Fluoride Optical Fibres Fluoride Glass Optical Fibres

by

P.W. FRANCE British Telecom Research Laboratories Ipswich, UK

with co-authors M.G. DREXHAGE Galileo Electro- Corporation Massachussetts, USA J.M. PARKER School of Materials University of Sheffield, UK M.W. MOORE S.F. CARTER and J.V. WRIGHT British Telecom Research Laboratories Ipswich, UK

Blackie Glasgow and London Published in the USA and Canada by CRC Press, Inc. Boca Raton, Florida Blackie and Son Ltd Bishopbriggs, Glasgow G64 2NZ and 7 Leicester Place, London WC2H 7BP Published in the USA and Canada by CRC Press, Inc. 2000 Corporate Blvd, N.W., Boca Raton, FL 33431

CD 1990 Blackie and Son Ltd First published 1990 Softcover reprint of the hardcover 1st edition 1990 All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means-graphic, electronic or mechanical, including photocopying, recording, taping-without the written permission of the Publishers

British Library Cataloguing in Publication Data

France, p, W. Fluoride glass optical fibres. \. Infrared fibre optics l. Title 621.36'92

ISBN-13: 978-94-011-6867-0

Library of Congress Cataloging-in-Publication Data

Fluoride glass optical fibres/edited by P.W. France. p. cm. ISBN-13: 978-94-011-6867-0 e-ISBN-13: 978-94-011-6865-6 DOl: 10.1007/978-94-011-6865-6 I. Optical fibres, 2. Infrared technology, I. France. P,W. TAI800.F57 1989 89-22246 621.36' 92 -dc20 CIP

Phototypesetting by Thomson Press (India) Limited, New Delhi Preface

One of the most exciting prospects for optical fibres made from fluoride is the possibility of providing long distance optical communication systems without the need for repeaters. This objective has stimulated much of the work into fluoride glasses over the past ten years, and has prompted the writing of this book. It has also emerged that fluoride fibres can transmit both visible and infrared energy (from about 0.5 to 5 ,urn) and that they have many applications outside the field of telecommunications. These include optical fibre sensors (particularly in remote infrared spectroscopy), laser surgery and fibre lasers. Several companies are now established in the field, and good quality fluoride fibres are available from sources throughout the USA, Europe and Japan. Moreover, the first commercial instruments based on fluoride fibres are finding their way to the market place and these fibres will undoubtedly form the basis of many more instruments yet to be developed. The work presented in this book represents the field both from an academic understanding of the materials and ways to convert them into fibre, and from a practical and commercial viewpoint. The principal author and some of the co• authors are based at the British Telecom Research Laboratories in the UK. These authors have been able to follow the technology from basic research to manufacture and are aware ofthe many non-telecom applications. Many close and personal links have been established with universities and industry and these are represented by the contributions from Dr John Parker of the University of Sheffield and Dr Martin Drexhage ofthe Galileo Electro-Optics Corporation. Dr Drexhage has been a leading pioneer of the field within the USA and is now overseeing the development of the technology into manufacturing. Chapter 1 presents an overview and provides a historical summary of the development ofthe field. Chapter 2 describes the general properties of fluoride glasses, both physical and optical, and is a useful reference chapter. Chapter 3 discusses the propagation of power in optical fibres, placing particular emphasis on longer wavelengths. This leads on to a detailed discussion of manufacturing techniques for fluoride glasses and ways to convert them into fibre in Chapter 4. Chapters 5,6 and 7 discuss the optical losses in fibre, both intrinsic and extrinsic, and Chapter 8 presents the most up to date information on measured losses and measurement techniques. Chapter 9 presents data on mechanical properties, strength and durability. Finally, Chapter 10 discusses applications of fluoride fibres, including long wavelength communications systems, fibre lasers and amplifiers, laser surgery, and infrared fibre sensors. VI PREFACE The book is aimed at anyone working in the field of optical fibres made from fluoride glasses, from academic researchers to engineers attempting to devise instruments based on the fibres, and will be of interest to scientists and engineers who are curious to hear of developments in a new and exciting field. The authors would like to acknowledge their respective establishments for support and advice whilst writing this book, and wish to acknowledge the whole community of scientists and engineers who have helped to advance the field and to whom many references have been made throughout the book.

P.W.F. Authors

S.F. Carter British Telecom Research Laboratories Martlesham Heath Ipswich IPS 7RE, UK

M.G. Drexhage Galileo Electro-Optics Corporation Galileo Park PO Box 550 Sturbridge MA 01566 USA

P.W. France British Telecom Research Laboratories Martlesham Heath Ipswich IPS 7RE, UK

M.W. Moore British Telecom Research Laboratories Martlesham Heath Ipswich IPS 7RE, UK

J.M. Parker School of Materials University of Sheffield Elmficld Northumberland Road Sheffield S 10 2TZ, UK

J.V. Wright British Telecom Research Laboratories Martlesham Heath Ipswich IPS 7RE, UK. Contents

1 Perspective and overview 1 M.G. DREXHAGE

1.1 IntroductIon 1.2 OptIcal fihre fundamentals 2 1.3 Loss mechanisms in optical fibre materials 4 1.3.1 Power losses and UnIts 4 I 32 Extnnslc and intrinsic loss mechanisms 5 1.3.3 Intnnslc transparency: mechanIsms and models 6 14 Long wavelength fibre materials 9 1.4.1 Materials. choices and sources of InformatIOn 9 14.2 Silica-based fibres 11 1.43 Crystalline fibre materials 13 1.4.4 ChalcogenIde glasses 15 1.4.5 Hollow Infrared waveguIdes 16 1.5 Fluonde glasses and optical fibres 16 1.5.1 Concepts. definitions and materials 16 1.5.2 Fluonde fibre technology: a brief personal history 20 1.6 ApplIcatIons for Infrared optIcal fibres 25 1.7 Future prospects 27 References 29

2 Properties of fluoride glasses 32 1.M. PARKER and P.W. FRANCE

2.1 IntroductIOn 32 2.2 Glass-forming systems. structure and crystallization 32 2.2.1 Structural models 32 2.2.2 Crystallization behaVIOur 36 2.3 Thermal propertIes 42 2.3.1 Viscosity 42 2.3.2 Thermal expansIOn behaviour 45 2.3.3 Diffusion 48 2.3.4 Thermal conductivity 48 2.3.51 Heat transfer 49 2.4 Other properties 51 2.4.1 Density 51 2.4.2 Gas solubility 52 2.4.3 Elastic moduli 53 2.4.4 Microhardness 54 2.5 Optical properties 54 2.5.1 Infrared absorption 54 2.5.2 Ultraviolet absorptIOn 61 2.5.3 Intrinsic scattering loss 62 2.5.4 Minimum intrinsic losses 64 2.5.5 and 66 2.5.6 Fluorescence 69 References 70 X CONTENTS 3 Propagation in optical fibres 75 J.V. WRIGHT

3.1 Introduction 75 3.2 Multimode fibres 76 3.2.1 Propagation in multi mode fibres 76 3.2.2 Attenuation 83 3.2.3 Real fibres and fibre links 83 3.3 Monomode fibres 85 3.3.1 Propagation in monomode fibres 85 3.3.2 Optimized fibre design 94 3.3.3 Non-linear elTects 97 References 98

4 Manufacture of infrared fibres 100 P.W. FRANCE

4.1 Materials preparation 100 4.1.1 Introduction 100 4.1.2 Synthetic routes 101 4.1.3 Purification 103 4.2 Melting techniques 105 4.2.1 Melting environments 105 4.2.2 Containment vessels 105 4.2.3 Reactive atmosphere processing 106 4.2.4 Homogenization and fining 107 4.2.5 Crystallization 107 4.3 Fibre fabrication 108 4.3.1 Glass compositions 108 4.3.2 Glass melting 109 4.3.3 Preform fabrication 113 4.3.4 Fibre drawing 114 4.3.5 Other techniques 116 4.3.6 Monomode fibre 117 4.4 Problems 118 4.5 Fibre results 120 References 121

5 Intrinsic loss measurements 122 P.W. FRANCE

5.1 Introduction 122 5.2 Rayleigh scattering 122 5.3 IR multi phonon edge 124 5.4 Minimum intrinsic loss 126 5.5 Longer wavelength transmitting fibres 127 References 130

6 Extrinsic absorption 132 P.W. FRANCE

6.1 Introduction 132 6.2 Spectrometer measurements 133 6.2.1 Nomenclature 133 6.2.2 Experimental 134 CONTENTS XI

6.3 Absorption due to water 136 6.3.1 OH - in oxide glasses 136 6.3.2 OH - in fluoride glasses 138 6.3.3 OH - in ZrF4 IR fibres 140 6.4 Absorption due to transition metal ions 143 6.4.1 Introduction 143 6.4.2 Oxidation-reduction equilibrium 144 6.4.3 Ligand field theory 145 6.4.4 Absorption spectra 147 6.4.5 Discussion 157 6.5 Absorption due to rare earth ions 159 6.5.1 Introduction 159 6.5.2 Oxidation states 159 6.5.3 Electronic spectra 160 6.5.4 Absorption spectra 162 6.5.5 Discussion 179 6.6 Other impurities 184 6.6.1 Dissolved gases 184 6.6.2 Molecular ions 182 6.6.3 Reduced species 182 6.6.4 Oxide absorption 182 References 184

7 Extrinsic scattering 186 P.W. FRANCE 7.1 Introduction 186 7.2 Identification of scattering centres 187 7.2.1 Large crystals 187 7.2.2 Gas bubbles 187 7.2.3 Sub-micron centres 188 7.2.4 Deviations of fibre geometry 188 7.3 Scattering theory 188 7.4 Scattering solutions for limiting cases 189 7.4.1 Small spheres, intermediate index (Rayleigh scattering) 189 7.4.2 Small high index spheres 190 7.4.3 Intermediate size spheres with m close to 1 (Rayleigh-Gans) 191 7.4.4 Intermediate size sphere with higher refractive index 191 7.4.5 High index (conducting) spheres of small size 193 7.4.6 High index spheres of intermediate size 194 7.4.7 Large spheres with index close to 1 194 7.4.8 Large spheres with high refractive index 194 7.5 Solutions for metallic (absorbing) spheres 195 7.6 Applications to practical examples 197 7.7 Calculations of scattering loss 199 7.8 Scattering loss in fibres 202 7.9 Conclusions 203 References 204

8 Measured losses in fibres 205 M.W. MOORE

8.1 Introduction 205 8.2 Available techniques 206 6.2.1 Sources and detectors 206 8.2.2 Totalloss 207 8.2.3 Absorption 207 8.2.4 Scattering 208 xii CONTENTS

8.3 Total loss 208 8.4 Absorption 210 8.5 Scattering 211 8.6 Recent results 212 8.7 Predicted minimum loss 216 References 217

9 Mechanical properties 219 S.F. CARTER

9.1 Introduction 219 9.2 Fracture and glass parameters 219 9.3 Strength measuring methods 221 9.3.1 What is strength? 221 9.3.2 Tensile measurements 222 9.3.3 Bend measurements 223 9.4 Fibre strengths 224 9.4.1 Mechanical polishing 224 9.4.2 Chemical polishing 224 9.5 Identification of flaws and effect on strength 227 9.6 Ultimate strength and maximum practical strength 230 9.7 Strength and homogeneous crystal growth 231 9.8 Durability 232 9.8.1 Zero stress aging 232 9.8.2 Dynamic fatigue 233 9.8.3 Hermetic coatings 235 9.9 Conclusions 236 References 236 to Applications 238 P.W. FRANCE

10.1 Mid-IR communication systems 238 10.1.1 System demonstrations 239 10.2 Active fibres 240 10.2.1 Fibre devices 241 10.2.2 Absorption and fluorescence spectra 243 10.2.3 Fluoride fibre lasers 247 10.2.4 U peon version 251 10.3 Laser surgery 251 10.4 Infrared fibre sensors 253 10.4.1 IR fibre chemical sensors 255 10.4.2 IR fibre non-chemical sensors 259 References 263