, fiber certification kits, mini-OTDRs, fiber inspection scopes, fiber optic talk sets, visible fault and fibre optic identifiers. www.nmi.org.uk" Primarily aimed at the semiconductor industry it provides a mechanism for collaboration between its members, educational organizations, regional bodies and government. Has established a UK-wide education and training strategy for the industry. www.opticspages.com: Optics P a g e s - 'the quickest Optics Industry Directory Online'. It has detailed and searchable lists of companies, products and relevant publications. www.optoelectronics.org.uk: Organizes events and meetings. www.osda.org.uk: OSDA is the successor to the DTI-funded LINK Photonics programme to support collaborative research between industry and academia on the application of optical techniques and devices. The five year, s million OSDA programme runs until 2005 and will focus on optical communications, imaging and sensors, high efficiency lasers, and optics in computers and displays. The site has links to descriptions of projects supported under the LINK Photonics programme. www.photonics.ca/cpc: This site contains detailed on-line publications dealing with the Canadian photonics industry, along with an events diary. www.photonics.crc.org.au: Run by the Australian Photonics Cooperative Research Centre, a joint venture between five Australian universities and industry and business, this site concentrates on photonics for the telecommunications industry. It describes current R&D projects and has an excellent education section with on-line mini tutorials in communications and basic photonics. www.pro.on.c a: This site contains up-to-date news items of general interest as well as those relating specifically to the Canadian industry. There are also on-line newsletters and annual reports. www.qaphotos.com: Jim Byrne and Diana Craigie founded QA as a photographic company specialising in tunneling and large construction projects. Jim goes world-wide taking photos for clients and Diana now runs the photographic library. www.royalsignals.army.org.uk: The Royal Corps of Signals Museum (located in Blandford Camp, Dorset) is the national museum of Army communications and contains exhibits and displays showing the part that communications have played in the many wars and campaigns during the last 150 years. The Museum collection is regarded as being of national importance and the excellent Archives are recognized by the Public Records Office. www.scottyweb.co.u k: David Scott is a freelance photographer based in Manchester, UK. The site, run by a small team with sponsorship from

304

Optoelectronicsand Fiber Optic Technology

small UK businesses, provides a platform for promoting diverse and dynamic photographic content from around the world. www.sepnet.org.uk: SEPNET is a group of businesses, research organizations, universities and support bodies in Southeast England aimed at developing the commercial application of photonics, optoelectronics, laser machining and microsystems. www.sid.org.uk: SID is a global non-profit voluntary technical society dealing with display technologies and applications. It has an active UK chapter and provides links to many other related sites. www.spie.org" US-based organization, SPIE has technical groups and discussion forums on all aspects of optics, photonics, optical fibres and lasers. It runs meetings, conferences and technical programmes. It also has an on-line bookshop selling books, videos and CDs, and contains an excellent education section. SPIE also publishes OE magazine, a monthly news bulletin on optics and optoelectronics. [email protected]" StingRay Technical are available for commissions to research and write reference books (on most subjects); technical manuals, user handbooks, etc. All technical literature is aimed at being well structured and simple to read. www.terahertztechnologies.com 9A US-based site that provides links to sources of information on photonics and optoelectronics around the world. Information sources are classified into magazines, societies and organizations, universities, research centres and national laboratories www.tycoelectronics.com: Tyco Electronics offers a broad range of high quality electronic component products. Products from well-known brand names include connectors/interconnection systems, terminal blocks, relays, electronic modules, circuit protection devices, fiber optic components, wire and cable, switches, wireless components, sensors, printed circuit boards, touchscreens and application tooling. www.ukleo.org: UKLEO is the trade association for suppliers of optoelectronics and photonics equipment and services. It provides a single voice for the industry and lobbies on its behalf. It sponsors trade exhibitions and technical conferences. www.ukcpo.org.uk: UKCPO acts as the umbrella organization for photonics in the UK, linking trade associations, professional organizations, and manufacturers, researchers and users.

Index

Absorbtion, definition, 278 Acceptance angle (numerical aperture), 48-49, 50, 278 Acceptance cone, 47-48, 278 ACOST see UK Advisory Council on Science and Technology Acrylic adhesive, 168 Active area, definition, 278 Admiralty shutter telegraph, 3-5 AEG Telefunken, 89 Aeschylus, 2 Agere, 236 Agilent, 235 Airplane design using fiber optics, 260 A1GaAs see Aluminium gallium arsenide A l m a g e s t (Ptolemy), 37 Aluminium gallium arsenide, 113-14 American Standard Code for Information Exchange (ASCII), definition, 279 American Telephone and Telegraph (AT&T), 277 Amplifier design, 138-41 Amplitude modulation, definition, 278 Anaerobic adhesive, 167-68 Analog, definition, 278 Andromeda computer, 251 Angle of deviation, definition, 278 Angle of incidence, 45, 278 Angular misalignment loss, definition, 278 Angular tilt, definition, 278 Anthropological telegraph, 8 APD see Avalanche photodiode Application Specific Optical Component (ASOC), 236

Aramid yarn, definition, 278 Argon/KTP lasers, 259 Armiento, Craig, 129 Armour, definition, 278 ASCII s e e American Standard Code for Information Exchange ASOC s e e Application Specific Optical Component Asynchronous transfer mode (ATM), 194, 279 AT&T s e e American Telephone and Telegraph Attenuation, definition, 279 Attenuation test equipment, 217 Attenuator, definition, 279 AURIGA (Europe) PLC, 76, 162, 163, 178 Avalanche photodiode (APD), 132, 135, 248, 279 Average power, definition, 279 Axial and angular fiber misalignment losses in couplers, 159 Axial ray, definition, 279

Babinet, Jacques, 10 Back reflection, definition, 279 Backscattering, definition, 279 Bainoski, Dr Michael, 225 Baird, John Logie, 14, 276 Ballistic transport of electrons, 237 Balloon signalling, 10 Bandpass, definition, 279 Bandwidth: definition, 280 measurement equipment, 217, 231-32 of optoelectronic transmission systems, 62-65

306

Index

Bandwidth-limited operation, definition, 280 Baseband, definition, 280 Baud, definition, 280 Beam splitter, definition, 280 Bell, Alexander Graham, 10, 11-13, 276 Bell Laboratories, 18, 128, 249, 253, 277 Bend loss, definition, 280 Bend radius, definition, 280 Bending diameter of cables, 92-93 BER s e e Bit error rate BH s e e Buried heterostructure of laser diodes BICC s e e British Insulated Callender Cables Biconical tapered transmissive mixer, 71,280 Binary code, definition, 280 Bit error rate (BER), definition, 280 Blandford Camp shutter telegraph station, 4, 5 Blown optical fiber, 211-15 advantages, 212, 214 Blolite system, 215 equipment, 214-15 Bookham Technologies, 236 Bragg mirror, 127, 128, 240 Break out cable, definition, 281 British Insulated Callender Cables (BICC) Brand-Rex, 55, 96, 99, 104, 175, 177, 183, 211,213, 215 British Post Office, 17, 277 British Telecom, 139, 211,277 Broadband, definition, 281 Broadcast, definition, 281 Buffer, definition, 281 Buffer coating, definition, 281 Buffer tube in cable design, 93-95, 281 Buffered fiber, definition, 281 Bundle, definition, 281 Buried heterostructure of laser diodes, 124 Bus, definition, 281 Bus network, definition, 281 Byte, definition, 281

Cable: assembly, definition, 281 bend radius, definition, 282 construction, 98-99 definition, 281 ducts, 97 jointing s e e Jointing/splicing of fibers routing s e e Routing of optical fiber cables types, 95-97 Cable Talk Pty Ltd, 20 Campus cabling for fiber optics, 197-99 Cap sealing techniques, 90-91 Car design using fiber optics, 260 Carbon dioxide laser, 259 Carrier sense multiple access (CSMA), definition, 282 Catastrophic optical damage (COD), 114 Central member, definition, 282 Central office, definition, 282 Centre wavelength, definition, 282 Centro-symmetrical reflective optics, definition, 282 Channel, definition, 282 Channelled substrate planer, 124 Chapp6, Claude, 5, 6, 276 Chromatic bandwidth, definition, 282 Chromatic dispersion, definition, 282 Chronology of fiber optics, 276-77 Cladding of an optical fiber, 47-50, 92, 282 Cleavers for fiber optics, 160-62, 283 Coating, definition, 283 COD s e e Catastrophic optical damage Coherent light, 109, 283 Collodon, Daniel, 10 Colour coding of optical fibers, 97 Communication systems, 182-214: s e e a l s o Fiber optics; Future developments; Optoelectronics; Technology of fiber optics data networks, 188-91 local area network (LAN), 189, 194 metropolitan area network (MAN), 189-90

Index

wide area network (WAN), 190-91, 194 design of an optical fiber system, 191-201 alternative types compared, 195, 196 asynchrous transfer mode (ATM), 194 cable routing see Routing of optical fiber cables comparison with copper cable, 192-93 digital or analog, 193-94 distribution, 194 ethernet, 194 fiber losses, 196-97, 201-203 multiplexing, 194, 196 system loss budget (SLB), 201-203 local networks, 184-87 comparison with coaxial system, 186 line terminating unit (LTU), 185-86 optical fibers advantages, 184-85 long distance networks, 187 telephone networks, 187-88 Communications and fiber optics, 261-62 Comparisons of types of fiber, 83, 84 Computing and optical technology, 251-52, 260-61 Concatenation of cables, 152 Concentrator, definition, 283 Cond6 telegraph post, 6 Conduit, definition, 283 Connectors, 1 4 2 - 5 2 : s e e also Couplers; Jointing/splicing; Receivers; Transmitters; attachment to a fiber, 160-62 attenuation, 144-45 definition, 142, 283 for fiber bundles, 149-50 for hybrid cables, 151-52 insertion loss, 145 quality, 143 requirements, 143 for single fibers, 148-49

307

stick and turn (ST), 147, 148 sub multiple assembly (SMA), 145, 148 types available, 145-46 Converting light into electrical energy, 132-34 Copernicus, Nicolaus, 37 Core eccentricity, definition, 283 Core of an optical fiber, 47-50, 283 Corguide dispersion-flattened single-mode fiber, 116 Corning Cable Systems GmbH, 19, 33, 96, 101, 102, 103, 143, 150, 174, 176, 197, 198, 200, 202, 211, 237 Corning Glass Works, 17-18, 60, 82, 116, 179, 243 Couplers, 152-64 concatenation, 152 definition, 283 end fire, 152-53, 163-64 evanescent wave, 164 insertion losses, 145, 158-60 angular and axial misalignment, 159 Fresnel losses, 160 lens, 154 multiport, 154-58 four-port, 156-57 as signal dividers, 158-59 as switches, 157 symmetrical and asymmetrical, 155-56 in transmissive or reflective mode, 158 Coupling efficiency, definition, 283 Coupling losses, definition, 283 Coupling ratio, definition, 283 Coxwell, Henry, 10 Crimping of fibers, 168 Critical angle, 46-47, 283 Crosstalk, definition, 283 CSMA Carrier sense multiple access, definition, 283 CSP see Chanelled substrate planer Cured epoxy for jointing, 166-67 Curvature sensors using fiber optics, 259 Cut-off frequency of a fiber, 63 Cut-off wavelength, definition, 283 Cyanoacrylate adhesive, 167

308

Index

Dark current noise, 69, 283 DARPA see U.S. Defence Advanced Research Projects Agency Data link, definition, 283 Data networks, 188-91 local area network (LAN), 189, 194 metropolitan area network (MAN), 189-90 wide area network (WAN), 190-91, 194 Data rate, definition, 283 Datacom Ltd, 222 Decibel (dB), definition, 283 Decker, C.D., 129 Decorations using fiber optics, 257 Deflecting diameter of cables, 92-93 Demodulation, definition, 283 Demultiflex, definition, 283 Dense wavelength division multiplexing (DWDM), 74, 229, 238, 240, 243, 249 Department of Trade and Industry (DTI), 233 Department of Transport (DOT), 258 Descartes, Ren6, 42 Despencer, Lord le, 3 Detector, definition, 283 Deutsche Bundespost, 246 DFB see Distributed feedback laser Diameter-mismatch loss, definition, 284 Diamond connector, definition, 284 Dichroic filter, definition, 284 Diffraction grating, definition, 284 Diffused gallium arsenide, 112-13 Digital, definition, 284 Digital signal, definition, 284 Diode, definition, 284 Directional coupler, definition, 284 Directivity, definition, 284 Dispersion losses in fibers, 58-60 chromatic dispersion, 59-60 definition, 284 dispersion-shifted fiber, 60 material dispersion, 58 waveguide dispersion, 58-59 Distributed feedback laser (DFB), 240

Distributed system, definition, 284 Distribution frames for fiber optics (patch panels), 201, 210-11 Dopant, definition, 284 Doppler effect in fiber optics, 239 Dorset Police, fiber optic link, 277 DOT see Department of Transport Drawing, definition, 284 DTI see Department of Trade and Industry Duplex cable, definition, 284 Duplex transmission, definition, 284 DWDM see Dense wavelength division multiplexing

EDFA see Erbium doped fiber amplifier EDFFA see Erbium doped fluoride fiber amplifier Edge emitting diode (ELED), 111-12, 119, 284 EDTFA see Erbium doped tellurite fiber amplifier EFTA see European Free Trade Association Elastomeric splicer, 173 Electromagnetic spectrum, 19-20, 284 E l e c t r o n i c s , 15 ELED see Edge emitting diode E l e m e n t s (Euclid), 36 Emitter, definition, 284 EN Standards, 263-67 Encapsulated fibers, 244-45 Encapsulating lasers, 241-43 Enclosure design for jointing fibers, 179-80 Encoding, definition, 284 End fire couplers, 152-53 End fire mixers, 70-71 End separation, definition, 285 End-to-end loss, definition, 285 Endoscope, definition, 285 Epitaxial layer, 122 Epoxy splice procedure, 166 Equilibrium mode distribution, definition, 285

Index

Erbium doped: fiber amplifier (EDFA), 139-40, 236, 238, 240, 243, 285 fluoride fiber amplifier (EDFFA), 140 tellurite fiber amplifier (EDTFA), 140-41 Er:YAG laser, 259 Ethernet, 189, 194, 285 Euclid, 36 European Community, 233-34 European Free Trade Association (EFTA), 234 European optical fiber market, 235 Evanescent wave coupling, 164, 285 Expanded beam connector, definition, 285 External ducted cable, 99 External vapour deposition in manufacturing, 86-87 Extrinsic losses, definition, 285

Fabrey-Perot cavity, 128, 246 Fast ethernet, 194 FDM s e e Frequency division multiplex signals Ferrul, definition, 285 FET s e e Field effect transistor amplifier Fiber distributed data interface (FDDI), definition, 285 Fiber Industry Association (FIA), 208 Fiber optics: s e e a l s o Future developments; Optoelectronics; Receivers; Technology of fiber optics advantages, 23-28 analog and digital transmission, 28 electrical insulation, 27 flexibility, 26-27 immunity to electrical fields, 24, 27, 28 low attenuation, 25 receiver sensitivity, 28 small size and weight, 25-26 wide transmission bandwidth, 25

309

amplifier, definition, 285 background and history, 1-19 bandwidth, definition, 285 basic design principles, 15, 19-22 bundle, definition, 285 cable, 22 capacity, 19 definition, 1,285, 286 disadvantages, 29-31 cost, 29 energy losses, 30-31 jointing and testing, 29-30 drawing during manufacture, 88-89 future applications, 233-62 gyroscope, 255-56 inter-repeater link, 285 loss, definition, 285 optical waveguide, 16-17, 18 receiver, 22 regenerator, 22 total internal reflection (TIR), 13-15 tracers, 221-23 transmitter, 19 waveguide, definition, 286 Fiberless transmission of signals, 248-49 FiberMeter (Datacom), 224-25 FiberProbe (Datacom), 222 Field effect transistor amplifier (FET), 248 Field testers for fiber optics, 224-25 FLOD s e e Full length outside deposition Fluoride glasses for optical fibers, 237-38 Fotec, 166, 168, 221,222 DM3000 power meter, 222 Fiber U OTDR, 229-30 $660 Fotracer, 222 $665 visual fault locator, 221 France, optical fiber market, 234 Frequency division multiplex signals (FDM), 67, 187, 286 Frequency modulation of optoelectronic transmission, 67, 286 Fresnel reflections, 160, 227, 228, 286

310

Index

Fujikura high precision fiber cleaver, 160-62, 176 Fujitsu, 277 Full length outside deposition (FLOD) in manufacture, 89-90 Fundamental mode, definition, 286 Fused coupler, definition, 286 Fusion splicing, 174-79, 286 graded index fibers, 177 multi-fibers, 178-79 procedure, 174-77 single-mode fibers, 178 Future developments, 233-62 ballistic transport, 237 car and airplane design, 260 commercial applications, 233 in communications, 261-62 curvature sensors, 259 distributed feedback laser (DFB), 240 encapsulated fibers, 244-45 fiber optic gyroscope, 255-56 fiberless transmission, 248-49 fluoride glasses, 237-38 global market, 233-35 increased bandwidth, 246-48 internet, 260 laser dazzler, 253-54 leak detection, 250 medical uses, 258-59 military applications, 253-56 mining, 261 modular lasers, 241-43 moulded-clad single-mode fibers, 243 moveable fibers, 246 multi-core fibers, 243-44 office lighting, 256-57 optical, bus, 246 computing, 251-52, 260-61 ether, 249-50 multiplexers, 252 processing, 260-61 radio, 250 sensors, 256 opto-isolators, 252 optoelectronic switches, 252-53

plastic opto-chips, 254-55 polarization of light, 238 porous silicon, 237 rare earth doped optical fibers, 240-41 solitron pulses, 238-39 synchronous optical network (SONET), 238 temperature monitor using optical fiber, 251 tunable lasers, 240 UK market, 235-36

GaA1As s e e Gallium aluminium arsenide GaAs s e e Gallium arsenide Gain-guided laser diodes (GLD), 123, 124-26 Gallium aluminium arsenide, 126-27, 130, 247 Gallium arsenide, 19, 107-9, 112-15, 127 Genghis Khan, 2, 3 Germanium, 61,134, 136-37, 248, 277 Germany, optical fiber market, 234 Glasgow University, 252 Glass preform in manufacture, 85 GLD s e e Gain-guided laser diodes Graded index fibers, 52, 81-82, 177, 286 Graded index profile, definition, 286 GTE Laboratories, 129

Hansell, Clarence, 14, 276 Hansen, Holger, 14 Hard-clad silica, definition, 286 Hazardous environments and ducted cables, 103-4 Heel, Abraham van, 14 Heliograph, 8-10 Hermaphrodite connector, definition, 286 Hockham, George, 16, 276 Hopkins, Harold Horace, 276 Horizontal cabling for fiber optics, 199-201

Index

Ho:YAG laser, 259 Hybrid cable, definition, 286 Hydrogen losses, definition, 286

IEC Standards, 268-73 ILD see Index-guided laser diodes ILI see Infonorme London Information Imperial Chemical Industries (ICI), 211 Imperial College of Science and Technology, 276 Incident angle, definition, 287 Index-guided laser diodes (ILD), 124-26 Index matching material, definition, 287 Indium phosphide, 114-16, 128 IndiumGalliumArsenidePhosphide, 114-15, 128, 134, 137, 236 lnfonorme London Information (ILl), 263 Infrared, definition, 287 lnGaAsP see lndiumGalliumArsenidePhosphide Injection laser, definition, 287 Injection luminance, 62 InP see Indium phosphide Insertion loss of connectors, 145, 158-60, 287 Installation of fiber optic cables, 104-5, 204-15 cable selection, 204 ducting, 207 laying, 206-7 minimum bend radius, 209 on power line pylons, 208 storage, 205 vertical risers, 209-10 Institut f6r Festkorperelectronik, Wein, 127 Institution of Electrical Engineers, 16, 276 Institution of Electrical Engineers, Proceedings, 276

Integrated optoelectronics, definition, 287

311

Integrated Services Digital Networks (ISDN), 233 Intensity (power) modulation of optical transmitters, 67-68 Internal vapour deposition in manufacturing, 85-86 International Standards Organisation (ISO) Standards, 273-75, 287 Internet use of fiber optics, 260 Intrinsic losses, definition, 287 ISDN see Integrated Services Digital Networks ISO see International Standards Organisation Isolator, definition, 287 ITT Corning, 233

Jacket, definition, 287 Jane's Defence Weekly, 253

JDS-Uniphase, 235 Jointing/splicing of fibers: acrylic adhesive, 168 advantages and disadvantages of methods, 170 alignment errors during splicing, 171 anaerobic adhesive, 167-68 crimping, 168 cured epoxy, 166-67 cyanoacrylate adhesive, 167 effect of dirt, 165 elastomeric splicer, 173 enclosure design, 179-80 epoxy splice procedure, 166 fusion splicing, 174-79 graded index fibers, 177 multi-fibers, 178-79 procedure, 174-77 single-mode fibers, 178 no epoxy no crimping, 169 pre-injected epoxy, 166 splice losses, 169, 171 UV adhesive, 167, 173-74 V-groove splicer, 172 Jumper cable, definition, 287 Junction laser, definition, 287

312

Index

Kao, Charles K., 16-17, 276 Kapany, Narinda S., 276 Karbowiak, Antoni E., 276 KD Optics, 220 KDX1000 loss meter, 220 LS20DV LED source, 225 PM23 optical power meter, 225 Keck, Donald, 17, 277 Kessler Marketing Intelligence, 233 Kevlar, 95, 98, 287 Keyance Corporation Japan, 244, 245 Korea, Republic of, 253

Lamberton source, 57 Lamm, Heinrich, 14, 276 LAN see Local area network Large core fiber, definition, 287 Larner, David, 251 Laser dazzler, 253-54 Laser diodes: see also Fiber optics; Light emitting diodes; Optoelectronics; Receivers; Technology of fiber optics; Transmitters in 1300 to 1600 nm region, 128 advantages, 119-20 with built-in waveguide, 124 buried heterostructure (BH), 124 and catastrophic optical damage, 114 channelled substrate planer (CSP), 124 comparison with LED, 108, 117, 118 with current-induced waveguide, 123 definition, 287 design and characteristics, 116-19, 122-24 disadvantages, 120-22 distributed feedback laser, 240 double heterostrucure characteristics, 121, 139 gain and index guided diodes compared, 124-26 gain-guided (GLD), 123, 124-26 index-guided (ILD), 124-26 invention of, 15-16 as light source, 19, 62, 106

medical uses, 258-59 metal clad ridge waveguide (MCRW), 124 microdisks, 128-29 modular lasers, 241-43 modulation, 65-68, 107 multiple signalling, 129-30 noise, 69 optical feedback sensitivity, 126 oxide stripe, 126-27 spatial radiation distribution, 125-26 spectral widths, 124-25 stripe geometry, 126 temperature, 125 threshold currents, 124 vertical cavity surface emitting (VCSEL), 127-28 Laser Focus, 1 6 - 1 7

Laser light sources, 220-21 LaslRvis, 110, 118, 132 Lateral displacement loss, definition, 287 Leak detection using optical fibers, 250 LED see Light emitting diodes Lens couplers, 154 Light, definition, 287 Light emitting diodes: see also Fiber optics; Laser diodes; Optoelectronics; Receivers; Technology of fiber optics; Transmitters advantages in optical fiber systems, 109-10 aluminium gallium arsenide, 113-14 comparison with laser, 108 connecting to an optical fiber, 163 definition, 288 design, 107-10 diffused gallium arsenide, 112-13 edge emitting diode (ELED), 111-12, 119 high radiance, 112 indium phosphide, 114-16 and injection luminance, 62 as light source, 19, 57, 59, 106 modulation, 65-68, 107 preparation, 110-11

Index

Light source, definition, 288 Light waves, definition, 288 Lightguide, definition, 288 LIGHTRAY OFX optical fiber circuit, 244-45 Line terminating unit (LTU), 185-86 Linear modulation of light, 66-67 Link, definition, 288 Lithium niobate, 254-55 Liverpool University, 256 Local area network (LAN), 52, 189, 194, 233, 288 Local government applications for optoelectronics, 256-58 Local loop, definition, 288 Local networks, 184-87 Loffe Physical Institute, St Petersberg, 18 Long distance networks, 187 Long-haul network, definition, 288 Long wavelength, definition, 288 Loose cable type, 95, 99 Loose tube, definition, 288 Loss, definition, 288 Loss measurements for fiber optic systems, 220-21 Losses in optical fiber systems, 196-97, 201-203 Losses in transmission, 53-57 absorption, 53-54 attenuation, 53 bending losses, 55-57 scattering, 54-55 Lucent Technologies, 236, 249

Macdonald, Colonel John, 7, 8 Macrobend, definition, 288 Macrobending loss in fibers, 56-57 Magnetic resonance imaging (MRI), 258 MAN s e e Metropolitan area network Manufacturing techniques, 82, 85-91 cap sealing, 90-91 external vapour deposition, 86-87 fiber drawing, 88-89 full length outside deposition (FLOD), 89-90

313

glass preform, 85 internal vapour deposition, 85-86 vapour axial deposition, 87 Marconi, 236 Margin, definition, 288 Material dispersion, definition, 288 Material scattering, definition, 288 Maurer, Robert, 17, 277 MBE s e e Molecular beam epitaxy Mechanical splice, definition, 288 MEMS s e e Micro electromechanical systems Metal organic chemical vapour deposition (MOCVD), 127 Metallic armour for ducted cable, 102-3 Metropolitan area network (MAN), 189-90 Micro electromechanical systems (MEMS), 253 Microbending loss in fibers, 56, 288 Mie scattering in fibers, 55 Military applications of fiber optics, 253-56 Mining, uses for fiber optics, 261 Minitel, 184 Misalignment loss, definition, 288 Mixers used with fiber optics, 70-71 MOCVD s e e Metal organic chemical vapour deposition Modal bandwidth, definition, 288 Modal dispersion, definition, 289 Modal noise in multimode fibers, 69-70, 289 Mode: coupling, definition, 289 definition, 289 field diameter, definition, 289 partition noise, 69 stripper, definition, 289 Modes of propagation, 51-53 multimode, graded index, 52 step index, 51 single mode step index, 52-53 Modular lasers, 241-43 Modulation of optical systems, 65-68, 289

314

Index

Molecular beam epitaxi thin film technology, 236 Molecular beam epitaxy (MBE), 127 Muffling, General Freiherr von, 7 Muldex couplers, 74, 186 Multi-core fibers, 243-44 Multimode fiber, 79-82, 289 graded index, 81-82 Multimode fiber design, 79-82 Multiplayer dialectric filter, 73 Multiplexing types, 71-75, 289 dense wavelength division (DWDM), 74, 229 electrical, 72 fiber, 72 time division (TDM), 74-75, 186, 194 wavelength division (WDM), 72-74, 139, 196 Multiport couplers, 154-58 Murray, Reverend Lord George, 3

NA see Numerical aperture Nanometre, definition, 289 Napoleon I, Emperor of France, 7, 8 National Institute of Justice (NIJ), 253-54 Nature, 14-15 Nd:YAG laser, 259 Nettest/PK Technology: Model 8000 OTDR, 228 Model CMA4000 OTDR, 229 Network, definition, 289 N e w Electronics, 251

NIJ see National Institute of Justice Nippon Telegraph and Telephone (NTT), 237, 277 Norris, John, 2 Nortel, 235 Noynes: visual fault finder, 222 WSP 100 Multi-wavelength Selective Power Meter, 231-32 NTT see Nippon Telegraph and Telephone Numerical aperture (acceptance angle), 48-49, 50, 289

Office lighting using fiber optics, 256-57 OFL see Optical fault locator On-off modulation of light, 65 Optic connection panel, 55 OpticAir fiberless transmission, 249 Optical: amplifier, definition, 289 bus, 246 cable, definition, 289 computing, 251-52 ether, 249-50 fault locator (OFL), 230-31 fiber, bandwidth, 63-65 colour coding, 97 coupler, definition, 289 definition, 289 frequency response, 64 structure, 49-50 waveguides, 16-17, 18, 289 link, definition, 289 mechanisms in semiconductors, 61-62 multiplexers, 252 power density, 57-58 radio, 250 receiver thermal noise, 69 sensors, 256 signal-to-noise ratio (OSNR), 68-70, 232 switch, definition, 289 telephone, 10, 11 window, definition, 289-90 Optical Fibres Ltd, 211 Optical time domain reflectometer (OTDR), 27, 217, 225-30, 289 advantages and disadvantages, 224, 230-31 compared to optical fault locator, 231 dead zone, 227-28 dynamic range, 228 equipment, 228-30 ghost reflections, 228 test outputs, 226-27 uses, 225-26 Opto-isolators, 252 Optoelectrical converter, defined, 290

Index

Optoelectronic switches, 252-53 Optoelectronics:see a l s o Couplers; Fiber optics; Laser diodes; Light emitting diodes; Receivers; Technology of fiber optics; Transmitters; all-optical network, 33 basic theory, 36-75 acceptance angle, 48-49 acceptance cone, 47-48 angle of incidence, 45 bandwidth, 62-65 critical angle, 46-47 dispersion losses see Dispersion losses in fibers fiber structure, 49-50 losses s e e Losses in transmission mixers, 70-71 modes s e e Modes of propagation modulation, 65-68 multiplexing s e e Multiplexing types numerical aperture, 49 optical power density, 57-58 optical signal-to-noise ratio (OSNR), 68-70, 232 reflection and refraction, 36-39, 42-45 refractive index, 39-40 semiconductors, 61-62 Snell' s Law, 41-43 total internal reflection (TIR), 13-15, 40-45, 50 in communication networks, 33 definition, 289 early signalling devices, 2, 7 in the future, 33-35 local government applications, 256-58 practical applications, 32-33 OSNR see Optical signal-to-noise ratio OTDR see Optical time domain reflectometer

Passive coupler, definition, 290 Patch panels (distribution frames), 201,210-11,290

315

Patchcord, definition, 290 PCM s e e Pulse code modulation PDFFA s e e Praseodymium-doped fluoride fiber amplifier Peak wavelength, definition, 290 Peltier coooler, 242, 247 Persenick, Dr Stuart, 225 PET see Position emission tomography Phase modulation, definition, 290 Photocurrent, definition, 290 Photodetector, 22, 23, 290 Photodiode, 62 definition, 290 design, 132-34 Photon, definition, 290 Photonics, definition, 290 Photophone, 11-13, 276 Pigtail, definition, 290 PIN photodiode, 132, 134-35, 139, 240, 248, 290 Planar waveguide, definition, 290 Plastic-clad silica fiber (PCS), 290 Plastic optical fiber (POF), 91-92, 290 Plastic opto-chips, 254-55 Plenum, defined, 291 PMMA s e e Polymethylmethacrylate POF s e e Plastic optical fiber Point-to-point, defined, 291 Polarization of light in fiber optics, 238, 291 Polarization stability, defined, 291 Polishing, defined, 291 Polybius, 2 Polymethylmethacrylate (PMMA), 91 Polytetrafluoroethylene (PTFE) (Teflon), 88 Porous silicon, 237 Position emission tomography (PET), 258 Power and attenuation measurement, 221,223-24 Power budget, defined, 291 Power meter, defined, 291 Praseodymium-doped fluoride fiber amplifier (PDFFA), 141 Pre-injected epoxy for jointing fibers, 166

316

Index

Preform, defined, 291 Prefusing, defined, 291 Primary coating, defined, 291 PTFE s e e polytetrafluoroethylene Ptolemy, Claudius, 36, 37 Pulse code modulation (PCM), 187, 291 Pulse dispersion, defined, 291 Pulse spreading, defined, 291

QA Photos Ltd, 24 Quantum efficiency, 112, 114, 291 Quantum noise of light, 69 Quaternary, defined, 291

Radiance, defined, 291 Radiation pattern, defined, 291 Rare earth doped optical fibers, 90, 240-41 Rayleigh backscattering, 30-31, 54-55, 225, 226, 227, 291 Receivers, 132-40, 291: see also Couplers; Fiber optics; Laser diodes; Light emitting diodes; Technology of fiber optics; Transmitters additional stages, 138 amplifier design, 138-41 analog fibre optic, 137 converting light into electrical energy, 132-34 detector composition and wavelength, 134 digital fibre optic, 137-38 erbium doped, fiber amplifier (EDFA), 139-40, 236 fluoride fiber amplifier (EDFFA), 140 tellurite fiber amplifier (EDTFA), 140-41 laser amplifier repeater, 139 photodiode design, 132-34 avalanche (APD), 132, 135 germanium, 136-37 indium gallium arsenide, 137

PIN, 132, 134-35, 139 silicon, 135-37 praseodymium-doped fluoride fiber amplifier (PDFFA), 141 semiconductor optical amplifier, 141 Reeves, Alec, 276 Reflectance, definition, 292 Reflection and refraction, 36-39, 42-45 Refraction of light, 37-38, 292 Refractive index (RI), 39-40, 51-52, 292 Repeator and regenerator spacing, 180-82, 292 RI see Refractive Index Ribbon cable, definition, 292 Ring network, definition, 292 Rise time, definition, 292 Riser, definition, 292 Road signs using fiber optics, 257-58 Rodent protection of ducted cable, 101-102 Roth and Reuss, Vienna, 13 Routing of optical fiber cables, 197-201 campus cabling, 197-99 distribution frames, 201 fiber to the customer, 198 horizontal cabling, 199-201 centralised, 200-201 conventional, 199-200 installation see Installation of fiber optic cables riser area cabling, 199 Royal Corps of Signals, 3, 4, 5, 8, 9

Saint-Ren6, Henry, 13 Sanyo Semiconductor Corporation, 129-30 Scattering, definition, 292 Schultz, Peter, 17, 277 SDI Research, 251 Semiconductor optical amplifier, 141, 292 Semiconductors in optoelectronics, 61-62 Sensitivity, definition, 292

Index 317

Serial transmission, definition, 292 Sheath, definition, 292 Short wavelength, definition, 292 Shot noise, definition, 292 Sifam Fiber Optics, 235 Signal-to-noise ratio of transmission systems, 68-70, 292 Silica glass, definition, 292 Silicon, 61, 134, 135-36 Simplex, definition, 292 Single-mode fiber, 77-79, 293 SLB s e e System loss budget Slotted core cables, 95-97 SMA s e e Sub multiple assembly connectors Smith, David, 13 Snell, Willebrord van Roijen, 41-42 Snell's Law, 41-43 Solitron pulses in fiber optics, 238-39, 293 SONET see Synchronous optical network Source, definition, 293 Southampton Photonics, 236, 240, 243 Southampton University Optoelectronics Research Centre, 236 Spectral attenuation, definition, 293 Spectral bandwidth, definition, 293 Speed of light, definition, 293 Splice losses in jointing fibers, 169, 171,293 Splicing see Jointing/splicing Standard Telecommunications Laboratories (STL), 16-17, 276 Standard Telephone & Cable (STC), 149 Standards: EN, 263-67 IEC, 268-73 ISO, 273-75 Star network, definition, 293 Station, definition, 293 STC s e e Standard Telephone & Cable Step index, 51, 52-53, 79-81,293 Steradian, 57 Stick and turn connectors (ST), 147, 148

STL

s e e Standard Telecommunications Laboratories Strain member, definition, 293 Strength member, definition, 293 Stripping, definition, 293 Sub multiple assembly connectors (SMA), 145, 148 Subscriber loop, definition, 294 Super lattice thin film technology, 236 Surface emitting diode, definition, 294 Synchronous optical network (SONET), 238 Synchronous transmission, definition, 294 System loss budget (SLB), 201-203, 288

T-type telegraph, 5, 6 Tainter, Sununer, 10 Tap loss, definition, 294 TDM s e e Time division multiplexing Technical University of Delft, 14 Technology of fiber optics, 76-105: s e e a l s o Couplers; Jointing/splicing of fibers; Laser diodes; Light emitting diodes; Receivers; Transmitters; basic design, 76-77 cable characteristics, 92-95 bending diameter, 92-93 buffer tube, 93-95 deflecting diameter, 92-93 tensile strength, 93 cable construction, 98-99 cable ducts, 97 cable types, 95-97 loose, 95, 99 slotted core, 95-97 tight buffered, 95 claddings, advantages and disadvantages, 92 comparisons of types of fiber, 83, 84 ducted cable, 99-104 external, 99 for hazardous environments, 103-4 installation, 104-5

318

Index

Technology of fiber optics ( c o n t . ) ducted cable ( c o n t . ) metallic armour, 102-3 rodent protection, 101-102 universal, 100 universal ruggedised, 100 water protection, 101 jointing see Jointing/splicing Tee coupler, definition, 294 Teflon see polytetrafluoroethylene (PTFE) Telecommunication systems, defined, 294 Telephone networks, 187-88 Temperature monitor using optical fiber, 251 Tensile strength of cables, 93 Ternary, defined, 294 Testing optoelectronics, 215-32 equipment, 221-32 bandwidth measurement, 231-32 defined, 294 fiber optic tracers, 221-23 field testers, 224-25 laser light sources, 220-21 loss measurements, 220-21 optical fault locator (OFL), 230-31 OTDR see Optical time domain reflectometer power and attenuation measurement, 223-24 power meters, 221 installation and field tests, 218-19 during manufacture, 215-18 attenuation, 217 bandwidth, 217 physical characteristics, 215-16 Thermal noise, defined, 294 Thermal stability, defined, 294 Thermistor, 242 Threshold current, defined, 294 Throughput loss, defined, 294 Tight buffered cables, 95, 294 Time division multiplexing (TDM), 74-75, 186, 194, 294 Time Warner Telecom, 260 TIR see Total internal reflection

Token, defined, 294 Total internal reflection (TIR), 13-15, 40-45, 50, 294 Transceiver, defined, 295 Transducer, defined, 295 Transmission loss, defined, 295 Transmission media, defined, 295 Transmissive mixer, 71 Transmitters, 106-30, 295: see also Fiber optics; Laser diodes; Light emitting diodes; Receivers laser diodes, in 1300 to 1600 nm region, 128 advantages, 119-20 with built-in waveguide, 124 buried heterostructure (BH), 124 and catastrophic optical damage, 114 channelled substrate planer (CSP), 124 comparison with LED, 108, 117, 118 with current-induced waveguide, 123 design and characteristics, 116-19, 122-24 disadvantages, 120-22 double heterostrucure characteristics, 121, 139 gain and index guided diodes compared, 124-26 gain-guided (GLD), 123, 124-26 index-guided (ILD), 124-26 invention of, 15-16 as light source, 19, 62, 106 metal clad ridge waveguide (MCRW), 124 microdisks, 128-29 modulation, 65-68, 107 multiple signalling, 129-30 noise, 69 optical feedback sensitivity, 126 oxide stripe, 126-27 spatial radiation distribution, 125-26 spectral widths, 124-25 stripe geometry, 126 temperature, 125

Index

threshold currents, 124 vertical cavity surface emitting (VCSEL), 127-28 light emitting diode (LED), advantages in optical fiber systems, 109-10 aluminium gallium arsenide, 113-14 comparison with laser, 108 design, 107-10 diffused gallium arsenide, 112-13 edge emitting diode (ELED), 111-12, 119 high radiance, 112 indium phosphide, 114-16 and injection luminance, 62 as light source, 19, 57, 59, 106 modulation, 65-68, 107 preparation, 110-11 Transparency, defined, 295 Tree coupler, defined, 295 Tunable lasers in fiber optics, 240 Turbotest 400 (Noynes), 223 Tyco Electronics, 244 Tyndall, John, 10, 11,276

UK Advisory Council on Science and Technology (ACOST), 233 UK market in fiber optics, 235-36 Ultraviolet, defined, 295 Uniformity, defined, 295 United Kingdom, optical fiber market, 235 Universal (ruggedised) ducted cable, 100 University of New South Wales, 276 University of Southern California, 254 University of Washington, 254 U.S. Army Research Office, 237

319

U.S. Defence Advanced Research Projects Agency (DARPA), 253-54 UV adhesive, 167, 173-74 V-groove splicer, 172 Vanzetti Systems, 251 Vapour axial deposition in manufacturing, 87 Vertical cavity surface emitting laser diodes(VCSEL), 127-28 Videophone, defined, 295 Virtual reality, defined, 295 Virtually lossless network (VLON), 246 Visible light, defined, 295 WAN s e e Wide area network Water protection of ducted cable, 101 Waveguide fiber, 16-17, 18, 295 Wavelength, defined, 295 Wavelength division multiplexing (WDM), 72-74, 139, 196, 235, 246, 254, 295 Wheeler, William, 13, 276 Whiteman Controls Corporation, 250 Wide area network (WAN), 190-91, 194, 295 Worcester, Marquis of, 2, 3 Working margin, defined, 295 Xerox Corporation, 189 Ytterbium, 140 Zheleg, Kathy, 249

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