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Front Matter
This front matter contains an introduction, table of contents, and glossary of symbols.

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



Published by


P.O. Box 10

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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 Series

Welcome 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 Series

Keep 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 Lasers

Within 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 Contents

Glossary 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 Symbols


area (e.g., the cross section of a laser beam)


brightness (radiance) of a laser beam


velocity of light in a vacuum


electric field strength


saturation energy (e.g., of a laser medium)


focal length (e.g., of a thermal lens)


relaxation oscillation frequency


fluence (energy per area) of a pulse


saturation fluence (e.g., of a laser medium)


gain coefficient


small-signal gain coefficient or initial gain


power amplification factor (= exp(g))


Planck’s constant


optical intensity (power per unit area)


saturation intensity (e.g., of a laser medium)


wave number (= 2π/ λ)


loss coefficient

(e.g., for round-trip losses of a resonator)


length (e.g., of a laser medium)


beam quality factor


refractive index


number density of ions in energy level 2


numerical aperture


optical power (e.g., of a laser beam)


radial position (= distance from beam axis)


radius of curvature (e.g., of wavefronts)


round-trip time of a resonator


output coupler transmission


beam radius


beam radius at the beam waist


position coordinate along a laser beam


Rayleigh length of a laser beam


linewidth enhancement factor


optical phase or azimuthal angle


divergence angle


thermal conductivity




optical frequency


optical bandwidth


absorption cross section


emission cross section


upper-state lifetime


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