Hanlon, J.

Jacobs, S. F.

English

QC 351 A7 no. 12The technique of coherent detection has been used to explore the problems involved in detecting extremely low power levels. An input signal power level of 5 x 10^-19 watts of 3.39u radiation was detected with voltage S/N of 2, in good agreement with theory. The major experimental problem was elimination of feedback from the local oscillator into the laser source. Narrowness of bandwidth was limited by instability in detector bias. Neither of these difficulties presents a fundamental limitation to a well designed receiver of light from a distant source.This title from the Optical Sciences Technical Reports collection is made available by the College of Optical Sciences and the University Libraries, The University of Arizona. If you have questions about titles in this collection, please contact repository@u.library.arizona.edu

The University of Arizona

Narrowband Optical Heterodyne Detection

NASA Technical Reports Server

I OPTI[CAL SCIENCES 
THE UNIVERSITY OF ARIZONA 
TUCSON, ARIZONA 
TECHNICAL R E P O R T  NO. 12 
NARROWBAND OPTICAL HETERODYNE DETECTION 
J. HANLON 
S. F. JACOBS 
January 1967 
https://ntrs.nasa.gov/search.jsp?R=19670017186 2020-03-12T11:20:50+00:00Z
ABSTRACT 
The technique of coherent  de t ec t ion  has  been used t o  explore  t h e  
problems involved i n  de t ec t ing  extremely low power l e v e l s .  
s i g n a l  power l e v e l  of 5 x w a t t s  of 3.3911 r a d i a t i o n  was  de tec ted  with 
vo l t age  S/N of 2, i n  good agreement with theory.  
An input  
The major experimental  problem was e l imina t ion  of feedback from t h e  
l o c a l  o s c i l l a t o r  i n t o  t h e  l a s e r  source.  Narrowness of bandwidth w a s  
l imi t ed  by i n s t a b i l i t y  i n  de t ec to r  b i a s .  
p re sen t s  a fundamental l i m i t a t i o n  t o  a w e l l  designed r ece ive r  of l i g h t  
from a d i s t a n t  source.  
Nei ther  of t hese  d i f f i c u l t i e s  
TABLE OF CONTENTS 
Pane 
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 
Equipment and Method . . . . . . . . . . . . . . . . . . . . . . . . .  4 
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6 
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8 
Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8 
References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . .  9 
Background Reading . . . . . . . . . . . . . . . . . . . . . . . . . .  9 
In t roduc t ion  
Coherent o r  heterodyne d e t e c t i o n  of l i g h t  i s  similar t o  t h e  super- 
heterodyne d e t e c t i o n  of r a d i o  waves. A weak, incoming s igna l -car ry ing  beam 
is superimposed on a s t rong  beam from a l o c a l  o s c i l l a t o r .  
mixing of t h e  f i e l d s  on t he  s u r f a c e  of a photodetector  r e s u l t s  i n  a photo- 
electric c u r r e n t  which may be  i n t e r p r e t e d  as a bea t  between components of 
t h e  electromagnet ic  f i e l d .  The process has  two important a t t r i b u t e s :  
no ise- f ree  ampl i f i ca t ion  of t he  weak s i g n a l  beam, and pos t -de tec t ion  
The nonl inear  
e l e c t r i c a l  f i l t e r i n g  of unwanted f requencies  t o  extremely narrow bandwidths 
(much narrower than can be obtained by prede tec t ion  o p t i c a l  f i l t e r i n g ) .  I n  
a d d i t i o n ,  phase- and frequency-modulated c a r r i e r s  can be demodulated. 
With t h e  advent of t he  laser, coherent d e t e c t i o n  of l i g h t  became con- 
v e n i e n t l y  poss ib l e .  However, t he  requirements f o r  i d e a l  performance a r e  
sever  e : 
1. The l o c a l  o s c i l l a t o r  m u s t  not  produce no i se  exceeding 
t h a t  which has  been ca l l ed  "shot noise." 
No s i g n i f i c a n t  wavefront d i s t o r t i o n s  may b e  t o l e r a t e d  
along t h e  t ransmission path be fo re  t h e  beams combine. 
2. 
3. I f  narrowband f i l t e r i n g  is  t o  be exp lo i t ed ,  a high 
degree of laser s t a b i l i t y  o r  c o n t r o l  i s  necessary.  
The l a t te r  two areas are today rece iv ing  cons iderable  r e sea rch  a t t e n t i o n .  
However, except  f o r  s p e c i a l  app l i ca t ions ,  t h e  complexi t ies  of coherent  
d e t e c t i o n  make i t  less p r a c t i c a b l e  than incoherent  d e t e c t i o n  ( i . e . ,  narrow- 
band o p t i c a l  f i l t e r  followed by photodetector) .  
pho tomul t ip l i e r s  are capable  of de t ec t ing  a few photons per  second, which 
is e x c e l l e n t  performance. However, i n  t h e  i n f r a r e d ,  photodetectors  of any 
In t h e  v i s i b l e  reg ion ,  
type a r e  p re sen t ly  too noisy f o r  t h i s  kind of performance, and i t  is h e r e  
t h a t  coherent  d e t e c t i o n  appears t o  bold i t s  g r e a t e s t  promise. 
above-mentioned requirements f o r  coherent d e t e c t i o n  are more e a s i l y  m e t  i n  
t he  i n f r a r e d  than i n  the  v i s i b l e  region of t h e  spectrum. 
Happily, t he  
The r e sea rch  repor ted  i n  th i s  paper concerns t h e  d e t e c t i o n  of extremely 
low o p t i c a l  s i g n a l  l e v e l s  i n  the  in f r a red  reg ion .  
In  1961, an  equat ion w a s  published r e l a t i n g  t h e  power of t h e  inpu t  
s i g n a l  (Fs) t o  t h e  s ignal- to-noise  power r a t i o  f o r  coherent de t ec t ion .  
r e l a t i o n s h i p  w a s  v e r i f i e d  experimentally by s e v e r a l  workers down t o  l e v e l s  
of 3 x w a t t s .  It w a s  assumed t h a t  lower input  s i g n a l  power l e v e l s  
would a l s o  g ive  p r e d i c t a b l e  va lues  of s ignal- to-noise  power r a t i o .  
lower inpu t  powers had not  a c t u a l l y  been t e s t e d .  
known what d i f f i c u l t i e s  might be inherent  i n  de t ec t ing  such very low va lues  
This  
But 
Furthermore, it w a s  no t  
- 
of Ps. 
19 I n  t h e  p re sen t  work, input  s i g n a l  power l e v e l s  a s  low a s  5 x 10- 
w a t t s  were de tec t ed .  It  w a s  found t h a t  t h e  measured s ignal- to-noise  power 
r a t i o s  s t i l l  coincided wi th  the  pred ic ted  va lues .  
The main problems i n  de t ec t ing  very low input  s i g n a l  power turned ou t  
t o  be  (1) o p t i c a l  feedback from the  l o c a l  o s c i l l a t o r  i n t o  the  laser and 
(2) o v e r a l l  system i n s t a b i l i t y .  
the experimental  apparatus .  
The former can be prevented by a d j u s t i n g  
The l a t t e r  r equ i r e s  c a r e f u l  system design.  
Previous  Work 
Considerable  work has been done t o  v e r i f y  t h a t  t h e  s ignal- to-noise  
power r a t i o  achievable  by means of coherent d e t e c t i o n  agrees  wi th  t h e  
expres s ion  1 
- 2- 
- 
where q is  t h e  d e t e c t o r  quantum e f f i c i ency ,  Ps is  t h e  average input  s i g n a l  
power, hv is  t h e  energy per  photon, and Af is  t h e  receiver bandwidth. Good 
agreement has been found a t  r e l a t i v e l y  high levels of Fs.2y3 
It is  of f u r t h e r  i n t e r e s t  t o  i n v e s t i g a t e  t h e  d i f f i c u l t i e s  encountered 
i n  d e t e c t i o n  of t h e  smallest t h a t  i s  feasible--for  example, narrowband- 
ing t o  t h e  o rde r  of 1 cps.  
one t h i n g  o r  another ,  t o  va lues  of Fs considerably g r e a t e r  than could have 
been reached by a more f o r t u n a t e  choice of components. 
t h e  arrangement shown i n  Figure 1, Jacobs and Rabinowitz2 de tec t ed  a Ps of 
S 
To da te ,  a l l  measurements have been l i m i t e d ,  by 
Using e s s e n t i a l l y  
- 
w a t t s  wi th  a bandwidth of 1 cps.  The r e s u l t i n g  S/N agreed with t h e  
equat ion above and demonstrated, a t  A = 1 . 1 5 ~ ~  d e t e c t i o n  two o rde r s  of 
magnitude below t h e  d e t e c t o r ’ s  incoherent NEP (no i se  equivalent  power). 
However, as f a r  as low P, w a s  concerned, t h e  choice of PbS d e t e c t o r  
(rl = 2.3 x w a s  unfortunate .  Recently, Goodwin and Pedinoff used a n  
I d s  photodiode (q = 0.3) and a Debye-Sears modulator both t o  phys i ca l ly  
- 
3 
s e p a r a t e  and t o  o f f s e t  t h e  frequency of t h e  s i g n a l  beam with r e s p e c t  t o  t h e  
l o c a l  o s c i l l a t o r  beam. They reported d e t e c t i o n  of 3 x w a t t s  w i th  
Af = 10  kc ( f i v e  o r d e r s  of magnitude below t h e  d e t e c t o r ’ s  incoherent NEP). 
The l i m i t a t i o n  on narrow bandwidth w a s  probably inhomogeneity i n  t h e  
modulator.  
I n  t h e  p re sen t  s tudy,  t h e  above work is extended t o  t h e  r e l a t i v e l y  
narrow bandwidth of 0.5 cps,  where a s i g n a l  level of 5 x 1O-l’ w a t t s  has 
been d e t e c t e d  w i t h  t h e  t h e o r e t i c a l l y  predicted S/N power r a t i o  of 4 .  
-3- 
Equipment and Method 
Figure 1 shows t h e  arrangement used i n  t h e  p re sen t  work. 
DETECT0 PREAMP SPECTRUM /-ANALYZER 
- , (LOCK-IN) 
~4 DC SUPPLY 
TRANSLATABLE 
MIRROR 
Figure 1. Experimental arrangement f o r  measurement 
of S/N v o l t a g e  i n  coherent de t ec t ion .  
The d e t e c t o r  w a s  a n  InAs photodiode (Phi lco L4530). The s t a b i l i t y  oL t h e  
p i e z o e l e c t r i c  modulator made i t  poss ib l e  t o  narrowband and coherently d e t e c t  
a s i g n a l  beam power t e n  o rde r s  of magnitude below t h e  d e t e c t o r ' s  incoherent  
NEP. As i n  t h e  preceding work, t h e  method involved ob ta in ing  a s ing le -  
frequency laser beam, f r e e  from excess n o i s e  and of s u f f i c i e n t  power t o  
gene ra t e  a photocurrent  whose sho t  noise  exceeded a l l  o the r  system noise.  
-4- 
I L A S E R  7 FOCUSING L E N S  
2 B E A M  S P L I T T E R  8 D E T E C T O R  
3 COMPENSATORS 9 D E T E C T O R  POSITIONER 
4 M I R R O R  IO WIRES FOR E L E C T R I C A L  
5 ATTENUATORS MODULATION SIGNAL .. . - 
6 MIRROR I I  WIRES FOR E L E C T R I C A L  
P H A S E  A D J U S T M E N T  
Figure 2 shows t h e  va r ious  n o i s e  l e v e l s  p re sen t .  
- L  
IO. 
n 
v) 
0 > 
v, 
5 
LL: = 12 
Y 
I- 
3 
I- 
3 
0 
E 
0 
I- - 7  
0 IO 
W 
I- 
W 
I- 
W 
v) 
z 
a 
n 
a 
6 
IO4 
---_.- 
IOOpa SHOT 
( LASER 
’ (LASER 
PREAMP 
ON 1 
OFF) 
- .- - _ _ _  - - _ _ _ - _ _ _ _ _ _ _  NOISE 
5~ JOHNSON NOISE LEVEL 
I & 4 A 
I KC 10 KC OPERATiNG 100 KC 
FR E OU E NCY 
F R E Q UE N CY (c p s ) 
Figure 2. System n o i s e  spectrum ( A f  = 6 cps) . 
A t  low f r equenc ie s ,  l / f  n o i s e  of t h e  de t ec to r  w a s  t h e  l a r g e s t  system noise;  
i t  l e s sened  a t  h ighe r  f r equenc ie s ,  eventual ly  dropping below s h o t  no i se  
beyond 20 kc. 
ana lyze r  (Hewlett-Packard Model 302A).  
l 
The maximum frequency was set by t h e  a v a i l a b l e  spectrum 
-5- 
The laser w a s  He-Ne, operated a t  3.39~1 wi th  r f  e x c i t a t i o n ,  i n  a 6-mm 
plasma tube. 
single-mode power. The r e sona to r  consis ted of an aluminized f l a t ,  separated 
60 cm from a 60-cm-radius concave quartz  r e f l e c t o r ,  t h e  c e n t r a l  0.090-inch 
diameter of which w a s  a l s o  aluminized. 
D i f f r a c t i o n  output  coupling4 w a s  used t o  o b t a i n  500p w a t t s  of 
By position-modulating t h e  mi r ro r  with a p i e z o e l e c t r i c  t ransducer ,  50 
S igna l  power w a s  v a r i e d  by kc phase modulation w a s  impressed on one beam. 
i n s e r t i n g  a t t e n u a t i o n  A (plane p a r a l l e l  aluminized qua r t z  r e f l e c t o r s ,  
cocked 15' from normal t o  the beam). For phase angles  8 < 0.2  r a d i a n s ,  t h e  
modulated s i g n a l  beam power Fs is r e l a t e d  t o  t h e  unmodulated beam power Fo 
through t h e  r e l a t i o n  Ps = 0' To A .  - 
- - 
Phase modulation depth 0 w a s  measured, under condi t ions where Ps << Po, 
by a d j u s t i n g  t h e  in t e r f e romete r  f o r  maximum AM s i g n a l  (quadrature  s e t t i n g )  
and t ak ing  the r a t i o  of t h i s  peak-to-peak s i g n a l  t o  t h e  d i f f e r e n c e  between 
maximum and minimum DC levels (in-phase and out-of-phase s e t t i n g s ) .  Because 
of cu rva tu re  of t h e  wavefronts i t  w a s  necessary t o  keep t h e  in t e r f e romete r  
ams equal  i n  l eng th ,  
o p t i c a l  compensators i n  t h e  l o c a l  o s c i l l a t o r  beam each t i m e  an a t t e n u a t o r  
was  i n s e r t e d  i n t o  the s i g n a l  beam. 
This  w a s  done by p l ac ing  clear, plane p a r a l l e l  
F i n a l  narrowband measurements were made using lock-in d e t e c t i o n  
(P r ince ton  Applied Research 
v o l t  ages  w e r e  
R e s u l t s  
F i g u r e  3 
measured wi th  
shows t h e  S/N 
Model JB-6, w i th  v a r i a b l e  bandwidth). 
a d e t e c t i o n  t i m e  constant  of 1 second. 
S/N 
- 
vo l t ages  measured a t  va r ious  values  of P,. 
-6- 
Because S/N w a s  a c t u a l l y  measured i n  terms of v o l t a g e  r a t h e r  than 
power, t h e  t h e o r e t i c a l  S / N  is expressed i n  terms of v o l t a g e  r a t h e r  than 
power. 
A s  shown by t h e  graph, t he  c o r r e l a t i o n  is  good between measured and 
t h e o r e t i c a l  va lues  of S / N .  
1 0 , ~ -  
1000- 
Z 
35 
w 
s 100- 5 
0 > 
SIGNAL POWER, 9 (WATTS) 
Figure  3 .  Measured and t h e o r e t i c a l  S/N vo l t ages  
v s .  i nc iden t  s i g n a l  beam power. 
-7- 
Discussion 
The major problem encountered i n  achieving t h e o r e t i c a l  S/N w a s  o p t i c a l  
feedback from the l o c a l  o s c i l l a t o r  i n t o  t h e  laser, which caused laser power 
f l u c t u a t i o n s  a t  t h e  s i g n a l  frequency. It  w a s  necessary t o  tilt t h e  
a t t e n u a t o r s ,  t h e  d e t e c t o r ,  and t h e  laser o p t i c  a x i s  s u f f i c i e n t l y  nonnormal 
t o  t h e  in t e r f e romete r  m i r r o r s  so  t h a t  no r e f l e c t e d  l i g h t  r een te red  t h e  
laser. This problem would not  occur i n  a system with a d i s t a n t  s i g n a l  
source.  
Longer t i m e  cons t an t s  were precluded by t h e  b i a s  i n s t a b i l i t y  of t he  
p a r t i c u l a r  d e t e c t o r  used. I n  o rde r  t o  a s s u r e  t h a t  photocurrent s h o t  n o i s e  
w a s  much g r e a t e r  than d e t e c t o r  no i se ,  and with l imi t ed  l o c a l  o s c i l l a t o r  
power a v a i l a b l e ,  i t  was necessary t o  keep t h e  d e t e c t o r  n o i s e  as s m a l l  as 
p o s s i b l e .  This w a s  done by ope ra t ing  t h e  d e t e c t o r  back biased,  c l o s e  t o  
zero v o l t s  (-50 mv), where n o i s e  is low b u t  s i g n a l  is  very bias-dependent 
and hence unsteady. 
quency ope ra t ion  o r  a q u i e t e r  de t ec to r )  o r  a more powerful l o c a l  o s c i l l a t o r  
laser, t h e  d e t e c t o r  could be  operated a t  l a r g e r  nega t ive  b i a s  levels where 
s i g n a l  i s  nea r ly  independent of b i a s  o r ,  a l t e r n a t i v e l y ,  b e t t e r  r e g u l a t i o n  
of bias  vo l t age  could b e  u t i l i z e d  f o r  work a t  narrower bandwidths. 
With e i t h e r  a lower system n o i s e  l e v e l  (higher f r e -  
Acknowledgment 
T h i s  work w a s  p a r t i a l l y  supported by NASA. 
-8- 
References Cited 
1. B. M. O l ive r ,  "Signal-to-noise r a t i o s  i n  p h o t o e l e c t r i c  mixing": Proc. 
I R E  49, 1960-1961 (1961). 
2. S .  F. Jacobs and P. J .  Rabinowitz, Quantum E lec t ron ic s  Conference, 
Feb. , 1963 , P a r i s  (Proceedings published by Columbia Universi ty  
P r e s s ,  New York, 1964). 
3. F. E. Goodwin and M. E .  Pedinoff ,  "An I R  o p t i c a l  heterodyne receiver 
system," Paper 7C-4 in 1966 I n t e r n a t i o n a l  Quantum E lec t ron ic s  
Conference, Digest  of Technical Papers,  A p r i l ,  1966, Phoenix: 
IEEE Jour .  of Quantum E lec t ron ic s ,  QE-2(4), 1 (1966). 
4. J .  T .  LaTourrette,  S .  F. Jacobs,  and P. Rabinowitz, "Improved laser 
angular  b r igh tness  through d i f f r a c t i o n  coupling": Applied Optics 
- 3, 981-982 (1964). 
Background Reading 
S . Jacobs , "The o p t i c a l  heterodyne; key t o  advanced space signaling": 
E lec t ron ic s ,  x ( 2 8 1 ,  29-31 ( Ju ly  12, 1963). 
G. Lucovsky, M. E .  Lasser, and R.  B. Emmons, "Coherent l i g h t  d e t e c t i o n  i n  
s o l i d - s t a t e  photodiodes": Proc. IEEE 5l, 166-172 (1963). 
L.  Mandel, "Heterodyne d e t e c t i o n  of a weak l i g h t  beam": JOSA 56, 1200- 
1206 (1966). 
A,. E. Siegman, S .  E .  Harris,  and B.  J .  McMurtry, "Optical  heterodyning and 
o p t i c a l  demodulation a t  microwave frequencies":  Proceedings of 
t h e  Symposium on O p t i c a l  Masers, New York, Apr i l ,  1963; Micro- 
wave Research I n s t i t u t e  Symposia S e r i e s ,  Vol. X I I I ,  pp. 511-527 
(Polytechnic P r e s s ,  Brooklyn, N .  Y . ,  1963). 
-9- 


Narrow band optical heterodyne detection

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