Table Of Content~ 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
