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Library of Congress Cataloging-in-Publication Data Paschotta, Rüdiger. Field guide to lasers/Rüdiger Paschotta. p. cm. -- (The field guide series; v. FG12) Includes bibliographical references and index. ISBN 978-0-8194-6961-8 1. Lasers. I. Title. QC688.P37 2007 621.36′6--dc22 2007031117 Published by SPIE P.O. Box 10 Bellingham, Washington 98227-0010 USA Phone: 1 360 676 3290 Fax:+1 360 647 1445 Email: spie@spie.org Web: http://spie.org Copyright © 2008 The Society of Photo-Optical Instrumentation Engineers All rights reserved. No part of this publication may be reproduced or distributed in any form or by any means without written permission of the publisher. The content of this book reflects the work and thought of the author. Every effort has been made to publish reliable and accurate information herein, but the publisher is not responsible for the validity of the information or for any outcomes resulting from reliance thereon. Printed in the United States of America.
Introduction to the SeriesWelcome to the SPIE Field Guides—a series of publications written directly for the practicing engineer or scientist. Many textbooks and professional reference books cover optical principles and techniques in depth. The aim of the SPIE Field Guides is to distill this information, providing readers with a handy desk or briefcase reference that provides basic, essential information about optical principles, techniques, or phenomena, including definitions and descriptions, key equations, illustrations, application examples, design considerations, and additional resources. A significant effort will be made to provide a consistent notation and style between volumes in the series. Each SPIE Field Guide addresses a major field of optical science and technology. The concept of these Field Guides is a format-intensive presentation based on figures and equations supplemented by concise explanations. In most cases, this modular approach places a single topic on a page, and provides full coverage of that topic on that page. Highlights, insights, and rules of thumb are displayed in sidebars to the main text. The appendices at the end of each Field Guide provide additional information such as related material outside the main scope of the volume, key mathematical relationships, and alternative methods. While complete in their coverage, the concise presentation may not be appropriate for those new to the field. The SPIE Field Guides are intended to be living documents. The modular page-based presentation format allows them to be easily updated and expanded. We are interested in your suggestions for new Field Guide topics as well as what material should be added to an individual volume to make these Field Guides more useful to you. Please contact us at fieldguides@SPIE.org. John E. Greivenkamp, Series Editor Optical Sciences Center The University of Arizona The Field Guide SeriesKeep information at your fingertips with all of the titles in the Field Guide series: Field Guide to Geometrical Optics, John E. Greivenkamp (FG01) Field Guide to Atmospheric Optics, Larry C. Andrews (FG02) Field Guide to Adaptive Optics, Robert K. Tyson & Benjamin W. Frazier (FG03) Field Guide to Visual and Ophthalmic Optics, Jim Schwiegerling (FG04) Field Guide to Polarization, Edward Collett (FG05) Field Guide to Optical Lithography, Chris A. Mack (FG06) Field Guide to Optical Thin Films, Ronald R. Willey (FG07) Field Guide to Spectroscopy, David W. Ball (FG08) Field Guide to Infrared Systems, Arnold Daniels (FG09) Field Guide to Interferometric Optical Testing, Eric P. Goodwin & James C. Wyant (FG10) Field Guide to Illumination, Angelo V. Arecchi; Tahar Messadi; R. John Koshel (FG11) Field Guide to LasersWithin the nearly five decades since the invention of the laser, a wide range of laser devices has been developed. The primary objectives of this Field Guide are to provide an overview of all essential lasers types and their key properties and to give an introduction into the most important physical and technological aspects of lasers. In addition to the basic principles, such as stimulated emission and the properties of optical resonators, this Field Guide discusses many practical issues, including the variety of important laser crystal properties, the impact of thermal effects on laser performance, the methods of wavelength tuning and pulse generation, and laser noise. Practitioners may also gain valuable insight from remarks on laser safety (emphasizing real-life issues rather than formal rules and classifications) and obtain new ideas about how to make the laser development process more efficient. Therefore, this Field Guide can be useful for researchers as well as engineers using or developing laser sources. I am greatly indebted to my wife, who strongly supported the creation of this Field Guide, mainly by improving the majority of the figures. Dr. Rüdiger Paschotta RP Photonics Consulting GmbH Zürich, Switzerland Table of ContentsGlossary of Symbols xi Basic Principles of Lasers 1 Principle of a Laser 1 Spontaneous and Stimulated Emission 2 Optical Pumping: Three- and Four-Level Systems 3 Cross Sections and Level Lifetimes 4 Transition Bandwidths 5 Calculating Laser Gain 6 Gain Saturation 7 Homogeneous vs. Inhomogeneous Saturation 9 Spatial Hole Burning 10 Threshold and Slope Efficiency 11 Power Efficiency 13 Amplified Spontaneous Emission 14 Characteristics of Laser Light 15 Laser Beams 16 Temporal Coherence of Laser Radiation 16 Spatial Coherence 17 Gaussian Beams 18 Laser Beam Quality 20 Brightness or Radiance of Laser Beams 21 Optical Resonators 22 Basic Structure of an Optical Resonator 22 Resonator Modes 23 Resonance Frequencies 24 Bandwidth and Finesse of a Resonator 25 Stability Zones of a Resonator 26 Unstable Resonators 27 Resonator Design 28 Waveguides 29 Principle of Waveguiding 29 Waveguide Modes 30 Optical Fibers 31 Planar and Channel Waveguides 32 Semiconductor Lasers 33 Semiconductor Lasers 33 Light Amplification in Semiconductors 34 Low-Power Edge-Emitting Laser Diodes 35 External-Cavity Diode Lasers 36 Broad-Area Laser Diodes 37 Diode Bars 38 Diode Stacks 39 Vertical-Cavity Surface-Emitting Lasers 40 Vertical-External-Cavity Surface-Emitting Lasers 41 Fiber-Coupled Diode Lasers 42 Properties of Diode Lasers 44 Quantum Cascade Lasers 45 Solid-State Bulk Lasers 46 Solid-State Bulk Lasers 46 Rare-Earth-Doped Gain Media 47 Transition-Metal-Doped Gain Media 48 Properties of Host Crystals 49 Effective Cross Sections 50 Phonon Effects in Solid-State Gain Media 51 Quasi-Three-Level Laser Transitions 52 Lamp Pumping vs. Diode Pumping 53 Side Pumping vs. End Pumping 55 Linear vs. Ring Laser Resonators 56 Thermal Effects in Laser Crystals and Glasses 57 Rod Lasers 59 Slab Lasers 60 Thin-Disk Lasers 62 Monolithic Lasers and Microchip Lasers 63 Composite Laser Gain Media 64 Cryogenic Lasers 65 Beam Quality of Solid-State Lasers 66 Properties of Solid-State Bulk Lasers 68 Fiber and Waveguide Lasers 69 Fiber and Waveguide Lasers 69 Rare-Earth-Doped Fibers 70 Types of Fiber Laser Resonators 71 DBR and DFB Fiber Lasers 72 Double-Clad High-Power Fiber Devices 73 Polarization Issues 75 Other Waveguide Lasers 76 Upconversion Fiber Lasers 77 Properties of Fiber Lasers 78 Dye Lasers 79 Properties of Dye Lasers 80 Gas Lasers 81 Gas Lasers 81 Helium-Neon Lasers 82 Argon-Ion Lasers 83 Properties of Ion Lasers 84 Carbon-Dioxide Lasers 85 Properties of Carbon-Dioxide Lasers 86 Excimer Lasers 87 Properties of Excimer Lasers 88 Other Types of Lasers 89 Raman Lasers 89 Free-Electron Lasers 90 Chemically and Nuclear Pumped Lasers 91 Narrow-Linewidth Operation 92 Single-Mode vs. Multimode Operation 92 Intracavity Etalons and Other Filters 94 Examples of Single-Frequency Lasers 96 Injection Locking 97 Tunable Lasers 98 Principles of Wavelength Tuning 98 Tunable Diode Lasers 100 Tunable Solid-State Bulk and Fiber Lasers 101 Other Tunable Laser Sources 102 Q Switching 103 Active vs. Passive Q Switching 104 Gain Switching 105 Mode Locking 106 Active Mode Locking 106 Passive Mode Locking 107 Examples of Mode-Locked Solid-State Lasers 108 Cavity Dumping 109 Nonlinear Frequency Conversion 110 Frequency Doubling 110 Sum and Difference Frequency Generation 113 Frequency Tripling and Quadrupling 114 Optical Parametric Oscillators 115 Laser Noise 116 Forms and Origins of Laser Noise 116 Relaxation Oscillations and Spiking 117 Noise Specifications 118 Schawlow-Townes Linewidth 119 Laser Stabilization 120 Laser Safety 121 Overview on Laser Hazards 121 Safe Working Practices 122 Common Challenges for Laser Safety 123 Design and Development 124 Designing a Laser 124 Laser Modeling 125 The Development Process 126 Power Scaling 128 Equation Summary 130 Bibliography 134 Glossary of SymbolsA area (e.g., the cross section of a laser beam) B brightness (radiance) of a laser beam c velocity of light in a vacuum E electric field strength Esat saturation energy (e.g., of a laser medium) f focal length (e.g., of a thermal lens) fro relaxation oscillation frequency Fp fluence (energy per area) of a pulse Fsat saturation fluence (e.g., of a laser medium) g gain coefficient g0 small-signal gain coefficient or initial gain G power amplification factor (= exp(g)) h Planck’s constant I optical intensity (power per unit area) Isat saturation intensity (e.g., of a laser medium) k wave number (= 2π/ λ) l loss coefficient (e.g., for round-trip losses of a resonator) L length (e.g., of a laser medium) M2 beam quality factor n refractive index N2 number density of ions in energy level 2 NA numerical aperture P optical power (e.g., of a laser beam) r radial position (= distance from beam axis) R radius of curvature (e.g., of wavefronts) Trt round-trip time of a resonator Toc output coupler transmission w beam radius w0 beam radius at the beam waist z position coordinate along a laser beam zR Rayleigh length of a laser beam α linewidth enhancement factor φ optical phase or azimuthal angle θ divergence angle κ thermal conductivity λ wavelength ν optical frequency Δν optical bandwidth σabs absorption cross section σem emission cross section τ2 upper-state lifetime |
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