Laser dynamics, control, and stabilization of optical frequency radiatio

Harris, S. E.

English

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Investigation of laser dynamics, modulation and control by means of intra-cavity time varying perturbation  Semiannual status report

Laser dynamics, modulation, and control by intra-cavity time varying perturbatio

Investigation of laser dynamics, modulation and control by means of intra-cavity time varying perturbation  Semiannual status report, 1 Aug. 1967 - 1 Feb. 1968

Argon FM laser, parametric oscillator, and backward wave oscillator developmen

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INVESTIGATION OF LASER DYNAMICS, MODULATION AND CONTROL 
, BY MEANS OF INTRA-CAVITY TIME VARYING PERTURBATION 
under the  d i r e c t i o n  of 
S.  E. Harris 
Semi-Annual S t a t u s  Report No. 2 
f o r  
NASA Grant NGR-05-020-103 
Nat ional  Aeronautics and Space Administration 
Washington 25, D. C.  
f o r  t h e  per iod 
1 August 1966 - 31 January 1967 
M .  L. Report No. 1502 
January 1967 
-
M i  crowave Labor a t  ory 
W. W. Hansen Laborator ies  of Physics 
Stanford Univers i ty  
Stanford,  Ca l i fo rn ia  
i 
. -  
https://ntrs.nasa.gov/search.jsp?R=19670008690 2020-03-16T19:00:41+00:00Z
0 
STAFF 
NASA Grant NGR-05 -020 -103 
f o r  t h e  per iod 
1 August 1966 - 31 January 1967 
PRINCIPAL INVESTIGATOR 
S.  E. Har r i s  
PROFESSOR 
A .  E. Siegman 
RESEARCEI ASSISTANTS 
R. L .  Byer 
C .  E ,  Constantinou 
J. Fa lk  
M. K. Oshman 
L. M. Oster ink 
C.  G .  Someda 
INTRODUCTION 
The work under t h i s  Grant  i s  genera l ly  concerned with the  generation, 
control ,  and s t a b i l i z a t i o n  of op t i ca l  frequency rad ia t ion .  Previous work 
under t h i s  Grant  has  been l a rge ly  concerned with the  con t ro l  of normally 
multimode l a s e r s  by means of i n t e rna l  time varying per turbat ion.  The work 
i n  t h i s  a rea  i s  now nearly complete, and w i l l  be described b r i e f l y  below. 
The more recent  and upcoming work under t h i s  Grant i s  concerned with the  
generation of l i g h t  by means of parametric processes i n  nonl inear  media. 
The present  p ro jec t s  under t h i s  category include a combined t h e o r e t i c a l  
and experimental study of op t i ca l  parametric o s c i l l a t i o n  wherein the  non- 
l i n e a r  element i s  placed in t e rna l ly  t o  the  cavi ty  of t h e  pumping l a s e r ;  
and a second project.  whose goal i s  t h e  parametric generation of l i g h t  
without t h e  need f o r  mult iple  o p t i c a l  c a v i t i e s .  I n  t h e  following sec-  
t i o n s  we w i l l  review the  s t a tus  of each of t he  above p ro jec t s .  
During t h i s  period t h e  following publ ica t ions  have been submitted 
for publicat ion,  o r  published, and a re  included as Appendices: 
S .  E. Harris, "Threshold of Multimode Parametric Osc i l l a to r s , "  published 
i n  IEEE J. Quant. E lec t r . ,  October 1966, p. 701. 
L. M. Osterink and R. Targ, "Single Frequency Light from an Argon FM 
Laser," submitted t o  Appl. Phys. Le t t e r s .  
S. E.  Harris, "S tab i l i za t ion  and Modulation of Laser Osc i l l a to r s  by 
I n t e r n a l  Time-Varying Perturbation," published i n  Proc. IEEE and 
Appl. Optics, October 1966, p. 1639. 
- 1 -  
I 
The following o r a l  disclosure has  a l s o  been presented: 
S .  E. Harris, "Temporal Modulation of Light," Optical  Society of 
America Meeting, San Francisco, October 1966. 
- 2 -  
PRESENT STATUS 
1. FM Osc i l l a t ion  i n  t h e  Argon Laser and Single  Frequency Techniques 
(L.  Oster ink and S. E. Harris) 
During t h i s  period the  experimental work on t h e  argon FM l a s e r  has  
been completed. In  a j o i n t  experiment with R.  Targ of Sylvania  Elec t ronic  
Systems, a s ing le  frequency output power of 350 mW of s ing le  frequency 
l i g h t  a t  5143 2 has been obtained. 
power i n  a l l  s i d e  bands, was l e s s  than 0.2% of the  t o t a l  power. 
continuous hours of s ing le  frequency output were achieved with only oc- 
cas iona l  minor adjustment of the  ex te rna l  modulator d r ive .  
The d i s t o r t i o n ,  or t o t a l  r e s i d u a l  
Severa l  
Particiilai- eiiiphasls ha:: been devnterl t o  studying t h e  e f f e c t  of 
power dependent homogeneous broadening on t h e  d i s t o r t i o n  i n  t h e  FM 
i a s e r .  It was experimentally deterpined t h a t  as t h e  power of an argon 
l a s e r  i s  increased, w i t h  t h e  modulator d r ive  remaining f ixed ,  t he  d i s -  
t o r t i o n  of t he  FM o s c i l l a t i o n  i s  a c t u a l l y  reduced. I n  e f f e c t ,  t he  
homogeneous broadening tends t o  reduce the  d i f f e rence  between the  ne t  
s a tu ra t ed  gain and l o s s  of each mode of t h e  coupled o s c i l l a t i o n ,  and 
thereby reduces t h e  t o t a l  d i s t o r t i o n .  Extrapolat ion of t h i s  r e s u l t  
i nd ica t e s  t h a t  as the  power of t he  argon l a s e r  i s  f u r t h e r  increased,  
increas ingly  l e s s  modulator dr ive w i l l  be required,  thereby easing t h e  
problem of modulator construct ion.  The r e s u l t  a l s o  i n d i c a t e s  t h a t  i n  
some very homogeneous l a s e r  l i n e s ,  f o r  ins tance  YAG, neg l ig ib l e  modu- 
l a t o r  d r ive  may be required t o  achieve exce l l en t  FM o s c i l l a t i o n .  
- 3 -  
A s  noted above, t h e  experimental work on t h i s  p ro jec t  i s  now 
complete. 
be completed and a paper published. 
During t h e  coming period computer and a n a l y t i c a l  work w i l l  
We note t h a t  t he  present  work on t h i s  phase of t he  Grant i s  c lose ly  
coordinated with the  supermode work a t  the  Sylvania Advanced Technology 
Laboratory, which i s  supported t h e r e  under NASA Contract NAS-8-50558, 
funded from Huntsvi l le ,  Alabama. 
2. Parametric Osc i l l a t ion  a t  ODtical Freauencies 
( M .  K. Oshman, R.  Byer, C. Someda, and S .  E. Harris) 
The purpose of t h i s  p ro jec t  i s  t o  study, both experimentally and 
t h e o r e t i c a l l y ,  parametric generat ion a t  o p t i c a l  f requencies .  The i n i t i a l  
emphasis of t h i s  p ro jec t  has  been d i r ec t ed  toward an inves t iga t ion  of t he  
dynamics of an op,tical  pai-arleti--jc osc- j l la tor  ~ 7 h - v  e i n  t h e  mnl t r ,ear  ma te r i a l  
i s  s i t u a t e d  i n t e r n a l l y  t o  the  cav i ty  of the  pumping l a s e r .  
'The p r inc ip l e  r e s u l t s  of t h e  S n i t ' l a l  t h e o r e t i c a l  study of such an 
o s c i l l a t o r  pointed out the  following: 
(i) The i n t e r n a l  parametric o s c i l l a t o r  i s  capable of very e f f i c i e n t  
operation, 
t h e  s t rong pump f i e l d s  e x i s t i n g  in s ide  the  l a s e r  cavi ty  a r e  
use fu l  i n  overcoming t h e  required threshold  f o r  o s c i l l a t i o n ,  
and 
the  o s c i l l a t o r  can e x h i b i t  t h r e e  d i s t i n c t  modes of operat ion.  
During the  i n t e r v a l  s ince t h e  l a s t  r epor t ,  t h e  emphasis of t he  work 
(ii) 
(iii) 
on t h i s  p ro jec t  has been d i r ec t ed  toward t h e  experimental  aspec ts  of con- 
s t r u c t i n g  such an o s c i l l a t o r .  This  work c o n s i s t s  of design and performance 
I, 
- - t -  
. 
ca lcu la t ions  followed by experimental construct ion and t e s t i n g  of t he  
elements necessary f o r  t h e  o s c i l l a t o r .  
In  the  design calculat ions,  e s s e n t i a l l y  a l l  ava i lab le  l a s e r s  and 
nonl inear  mater ia l s  were considered. The r e s u l t s  of t h i s  inves t iga t ion  
l e d  t o  the  decis ion t o  use LiNbO as the  nonl inear  c r y s t a l  due t o  i t s  
high nonl inear i ty  and usefulness i n  t he  v i s i b l e  and near i n f r a red  r e -  
3 
gions of t he  spectrum. A s  the  pump l a s e r ,  t h e  r e l a t i v e l y  high power 
ionized gas l a s e r s  were chosen. 
An r f  pumped r ing  discharge configurat ion was chosen as the  design 
f o r  t h e  ion l a s e r .  A 10 MHz o s c i l l a t o r  capable of de l iver ing  approxi- 
mately 10 k i lowat t s  of power t o  t h e  l a s e r  on a continuous basis was 
constructed.  Several  l a s e r  tubes have been constructed and t e s t e d  with 
both argon and krypton gas as t h e  l a s e r  mater ia l .  With argon, t h e  l a s e r  
i s  capable of producing a cw output power i n  each s t rong  l i n e  of approxi- 
mately one w a t t ;  with krypton, one-quarter wat t .  
I n  order t o  subs t an t i a l ly  Inci-ease t h e  gain of the laser t o  the  
point  t h a t  t he  in se r t ion  of the nonl inear  c r y s t a l  w i l l  not s i g n i f i c a n t l y  
lower t h e  l a s e r  pump power, a second l a s e r  tube i s  being b u i l t  t o  run i n  
s e r i e s  with the  f i r s t  tube.  The second tube w i l l  have an independent r f  
pumping source. 
A g r e a t  dea l  of e f f o r t  has been d i r ec t ed  toward t e s t i n g  a f ab r i ca -  
t i o n  of LiNbO c r y s t a l s  f o r  use as the  nonl inear  element of t he  o s c i l -  
l a t o r .  Very high o p t i c a l  qua l i t y  c r y s t a l s  with very f l a t  a n t i - r e f l e c -  
3 
t i o n  coated ends a r e  necessary t o  prevent degradation of t h e  Q of t he  
o p t i c a l  cavi ty  f o r  s igna l ,  i d l e r ,  and pump. There a re  a number of 
problems associated with t h i s  requirement, a l l  of which have been e i t h e r  
overcome o r  should sooi be sclved. 
- 5 -  
E 
1 
The genera l  o p t i c a l  qua l i ty  of t he  c r y s t a l s  ava i l ab le  from outs ide 
supp l i e r s  i s  apparent ly  below acceptable l e v e l s  f o r  usefulness  with 
i n t e r n a l  o p t i c a l  parametric o s c i l l a t o r s .  For example, t he  bulk s c a t -  
t e r i n g  and absorption of c r y s t a l s  2 cm long was on the  order  of 5% a t  
1.15 p .  We bel ieve  t h i s  problem w i l l  be solved with improved c r y s t a l  
growing procedures which have been developed a t  Stanford.  
LiNbO 
i t s e l f .  To solve t h i s  problem a s t a b l e  oven has  been constructed which 
In  addi t ion,  
exh ib i t s  inhomogeneities developed by the  pumping l a s e r  l i g h t  3 
maintains the  c r y s t a l  a t  a s u f f i c i e n t l y  high temperature t o  reverse  the  
development of t h e  inhomogeneities a t  a r a t e  g r e a t e r  than they a r e  formed. 
In  conjunction with t h i s  portion of t he  work, techniques f o r  phase match- 
ing a t  t hese  e leva ted  temperatures were developed. 
The sur face  proper t ies  of t h e  LiNbO must be extremely good f o r  3 
t he  c r y s t a l  t o  be usefu l  ins ide  a resonator .  Successful  procedures 
have been developed f o r  obtaining very f l a t ,  s c ra t ch - f r ee  sur faces .  I n  
addi t ion ,  a technique which includes "vth E,reucter angle cut t . ing and 
a n t i - r e f l e c t i o n  coat ing has been designed s o  t h a t  r e f l e c t i o n s  can be 
reduced a t  a l l  t h ree  frequencies of the  o s c i l l a t o r .  
3 .  Backward Wave Osc i l l a t ion  i n  t h e  Far  Infrared* 
( C .  Wilkinson, J. Murray, C .  Someda, J. Falk, and S. E. Harris) 
The purpose of t h i s  p ro jec t  i s  t o  demonstrate a technique f o r  ob- 
t a i n i n g  l a r g e  amounts of far  I - R  power, r e a d i l y  tunable  over t h e  range 
of 100 t o  1000 p .  The bas i c  idea  i s  t o  make use of a backward wave 
* 
The work On t h i s  project  was p a r t i a l l y  supported by Contract 
AF 19( 638) -1429. 
c 
c 
I 1 
i n t e r a c t i o n  of t h ree  electromagnetic waves which are coupled by means of 
t h e  e l ec t ro -op t i c  nonl inear i ty .  Of p a r t i c u l a r  importance i s  t h e  f a c t  
t h a t  i n  a backward wave o s c i l l a t i o n ,  feedback i s  provided i n t e r n a l l y  t o  
t h e  nonl inear  process,  and the re fo re  o p t i c a l  c a v i t i e s  f o r  t he  s i g n a l  and 
i d l e r  modes a r e  not  required.  The technique a l s o  provides a means f o r  
continuously o f f - s e t t i n g  t h e  frequency of a pump l a s e r  by about 10 8. 
The proposed experiment makes use of a &-switched ruby l a s e r  and 
a nonl inear  c r y s t a l  of LiTaO 
taken a s  the  backward wave, and t h e  th ree  waves involved s a t i s f y  the  k 
matching condition, k + Ei = k . A s  a r e s u l t  of t h i s  k vec tor  
matching, t h e  s i g n a l  frequency i s  r e l a t e d  t o  t h e  pump frequency by t h e  
r e l a t i o n  
The f a r  I-R (lowest frequency wave) i s  
- 3' 
- - - 
S P 
v 
P vi + 7,  
where 'li , vp , and 7, are the i -e f rac t ive  ind ices  a t  t h e  ='ATP, s i g r ? d ,  
and i d l e r  f requencies ,  respec t ive ly .  By cont ro l ing  t h e  temperature of 
t he  c r y s t a l  over a range of about 100 degrees, t he  s i g n a l  frequency 
should be tunable  from 100 t o  1000 p.  
The ca lcu la ted  threshold pump power dens i ty  f o r  t h i s  process i s  
2 2 
135 MW/cm for a c r y s t a l  1 cm in  length,  and va r i e s  inverse ly  as ( l eng th )  . 
Assuming the  exis tence of a d i f f r a c t i o n  l imi t ed  l a s e r ,  optimum focusing 
considerat ions y i e l d  a pump spot a r ea  of about 0.02 cm 
2 f o r  a c r y s t a l  
1 cm i n  length,  and the re fo re  a required pump power of about 3 Mw. O u r  
l a s e r  is ,  however, very f a r  from being d i f f r a c t i o n  l imi ted ,  and the  
e f f e c t  of t h i s  divergence i s  n o t  y e t  understood. 
- 7 -  
Progress on t h i s  p ro jec t  during t h i s  qua r t e r  has  included: 
(1) De-bugging and s t a b i l i z a t i o n  of t h e  high power &-switched 
ruby l a s e r . '  The l a s e r  now furn ishes  between 60 t o  80 MW i n  a s ing le  
long i tud ina l  mode having a beam divergence of about 5 rad ians .  
(2) The construct ion of a Dewar system f o r  possible  operat ion a t  
l i q u i d  helium temperatures.  
( 3 )  The measurement of b i re f r ingence  as a funct ion of temperature 
3' i n  LiTaO 
A t  present ,  work on t h i s  p r o j e c t  i s  being badly hampered by our 
i n a b i l i t y  t o  obtain a properly poled c r y s t a l  of LiTaO Such c r y s t a l s  
are ,  however, ava i l ab le  a t  other  l a b o r a t o r i e s  and most l i k e l y  w i l l  be 
3' 
obtained during t h e  coming quar te r .  
APPENDICES 
A .  S. E .  Harr is ,  "Threshold of Multimode Parametric Osc i l l a to r s ,  
Microwave Laboratory Report No. 1458, published IEEE J. Quant. 
E lec t r .  - a - 2 ,  701 (October 1966). 
L.  M. Osterink and R.  Targ, "Single Frequency Light from an Argon 
FM Laser," Microwave Laboratory Report No. 1484, submitted t o  Appl. 
F'hys. Le t t e r s .  
B. 
C. S. E. Harr is ,  "S tab i l iza t ion  and Modulation of Laser O s c i l l a t o r s  
by I n t e r n a l  Time-Varying Perturbat ions,  I '  Microwave Laboratory 
Report N o .  1429, published i n  Appl. Optics 2, 1639-51 (October 1966). 


Investigation of laser dynamics, modulation and control by means of intra-cavity time varying perturbation  Semiannual status report no. 2, 1 Aug. 1966 - 31 Jan. 1967

Investigation of laser dynamics, modulation and control by means of intra-cavity time varying perturbation Semiannual status report

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