`
`
`
`Loser
`Engineering
`
`Kelin Kuhn
`University of Washington
`
`ASML 1217
`
`

`

`Library of Congress Cataloging-in-Publlcation Data
`Kuhn. Kelin 1.
`Laser engineering I Kelin J. Kuhn
`.
`cm.
`Includes index.
`ISBN 0-02—366921-7 (hardcover)
`
`I. Title.
`TA1675.K84
`
`1998
`
`97-53211
`CIP
`
`1.Lasers—Designandconstruction.2.Nonlinearoptics.
`
`Acquisition Editor: Eric Svendsen
`Editor-in-Chief: Marcia Horton
`Production Manager. Bayani Mendoza de Leon
`Editor-in—Chief: Jerome Grant
`Director of Production and Manufacturing: David W. Riccardi
`Manufacturing Manager: Trudy Pisciotti
`Full Service Coordinator: Donna Sullivan
`Composition/Production Service: ETP Harrison
`Editorial Assistant: Andrea Au
`Creative Director: Paula Maylahn
`Art Director: Jayne Conte
`Cover Designer: Bruce Kenselaar
`
`© 1998 by Prentice-Hall. Inc.
`A Pearson Education Company
`Upper Saddle River, N] 07458
`
`All rights reserved. No part of this book may be
`reproduced, in any form or by any means.
`without pen-nission in writing from the publisher.
`
`The author and publisher of this book have used their best efforts in preparing this book. These efforts
`include the development, research, and testing of the theories and programs to detemiine their effectiveness.
`The author and publisher make no warranty of any kind. expressed or implied, with regard to these programs
`or the documentaan contained in this book. The author and publisher shall not be liable in any event for
`incedental or consequential damages in connection with. or arising out of. the furnishing, performance, or
`use of these programs.
`
`Printed in the United States of America
`10
`9
`8
`7
`6
`5
`4
`3
`2
`
`ISBN D-UE-BEE‘IEl-7
`
`Prentice-Hall International (UK) Limited,London
`Prentice-Hall of Australia Pty. Limited, Sydney
`Prentice-Hall Canada Inc., Toronto
`Prentice-Hall Hispanoamericana, S.A., Mexico
`Prentice-Hall of India Private Limited, New Delhi
`Prentice-Hall of Japan, Inc., Tokyo
`Pearson Education Asia Pte. Ltd., Singapore
`Editora Prentice—Hall do Brasil, Ltda., Rio de Janeiro
`
`}
`
`2
`
`.9
`
`.J..,_.,,(;‘-_
`
`j
`
`
`
`

`

`PREFACE
`
`xi
`
`Organization
`
`xi
`
`Technical Background
`
`xii
`
`Pedagogy xii
`
`Scheduling
`
`xiii
`
`Acknowledgments
`
`xiv
`
`Part I Laser Fundamentals
`
`1
`
`INTRODUCTION TO LASERS
`
`2
`
`1.1
`
`1.2
`
`1.3
`
`1.4
`
`1.5
`1.6
`
`A Brief History
`
`2
`
`The Laser Market
`
`5
`
`Energy States in Atoms
`
`9
`
`10
`Basic Stimulated Emission
`1.4.1
`Transitions Between Laser States, 10
`1.4.2
`Population Inversion, 13
`Power and Energy
`14
`
`Monochromaticity, Coherency, and Linewidth
`
`15
`
`
`
`“
`
`1
`!
`g
`
`
`
`

`

`
`
`Contents
`
`1.7
`
`1.8
`
`1.9
`
`1.10
`
`1.11
`
`Spatial Coherence and Laser Speckle
`
`18
`
`The Generic Laser
`
`19
`
`Transverse and Longitudinal Modes
`
`20
`
`The Gain Profile
`
`22
`
`Laser Safety
`
`24
`
`Symbols Used in the Chapter
`
`25
`
`Exercises
`
`26
`
`2 ENERGY STATES AND GAIN
`
`34
`
`2.1
`
`2.2
`
`35
`Energy States
`2.1.1
`Laser States, 35
`2.1.2
`Multiple-State Laser Systems, 36
`2.1.3
`Linewidth and the Uncertainty Principle, 39
`2.1.4
`Broadening of Fundamental Linewidths, 41
`43
`
`Gain
`2.2.1
`2.2.2
`2.2.3
`
`Basics of Gain, 43
`Blackbody Radiation. 47
`Gain. 55
`
`Symbols Used in the Chapter
`
`58
`
`Exercises
`
`59
`
`3 THE FABRY-PEHOT ETALON
`
`62
`
`3.1
`
`3.2
`
`3.3
`
`62
`Longitudinal Modes in the Laser Resonant Cavity
`3.1.1
`Using an Etalon for Single Longitudinal Mode Operation, 64
`
`65
`Quantitative Analysis of a Fabry-Perot Etalon
`3.2.1
`Optical Path Relations in a Fabry-Perot Etalon, 65
`3.2.2
`Reflection and Transmission Coefficients in a Fabry-Perot Etalon, 67
`3.2.3
`Calculating the Reflected and Transmitted Intensities for a Fabry-Perot
`Etalon with the Same Reflectances, 70
`Calculating the Reflected and Transmitted Intensities for a Fabry—Perot
`Etalon with Different Reflectances, 72
`Calculating the Q and the Finesse of a Fabry-Perot Etalon. 73
`
`3.2.4
`
`3.2.5
`
`Illustrative Fabry-Perot Etalon Calculations
`
`73
`
`
`
`

`

`4 TRANSVERSE MODE PROPERTIES
`
`83
`
`4.1
`
`4.2
`
`4.3
`
`4.4
`
`4.5
`
`Introduction
`
`84
`
`84
`TEMI,y Transverse Modes
`4.2.1
`The Paraxial Approximation, 84
`4.2.2 Mathematical Treatment of the Transverse Modes, 86
`
`88
`TEMo‘o Gaussian Beam Propagation
`4.3.1
`The TEMolo or Gaussian Transverse Mode, 88
`4.3.2
`Properties of the TEMM Mode of the Laser, 94
`
`Ray Matrices to Analyze Paraxial Lens Systems
`4.4.1
`Ray Matrix for a Distance d, 103
`4.4.2
`Ray Matrix for a Lens, 104
`4.4.3
`ABCD Law Applied to Simple Lens Systems, 108
`
`101
`
`110
`Gaussian Beams in Resonant Cavities
`4.5.1 Modeling the Stability of the Laser Resonator, 113
`4.5.2 ABCD Law Applied to Resonators, 117
`
`Symbols Used in the Chapter
`
`122
`
`Exercises
`
`124
`
`5 GAIN
`
`SATURATION
`
`131
`
`5.1
`
`5.2
`
`5.3
`
`131
`Saturation of the Exponential Gain Process
`5.1.1
`Gain Saturation for the Homogeneous Line, 134
`5.1.2 Gain Saturation for the Inhomogeneous Line, 134
`5.1.3
`The Importance of Rate Equations, 134
`
`135
`Setting Up Rate Equations
`5.2.1
`Rate Equations for Four-State Lasers, 137
`
`142
`Laser Output Power Characteristics
`5.3.1
`Optimal Coupling, a Simple Approach, 142
`5.3.2
`Pom versus Pin. an Engineering Approach, 147
`5.3.3
`Pom versus P}... the Rigrod Approach, 152
`
`Symbols Used in the Chapter
`
`159
`
`Exercises
`
`161
`
`6 TRANSIENT PROCESSES
`
`153
`
`6.1
`
`6.2
`
`164
`Relaxation Oscillations
`6.1.1
`A Qualitative Description of Relaxation Oscillations. 164
`6.1.2 Numerical Modeling of Relaxation Oscillations, 165
`6.1.3
`Analytical Treatment of Relaxation Oscillations, 17]
`
`177
`Q—Switching
`6.2.1
`A Qualitative Description of Q-Switching, 177
`
`~
`
`Contents
`
`
`
`
`
`
`
`

`

`
`
`Contents
`
`vii
`
`6.3
`
`6.4
`
`6.2.2 Numerical Modeling of Q—Switching, 177
`6.2.3 Analytical Treatment of Q-Switching, 178
`
`182
`The Design of Q-Switches
`6.3.1 Mechanical Q-Switches, 183
`6.3.2
`Electrooptie Q-Switches, 184
`6.3.3
`Acousto—Optic Q-Switehes, 190
`6.3.4
`Saturable Absorber Dyes for Q-Switching, 191
`
`193
`Mode-Locking
`6.4.1
`A Qualitative Description of Mode-Locking, 193
`6.4.2 Analytical Description of Mode-Locking. 195
`6.4.3
`The Design of Mode-Locking Modulators, 198
`
`Symbols Used in the Chapter 202
`
`6.5
`
`Exercises
`
`204
`
`7 INTRODUCTION TO NONLINEAR OPTICS
`
`207
`
`7.1
`
`7.2
`
`7.3
`
`7.4
`
`7.5
`
`7.6
`
`Nonlinear Polarizability
`
`208
`
`209
`Second Harmonic Generation
`7.2.1
`The Process of Conversion, 210
`7.2.2
`Phase Matching, 215
`7.2.3 Design Techniques for Frequency-Doubling Laser Beams, 220
`
`Optical Parametric Oscillators
`
`221
`
`Stimulated Raman Scattering
`
`226
`
`Self-Focusing and Optical Damage
`
`231
`
`233
`Nonlinear Crystals
`7.6.1 Major Crystals. 233
`7.6.2 Other Crystals Used in Nonlinear Optics. 235
`
`Symbols Used in the Chapter
`
`236
`
`Exercises
`
`238
`
`8 SUPPORTIVE TECHNOLOGIES
`
`241
`
`8.1
`
`8.2
`
`Introduction
`
`242
`
`242
`~Multilayer Dielectric Films
`8.2.1
`The Fundamentals of Multilayer Film Theory, 243
`8.2.2 Anti-Reflection Coatings from Multilayer Films. 245
`8.2.3 High-Reflectance Coatings from Multilayer Films, 248
`
`
`
`:3a;
`
` i 1
`
`

`

`Contents
`
`vill
`
`8.4
`
`261
`Photodetectors
`8.4.1
`Thermal Detectors, 261
`8.4.2
`Photoelectric Detectors, 262
`8.4.3
`Photoconductors, 263
`8.4.4
`Junction Photodetectors, 265
`8.4.5 MOS Capacitor Devices, 268
`
`Symbols Used in the Chapter
`
`269
`
`Part II Design of Laser Systems
`
`273
`
`9 CONVENTIONAL GAS LASERS
`
`274
`
`9.1
`
`9.2
`
`274
`HeNe Lasers
`9.1.1 History of HeNe Lasers, 274
`9.1.2 Applications for HeNe Lasers, 276
`9.1.3
`The HeNe Energy States, 280
`9.1.4 Design of a Modem Commercial HeNe Laser, 283
`
`288
`Argon Lasers
`9.2.1
`History of Argon- and Krypton-Ion Lasers, 289
`9.2.2 Applications for Argon- and Krypton-Ion Lasers, 290
`9.2.3 Argon and Krypton Laser States, 292
`9.2.4 Design of a Modern Commercial Argon-Ion Laser, 294
`Exercises
`300
`
`10 CONVENTIONAL SOLID-STATE LASERS
`
`302
`
`10.1
`
`History
`
`303
`
`10.2
`
`10.3
`
`Applicau'ons
`
`307
`
`Laser Materials
`
`308
`
`10.4
`
`10.5
`
`10.3.1 Crystalline Laser Hosts, 309
`10.3.2 Glass Laser Hosts, 310
`10.3.3 The Shape of the Solid-State Laser Material, 311
`The Laser Transition In Nd:YAG 312
`
`315
`Pump Technology
`10.5.1 Noble Gas Discharge Lamps as Optical Pump Sources for Nd:YAG
`Lasers, 316
`
`10.5.2 Power Supplies for Noble Gas Discharge Lamps, 321
`10.5.3 Pump Cavities for Noble Gas Discharge Lamp—Pumped Lasers, 324
`10.5.4 Spectra-Physics Quanta-Ray GCR Family, 327
`10.5.5 Semiconductor Lasers as Solid-State Laser Pump Sources, 329
`10.5.6 Pump Cavities for Diode Laser Pumped Solid-State Lasers, 333
`10.5.7 Coherent DPSS 1064 Laser Family, 337
`Exercises
`338
`
`
`
`,
`4}
`g
`
`
`
`it
`1.;
`
`
`
`

`

`
`
`Contents
`
`Ix
`
`11 TRANSITION-METAL SOLID-STATE LASERS
`
`344
`
`11.1
`
`11.2
`
`11.3
`
`11.4
`
`11.5
`
`History
`
`345
`
`Applications
`
`348
`
`348
`Laser Materials
`11.3.1 Ruby—Primary Line at 694.3 nm, 349
`11.3.2 Alexandrite—Tunable from 700 nm to 818 nm, 351
`11.3.3 Ti:Sapphire——Tunable from 670 nm to 1090 nm, 353
`11.3.4 Comparison between Major Solid-State Laser Hosts, 355
`
`Ti:Sapphire Laser Design
`11.4.1 Ring Lasers. 356
`11.4.2 Birefringent Filters, 362
`11.4.3 Coherent Model 890 and 899 Ti:Sapphire Lasers, 365
`
`356
`
`370
`Femtosecond Pulse Laser Design
`11.5.1 Dispersion in Femtosecond Lasers, 370
`11.5.2 Nonlinearities Used to Create Femtosecond Pulses, 371
`11.5.3 Measuring Femtosecond Pulses, 373
`11.5.4 Colliding Pulse Mode-Locking, 373
`11.5.5 Grating Pulse Compression, 374
`11.5.6 Solitons, 375
`11.5.7 Kerr-Lens Mode-Locking (KLM) in Ti:Sapphire, 376
`11.5.8 Coherent Mira Femtosecond Lasers. 377
`
`Exercises
`
`380
`
`12 OTHER MAJOR COMMERCIAL LASERS
`
`384
`
`12.1
`
`12.2
`
`12.3
`
`385
`The Design of Carbon Dioxide Lasers
`12.1.1
`Introduction to C02 Laser States. 386
`12.1.2 The Evolution of CO; Lasers, 389
`12.1.3 Waveguide C02 Lasers, 393
`12.1.4 A Typical Modern C02 Industrial Laser, 394
`12.1.5 Optical Components and Detectors for C02 Lasers, 403
`
`404
`The Design of Excimer Lasers
`12.2.1
`Introduction to Excimer Laser States, 405
`12.2.2 The Evolution of Excimers. 408
`12.2.3 General Design Background, 409
`12.2.4 A Typical Modern Excimer Laser, 414
`12.2.5 Laser Beam Homogenizers, 417
`12.2.6 Application Highlight, 418
`
`421
`Overview of Semiconductor Diode Lasers
`12.3.1 History of Semiconductor Diode Lasers, 421
`12.3.2 The Basics of the Semiconductor Diode Laser, 424
`
`

`

`x
`
`APPENDIX
`
`441
`
`Contents
`
`A.1
`
`A2
`
`A3
`
`AA
`
`A.5
`
`A6
`
`A7
`
`AS
`
`441
`Laser Safety
`A. 1 .1
`Electrocution, 441
`A.1.2 Eye Damage, 444
`A.1.3 Chemical Hazards, 446
`A.1.4 Other Hazards. 447
`
`Significant Figures
`
`450
`
`450
`
`The Electromagnetic Wave Equation
`A.3.1 Maxwell's Equations, 450
`A32 A General Wave Equation for Light Propagation in a Material, 452
`A33 Light Propagation in a Vacuum. 453
`A.3.4 Light Propagation in a Simple Isotropic Material with No Net Static
`Charge, 454
`A.3.5 Light Propagation in a Simple Laser Material with No Net Static
`Charge, 454
`A.3.6 A One-Dimensional Wave Equation for a Less Simple Isotropic
`Material, 454
`
`Lenses and Telescopes
`A.4.l
`Lenses, 456
`A.4.2 Classical Lens Equations, 457
`A.4.3 Telescopes, 459
`
`456
`
`Reflection and Refraction
`A.5.1 Nomenclature, 461
`A52 Snell's Law, 462
`
`461
`
`A.5.4 Brewster's Angle, 462
`
`Total Internal Reflection, 462
`A.5.3
`Fresnel Equations
`463
`
`The Effective Value of the Nonlinear Tensor 465
`
`Projects and Design Activities
`A.8.1 Gas Laser Activities, 466
`A.8.2 Nd:YAG Laser Activities, 472
`A.8.3 Transition Metal Laser Activities, 473
`A.8.4
`Successful Student Projects, 474
`
`466
`
`A9
`
`Laser Alignment
`
`475
`
`A.10 Glossary of Basic Laser Terms
`
`477
`
`INDEX
`
`483
`
`CONSTANTS USED IN BOOK
`
`498
`
`
`
`i
`
`#
`1
`
`_..--—x—u—-—_.+L__M...V.,-
`
`
`
`

`

`
`
`i;
`
`'
`
`wee-u» i h»‘:.
`
`h
`
`o'mrhercidl i
`Losers
`
`Objectives
`
`Carbon dioxide lasers
`
`0 To summarize the generic characteristics of the C0; laser.
`a To describe the various energy states of the C02 laser and to summarize how these
`states interact with each other.
`
`0 To summarize the sequence of historical events leading to the development of the
`C02 laser.
`
`0 To describe the major characteristics of waveguide versus free space C02 lasers.
`0 To describe the construction of a commercial waveguide C02 laser.
`Excimer lasers
`
`0 To summarize the generic characteristics of the excimer laser.
`c To describe the various energy states of the excimer laser and to summarize how
`these states interact with each other.
`0 To summarize the sequence of historical events leading to the development of the
`excimer laser.
`
`0 To describe the general design principles underlying excimer lasers. These include
`preionization, corona discharge circuitry, and main discharge circuitry.
`c To describe the construction of a commercial excimer laser.
`Semiconductor diode lasers
`
`g
`
`.
`
`‘
`a.
`
`c;
`‘
`7 .
`
`0 To summarize the sequence of historical events leading to the development of the
`semiconductor laser.
`_
`.
`a To describe the energy band structure of the semiconductor diode laser.
`
`_
`
`.
`;"
`
`334
`
`‘
`
`

`

` Sec. 12.1
`
`
`
`The Design of Carbon Dioxide Lasers
`
`385
`
`
`
`3:]
`“71-.
`”I.
`
`
`
`.
`_-
`
`:
`
`f.
`I
`
`0 To summarize the process of pumping the semiconductor diode laser with a PN-
`junction.
`0 To describe the process of creating a semiconductor laser cavity by cleaving the
`semiconductor material.
`
`0 To describe the similarities and differences between homostructure and heterostruc-
`ture semiconductor diode lasers.
`
`0 To describe the importance of vertical and horizontal confinement in designing
`semiconductor laser structures.
`
`a To describe the major vertical and horizontal confinement structures.
`e To describe the general physical principles governing the design of quantum wells,
`with special emphasis on the importance of the width of the quantum well
`in
`determining the optical properties of quantum well laser diodes.
`
`12.1 THE DESIGN OF CARBON DIOXIDE LASERS
`
`C02 lasers operate over a series of vibrational and rotational bands in the regions 9.4 and 10.6
`pm. They are both high—average-power and high-efficiency laser systems. Commercially
`available cw C02 lasers range in power from 6 watts to 10,000 watts, and custom lasers are
`available at even higher powers. Small (2 to 3 feet long) C02 lasers can produce hundreds
`of watts of average power at an efficiency of 10%. Larger C02 lasers can produce many
`kilowatts of cw power. CO2 lasers are widely used in such diverse commercial applications
`as marking of electronic components, wafers, and chips; marking on anodized aluminum;
`trophy engraving; acrylic sign making; rapid prototyping of 3D models; cutting of ceramics,
`textiles, and metals; carpet, sawblade, and sail cutting; drilling; thin film deposition; and
`wire shipping (see Figure 12.1). They find application in the medical field for laser surgery,
`and in research for spectroscopy and remote sensing. Military applications include imaging,
`mapping, and range-finding. They have also been used in inertial confinement fusion as an
`alternative to large glass lasers.
`C02 is a laser material totally unlike the materials discussed so far in this text. Con-
`ventional lasers lase off of electronic transitions between various atomic states. C02 lasers
`lase off molecular transitions between the various vibrational and rotational states of C02.
`Among other things, this means that CO2 lasers have a longer wavelength and higher effi-
`ciency than most conventional lasers. Additional information on CO2 lasers can be found
`in Cheo,1 Duley,2 Tyte,3 and Witteman.4 Additional information on high peak power and
`gas dynamic C02 lasers can be found in Anderson,5 Beaulieu,6 and Losev.7
`
`
`1Peter K. Cheo, Handbook of Molecular Lasers (New York: Marcel-Dekker Inc., 1987).
`2W. W. Duley, C02 Lasers: Effects and Applications, (New York: Academic Press. 1976).
`3D. C. Tyte, Advances in Optical Electronics, Vol. 1, ed D. W. Goodwin, (New York: Academic Press.
`1970), pp. 129—198.
`
`4W. J. Witteman, The C02 laser (Berlin: Springer-Verlag, 1987).
`slohn Anderson. Gasdynamz’c Lasers: An Introduction (New York: Academic Press, 1976).
`
`

`

`386
`
`Other Major Commerciai Lasers
`
`Chap. 1
`
`
`
`

`

`
`
`Sec. 12.1
`
`The Design of Carbon Dioxide Lasers
`
`387
`
`W. Carbon dioxide molecule
`
`W Symmetric stretch mode
`
`WM Bendingmode
`W Asymmetric stretch mode
`
`<—-——><———><—>
`
`Figure 12.2 Normal modes of the carbon dioxide molecule.
`
`The C02 molecules can also rotate, resulting in a series of closely spaced states char-
`acterized by the rotational quantum number J. The rotational energies of a given vibrational
`state 1' relative to the J = 0 level are given as
`
`EN = hcoBiJU +1) — hcaDJ2(J +1)2
`
`(12.2)
`
`where B, and D are constants.9
`The principal laser transitions in C02 are the (001) to (100) 10.6 um transitions and
`the (001) to (020) 9.4 nm transitions (see Figure 12.3). Each of the levels (001), (100), and
`(020) consists of a series of rotational states. Transitions in C02 occur between states where
`Judd —> (J + Dam, (termed the P-branch) and Judd —> (J — Dem, (termed the R-branch).
`(See Figure 12.4.)
`If no wavelength discrimination is provided in the cavity, the P-branch of the (001) to
`(100) 10.6 urn transition will dominate. However, if wavelength selection is provided (by a
`grating, for example), it is possible to lase on any of the allowed P- or R-branch transitions.
`Notice, however, that since both the (001)—>(100) and the (001)—>(020) transitions share
`the same upper laser level,
`then the (001)—>000) transition must be suppressed for the
`(001)—>(100) transition to lase.
`The majority of CO2 lasers contain a mixture of three gases (CO2, N2, and He) in a
`roughly 0.8:l:7 ratio.10 The C02 is the laser gain material. The N2 has only one excited
`mode (the symmetrical stretch mode) and the energy of the (1) N2 vibration nicely aligns
`with the (001) upper state of the C02 molecule (see Figure 12.3). Since the N2 vibrational
`states are metastable (very long lifetimes) the energy in the (1) N2 transition (plus a little
`kinetic energy) can be transferred to a C02 molecule as a means of populating the (001)
`
`

`

`388
`
`Other Major Commercial Lasers
`
`Chap, 12
`
`I
`<——~~co2
`o
`I
`$300639
`g
`4-
`.
`.
`Asymmetric stretch I
`
`e0 9 are
`<-
`—>
`.
`Symmetric stretch
`
`r
`
`Bending
`
`I
`N2———>I<—He—>
`0 one:
`<—
`—>
`
`;
`
`r
`f
`s
`‘
`
`.
`
`II
`
`I
`I
`I
`I
`I
`I
`I
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`I
`helium is 67.7 times {ha
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`v = 0 to v = 1 spacing
`I
`in nitrogen
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`>
`E‘
`
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`
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`
`
`
`
`
`Absorption(percent)
`
`Figure 12.3 Laser states of the carbon dioxide molecule. (From LASER ELECTRONICS 2E. by
`VERDEYEN, J.T. @1989. Figure 10.14, p. 336. Adapted by permission of Prentice-Hall. Inc.,
`Upper Saddle River, NJ.)
`
`P B
`
`h
`
`ram
`
`”20)
`
`P(10)
`
`Fl Branch K
`
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`
`930
`
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`
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`
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`
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`
`960
`
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`
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`
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`
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`
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`HUG)
`\
`
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`
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`
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`
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`
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`
`'
`
`1030
`
`1040
`
`1050
`
`1060
`
`1070
`
`1080
`
`Figure 12.4 Absorption spectrum of the carbon dioxide molecule. (From E. F. Barker
`and A. Adel, Phys. Rev. 44:185 (1933))
`
`Frequency (cm‘l)
`
`
`
`

`

`
`
`Sec. 12.1
`
`The Design of Carbon Dioxide Lasers
`
`389
`
`upper CO2 level (notice that the N2—CO2 energy transfer is very similar to the He—Ne en—
`ergy transfer in HeNe lasers; see Section 9.1.3). The helium in the gas mixture provides
`cooling by means of thermal transfer to the walls (helium is a very thermally conductive
`gas). Helium also plays a role in optimizing the kinetic energy of the N2 molecules for
`maximum energy transfer between the N2 and CO2.
`Because of the metastable N2 and the match between the (1) N2 level and the (001)
`C02 level, the conversion efficiency between input electrical power to power in the upper
`laser state is 50 to 70%. Since the quantum efficiency is roughly 45%, this means that C02
`lasers can operate at extremely high efficiencies (10 to 35%).
`
`12.1.2 The Evolution of CO2 Lasers
`
`in 1964.1“2’13
`The first demonstration of laser action from CO2 was reported by Patel
`The concept of using N2 to transfer vibrational energy from the electrical discharge to the
`CO2 was recognized by Legay and Legay-Somrnaire in the same year14 and the idea of
`incorporating helium for cooling was first proposed by Patel a year later.15 During this
`period of rapid development on the CO2 laser, Patel and other researchers were able to
`improve Patel’s original 1 mW output to roughly 100 watts.16'l7'”3
`The first CO2 lasers were constructed from long tubes of glass where the desired laser
`mixture flowed through the glass tube (see Figure 12.5). Electrodes in the gas generated
`a plasma arc to excite the N2 molecules into their symmetrical stretch mode. Although
`the very first demonstration of laser action from CO2 used RF excitation, systems soon
`converted to DC excitation for increased power.19
`The original glass tube CO2 lasers operated at low pressures with the electrical dis-
`charge running longitudinally down the cavity. As a consequence, operating pressures were
`low due to the necessity to create and maintain a plasma over a long distance. However, in
`1970, Beaulieu20 first reported operation of an atmospheric pressure CO2 laser by exciting
`the discharge transversely to the cavity (see Figure 12.6). These Transverse Excited At—
`mospheric (TEA) lasers offered higher gains and greater output powers than longitudinally
`excited lasers.
`
`
`
`“c. KN. Patel. Phys. Rev. Len. 12:588 (1964).
`l2C. K. N. Patel, Phys. Rev. Len. 13: 617 (1964).
`13c. K. N. Patel, Phys. Rev. 136:A1187 (1964).
`l4F. Legay and N. Legay-Sornmaire. C. R. Acad. Sci. 259Bz99 (1964).
`15c. K. N. Patel, P. K. Tien, and I. H. McFee,Appl. Phys. Lett. 7:290 (1965).
`16c. K. N. Patel, Phys. Rev. 136:A1187 (1964).
`17N. Legay-Sommaire, L. Henry, and F. Legay, CR. Acad. Sci. 260Bz3339 (I965).
`
`

`

`390
`
`IR
`TRANSMITTING
`WINDOW
`I
`CONCAVE
`
`Other Major Commercial Lasers
`
`Chap. 12
`
`_'
`
`C02
`
`
`
`RF
`
`GENERATOR
`
`(27 Me/s)
`
`
`
`
`
`
`
`MICROMETERS
`
`FOR ALIGNMENT
`PUMP
`
`7/”
`
`
`
`
`NOTE THAT THERE
`
`IS NO DISCHARGE
`IN THE INTERACTION
`REM” ‘
`
`
`Figure 12.5 Early carbon dioxide laser construction. (From C. K. N. Patel, Phys. Rev. Lett. 13:
`617 (1964). Reprinted with the permission of the author.)
`
`The C02 laser Q-switches exceptionally well and Q—switched operation was reported
`in 1966 by a number of researchers including Flynn,”22 Kovacs,23 Bridges.24 and Patel.25
`However,
`the narrow bandwidth of C02 (approximately 50 MHz), means that physically
`long lasers are required to effectively demonstrate mode—locking. In spite of this difficulty,
`the first mode-locking of a conventional CO2 laser was reported in 1968 by Caddes,26 and
`Wood and Schwartz.27 High-peak power can also be obtained from C02 lasers by pulsing
`or gain switching the lasers.28 TEA lasers are especially well-suited for production of
`high-peak pOWer CO2 laser pulses.29
`In a conventional C0; laser, the output power will increase as the gas flow is increased.
`This increased power is thought to be due to enhanced cooling and more effective removal
`of dissociation products such as CO and 02 from the CO2 discharge.30 However, in many
`applications,
`it is not possible to support the peripheral equipment for handling flowing
`gases and a sealed laser configuration is required.
`In a sealed laser, the lack of gas flow
`means that some mechanism must be provided to regenerate the dissociated gas products
`R
`21(3. W. Flynn. M. A. Kovacs, C. K. Rhodes. and A. Javan, Appl. Phys. Lett. 8:63 (1966).
`22G. W. Flynn. L. O. Hooker, A. Javan. M. A. Kovacs. and C. K. Rhodes. IEEE J. Quan. Elec. QE-2:378
`(1965).
`23G. W. Flynn. L. 0. Hooker, A. Javan, M. A. Kovacs, and C. K. Rhodes, IEEE J. Quart. Elec. QE—2:378
`(1966).
`24T. J. Bridges. Appl. Phys. Lett. 9:174 (1966).
`25C. K. N. Patel, Phys. Rev. Lerr. 162613 (1966).
`26D. E. Caddes. L. M. Osterink, and R. Targ. Appl. Phys. Len. 12:74 (1968).
`270.11. Wood and s. E. Schwartz, Appl. Phys. Len. 12:263 (1968).
`23A. E. Hill, Appl. Phys. Lett. 12:324 (1968).
`29W. W. Duley, C02 Lasers: Effects and Applications (New York: Academic Press, 1976), Chapter 2.
`3°Tyte. D. C., in Advances in Optical Electronics, Vol 1. ed D.W. Goodwin (New York: Academic Press.
`1970). pp. 167—168.
`
`.-
`l.-
`7
`.'
`.
`T l
`
`4-
`
`.
`
`_
`"'
`
`in:
`.1
`
`
`
`

`

`
`
`The Design of Carbon Dioxide Lasers
`
`391
`
`
`
`Sec. 12.1
`
`Anode
`
`A schematic of an early
`Figure 12.6
`Transverse Excited Atmospheric (TEA)
`laser. (Reprinted with permission from A. J.
`Beaulieu, Appl. Phys. Lett. 16:504 (1970).
`©1970 American Institute of Physics.)
`
`NaCl window
`
`/ H
`
`irror
`
`M
`
`/%
`
`Cathodes
`
`
`
`Detector
`
`(particularly the oxygen species) back into C02. If these products are permitted to react
`with the tube walls,
`the chemical equilibrium of the plasma is disturbed and additional
`dissociation products are formed. Various regeneration methods include adding additional
`gases, periodically heating the tube, or incorporating catalyst alloys on the electrodes. Sealed
`lasers demonstrating such regeneration methods were first developed by Wittman in 196531
`and further developed by Wittman32 and Carbone.33
`The initial use of flowing gases to improve the output performance of C02 lasers led
`to the development of another fascinating way to pump C02. The basic idea is to begin
`with a hot equilibrium gas mixture and then to expand the mixture through a supersonic
`nozzle. This lowers the temperature and pressure of the gas mixture in a time short compared
`to the upper state lifetime. When this occurs, the upper laser level cannot track with the
`temperature and pressure changes and so remains at its initial values. In contrast, the lower
`level population drops dramatically. The result is a population inversion that extends some
`distance downstream of the supersonic nozzle (see Figure 12.7). Lasers using this type
`of pumping are called gas dynamic lasers and were first suggested by Konyukhov and
`Prokhorov34 in 1966 and demonstrated by Gerry35 and Konyukhov.36
`The most spectacular forms of gas dynamic lasers are those run using jet or rocket
`engines as the pump source. The basic idea is to create a laser gas mixture by burning some
`type of fuel that generates the C02. The fuel source is often ignited with a methanol burner,
`
`31w.r. Witteman. Phys. Lett. 18:125 (1965).
`32W. J. Witteman. IEEE J. Quart. Electron. QE-5:92 (1969).
`33R. .1. Carbone, [EEE J. Quart. Electron. QE—5:48 (1969).
`34V. K. Konyukhov and A. M. Prokhorov. JETP Lett. 3:286 (1966).
`355. T. Gerry, IEEE Spectrum 7:51 (1970).
`
`
`
`

`

`392
`
`Other Major Commercial Lasers
`
`
`
`Subsonic section
`
`—>f Expansion nozzle —>{
`
`
`
`Lower laser level
`
`
`Upper laser level
`
`
`inversion
`
`g a population inversion via gas
`expansion through a nozzle. (From E. T. Geny. "Gasdynamic Lasers." IEEE Spectrum
`7:51—58 (1970). ©1970 IEEE.)
`
`
`
`
`
`
`
`
`
`
`
`
`Quick-freeze
`
`
`
`

`

`
`
`The Design of Carbon Dioxide Lasers
`
`393
`
`/4kv,3mA
`
`PL AT/NUM
`ANODE
`
`K01/111?
`
`CATHODE
`
`
`
`
`COOLANT
`IN
`
`COOLAN 7'
`DU 7'
`
`i._ zscm. ———+——/2.25cm.——+— 7. 5cm.—-i
`
`Figure 12.9 The construction of an early waveguide carbon dioxide laser. (Reprinted
`with permission from T. J. Bridges. E. G. Burkhardt. and P. W. Smith, Appl. Phys. Lett.
`20:403 (1972). ©1972 American Institute of Physics.)
`
`12.1.3 Waveguide 002 Lasers
`
`One very good method for improving C02 laser performance is to decrease the bore size
`of the laser. This increases the number of gas collisions with the bore and significantly
`enhances the cooling rate (see Figure 12.9). If the electrodes are located transversely (rather
`than longitudinally) in the laser cavity, then the possibility also exists of using the electrodes
`themselves as an optical waveguide, thus permitting an even smaller bore size. The use of
`such a waveguide allows increased gas pressure with the attendant advantages of improved
`gain and larger linewidth. Operation in a waveguide mode also offers some additional
`advantages in alignment stability. The concept of a waveguide C02 laser was first proposed
`in 1964 by Marcatili and Schmeltzer38 and later demonstrated by Steffen and Kneubuhl39
`and Smith.40 Transverse-excited waveguide lasers are disclosed by Smith in US Patent
`#3,815,047.‘”
`Waveguide lasers use a small bore to confine the laser beam. The bore is itself an
`optical element, composed of two or four optically reflecting walls. Conventional mirrors
`are placed on either end of the cavity, but (unlike a conventional free space laser) these
`mirrors do not define a Gaussian beam in the cavity. Instead, the laser establishes various
`stable modes inside the bore (not unlike the modes in a laser fiber or a zig—zag slab laser).
`It is also possible to control the mode formation by introducing artifacts inside the bore that
`force the development of stable reflecting points.42
`
`33E. A. J. Marcatili and R. A. Schmeltzer. Bell Sys. Tech. J. 43:1783 (1964).
`39H. Steffen and F. K. Kneubuhl. Phys. Lett. 27A2612 (1968).
`40R w. Smith.App1. Phys. Len‘. 19:132 (1971).
`“Peter W. Smith. "Transverseiy Excited Waveguide Gas Laser." U.S. Patent #3.815.047. June 4. 1974.
`42Peter Laakmann. “Sealed Off RF-Excited Gas Lasers and a Method for Their Manufacture." US. Patent
`
`P: 8cm
`
`: 6/15 //v
`
`6.45 our:
`
`PART/AL REFLECTOR
`
`[WAVEGUIDE
`
`Ii
`
`R: 8cm
`
`i—
`
`/00 96 EFL- CTOF?
`
` Sec. 12.1
`
`

`

`
`
`state population increases, and the laser performance drops. With good heat sink design,
`sealed C02 lasers can be operated in air-cooled mode up to approximately 25 watts. Past
`that power level, water—cooling is typically required.45
`
`f_
`i"
`
`The basic series 48 module
`Design and manufacture of the series 48 module.
`is described in US. Patent #5,06S,405 (Peter Laakmann, “Sealed Off RF-Excited Gas Lasers
`and a Method for Their Manufacture,” November 12, 199]) and the technology is discussed
`in US. Patent #4,805,182 (Peter Laakmann, "RF-Excited All-Metal Gas laser," February 14,
`3Peter Laakmann, ”Sealed Off RF-Excited Gas Lasers and a Method for Their Manufacture." US. Patent
`44Peter Laakmann. "Usmg Low Power C02 Lasers in Industrial Applications," Synrad Application Note.
`451"eter Laakmann, “Using Low Power C01 Lasers in Industrial Applications." Synrad Application Note.
`
`\ 4
`
`#5,06S,405. November 12, 1991.
`
`
`
`

`

`
`
`Sec. 12.1
`
`The Design of Carbon Dioxide Lasers
`
`395
`
`
`
`Figure 12.10 Typical products marked by a carbon dioxide laser. (Courtesy of Synrad)
`
`1989). The key points of the design and manufacturing are described below and additional
`details may be found in the patents.
`The basic series 48 module consists of two extruded aluminum electrodes and two
`extruded aluminum ground plane strips (see Figure 12.12). The inner surfaces of the elec—
`trodes and ground strips are optically reflective at 10.6 run.
`(The electrodes are typically
`anodized with a 5 gm hard anodization to improve discharge stability and RF breakdown
`characteristics“) The top and bottom electrodes are identical and measure approximately
`1 cm by 2 cm by 40 cm long. The left and right ground plane strips are also identical and
`measure approximately 2 cm by 4 cm by 40 cm long. To reduce costs. the overall shape
`of the electrodes and ground planes is predefined by the extrusion process and only minor
`post-extrusion machining operations need to be performed.
`The inner surfaces of the electrodes and the ground strips define the optical cavity of
`the laser. The bore of this cavity measures roughly 5 mm square, which gives the overall
`
`
`

`

`396
`
`OtherMajorCommercialLasers
`
`
`
`Chap_ 12
`
`
`
`lnner, optical
`quality surface
`
`
`
`Inner. optical
`quality surface
`
`
`
`
`Basic extruded
`ground plane strip
`
`Basic extruded electrode
`
`Figure 12.12
`The Synrad electrodes and
`ground plane strips.
`
`
`
`

`

`
`
`Sec. 12.1
`
`The Design of Carbon Dioxide Lasers
`
`397
`
`Electrodes
`
`Outer case
`
`
`
`
`Ceramic
`
`Ceramic disk
`
`Laser bore
`
`
`
`
`Ground plane strips
`
`Figure 12.13 The Synrad series 48 cross—section.
`
`front mirror. The taper angle is quite small, typically less than a milliradian. The second
`artifact consists of introducing small, sharp bends in the optical surfaces. The bends can
`be in one electrode and its adjacent ground strip, or in two opposing electrodes (or ground
`strips).
`If the bend is introduced into the electrode and its adjacent ground strip, then it
`is a bend on the order of 5 to 10 milliradians.
`If the bend is introduced into opposing
`electrodes (or ground strips), then it is on the order of 1 milliradian. These bends prevent
`reflections off all four walls from adding in phase to produce competing modes and parasitic
`oscillations.
`The two electrodes and two ground planes are assembled in a clean room. The
`electrodes and ground plane strips are slipped into an outer case as shown in Figure 12.13.
`Small ceramic beads are used to isolate the electrodes from the ground plane strips. Larger
`ceramic disks are used to isolate the electrodes from the outer case. Electrical feedthroughs
`(which also serve as gas fill ports) are provided in the outer case.
`A common cause of short lifetimes in C02 lasers is the accumulation of water va-
`por. Water vapor can migrate through the o-ring mirror seals, or be formed by hydro-
`gen combining with dissociated oxygen. An effective way to minimize contamination
`due to water vapor is to introduce a getter into the cavity. A molecular sieve getter for