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FUNDAMENTALS OF LIGHT SOURCES AND LASERS FUNDAMENTALS OF LIGHT SOURCES AND LASERS Mark Csele A JOHN WILEY & SONS, INC., PUBLICATION Copyright # 2004 by John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. 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, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400, fax 978-646-8600, or on the web at www.copyright.com. 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For general information on our other products and services please contact our Customer Care Department within the U.S. at 877-762-2974, outside the U.S. at 317-572-3993 or fax 317-572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print, however, may not be available in electronic format. Library of Congress Cataloging-in-Publication Data: Csele, Mark. Fundamentals of light sources and lasers/Mark Csele. p. cm. “A Wiley-Interscience publication.” Includes bibliographical references and index. ISBN 0-471-47660-9 (cloth : acid-free paper) 1. Light sources. 2. Lasers. I. Title. QC355.3.C74 2004 621.36 0 6- -dc22 2004040908 Printed in the United States of America 10987654321 To my parents for fostering and encouraging my interest in science & CONTENTS Preface xiii 1. Light and Blackbody Emission 1 1.1 Emission of Thermal Light 1 1.2 Electromagnetic Spectrum 2 1.3 Blackbody Radiation and the Stefan –Boltzmann Law 2 1.4 Wein’s Law 4 1.5 Cavity Radiation and Cavity Modes 6 1.6 Quantum Nature of Light 9 1.7 Electromagnetic Spectrum Revisited 10 1.8 Absorption and Emission Processes 10 1.9 Boltzmann Distribution and Thermal Equilibrium 13 1.10 Quantum View of Blackbody Radiation 14 1.11 Blackbodies at Various Temperatures 15 1.12 Applications 17 1.13 Absorption and Color 18 1.14 Efficiency of Light Sources 18 Problems 19 2. Atomic Emission 21 2.1 Line Spectra 21 2.2 Spectroscope 22 2.3 Einstein and Planck: E ¼ h n 26 2.4 Photoelectric Effect 27 2.5 Atomic Models and Light Emission 28 2.6 Franck–Hertz Experiment 31 2.7 Spontaneous Emission and Level Lifetime 34 2.8 Fluorescence 35 2.9 Semiconductor Devices 37 2.10 Light-Emitting Diodes 44 Problems 48 3. Quantum Mechanics 49 3.1 Limitations of the Bohr Model 50 3.2 Wave Properties of Particles (Duality) 50 vii 3.3 Evidence of Wave Properties in Electrons 52 3.4 Wavefunctions and the Particle-in-a-Box Model 53 3.5 Reconciling Classical and Quantum Mechanics 55 3.6 Angular Momentum in Quantum States 56 3.7 Spectroscopic Notation and Electron Configuration 57 3.8 Energy Levels Described by Orbital Angular Momentum 60 3.9 Magnetic Quantum Numbers 62 3.10 Direct Evidence of Momentum: The Stern –Gerlach Experiment 63 3.11 Electron Spin 65 3.12 Summary of Quantum Numbers 67 3.13 Example of Quantum Numbers: The Sodium Spectrum 69 3.14 Multiple Electrons: The Mercury Spectrum 71 3.15 Energy Levels and Transitions in Gas Lasers 72 3.16 Molecular Energy Levels 73 3.17 Infrared Spectroscopy Applications 77 Problems 79 4. Lasing Processes 83 4.1 Characteristics of Coherent Light 84 4.2 Boltzmann Distribution and Thermal Equilibrium 86 4.3 Creating an Inversion 87 4.4 Stimulated Emission 90 4.5 Rate Equations and Criteria for Lasing 92 4.6 Laser Gain 98 4.7 Linewidth 101 4.8 Thresholds for Lasing 104 4.9 Calculating Threshold Gain 106 Problems 113 5. Lasing Transitions and Gain 117 5.1 Selective Pumping 117 5.2 Three- and Four-Level Lasers 119 5.3 CW Lasing Action 124 5.4 Thermal Population Effects 127 5.5 Depopulation of Lower Energy Levels in Four-Level Lasers 128 5.6 Rate Equation Analysis for Atomic Transitions 130 5.7 Rate Equation Analysis for Three- and Four-Level Lasers 136 5.8 Gain Revisited 143 5.9 Saturation 146 5.10 Required Pump Power and Efficiency 149 viii CONTENTS 5.11 Output Power 154 Problems 156 6. Cavity Optics 159 6.1 Requirements for a Resonator 159 6.2 Gain and Loss in a Cavity 160 6.3 Resonator as an Interferometer 162 6.4 Longitudinal Modes 164 6.5 Wavelength Selection in Multiline Lasers 166 6.6 Single-Frequency Operation 169 6.7 Characterization of a Resonator 174 6.8 Gaussian Beam 176 6.9 Resonator Stability 178 6.10 Common Cavity Configurations 180 6.11 Spatial Energy Distributions: Transverse Modes 185 6.12 Limiting Modes 186 6.13 Resonator Alignment: A Practical Approach 187 Problems 190 7. Fast-Pulse Production 193 7.1 Concept of Q-Switching 193 7.2 Intracavity Switches 195 7.3 Energy Storage in Laser Media 196 7.4 Pulse Power and Energy 198 7.5 Electrooptic Modulators 202 7.6 Acoustooptic Modulators 206 7.7 Cavity Dumping 211 7.8 Modelocking 212 7.9 Modelocking in the Frequency Domain 215 Problems 217 8. Nonlinear Optics 219 8.1 Linear and Nonlinear Phenomena 219 8.2 Phase Matching 223 8.3 Nonlinear Materials 227 8.4 SHG Efficiency 229 8.5 Sum and Difference Optical Mixing 230 8.6 Higher-Order Nonlinear Effects 231 8.7 Optical Parametric Oscillators 232 Problems 233 CONTENTS ix 9. Visible Gas Lasers 235 9.1 Helium–Neon Lasers 235 9.2 Lasing Medium 236 9.3 Optics and Cavities 237 9.4 Laser Structure 239 9.5 HeNe Power Supplies 241 9.6 Output Characteristics 245 9.7 Applications 246 9.8 Ion Lasers 247 9.9 Lasing Medium 247 9.10 Optics and Cavities 251 9.11 Laser Structure 252 9.12 Power Supplies 256 9.13 Output Characteristics 258 9.14 Applications and Operation 259 10. UV Gas Lasers 261 10.1 Nitrogen Lasers 261 10.2 Lasing Medium 262 10.3 Gain and Optics 264 10.4 Nitrogen Laser Structure 265 10.5 Output Characteristics 269 10.6 Applications and Practical Units 269 10.7 Excimer Lasers 270 10.8 Lasing Medium 271 10.9 Gain and Optics 274 10.10 Excimer Laser Structure 274 10.11 Applications 277 10.12 Practical and Commercial Units 278 11. Infrared Gas Lasers 283 11.1 Carbon Dioxide Lasers 283 11.2 Lasing Medium 283 11.3 Optics and Cavities 285 11.4 Structure of a Longitudinal CO 2 Laser 286 11.5 Structure of a Transverse CO 2 Laser 289 11.6 Alternative Structures 290 11.7 Power Supplies 290 11.8 Output Characteristics 292 11.9 Applications 292 11.10 Far-IR Lasers 293 x CONTENTS 12. Solid-State Lasers 295 12.1 Ruby Lasers 295 12.2 Lasing Medium 296 12.3 Optics and Cavities 297 12.4 Laser Structure 298 12.5 Power Supplies 299 12.6 Output Characteristics 300 12.7 Applications 301 12.8 YAG (Neodymium) Lasers 301 12.9 Lasing Medium 302 12.10 Optics and Cavities 302 12.11 Laser Structure 303 12.12 Power Supplies 306 12.13 Applications, Safety, and Maintenance 308 12.14 Fiber Amplifiers 309 13. Semiconductor Lasers 313 13.1 Lasing Medium 313 13.2 Laser Structure 315 13.3 Optics 319 13.4 Power Supplies 320 13.5 Output Characteristics 321 13.6 Applications 324 14. Tunable Dye Lasers 327 14.1 Lasing Medium 327 14.2 Laser Structure 330 14.3 Optics and Cavities 334 14.4 Output Characteristics 334 14.5 Applications 335 Index 337 CONTENTS xi & PREFACE The field of photonics is enormously broad, covering everything from light sources to geometric and wave optics to fiber optics. Laser and light source technology is a subset of photonics whose importance is often underestimated. This book focuses on these technologies with a good degree of depth, without attempting to be overly broad and all-inclusive of various photonics concepts. For example, fiber optics is largely omitted in this book except when relevant, such as when fiber amplifiers are examined. Readers should find this book a refreshing mix of theory and practical examples, with enough mathematical detail to explain concepts and enable predic- tion of the behavior of devices (e.g., las er gain and loss) without the use of overwhel- mingly com plex calculus. Where possible, a graphical approach has been taken to explain concepts such as modelocking (in Chapter 7) which would otherwise require many pages of calculus to develop. This book, targeted primarily to the scientist or engineer using the technology, offers the reader theory coupled with practical, real-world examples based on real laser systems. We begin with a look at the basics of light emission, including black- body radiation and atomic emission, followed by an outline of quantum mechanics. For some readers this will be a basic review; however, the availability of background material alleviates the necessity to refer back constantly to a second (or third) book on the subject. Throughout the book, practical, solved examples founded on real-world laser systems allow direct application of concepts covered. Case studies in later chap- ters allow the reader to further apply concepts in the text to real-world laser systems. The book is also ideal for students in an undergraduate course on lasers and light sources. Indeed, the original design was for a textbook for an applied degree course (actually, two courses) in laser engineering. Unlike many existing texts which cover this material in a single chapter, this book has depth, allowing the reader to delve into the intricacies. Chapter problems assist the reader by challenging him or her to make the jump between theory and reality. The book should serve well as a text for a single course in laser technology or two courses where a laboratory com- ponent is present. Introductory chapters on blackbody radiation, atomic emission, and quantum mechanics allow the book to be used without the requirement of a second or third book to cover these topics, which are often omitted in similar texts. It is assumed that students will already have a grasp of geometric and wave optics (including the concepts of interference and diffraction), as well as b asic first-year physics, including kinematics. Chapter 1 begins with a look at the most basic light source of all, blackbody radi- ation, and includes a look at standard applications such as incandescent lighting as xiii [...]... e-mail me at mcsele@ieee.org Since I get a large volume of e-mail (and spam), please refer to the book in the subject line MARK CSELE &CHAPTER 1 Light and Blackbody Emission As a reader of this book, you are no doubt familiar with the basic properties of light, such as reflection and interference This book deals with the production of light in its many forms: everything from incandescent lamps to lasers In... reciprocal of the wavelength (1/l) was a function of a constant (the Rydberg constant) and an integer n Neither of these relations (Balmer or Rydberg) detail the mechanism of light emission in the atom Fundamentals of Light Sources and Lasers, by Mark Csele ISBN 0-471-47660-9 Copyright # 2004 John Wiley & Sons, Inc 21 22 ATOMIC EMISSION but rather, attempt to predict its behavior To fully understand the... complete without a discussion of efficiency of various sources Most practical sources of light involve a heated medium, and as such, most of the radiation emitted is in the infrared region of the spectrum at wavelengths too long to be usable by the eye as light Some sources are more efficient than others at emitting light at visible wavelengths To compare light output by various sources, we use a unit called... which govern the operation of other light sources, such as lasers 1.1 EMISSION OF THERMAL LIGHT We have all undoubtedly encountered thermal light in the form of emission of light from a red-hot object such as an element on an electric stove Other examples of such light are in the common incandescent electric lamp, in which electrical current flowing through a thin filament of tungsten metal heats it... This light source did, however, find its way into theaters, where it was used as a Fundamentals of Light Sources and Lasers, by Mark Csele ISBN 0-471-47660-9 Copyright # 2004 John Wiley & Sons, Inc 1 2 1 km LIGHT AND BLACKBODY EMISSION 1m 1 mm Figure 1.2.1 1 mm 1 nm Wavelength Electromagnetic spectrum spotlight which replaced the particularly dangerous open gas flames used at the time for illumination, and. .. have broad energy bands that allow absorption (or emission) over a wide spectrum of wavelengths This is why liquids absorb a range of wavelengths (such as all red and orange light) instead of a specific wavelength such as a low-pressure gas would This entire concept of atomic emission is dealt with in detail in Chapter 2 1.14 EFFICIENCY OF LIGHT SOURCES Since the majority of light sources (natural or... lasers In this chapter we examine the fundamental nature of light itself as well as one of the most basic sources of light: the blackbody radiator Blackbody radiation, sometimes called thermal light since the ultimate power source for such light is heat, is still a useful concept and governs the workings of many practical light sources, such as the incandescent electric lamp In later chapters we shall see... the power of light emitted by a source PROBLEMS 19 In terms of a practical light source, we measure efficiency as a function of light output to power input using the unit lumens per watt, which measures the number of lumens a light source emits for 1 W of input power In theory, a perfect light source emitting at the peak sensitivity of the human eye could deliver 622 lm/W Practical light sources fall... original incandescent lamp was developed in 1825 for use in surveying Ireland and was later used in lighthouses The lamp worked by spraying a mixture of oxygen and alcohol (which burns incredibly hot) at a small piece of lime and igniting it The lime was placed at the hottest part of the flame and heated until it glowed white-hot, emitting an immense quantity of light It was the brightest form of artificial... various lasers are outlined with respect to the PREFACE xv lasing process involved (including quantum mechanics, energy levels, and transitions), details of the laser itself (lasing medium, cooling requirements), power sources for the laser, applications, and a survey of commercially available lasers of that type Visible gas lasers, including helium –neon and ion lasers, are covered in Chapter 9 UV gas lasers . FUNDAMENTALS OF LIGHT SOURCES AND LASERS FUNDAMENTALS OF LIGHT SOURCES AND LASERS Mark Csele A JOHN WILEY & SONS, INC.,. Data: Csele, Mark. Fundamentals of light sources and lasers/ Mark Csele. p. cm. “A Wiley-Interscience publication.” Includes bibliographical references and