~ I 20887 DIODE LASERS by Ralph W. Campbell and Forrest M. Mims, III Semiconductor Diode Lasers by Ralph W. Campbell & Forrest ~. ~;ms III () I HOWARD W. SAMS & CO., INC. liIIIlr. THE BDBBS-MERRILL CO., INC. INDIANAPOLIS • KANSAS CITY • NEW YORK FIRST EDITION FIRST PRINTING-1972 Copyright © 1972by Howard W.Sams Co., Inc.•Indianapolis. Indiana 46268. Printed in the United States of America. All rights reserved. Reproduction or use, without express per mission, of editorial or pictorial content, in any manner, is prohibited. No patent liabilityisassumedwith respectto the use of the information contained herein. While every precaution has been taken in the preparation of this book, the publisher assumesno responsibilityfor errorsor omissions.Neither isany liability assumed for damages resulting from the use of the information contained herein. International Standard Book Number: 0-672-20887-3 Library of Congress Catalog Card Number: 77-182873 Preface The purpose of this book is to provide experimenters and design engineers with a broad introduction to one of the most unique semi conductor devicesin electronics-the injection laser. In order to ap peal to a widecross section of readers, thet theory of lasers has been greatly simplified.Additionally, numerous circuits for operating and detecting lasers, some never before in print, have been included. The book is rounded out by a wideselection of conventional and infrared photographs and three appendices. The authors feelthat the injection laser offersnumerous challenges to ambitiousexperimenters.The fieldof radiocommunications, for ex ample, is old and crowded, but practical light-beam communication on alargescaleisstillyears in thefuture. Withthecurrent availability of inexpensive injection lasers, experimenters are presented with the unique opportunity of making important contributions to the state of the art in this vitallyimportant field. This book could not have been completed without the assistance of many individuals and companies. Sample components and abun dant technical information were provided by dozens of firms. Par ticularly important assistance was provided by Richard S. Myers, Dr. Henry Kressel, Robert Felmly, Walter Dennen, and Stanley Katz of RCA. Contributions werealsoprovided byLaser DiodeLaboratories, United Detector technology,EG&G, Texas Instruments, and Metrol ogic Instruments. Additional support was furnished by Optical En gineering, Eastman Kodak, Teledyne Crystalonics, and Corlon In struments. Dr. Izuo HayashiofBellTelephoneLaboratories providedinforma tion and photographs concerning his important work in the develop ment of continuously operating injection lasers. Gary Royster of Hewlett-Packard supplied a high speed HP-180 oscilloscopefor use in testing and evaluating many of the circuits included in the text. Dr. Malvern Benjamin of BionicInstruments contributed information on his work with injection-laser mobility aids for the blind. Finally, WilliamGaumer, an optics specialist currently conducting laser research, reviewed portions of the text and supplied numerous helpful suggestions. Duncan Campbell of Santa Barbara Research Center made available supplementary information on the design of laser transceivers.The finalmanuscript and much of the drafting was typed by Minnie C. Mims. Mabel W. Campbell helped finance and encourage the project. To these many individualsand firms,we both expressour gratitude for such generous assistance. RALPH W. CAMPBELL FORRESTM. MIMSIII Contents CHAPTER J LIGHT, SEMICONDUCTORS, AND LASERS • 7 Light - Practical Light Sources- Light-Emitting Diodes- The The Injection Laser - Injection-Laser Theory- Other Lasers CHAPTER 2 INJECTION LASERS ANDTHEIR PROPERTIES • 25 Homostructure Injection Lasers- Heterostructure Injection Lasers - Double-Heterostructure InjectionLasers- Comparing theMajor Structures - Other Laser Structures - Electrical Properties of In jection Lasers - Optical Properties of Injection Lasers - Coher ence of Injection Lasers - Visible Injection Lasers - Injection Lasers - Injection Laser Degradation - Laser Interactions CHAPTER 3 FABRICATION AND COMMERCIAL DEVICES 53 Crystal Growth - Wafer Formation - Junction Formation Metalization- Formation of SingleLaser Diodes- Laser Arrays - Fabrication of other Semiconductor Lasers - Commercial In jection Lasers - Lifetime of Commercial Lasers CHAPTER 4 PULSE GENERATORS, MODULATORS,ANDPOWERSUPPLIES • 75 Pulse Requirements - Laser Degradation - Pulse Generators Silicon Controlled Rectifier - Two General-Purpose SCR Pulse Generators - Other SCR Pulse Generators - Thyratron Drivers - Transistor Driver for Cryogenic Operation - Cryogenic Laser Assembly- DrivingLaserArrays - Commercial PulseGenerators - Modulators for Voice Communications - Power Supplies CHAPTER 5 DETECTORS ANDRECEIVERS • 111 Detectors - Photomultiplier Tubes - Commercial Photomulti pliers - Silicon Detectors - Solar Cells - Phototransistors Field-Effect Phototransistors - PIN Photodiodes - Commercial PIN Photodiodes - Avalanche Photodiodes - Commercial Ava lanche Photodiodes - Photodetector Trade-Offs - Receivers Two "Slow"Receivers- A Basic"Fast" Receiver- FotofetLaser Receiver - Photodiode Laser Receiver - Receiver for Voice Modulated Laser - Integrated Circuits for Laser Receivers Commercial Receivers CHAPTER 6 OPTICS ANDVIEWING DEVICES 137 Lenses- Infrared Filters - Fiber Optics - Other Optical Com ponents - Mounting Optical Components - Commercial Sources for Optics - Viewing Devices - Infrared-Sensitive Phosphor Screens - Infrared Image-Converter Tubes - Image-Converter Safety Procedures - Commercial Image Converters - Infrared Photography CHAPTER 7 ApPLICATIONS · 167 Communications - Rangefinders- Intrusion Alarms - Illumina tion - Mobility Aids for the Blind - Holography - Interfer ometry - Optical Computers - Laser Pump Source - Other Applications APPENDICES A. LASER SAFETY · 183 B. RANGE EQUATIONS · 185 C. ADDRESSES OF MANuFACTURERS · 188 INDEX · 190 1 Ligh~ Semiconductors, and Lasers The laser is one of the most remarkable creations of modem sci ence. First predicted by Drs. Schawlowand Townes in 1958, the first working laser was constructed by Theodore Maiman in 1960. Mai man's firstcrude laser emitted brief pulses of brilliant red light when a ruby rod withparallel, silveredends wasexcited by a powerful flash lamp similar to those used by photographers. The first laser satisfied the requirements for lasing set forth in the original proposal bySchawlowand Townes: a readily excited fluores cent material of good optical quality, a method for stimulating the material to an excited state, and an optical resonating cavity. Almost all lasers developed since the first have been based on these funda- mental requirements. . Soon after Maiman demonstrated his first laser, several other de viceswere assembled. Most of them used ruby or someother fluores cent crystal, but a major accomplishment occurred a year later at Bell Telephone Laboratories with the development by Ali Javan of the gaslaser. Another major step forward occurred with the develop ment of the semiconductor injection laser in 1962. This and subsequent chapters willdiscuss the physics, fabrication, and application of the injection laser at somelength. But firstweshall digresswith a discussion on the subject of light, since an understand ingofitsoriginisessentialto understanding the injectionlaser. 7 LIGHT Many forms of light are produced by energy transitions of elec trons. Basicphysicstells us that atoms consistof a central, positively charged region surrounded by a negatively charged cloud of elec trons. Normally, an electron must occupy a specific energy level within the cloud. But those electrons near the outer region of the cloud may temporarily be excited to its perimeter by the application of energy from an external source. The energy may take the form of light, heat, a beam of electrons, an electrical current, or even a chemical reaction. Physicists have developed a special vocabulary to describe the various events that may occur within an atom. For example, the spacevacatedbyanexcitedelectroniscalledahole.Whenan electron recombineswith a hole, it usuallygivesoffits absorbed energyin the form of either heat or light. When a recombination results in the emissionof a photon, a packet of electromagnetic energy, the event is referred to as radiative recombination. The outermost level that electronscan occupywithinthecloudaround a nucleusisthe conduc tion band. The next highest level, the highest point that unexcited electronsmayoccupy,isthevalenceband. The regionbetweenthetwo bands iscalled the forbidden gap, as electrons normally pass directly betweenthe two levels,possiblywith brief pauses alongthe way. EXCITEDELECTRON "* ~CONDUCTION ~ ~ND / RADIATIVE RECOMBINATION VALENCE BAND Fig. 1-1.Portion ofa hypotheticalatom. Some materialspermit transitioning electrons to pause at one or more points in the forbidden gap. These are calledindirect band gap materials. Materials that do not permit electrons to occupy points in the forbidden gap are said to have a direct band gap. A hypothetical atom with severalof these terms labeled is shown in Fig. 1-1. The band gap is important as it is related to the wavelengthof an emitted photon by 8 A= hc 1.1 E where, h is Planck's constant(6.63 X 10-34 joule-seconds), c is the velocity of light (3 X 1014 micrometers per second), E is the energyin joules that separates the valence and conduction bands. The equation may be simplifiedby using electron-volts instead of joules. We then obtain ti = 12 nanomet~rs) A E 1.2 eectron-vo ts g A material with a band-gap separation of 1.36 electron-volts would then emit photons with a wavelengthof 909 nanometers. The term nanometer defines the wavelength of light in relation to the meter (39.37 inches); one nanometer is one-billionth of a meter. To better understand the designation of light in terms of wavelength, refer to Fig. 1-2. Note thatvisiblelightfalls roughly between 400 and 700nanometers, though as isdiscussedin a later chapterit is possible for many individuals to see light that has a wavelength of more than 900 nanometers. Fig. 1-2 also shows the relationship to one another of radio waves,ultraviolet, and other divisionsof the electromagnetic spectrum. It is important to note that the band gap is not necessarily a fixed number. Many materials willallowelectrons to occupy slightlydiffer ent levels within the conduction and valence band. Therefore, the wavelengthofphotons emitted during radiative recombination may be smeared overasmuch asseveraltens of nanometers. Bl~ ~, 1.1 GREEN ORANGE VISIBLE Fig.1-2. The electromagneticspectrum. PRACTICAL LIGHT SOURCES Nowthat weknowsomeofthebasicsoflightgeneration, how is an efficientlightemitter fabricated? There are several techniques, and 9