Electro-optic modulation, crystal growth and properties, and design equations and performance for 10.6-micron laser modulator of gallium arsenid

Walsh, T.

English

NASA Technical Reports Server

2 DEVELOPMENT OF 
A 10.6-MICRON LASER MODULATOR 4 
Prepared by 
FINAL REPORTJ 
--I / 
February 18 t a u l y  18,1966 6 EGcu 
Covering the Period 
under 
Contract NASS-101 4,2q c\l 
I 
7 5  Approved 
(G T.Walshq F.'St e r z e r 
for 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
Goddard Space Flight Center 
? RADIO CORPORATION OF AMERIC ELECTRONIC COMPONENTS AND DEVICES 
-a_MICROWAVE APPLIED RESEARCH L A B O R A t O R J $ 3  
DAVID SARNOFF CENTER ' a R m ,  NEW JERSEY 
https://ntrs.nasa.gov/search.jsp?R=19670029537 2020-03-12T12:13:25+00:00Z
TABLE OF CONTENTS 
Paragraph 
I 
I1 
I11 
N 
v 
Page 
Objectives. . . . . . . . . . . . . . . . . 1 
Electro-optic Modulation. . . . . . . . . . . 1 
Crystal Growth and Properties , . . . , . , , e . 2 
A. G r o w t h . . . . . . .  . , , . O . . O e . . . .  2 
B. %t i ca l  Properties. . . . . . . . . . . . . . 5 
C. Electro-optic Coefficient . . . . . . , . e . 9 
Design Equations and Modulator Performance. e . . 11 
A. &sign Equations. . . . . . e . e . . . 11 
B. Modulator Construction and Performance, . . 14 
Conclusions . . . . . . . . , e . . . . . . . . . 18 
References. . . . . . . , . . , , . . , . 19 
ii 
LIST OF ILLUSTRATIONS 
Figure Page 
Three o p t i c a l  systems f o r  e l e c t r i c a l l y  modulating 1 
a l i g h t  beam using an e l ec t ro -op t i c  c r y s t a l .  . . e 3 
2 A gallium-arsenide ingo t  and a modulator c r y s t a l  
c u t  from t h e  ingo t  . . . e . . e . e . . e 4 
I n f r a r e d  photograph of a gall ium-arsenide crystal  
r e s t i n g  on a metal s c a l e  e . e . . e . . . 6 3 
4 Absorption c o e f f i c i e n t  of ga l l ium a r sen ide  a s  a 
func t ion  of wavelength . . . . . . ., . . e a . 7 
Index of r e f r a c t i o n  of gall ium arsenide  as a 
func t ion  of wavelength . . . . . . . . . . ., . . 8 5 
Effec t  of a n t i - r e f l e c t i o n  coa t ing  on t h e  modulator 
t ransmi t tance  of a gall ium-arsenide c r y s t a l .  . e 10 6 
7 Elec t ro-opt ic  c o e f f i c i e n t  of gal l ium a r sen ide  a s  
a func t ion  of wavelength . . . . . . . . e 12 
8 RCA 5-2036A s o l i d - s t a t e  e l ec t ro -op t i c  modulator. . e 15 
9 RCA J-2O36A s o l i d - s t a t e  e l ec t ro -op t i c  modulator. 
Depth of modulation vs wavelength. . . . e . . . 17 
10144 F 
DEVELOPPENT JF 
A 10.6-1CLCRON L.'.SSH MODJUTOH 
I. Object ives  
The o b j e c t i v e  of t h i s  program w a s  t o  develop a G a l l i u m  Arsenide 
e l ec t ro -op t i c  modulator capable of modulating t h e  output  from a 10.6 
micron laser. A depth of modulation g r e a t e r  than 35% w a s  required f o r  
an applied vo l t age  of 1000 v o l t s  nns over a bandwidth from d c  t o  over 
10 MHz. A modulator similar t o  t h i s  was developed f o r  t h e  wavelength 
range 0.9 t o  3.0 microns under Contract NAS 5-9620. The extension of 
t h e  ope ra t ing  wavelength t o  10.6 microns required t h e  development of 
p o l a r i z e r s  and waveplates f o r  t h i s  wavelength. 
11. Electro-opt ic  Modulation 
The e l ec t ro -op t i c  e f f e c t  i s  an e lec t r ic  f i e l d  induced change 
i n  the  o p t i c a l  i n d i c e s  of r e f r a c t i o n  of a c r y s t a l .  There i s  no satis- 
f a c t o r y  microscopic theory which p r e d i c t s  t h e  magnitude of t h e  e f f e c t  
from t h e  c r y s t a l  p r o p e r t i e s ,  although H e i l m e i e r ( ' )  has presented a 
theo ry  f o r  t h e  molecular crystal  Hexamethylenetetramine giving good 
agreement wi th  t h e  observed value. There i s ,  however, a complete 
phenomenological theory(2) so t h a t  once t h e  e l ec t ro -op t i c  c o e f f i c i e n t s  
of a material are measured, t h e  e f f e c t  of an e l e c t r i c  f i e l d  i n  t h e  
c r y s t a l  on t h e  t ransmit tance of a l i g h t  beam through t h e  c r y s t a l  can be 
predicted f o r  any o r i e n t a t i o n  of t h e  c r y s t a l  axes with r e spec t  t o  both 
t h e  e l e c t r i c  f i e l d  and t h e  l i g h t  beam. A review of the  theory f o r  
( 3 )  
cubic  c r y s t a l s  such as Gallium Arsenide i s  given by Namba and by 
( 4 )  S t e r z e r  e t  a1 . 
10144 F 
- 2 -  
A cubic  e l ec t ro -op t i c  c r y s t a l  l i k e  GaAs i s  o p t i c a l l y  iso-  
t r o p i c  i n  t h e  absence of an e lectr ic  f i e l d .  The app l i ca t ion  of an 
e l ec t r i c  f i e l d  along c e r t a i n  c r y s t a l  d i r e c t i o n s  causes t h e  c r y s t a l  t o  
became b i r e f r i n g e n t  by an amount propor t iona l  t o  the  e l e c t r i c  f i e l d .  
The c r y s t a l  i s  the re fo re  an op t i ca l  wave p l a t e  wi th  a vo l t age  con t ro l l ed  
r e t a r d a t i o n  and can be used  i n  var ious  o p t i c a l  systems t o  e l e c t r i c a l l y  
modulate t h e  i n t e n s i t y ,  phase, o r  po la r i za t ion  of a l i g h t  beam. Three 
such conf igura t ions  are shown i n  Figure 1. 
111. Crvstal Growth and ProDerties 
A.  Growth 
The c r y s t a l s  are boat grown i n  a ho r i zon ta l  Bridgeman furnace 
from elemental  G a l l i u m  and Arsenic. They are doped wi th  Iron during 
growth t o  ra ise  t h e i r  r e s i s t i v i t y  above lo6 ohm cm over t h e  e n t i r e  
length  of t h e  ingot .  A t y p i c a l  ingot and a modulator crystal  c u t  from 
t h e  ingot  are shown i n  Figure 2. The ingot  i s  1 1/2 c m  i n  diameter and 
8 cm long. It has  been seeded  t o  make most of i t s  length use fu l  f o r  
modulator c r y s t a l s .  The c r y s t a l s  a r e  hard,  non-hygroscopic, and possess  
a r e l a t i v e l y  high thermal conductivity.  They a r e  v i r t u a l l y  s t r a i n  f r e e  
as grown and are e a s i l y  cu t  and polished f l a t  t o  1/10 wavelength of 
v i s i b l e  l i g h t  without  introducing apprec iab le  s t r a i n .  The e x t i n c t i o n  
r a t i o  of 1 c m  t h i c k  c r y s t a l s  between crossed p o l a r i z e r s  i s  g r e a t e r  than 
100 t o  1 a f t e r  a l l  c u t t i n g  and pol ishing opera t ions  and they  can be 
handled r a t h e r  c a r e l e s s l y  without in t roducing  add i t iona l  s t r a i n .  Since 
t h e  c r y s t a l s  are inso luble  i n  water and d i s s o c i a t e  a t  8OO0C, no spec ia l  
p recaut ions  are necessary with regard t o  ambient temperature and 
humid it y . 
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B. Optical  P r o p e r t i e s  
The c r y s t a l s  are opaque i n  t h e  v i s i b l e ,  b u t  are t r a n s p a r e n t  i n  
t h e  in f r a red  between 0.9 and 16 microns. 
of t h e  c r y s t a l s  i n  t h e  in f r a red  i s  i l l u s t r a t e d  i n  Figure 3 which i s  
a photograph of a G a l l i u m  Arsenide c r y s t a l  on a metal scale. The 
photograph w a s  taken with t h e  a i d  of an in f r a red  image converter  sen- 
s i t ive  t o  wavelengths near  1 micron. 
t o  t h a t  of good o p t i c a l  g l a s s .  
The e x c e l l e n t  o p t i c a l  q u a l i t y  
The o p t i c a l  q u a l i t y  i s  comparable 
The o p t i c a l  absorpt ion c o e f f i c i e n t  k w a s  determined by measuring 
t h e  t ransmit tance of a c r y s t a l  which w a s  polished on f o u r  f a c e s  t o  
provide two unequal path lengths  through t h e  c r y s t a l .  
c o r r e c t i o n  f o r  t h e  r e f l e c t i o n  lo s ses  which are equal  f o r  t h e  two 
d i r ec t ions .  The absorption c o e f f i c i e n t  determined i n  t h i s  manner i s  
shown as a funct ion of wavelength i n  Figure 4. 
c o e f f i c i e n t  which could be measured on t h e  spectrometers u s e d  w a s  
This  permits  
The smallest absorption 
A t  those wavelengths where n e g l i g i b l e  absorpt ion e x i s t s ,  re- 
f l e c t i o n  l o s s e s  l i m i t  t h e  ex te rna l  t r ansmi t t ance  of a p l ane -pa ra l l e l  
p l a t e  of GaAs to  55%. 
of r e f r a c t i o n  of GaAs, shown i n  Figure 5. The e x t e r n a l  t ransmit tance 
T i s  given i n  terms of t h e  r e f r a c t i v e  index Nby  t h e  expression 
This low t ransmit tance i s  d u e  t o  t h e  high index 
2 N  T=- 
Nz+1 
10144 F 
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Fig. 3 - Infrared photograph of a gallium-arsenide crystal resting 
on a metal scale, 
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The t ransmi t tance  a t  10.6 microns was increased t o  90% by an t i - re f lec-  
t i o n  coa t ing  t h e  modulator c r y s t a l  wi th  Zinc Su l f ide .  The e f f e c t  of 
t h e  coa t ing  on t h e  modulator t ransmi t tance  i s  shown i n  Figure 6. 
C. E lec t ro-opt ic  Coeff ic ien t  
The e l ec t ro -op t i c  c o e f f i c i e n t  of GaAs was measured  i n  t h e  wave- 
length  range 2.5 t o  12 microns by t h e  fol lowing method. The Gallium 
Arsenide c r y s t a l  was placed between p a r a l l e l  p o l a r i z e r s  and a t h i c k  
wave p l a t e  of Cadmium Su l f ide  was a l s o  placed between t h e  p a r a l l e l  
p o l a r i z e r s  and or ien ted  wi th  i t s  f a s t  axis a t  45' t o  t he  plane of 
passage of t he  p o l a r i z e r s .  The phase r e t a r d a t i o n  produced by t h e  CdS 
wave p l a t e  was - 5 wavelength a t  9.0 microns,  - 7 X a t  7.1 microns, - 9 h 
4 4 4 
a t  5.6 microns, and so on toward shor t e r  wavelengths. A t  t hese  wave- 
lengths ,  t h e  t ransmi t tance  of t h e  conf igura t ion  i s  given by 
= - 1 TO {l + COS [+ + (2M + 1) qi 
2 2 
2n' N% where = - 
i d e  c r y s t a l .  
i s  t h e  r e t a r d a t i o n  produced by t h e  Gallium Arsen- 4 17 X 
These equat ions  can be solved f o r  r41 i n  terms of t h e  ob- 
served change i n  t ransmit tance.  
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The va lues  of r41  obtained by t h i s  method a r e  shown i n  Figure 6. 
The measurements of r41  versus  wavelength shown i n  Figure 7 were 
made with dc vo l t ages  appl ied t o  t h e  c r y s t a l .  The dependence of r41  
on t h e  frequency of t h e  modulating e l e c t r i c  f i e l d  w a s  determined a t  t h e  
f ixed  in f r a red  wavelength of 1 micron using the  conf igura t ion  shown 
i n  Figure l ( a > .  The modulating frequency w a s  var ied  from dc t o  20 
MHz and t h e  modulation was detected wi th  a 7102 photomul t ip l ie r  t e r -  
minated with a 50 ohm r e s i s t o r .  The observed modulation w a s  constant  
over t h i s  frequency range except near t h e  p i e z o e l e c t r i c  resonances of 
t h e  c r y s t a l ,  which occurred a t  700 KHz. A t  a resonance, t h e  modulation 
increased by a f a c t o r  of about l o 3  over t h e  non-resonant e f f e c t .  This  
provides  a means f o r  ob ta in ing  s ingle  frequency on-off modulation of 
a C02 l a s e r  wi th  an applied vol tage  of only about 10 v o l t s .  
IV. Design Equations and Modulator Performance 
In  t h i s  s ec t ion  t h e  design equat ions f o r  e l ec t ro -op t i c  amplitude 
modulators are presented and t h e  performance da ta  of t h e  10.6 micron 
modulator are given. 
A. Design Equations 
The bas i c  quest ion concerning an e l ec t ro -op t i c  modulator i s  
what vo l t age  i s  required t o  produce a given change i n  t ransmit tance.  
12 
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For the  conf igura t ion  sham in Figure l ( a ) ,  t h e  t ransmi t tance  i s  given 
by 
where # - - 2rr vL i s  t h e  r e t a rda t ion  d u e  t o  the  e lec t ro-opt ic  r41 d
i s  t h e  s t a t i c  r e t a rda t ion  of t he  wave p l a t e .  The f a c t o r  
To inc ludes  t h e  e f f e c t s  of any r e f l e c t i o n  and absorpt ion l o s s e s  i n  
t h e  c r y s t a l ,  p o l a r i z e r s ,  and wave p l a t e .  The g r e a t e s t  change i n  T f o r  
small va lues  of V occurs  when = 2 .  This  can be seen by tak ing  t h e  
d e r i v a t i o n  of T wi th  respec t  to  V i n  equat ion 3 and then f ind ing  t h e  
2 
maximum value of t h i s  derivative as a func t ion  of $&’ . 
occurs  f o r  9 + 
t o  avoid d i s t o r t i o n  i n  the  modulation, t h i s  reduces t o  
va lue  of )o g ives  no t  only t h e  g r e a t e s t  s e n s i t i v i t y ,  bu t  a l s o  t h e  
g r e a t e s t  l i n e a r i t y  s ince  it i s  an i n f l e c t i o n  poin t  on the  T versus  
curve. With 9 = 2 ,  equat ion(3)  becomes 
This  maximum 
= t. For 9 small compared t o  )u , as it must  be 
- E. This  
y - 2  
2 
To T- -
2 = s i n  jb 
To 
2 
-
(4) 
The l e f t  s i d e  of equat ion (4) i s  t h e  modulation i n d e ~ ( ~ ) m  of 
the  amplitude modulated beam. In terms of t h e  c r y s t a l  parameters and 
t h e  vol tage ,  
l01h.b F 
Equation ( 5 )  shows t h a t  t he  depth of modulation decreases  a s  
t he  wavelength inc reases .  It a l s o  shows t h a t  t he  decrease may be of f -  
set by using a c r y s t a l  wi th  a l a rge r  length-to-width r a t i o  d / d .  
f i n a l  po in t  t o  be made i s  t h a t  t he  r e l a t i v e l y  low va lue  of r41  of G a A s  
i s  compensated f o r  by i t s  l a r g e  index of r e f r a c t i o n  which e n t e r s  cubed 
i n  equat ion ( 5 ) .  
The 
A. Modulator Construct ion and Performance 
The modulator i s  shown i n  Figure 8. The G a A s  c r y s t a l ,  3mm x 3mn 
i n  c r o s s  sec t ion  and 6.7 cm long, i s  held between spr ing  loaded p a r a l l e l  
p l a t e  e lec t rodes .  The e l e c t r o d e s  a re  in su la t ed  from t h e  aluminum body 
of t h e  modulator and t h e i r  pos i t i on  allows t h e  modulator t o  be con- 
nected d i r e c t l y  between t h e  p l a t e  te rmina ls  of two push-pull output 
tubes  of a dr iv ing  ampl i f ie r .  The t o t a l  capaci tance of t h e  modulator 
inc luding  t h e  c r y s t a l  i s  14 picofarads.  
The wave p l a t e ,  shown next to the  modulator body i n  Figure 8 ,  
i s  a s i n g l e  c r y s t a l  of Cadmium Sul f ide  whose th ickness  i s  such as t o  
provide a phase r e t a r d a t i o n  of f i v e  quarter-wavelengths a t  10.6 microns. 
Each p o l a r i z e r  c o n s i s t s  of two p l a t e s  of h igh  r e s i s t i v i t y  G e r m  
manium set a t  Brewster's angle. 
over 99% of the  inc iden t  r a d i a t i o n  because of t h e  high index of re- 
f r a c t i o n  of Germanim (N = 4).  The plates a r e  arranged so t h a t  t h e r e  
Two p l a t e s  are s u f f i c i e n t  t o  po la r i ze  
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i s  no de f l ec t ion  of t he  l a s e r  beam upon passage through t h e  polar-  
i z e r s .  
The modulator w a s  t e s t e d  a t  10.6 microns wi th  a C02 l a s e r  and 
a Mercury doped Germanium de tec tor .  The measured depth of modulation 
was l i n e a r  wi th  appl ied vo l t age  up t o  1000 v o l t s  peak, a t  which level 
the  depth of modulation was 61%. 
ance than required by t h e  spec i f i ca t ions .  
This  i s  cons iderably  b e t t e r  perform- 
The modulator was a l s o  t e s t ed  wi th  a spectrometer a t  those  
wavelengths where t h e  wave p l a t e  gave an odd m u l t i p l e  of a qua r t e r  
wave re t a rda t ion .  These resul ts ,  shown i n  Figure 9 ,  i nd ica t e  t h a t  t he  
modulator has  a range of operat ion extending from 2p (absorpt ion i n  
t h e  Germanium p o l a r i z e r s )  t o  12p (absorpt ion i n  t h e  G a A s  and CdS). 
The frequency response was t e s t e d  a t  1 micron using a 7102 
phototube terminated i n  a 50 ohm load a s  t h e  f a s t  de t ec to r ,  and a 
Xenon lamp a s  t h e  source. 
replaced wi th  Ca lc i t e  Glan-Thompson po la r i ze r s .  
l a t i o n  was constant  a s  a func t ion  of frequency from dc t o  over 20 MHz, 
except near  t h e  p i e z o e l e c t r i c  resonances i n  t h e  c r y s t a l ,  t h e  s t ronges t  
of which occurred a t  700 KHz. A t  t h i s  resonance, t he  pho toe la s t i c  
e f f e c t  a d d s  t o  t h e  modulation v i a  the  p i e z o e l e c t r i c  response of t h e  
c r y s t a l .  The ne t  modulation e f f e c t  i s  g r e a t l y  increased (about 1000 
times) over t h e  non-resonant e f f ec t .  100% modulation w a s  observed f o r  
1 v o l t  of d r i v e  s igna l  a t  t h i s  s ing le  frequency. 
For t h i s  tes t  the  Germanium p o l a r i z e r s  were 
The observed modu- 
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V. Conclusions 
A 10.6 micron GaAs modulator has  been constructed which g ives  
61% depth of modulation f o r  1000 v o l t s  peak modulating s igna l  over a 
bandwidth from d c  t o  over 20 MHz. E f f i c i e n t  and compact q u a r t e r  wave 
p l a t e  and p o l a r i z e r s  were developed f o r  use a t  t h i s  wavelength. The 
modulator can be used  a t  wavelengths from 2 t o  12 microns and w a s  used 
t o  measure t h e  e l ec t ro -op t i c  c o e f f i c i e n t  of GaAs  throughout t h e  range. 
. 
l -  
-19- 
Footnotes 
1. G. H e i l m e i e r ,  "The D i e l e c t r i c  and Electro-Optical  P rope r t i e s  of 
t h e  Molecular Crys ta l  Hexamine," Applied Optics, Vol. 3, No. 11, 
p. 1281, November 1964. 
2. W. P. Mason, "Optical Proper t ies  and t h e  Elec t ro-opt ic  and Photo- 
e l a s t i c  Effects i n  C r y s t a l s  Expressed i n  Tensor Form,'* B e l l  System 
Technical Journa l ,  Vol. 29, p. 161, Apr i l  1950. 
3. S. Namba, "Electro-Optical  Ef fec t  of Zincblende," Journa l  of t h e  
Optical Socie ty  of America, V o l .  51, No. 1, p. 76, January 1961. 
4. F. S t e r z e r ,  D. B la t tne r ,  S. Mini ter ,  ')Cuprous Chloride Light 
Modulators," Journa l  of t h e  Optical Soc ie ty  of America, Vol. 34, 
No. 1, p. 62, January 1964. 
5. F. Terman, "Electronic  and Radio Engineering," p. 523, McGraw- 
H i l l  Book Co., Inc., New York, 1955. 


Development of a 10.6-micron laser modulator Final report, 18 Feb. - 18 Jul. 1966

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