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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

Quantum Cascade Lasers 44

Solid-State Bulk Lasers 45

Solid-State Bulk Lasers 45

Rare-Earth-Doped Gain Media 46

Transition-Metal-Doped Gain Media 47

Properties of Host Crystals 48

Effective Cross Sections 49

Phonon Effects in Solid-State Gain Media 50

Quasi-Three-Level Laser Transitions 51

Lamp Pumping vs. Diode Pumping 52

Side Pumping vs. End Pumping 54

Linear vs. Ring Laser Resonators 55

Thermal Effects in Laser Crystals and Glasses 56

Rod Lasers 58

Slab Lasers 59

Thin-Disk Lasers 61

Monolithic Lasers and Microchip Lasers 62

Composite Laser Gain Media 63

Cryogenic Lasers 64

Beam Quality of Solid-State Lasers 65

Fiber and Waveguide Lasers 67

Fiber and Waveguide Lasers 67

Rare-Earth-Doped Fibers 68

Types of Fiber Laser Resonators 69

DBR and DFB Fiber Lasers 70

Double-Clad High-Power Fiber Devices 71

Polarization Issues 73

Other Waveguide Lasers 74

Upconversion Fiber Lasers 75

Dye Lasers 76

Gas Lasers 77

Gas Lasers 77

Helium-Neon Lasers 78

Argon-Ion Lasers 79

Carbon-Dioxide Lasers 80

Excimer Lasers 81

Other Types of Lasers 82

Raman Lasers 82

Free-Electron Lasers 83

Chemically and Nuclear Pumped Lasers 84

Narrow-Linewidth Operation 85

Single-Mode vs. Multimode Operation 85

Intracavity Etalons and Other Filters 87

Examples for Single-Frequency Lasers 89

Injection Locking 90

Tunable Lasers 91

Principles of Wavelength Tuning 91

Tunable Diode Lasers 93

Tunable Solid-State Bulk and Fiber Lasers 94

Other Tunable Laser Sources 95

Q Switching 96

Active vs. Passive Q switching 97

Gain Switching 98

Mode Locking 99

Active Mode Locking 99

Passive Mode Locking 100

Examples for Mode-Locked Solid-State Lasers 101

Cavity Dumping 102

Nonlinear Frequency Conversion 103

Frequency Doubling 103

Sum and Difference Frequency Generation 106

Frequency Tripling and Quadrupling 107

Optical Parametric Oscillators 108

Laser Noise 109

Forms and Origins of Laser Noise 109

Relaxation Oscillations and Spiking 110

Noise Specifications 111

Schawlow-Townes Linewidth 112

Laser Stabilization 113

Laser Safety 114

Overview on Laser Hazards 114

Safe Working Practices 115

Common Challenges for Laser Safety 116

Design and Development 117

Designing a Laser 117

Laser Modeling 118

The Development Process 119

Power Scaling 121

Bibliography 125

Index 126

Introduction

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