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OPTICAL FIBER SENSOR TECHNOLOGY
Optical and Quantum Electronics Series
Series editors
Professor G. Parry; University of Oxford, UK Professor R. Baets) University of GentJBelgium
This series focuses on the technology, physics and applications of optoelectronic systems and devices. Volumes are aimed at both research and development staff and engineers involved in the application of optical technologies. Graduate textbooks are included, giving tutorial introductions to the many exciting areas of optoelectronics. Both conventional books and electronic products will be published, to provide information in the most appropriate and useful form for users.
1 Optical Fiber Sensor Technology Edited by K.T.V. Grattan and B.T. Meggitt
2 Vision Assistant Software A practical introduction to image processing and pattern classifiers c.R. Allen and N.C. Yung
Optical Fiber Sensor Technology
Edited by
K. T. V. Grattan Head, Department of Electrical,
Electronic and Information Engineering City University
London, UK
and
B. T. Meggitt ERA Technology Limited
Leatherhead, UK
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V
First edition 1995
© 1995 Springer Science+Business Media Dordrecht Originally published by Chapman & Hall in 1995 Softcover reprint of the hardcover 1st edition 1995
ISBN 978-94-010-4530-8 ISBN 978-94-011-1210-9 (eBook) DOl 10.1007/978-94-011-1210-9 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page.
The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made.
A catalogue record for this book is available from the British Library
8Printed on acid-free text paper, manufactured in accordance with ANSI/NISO Z39.48-1992 (Permanence of Paper).
Contents
List of contributors xv
Preface xvii
1 Overview of fiber sensor developments 1 D. A. Jackson 1.1 Introduction 1 1.2 Current state of the art 5
1.2.1 External sensors 5 1.2.2 Intrinsic sensors 5 1.2.3 Extrinsic sensors 7
1.3 Future developments 8 1.4 Summary 9 References 9
2. Foundations of optical fiber technology 11 V. Handerek 2.1 Introduction 11 2.2 Optical guidance 11
2.2.1 Modes in dielectric waveguides 14 2.2.2 Propagation in optical fibers 16 2.2.3 Single mode fibers 18
2.3 Fiber dispersion 20 2.3.1 Intermode dispersion 21 2.3.2 Dispersion in single mode fibers 23
2.4 Commercially available optical fibers 26 2.5 Fiber fabrication and strength 26 2.6 Fiber attenuation 28
2.6.1 Intrinsic attenuation factors 28 2.6.2 Extrinsic attenuation factors 30
2.7 Modal noise 32 2.8 Power handling 32 2.9 Fiber handling 33
2.9.1 Endface preparation 33 2.9.2 Interfacing sources and detectors 33 2.9.3 Fiber joints 35
vi CONTENTS
2.10 Polarization behavior in optical fibers 37 2.10.1 Polarization in nominally isotropic single
mode fibers 37 2.10.2 Polarization-controlled fibers 40
2.11 Fiber components 42 References 43 Further Reading 44
3. Sources for optical fiber sensors 45 K. T. V. Grattan 3.1 Introduction 45 3.2 Basics of radiation sources 46 3.3 Incoherent sources 48
3.3.1 Thermal sources 48 3.3.2 Discharge lamps 51 3.3.3 Solid state incoherent sources - the light-
emitting diode 53 3.4 Coherent sources 54
3.4.1 Laser operation 55 3.4.2 Laser modes and laser spectra 58 3.4.3 Laser sources applied to optical sensors 59
3.5 Choice of photon detectors 70 3.6 Summary of laser sources 70 3.7 Conclusions 72
References 72
4 Optical detectors and receivers 75 J. D. C. Jones 4.1 Introduction 75
4.1.1 The function of detectors and receivers in optical sensors 75
4.1.2 Requirements for detectors 75 4.1.3 Classification of detectors 76 4.1.4 Overview 77
4.2 Photothermal detectors 77 4.2.1 Introduction 77 4.2.2 Thermoelectric detectors 77 4.2.3 Thermoresistive detectors 78 4.2.4 Golay cells 78 4.2.5 Pyroelectric detectors 79
4.3 Photoemissive devices 79 4.3.1 Basic principles 79 4.3.2 Photocathodes 79 4.3.3 Vacuum photodiodes 80 4.3.4 Photomultipliers 80
CONTENTS vii
4.3.5 Image intensifiers 80 4.4 Photoconductive detectors 81
4.4.1 Introduction 81 4.4.2 Performance 82 4.4.3 Materials 82
4.5 Photodiodes 83 4.5.1 Operation of junction detectors 83 4.5.2 Responsivity 84 4.5.3 Wavelength range of operation 85 4.5.4 Modes of operation 85 4.5.5 Depletion layer width and junction
capacitance 86 4.5.6 Speed of response 86 4.5.7 Avalanche multiplication 87 4.5.8 Materials 91 4.5.9 Device structures 91
4.6 Receivers 94 4.6.1 Front end designs 94 4.6.2 PET preamplifiers 96
4.7 Noise in photodiode receivers 96 4.7.1 Summary of noise sources 96 4.7.2 Dark current and shot noise 97 4.7.3 Thermal and amplifier noise 98 4.7.4 Signal-to-noise ratio 98
4.8 Conclusions 100 References 101 Further reading 103
5. Multimode optical fiber sensors 105 G. R. Jones, R. E. Jones and R. Jones 5.1 Introduction 105 5.2 Formal systems approach 106
5.2.1 Performance criteria 106 5.2.2 Formal representation of a fiber system 108
5.3 Source and fiber effects 111 5.3.1 Spectral emission of source (P(A.» 111 5.3.2 Wavelength-dependent fiber attenuation (F(A.» 112 5.3.3 Fiber modal effects 113
5.4 Some important modulation mechanisms 115 5.4.1 Extrinsic transmission-reflection modulation 116 5.4.2 Quasi-intrinsic modulation 120 5.4.3 Wavelength selective modulation 125
5.5 Signal processing and system architecture 132 5.5.1 Analog techniques 133 5.5.2 Broadband interferometric techniques 140
viii CONTENTS
5.5.3 Digital and time domain techniques 153 5.5.4 Full hybrid techniques 157
5.6 Conclusions 159 References 159
6 Multimode optical fiber chemical sensors 161 J. O. W. Norris 6.1 Introduction 161 6.2 Perceived advantages and disadvantages for chemical
sensing 162 6.2.1 Advantages 162 6.2.2 Disadvantages 163
6.3 Underlying principles of fiber optic chemical sensors 164 6.3.1 Optical effects 164 6.3.2 Chemical equilibria 169
6.4 Classifying fiber optic sensors for chemical sensing 171 6.5 Description of some illustrative sensors 172
6.5.1 Extrinsic species 172 6.5.2 Intrinsic species-specific sensors 184 6.5.3 Nonspecies-specific techniques 189 6.5.4 Indirect techniques 191
6.6 Conclusions 193 References 193
7 Single mode optical fiber sensors 197 V. Handerek 7.1 Introduction 197 7.2 Interferometer configurations 198
7.2.1 Two-beam interferometers 198 7.2.2 Multiple beam interferometers 199
7.3 Transfer functions of interferometers 201 7.3.1 Two-beam interferometers 201 7.3.2 Fabry-Perot interferometer 202 7.3.3 Ring Resonator 205 7.3.4 Grating reflector 206
7.4 Signal processing techniques 207 7.4.1 Active homodyne methods 207 7.4.2 Passive homodyne methods 209 7.4.3 Heterodyne methods 211 7.4.4 Synthetic heterodyne detection 212 7.4.5 Pseudo-heterodyne detection 213 7.4.6 Range enhancement techniques 215
7.5 Fiber interactions 217
CONTENTS ix
7.6 Applications 218 7.6.1 Fiber interferometer gyroscope 218 7.6.2 Hydrophones 220 7.6.3 Particle sizing 220
References 221
8 Optical fiber modulation techniques for single mode fiber sensors 223 R. P. Tatam 8.1 Introduction 223 8.2 Optical fiber phase modulators 224
8.2.1 Phase modulators 224 8.2.2. Polarization state modulators 229
8.3 Optical fiber frequency shifters 239 8.3.1 Basic principles 239 8.3.2 Extrinsic devices 242 8.3.3 Intrinsic devices - stimulated Brillouin scattering
(SBS) 250 8.4 In-line fiber intensity modulators 257
8.4.1 Acousto-optic devices 258 8.4.2 Active overlay devices 259 References 261
9. Fiber optic white-light interferometric sensors 269 B. T. Meggitt 9.1 Introduction 269
9.1.1 Source characteristics 270 9.1.2 Basic interferometry 271
9.2 Spectral domain processing 272 9.3 Phase domain processing 276
9.3.1 Operating characteristics 276 9.3.2 Wavelength stability 278 9.3.3 Fringe order ambiguity 279 9.3.4 Temporal fringe processing 279
9.4 Spatial domain processing 287 9.4.1 Electronically scanned technique 287 9.4.2 Fringe visibility 288 9.4.3 Central fringe identification 292 9.4.4 Methods of extending the dynamic range 297
9.5 Spatial to temporal fringe generation 302 9.5.1 Operating characteristics 302 9.5.2 Dynamic operation 303
9.6 Quasi-distributed sensor systems: multiplexing 305 9.7 Bragg-grating devices 307
References 310
x CONTENTS
10 Nonlinear effects in optical fibers 313 A. J. Rogers 10.1 Introduction 313 10.2 Parametric effects 315
10.2.1 General 315 10.2.2 Phase matching 315 10.2.3 Four-photon mixing 317 10.2.4 Intensity-dependent refractive index 318 10.2.5 Optical Kerr effect 319 10.2.6 Self-phase modulation (SPM) 322 10.2.7 Solitons 326 10.2.8 Photosensitivity 328
10.3 Inelastic scattering 331 10.3.1 Spontaneous scattering 331 10.3.2 Stimulated scattering 333 10.3.3 Raman processes 333 10.3.4 Brillouin processes 339
10.4 Conclusions 344 References 344
11 Distributed fiber optic sensors 347 A. H. Hartog 11.1 Introduction 347 11.2 Classification of distributed optical fiber sensors 349 11.3 Principles of operation 350
11.3.1 Optical time-domain reflectometry 350 11.3.2 Modulation of fiber loss 351 11.3.3 Polarization effects 352 11.3.4 Numerical aperture effects 353 11.3.5 Modulation of scattering loss 354 11.3.6 Inelastic scattering 357 11.3.7 Fluorescence 360 11.3.8 Nonlinear optical effects 360 11.3.9 Discrete signal sources - quasi-distributed
sensors 362 11.3.10 Forward scattering methods 363
11.4 Performance of distributed sensors and engineering aspects 364 11.4.1 Performance criteria in distributed sensors 364 11.4.2 Constraints in the engineering of distributed
sensors 365 11.4.3 Alternative methods of interrogation and signal
acquisition 367 11.5 Applications 371
11.5.1 Power supply industry 371 11.5.2 Petrochemicals 373
CONTENTS xi
11.5.3 Process industry 373 11.5.4 Tunnels 374 11.5.5 Construction 374 11.5.6 Transportation 375
11.6 Examples of practical implementations of distributed sensors 375 11.6.1 Herga pressure mats 375 11.6.2 Distributed cryogenic leak detection
system 376 11.6.3 Distributed temperature sensor based on Raman
back-scattering 377 11.6.4 Distributed cable strain monitor 377 11.6.5 Fiber optic hydrophone array 378
11.7 Safety of distributed sensors 378 11. 7.1 Explosion hazards 378 11. 7.2 Laser safety considerations 379
11.8 Future prospects 380 References 380
12 Schemes for referencing of intensity-modulated optical sensor systems 383 G. Murtaza and J. M. Senior 12.1 Introduction 383 12.2 Important design considerations 386 12.3 Referencing mechanisms 387 12.4 Spatial referencing 388
12.4.1 Optical bridge balancing 388 12.4.2 Optical signal tapping 392 12.4.3 Bypass fiber monitoring 393
12.5 Temporal referencing 394 12.5.1 Temporal signal recovery 395 12.5.2 Self-referenced mUltiplexing 396
12.6 Dual wavelength referencing 397 12.6.1 Basic configuration 398 12.6.2 Single photodetector configuration 399 12.6.3 Dual wavelength bridge 400 12.6.4 Single LED configuration 401
12.7 Comparative assessment 402 12.8 Summary 403 References 405
13A Hybrid optical fiber sensors 409 R. C. Spooncer and G. S. Philp 13A.l Introduction 409 13A.2 Sensor excitation 409
xii CONTENTS
13A.3 Sensor classification 411 13A.3.1 Nonresonant sensors 412 13A.3.2 Resonant sensors 414
13A.4 Hybrid actuators 416 13A.5 Hybrid sensor multiplexing 417 13A.6 Conclusions 418 References 419
13B Optical fiber current measurement 421 A. J. Rogers 13B.l Introduction 421 13B.2 Basic principle 421 13B.3 Design features 423
13B.3.1 The light source 423 13B.3.2 The fiber 423 13B.3.3 The detection system 425
13B.4 Evaluation of experimental devices 425 13B.4.1 'Fawley' device 425 13B.4.2 Siemens device 427 13B.4.3 Single-ended device 428 13B.4.4 Kema device 431
13B.5 Device analysis 432 13B.5.1 Vibration isolation 435 13B.5.2 Birefringence bias 435 13B.5.3 High circular birefringence fiber 436 13B.5.4 Birefringence normalization 437
13B.6 Conclusions 438 References 439
13C Fiber optic techniques for temperature measurement 441 K. T. V. Grattan 13C.l Introduction 441 13C.2 Sensor devices 442
13C.2.1 Classification 442 13C.2.2 Early work and its extension 442 13C.2.3 Extrinsic sensors 443 13C.2.4 Luminescent devices 445 13C.2.5 High temperature devices 448 13C.2.6 Frequency-dependent devices 450 13C.2.7 Interferometric devices 451 13C.2.8 Distributed devices 452
13C.3 Conclusions 455 References 456
CONTENTS Xlll
14 Advanced external fiber optic sensors 461 D. A. Jackson 14.1 Laser Doppler anemometry 461 14.2 Transit time (two-spot) anemometers (TT A) 465 14.3 Noncontact vibrometers (NCV) 465
14.3.1 Signal recovery 468 14.4 Holography and TV holography 469
14.4.1 Optical fiber holography 469 14.4.2 Optical fiber TV holography 469
14.5 Accelerometers and geophones 471 14.5.1 Diaphragm-based accelerometers 471 14.5.2 Compliant-cylinder-based accelerometer 474
14.6 Optical displacement 476 14.6.1 Conventional interferometry 476 14.6.2 Source modulation interferometry; frequency
modulation continuous wave 478 14.6.3 Optical radar techniques 482
14.7 Conclusions 486 References 487
Index 489
Contributors
K. T. V. Grattan Department of Electrical, Electronic & Information Engineering City University London, UK
A. Hartog York Ltd Hants, UK
V. Handerek Department of Electronic and Electrical Engineering King's College London, UK
D. A. Jackson Department of Physics University of Kent Canterbury, UK
G. R. Jones Department of Electrical Engineering and Electronics The University of Liverpool Liverpool, UK
J. D. C. Jones Department of Physics Heriot-Watt University Edinburgh, UK
R. Jones Cambridge Consultants Ltd Cambridge, UK
R. E. Jones Lucas Control Products Advanced Engineering Centre Solihull, UK
XVI
B. T. Meggitt ERA Technology Limited Leatherhead, UK
G. Murtaza
CONTRIBUTORS
Department of Electrical and Electronic Engineering The Manchester Metropolitan University Manchester, UK
J. O. W. Norris AEA Technology Harwell, Oxfordshire, UK
G. S. Philp The BruneI Centre of Manufacturing Metrology The University of West London Middlesex, UK
A. J. Rogers Department of Electronic and Electrical Engineering King's College London, UK
J. M. Senior Department of Electrical and Electronic Engineering The Manchester Metropolitan University Manchester, UK
R. C. Spooncer The BruneI Centre of Manufacturing Metrology The University of West London Middlesex, UK
R. P. Tatam Centre for Photonics and Optical Engineering School of Mechanical Engineering Cranfield University Bedford, UK
Preface
The field of measurement and instrumentation, and in particular that of sensor development, is one that has expanded rapidly in recent years. With the approach of the 21st century, the need for high quality sensors to be integrated into sophisticated measurement and control systems is clear. In parallel with the rapid advance in the development of sensors based on microelectronics, those based on optical techniques have expanded significantly over the last few years, particularly with the development and incorporation of optical fibers. As a result, optical techniques have been widely used for numerous measurement applications. The familiar and by now classical optical instruments, incorporating interferometry and pyrometry for example, have been expanded and adapted with the developments within the optoelectronics industry to produce the new class of optical sensing device, the optical fiber sensor. This book discusses optical fiber sensor technology with an emphasis upon the range of optical sensors that has been developed using optical fiber-based techniques.
Optical sensor schemes have been described widely in the scientific literature, and they exhibit such important characteristics as immunity to electromagnetic interference, a nonelectrical method of operation, small size and weight, in general low power consumption and in many cases comparatively low cost. The initial view of the technology had been of one which had broad applications across the wide spectrum of the need for sensors in industry, but this has largely been modified to emphasize the advantages of optical sensing for niche markets such as medical instrumentation, safety critical systems, defense and aerospace, environmental monitoring and more recently in construction. In this way the initial promise of optical fiber sensors can be more fully met in their applications in these areas.
The advancement of the field owes much to the development of the optical fibers and associated optoelectronic devices for the telecommunications industry. Many optical fiber sensors capitalize upon the use of low cost sources, detectors and the specialized optical fibers that have been developed for other purposes. With the worldwide expansion of the fiber optical telephone trunk network for voice and data communication, there is now a wide availability of high quality optical and associated electronic components at competitive prices. Coupled to this, the expansion of the optoelectronics market for domestic consumer products such as the CD player, and infrared remote control for many devices, has again led to an expansion of novel technology available to the sensor engineer and designer. It will be seen from this book that the combination of these areas of
xv III PREFACE
expertise has led to the new developments in optical fiber sensor technology, which are reported.
This text is aimed primarily at scientists and engineers with an interest in the rapidly expanding branch of optoelectronics that is optical fiber sensors. The purpose of the book is to be comprehensible to a wide variety of undergraduate and postgraduate students undertaking courses and research in optoelectronics, and associated measurement, instrumentation and sensor technology, as well as practising scientists and engineers who wish to acquaint themselves with both the fundamentals and new developments in this field. A prime objective in the writing of the text has been to keep it relatively simple and straightforward, edited and planned to be a systematic tour through the subject, with the minimization of the amount of mathematical knowledge required to understand the technology that is represented. In addition a wide variety of readily available references is quoted with each chapter to enable the reader interested in further detail to refer to the source material from the origin of the particular advance in the technology.
The stimulus for the preparation of this book came about initially following a short course for scientists and engineers at City University, London, organized by the editors and involving many of the authors. It was realized there was a need for a suitable book which presented the information disseminated at the course in a coherent and logical manner and as a result the present text was envisaged. It was clear that it would be unwise for the editors to attempt to write all the material themselves, as there were a number of outstanding experts in the field who were available to contribute to it. As a result, the editors invited the listed authors to produce their input to the overall structure of the book, based on their individual expertise and detailed knowledge of many aspects of the field. In that the aim has been, as with the editorship, to reflect a mix between academic and industrial experience in the choice of writers. The editors are particularly grateful to the contributors who have responded so enthusiastically in taking the time and effort to produce the chapters which make up this book. As far as possible authors were chosen on the basis of their leadership of various aspects of research in the field, to emphasize from their detailed personal knowledge and experience of research published widely in the literature in those areas, the new developments in the technology itself.
The text is organized into three major parts. The first four chapters form the first part, Chapters 1 and 2 dealing with an introduction and the basis of optical fiber sensors covering the technology of optical fibers themselves, and the background to optical fiber sensing with the fundamental physics involved, and Chapters 3 and 4 considering in detail the sources available for optical fiber sensors and detectors, and associated noise considerations in these devices. Thus, following an introductory overview of the evolution of the subject and its direction by David Jackson, Vince Handerek provides, in Chapter 2, an introduction to the basis of the optics of fiber technology itself, emphasizing the underlying physical principles. In Chapter 3, Kenneth Grattan provides a detailed consideration of the wide variety of sources that may be used in various optical and fiber sensor devices, emphasizing the strengths and weaknesses of conven-
PREFACE xix
tional and laser sources. In Chapter 4, Julian Jones complements the preceding chapter in providing detailed discussion of solid state and other detectors which are used in optical fiber sensors, and in particular the noise considerations which relate to the choice of a particular detector for a specific application.
The second part of the text deals with specific aspects of multimode and singlemode fiber sensors and other more specific and topical aspects of optical sensing, including nonlinear distributed sensors and multiplexing of optical fiber sensors. In Chapter 5, Gordon Jones, Roger Jones and Robert Jones provide a detailed analysis of the wide range of multimode optical fiber sensors that have been produced in recent years for physical measurands. This is complemented in Chapter 6 in the work of John Norris who produces a similar and comprehensive review of multimode devices used in optical chemical sensors, seen as a rapidly expanding field. In Chapter 7, singlemode optical fiber technology is introduced with Vince Handerek discussing optical fiber interferometry, developing the signal processing aspects of this work, reviewing pseudo-heterodyne sensors and the advantages that can be gained through the use of this technology. Ralph Tatam discusses, in Chapter 8, the use of optical modulation techniques in optical fiber sensors, again emphasizing the wide variety of technology that is available to the optical sensor designer. Beverley Meggitt, in Chapter 9, extends the use of optical interferometry for the more recent developments using white-light techniques where the diversity in application and inherent advantages in practice are considered in some depth, covering both single and multimode fiber use.
In Chapter 10, Alan Rogers gives an expert tutorial on nonlinear optical devices, a field of research in which he has worked for many years, and in Chapter 11, Arthur Hartog discusses the technology of distributed optical fiber sensors, an area where optical fibers offer almost unique advantages in sensor applications and where successful commercial devices have been available for some years. With the success of simple intensity-based sensors, there is a need to consider referencing and John Senior and Ghulam Murtaza have written a comprehensive review of this subject as Chapter 12.
The text concludes with the third part, Chapters 13 and 14, where a number of special application areas are reviewed. These are aspects of the technology where optical fiber sensors have been widely applied for a number of years or where there have been recent advances in the field and they thus merit specific and individual discussion. Chapter 13 is itself in three parts: Ron Spooncer and G. S. Philp (13A) discusses the uses of 'hybrid' sensors - optical fiber sensors that incorporate electronic or pneumatic components. In many cases the move away from conventional technology is such that these 'hybrid' sensors provide a bridge between the two and enable conventional applications to be approached using optical fibers. Alan Rogers (13B) discusses the field of current and voltage sensing which he pioneered at the Central Electricity Research Laboratories in England, and subsequently. The field of temperature sensors, where optical fiber techniques have been applied more widely than in many other sensing applications is discussed by Kenneth Grattan (13C). Finally, Chapter 14 is a view, from
xx PREFACE
David Jackson, of the mode of operation and performance of the most advanced external fiber optic sensor devices offering significant potential for a number of coherent and incoherent optical fiber sensing systems.
The authors would like to thank all those who have contributed to the development of this particular volume and helped in its production. It is hoped that it will prove useful to the readership and be of long term value as a source text in optical fiber sensors.
K. T. V. Grattan B. T. Meggitt
