PRENTICE HALL INTERNATIONAL SERIES INOPTOELECTRONICS Lasers Principles and Applications J. HAWKES ya ee tat ae tay ; * ; le t Lasers Principles and Applications ara Ie iM a hee ir: <2 2 wD ety + a 4. ¢ i te <4 has vais ah ae mor 5 ae by sae4 —< 4. ~ , ca i fips aps ~ ee ry’ Te thas© —*Tim? a= ; ie : ca ¥ ae at ” is ee ay ’ Mee oe 1 Co id y ¢ Cee ¥ ; 1 4 :é.
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Hawkes School of Physics Newcastle upon Tyne Polytechnic Prentice Hall New York London Sydney Tokyo First published 1987 by Prentice Hall International (UK) Ltd 66 Wood Lane End, Hemel Hempstead, Hertfordshire, HP2 4RG A division of Simon & Schuster International Group © 1987 Prentice Hall International (UK) Ltd 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, electronic, mechanical, photocopying, recording or otherwise, without the prior permission, in writing, from the publisher. For permission within the United States of America contact Prentice Hall Inc., Englewood Cliffs, NJ 07632. Printed and bound in Great Britain by A.
Wheaton and Co. Ltd, Exeter Library of Congress Cataloging-in-Publication Data Wilson, J. (John), 1939-— Lasers: principles and applications. (Prentice Hall international series in optoelectronics) Includes bibliographies and index.366 87-18683 ISBN 0-13-523705-X British Library Cataloguing in Publication Data Wilson, J.
Lasers: principles and applications.366 TA1675 ISBN 0-13-523705-X ISBN 0-13-523697-5 Pbk 345 91 90 89 88 Helse sesvO5—% = 13-Se3b1"=5: PBR Contents Preface ix Glossary of Symbols xi 1 Laser Fundamentals 1 1.1 The nature of light 1 1.2 Emission and absorption of light 9 1.3 Interaction of radiation and matter 11 1.4 The Einstein relations 13 1.5 The gain coefficient 16 1.6 Attainment of a population inversion 18 1.7 The optical resonator 21 1.8 Threshold gain coefficient 24 1.9 The lineshape function 26 1.2 Transverse modes 31 Problems 32 References 33 2 Operation of Practical Lasers 35 2.1 Doped insulator lasers 35 2.1 Impurity ion energy levels in solids 36 2.4 The Nd: YAG laser 43 2.5 Nd: glass lasers 46 2.6 The ruby laser 47 2. The alexandrite laser 48 2.8 | Color or ‘F’ center lasers 50 2.1 The HeNe laser 64 2.2 The copper vapor laser 67 vi Contents 2.1 The argon ion laser 68 2.2 The helium—cadmium laser 70 2.1 The carbon dioxide laser 71 Sealed-tube lasers 73 Gas flow lasers 74 Gasdynamic lasers 75 Transversely excited atmospheric (TEA) lasers 75 Optics 76 2.2 The nitrogen laser 76 2. The excimer laser 77 2.4 The chemical laser 78 2.5 Far infra-red lasers 79 2.4 Liquid dye lasers 80 Pe, The free electron laser 84 Problems 86 References 88 Properties of Laser Radiation 90 3.1 Laser linewidth 90 my! Laser frequency stabilization 94 333 Beam divergence 96 3.4 Beam coherence 102 a Brightness 106 3.6 Focusing properties of laser radiation 107 byl Q-switching 108 3.1 Methods of Q-switching 110 3.1 Rotating-mirror method 110 3.2 Electro-optic Q-switching 111 3.3 Acousto-optic Q-switching 112 3.1 Methods of mode locking 117 Joo Frequency doubling 119 3.10 Phase conjugation 121 Problems 125 References 126 Metrological and Scientific Applications 128 4.2 Measurement of distance 131 4.2 Refractive index correction 136 Contents vii 4.2 Surface topography and optical component testing 137 4.3 Beam-modulation telemetry 142 4.4 Pulse—Echo techniques 144 4.3 Laser Doppler velocimetry 145 4.4 Surface velocity measurements using speckle patterns 151 4.1 Molecular beam spectroscopy 158 4. Two-photon spectroscopy 159 4.7 Laser uranium enrichment 160 Problems 162 References 163 5 Industrial, Medical and Military Applications 165 oe Theoretical analysis 166 5.1 Temperature changes assuming no melting or vaporization 168 5.
Vaporization depth 175 Diz Beam transport and focusing 176 «ps. Materials-processing applications 179 5.2 Deep penetrating welding 188 = is) Laser-assisted machining 189 5.6 Laser cutting 190 “py Micromachining 193 5.8 Drilling, scribing and marking 194 5 Lasers in medicine 197 5.10 Very high-power laser uses 199 5.1 Laser-induced nuclear fusion 199 5.2 Laser weapons 201 Problems 201 References 203 6 Holography 204 6.2 Classification of holograms 207 6.3 A mathematical description of holography 212 6.4 Hologram efficiency 218 viii Contents 6.5 Applications of holography 219 6.1 Double exposure holographic interferometry 220 6.3 Real-time holography 221 6.4 Time-average holographic interferometry 223 6.2 Holographic optical components 227 6.1 Holographic optical elements (HOE) 228 6. Information storage and display 230 6.4 Character recognition 233 References 233 7 Optical Information Transmission and Storage 236 TA Optical communication 236 7.1 Light modulation schemes 237 7.2 The optical fiber 241 7.2 Graded-index fiber 248 7. Low-dispersion fibers 249 7.5 Fiber materials and manufacture 254 7.5 System design considerations 265 7.9 | Free-space communication 273 V2 Laser printing 275 es.
Optical disk systems 277 7. Data readout from optical disks 287 7.4 Erasable optical disks 289 Problems 290 References 292 Appendix 1 Answers to Problems 294 Appendix 2 Physical Constants 297 Appendix 3 Laser Safety 298 Index 302 Preface It is now just over twenty-five years since laser action was first successfully demonstrated using a crystal of ruby as the laser medium. In the time since then not only has the term /aser been adopted into our everyday speech but the laser has grown from the status of a scientific curiosity with a few poten- tial applications to that of being one of the most important inventions of our time. It is now a vital tool for areas as diverse as manufacturing industries and medicine, an essential component of communication and holographic systems and the basis of many scientific measurements and research programs.
The term laser is used here in a general way for there are many different types of laser with quite different characteristics. All lasers, however, emit radiation with special properties which enables the laser to be used in a much wider range of applications than conventional light sources. In view of the very wide range of applications, there is clearly a need for those other than physicists to have a good working knowledge of lasers so that they may better understand the particular application they are interested in and also appreciate the advantages and limitations of using lasers. This knowledge will hopefully enable them to make informed judgments on the purchase of appropriate lasers and associated com- ponents.
Such people might include mechanical, electronic, civil and telecommunications engineers, chemists, life scientists, surgeons, military personnel and artists. In writing this book, we have attempted to present a treatment of lasers appropriate to the needs of such a wide range of people. It is at an academic level equivalent to about the second year of a degree program and should also prove useful to students taking an introductory course in lasers. The text falls short of a comprehensive quantum-mechanical treatment of the theory of lasers but does present a much more detailed discussion than the rather brief reviews contained in many texts on modern optics.
We hope therefore that this text will provide a basis for the study of the more advanced books cited in the references. Chapter 1 covers the fundamentals of laser physics while Chapter 2 describes the principles of operation of a quite comprehensive selection of the various types of laser. Space does not allow an exhaustive coverage and X Preface doubtless we have omitted one or two lasers which will become important in the near future. We believe, however, that an understanding of these two chapters will enable the reader to appreciate the operation of new lasers as they appear.
Chapter 3, which discusses the properties of laser radiation, forms the basis for an understanding of why lasers are so useful and of many of their applications. We hope that it will help the reader to devise new applications in various fields. This chapter also describes some of the ways in which the laser output can be modified to enhance its usefulness. Because of the special properties of laser radiation most laser beams can be focused to a very small spot, in which there is a very high energy den- sity, and Chapter 5 covers some of the applications of lasers which depend on this.
These range from cutting and welding metal sheets to an alternative to the surgeon’s scalpel. In this chapter, perhaps more than in any other, we have quoted several useful equations. Space does not permit the deriva- tion of such equations and indeed in many cases this would not help in understanding the application to which they are related. Full references are provided to these derivations for the interested reader.
In Chapter 6 we have given a mathematical description of holography and presented some of its applications. After a rather slow start the range and number of these appli- cations is steadily increasing. Although a full discussion is beyond the scope of this text, it is hoped that some readers will be encouraged to delve deeper into this fascinating subject. Finally in Chapter 7 we discuss the role of lasers in optical communica- tions.
Topics covered include laser printers, optical disk systems and, most importantly, fiber-optic communications, in which there has been a dramatic growth in the last few years. We have interspersed a number of worked examples throughout the text, which we hope will illustrate the use of equations, provide typical values of various parameters and enhance the reader’s understanding and enjoyment of the book. There are also end-of-chapter questions with numerical solutions provided in Appendix 1. Appendix 2 provides a list of physical constants.
References have also been provided at the end of each chapter. These include suggestions for further reading as well as specific references to various points in the text. Finally we would like to thank our colleagues, particularly Dr I. Latimer, for their interest and many valuable discussions and suggestions, Mrs Pat Weddell for typing the manuscript and our families for their encouragement and forbearance.
Glossary of Symbols Wherever possible we have endeavored to use the commonly accepted symbols for the various physical parameters needed. Inevitably many symbols have duplicate meanings, and if in any doubt the reader should note carefully both the context and the dimensions. The following list of symbols does not include all the varieties formed by adding suffices, nor does it include all the (fairly frequent) cases where a symbol is used as a measure of physical dimensions. area, spontaneous transition rate (A2;) fiber radius Einstein coefficient (Bi2, B21), bit rate capacitance, specific heat capacity velocity of light Diameter, optical density distance electric field amplitude energy, bandgap (£;,) electron charge lens F number modulation frequency, focal length gain lineshape function (g(v)) DR TPMSARAHTQOBS THON heat flow per unit area Planck’s constant =<wh/ 27.
irradiance current, /(—1) molecular rotational quantum number total momentum diffraction factor thermal conductivity, film development parameter Se a ~ unit vector, small signal gain coefficient, threshold gain © coefficient (kin) Boltzmann’s constant, wave vector.
