Far infrared transmissive fiber optics as a component in the design of integrated far infrared focal plane array utilization is discussed. A tightly packed bundle of fibers is placed at the focal plane, where an array of infrared detectors would normally reside, and then fanned out in two or three dimensions to individual detectors. Subsequently, the detectors are multiplexed by cryogenic electronics for relay of the data. A second possible application is frequency up-conversion (v sub 1 + v sub 2 = v sub 3), which takes advantage of the nonlinear optical index of refraction of certain infrared transmissive materials in fiber form. Again, a fiber bundle is utilized as above, but now a laser of frequency v sub 1 is mixed with the incoming radiation of frequency v sub 1 within the nonlinear fiber material. The sum, v sub 2 is then detected by near infrared or visible detectors which are more sensitive than those available at v sub 2. Due to the geometrical size limitations of detectors such as photomultipliers, the focal plane dispersal technique is advantageous for imaging up-conversion

Goebel, J. H.

Breton 

NASA Technical Reports Server

INFRARED FIBER OPTIC FOCAL PLANE DISPERSERS 
John H. Goebel 
Ames Research Center 
Nat i onal Aeronaut i c s  and Space Admi n i  s t  r a t  i on 
SUMMARY 
Midd le  and f a r  i n f r a r e d  f o c a l  p lane d e t e c t o r s  a r e  c u r r e n t l y  l e s s  i n t e -  
g r a t e d  i n  t h e i r  des ign than a r e  those i n  t h e  near i n f r a r e d .  
o f  low temperature process ing s teps on t h e  u l t r a h i g h  p u r i t y  germani un d e t e c t o r  
m a t e r i a l ,  which i s  r e q u i r e d  i n  t h e  f a r  i n f r a r e d ,  l i m i t  t h e  r e a l i z a t i o n  o f  i n t e -  
g r a t e d  c i r c u i t  technology i n  t h e  near fu tu re .  
d iscuss  t h e  u t i l i z a t i o n  o f  f a r  i n f r a r e d  t r a n s m i s s i v e  f i b e r  o p t i c s  as a compon- 
e n t  i n  t h e  des ign o f  i n t e g r a t e d  f a r  i n f r a r e d  f o c a l  p lane a r r a y  u t i l i z a t i o n .  A 
t i g h t l y  packed bundle o f  f i b e r s  i s  p laced a t  t h e  f o c a l  p lane,  where an a r r a y  
o f  i n f r a r e d  d e t e c t o r s  would normai iy  r e s i d e ,  and then fanned o u t  i n  two o r  
t h r e e  dimensions t o  i n d i v i d u a l  d e t e c t o r s .  Subsequently, t h e  d e t e c t o r s  a r e  
m u l t i p l e x i e d  by c ryogen ic  e l e c t r o n i c s  f o r  r e l a y  o f  t h e  data.  
The d i f f i c u l t i e s  
As an a l t e r n a t i v e  approach, we 
A second p o s s i b l e  a p p l i c a t i o n  i s  f requency upconversion (vi + vf = v 2 ) ?  
which takes  advantage o f  t h e  n o n - l i n e a r  o p t i c a l  index  o f  r e f r a c  i o n  o c e r  a i n  
i n f r a r e d  t r a n s m i s s i v e  m a t e r i a l s  i n  f i b e r  form. Again, a f i b e r  bundle i s  u t i -  
l i z e d  as above, b u t  now a l a s e r  o f  f requency v1 i s  mixed w i t h  t h e  incoming 
r a d i a t i o n  o f  f requency v2 w i t h i n  t h e  n o n - l i n e a r  f i b e r  m a t e r i a l .  The sun, v3, 
i s  t h e n  de tec ted  by near  i n f r a r e d  o r  v i s i b l e  d e t e c t o r s  which a r e  more sens i -  
t i v e  than those a v a i l a b l e  a t  v . Due t o  t h e  geometr ica l  s i z e  l i m i t a t i o n s  of 
d e t e c t o r s  such as photomul t i p 1  f e r s ,  t h e  focal -p lane d i s p e r s a l  technique i s  
advantageous f o r  imaging upconversion. 
30 1 
https://ntrs.nasa.gov/search.jsp?R=19820008045 2020-03-21T09:52:39+00:00Z
INTRODUCTION 
C u r r e n t l y ,  r e a l i z a t i o n s  o f  i n s t r u n e n t s  based upon i n f r a r e d  upconversion 
a r e  l i m i t e d  by t h e  a v a i l a b i l i t y  o f  l a r g e  h i g h  q u a l i t y  n o n - l i n e a r  c r y s t a l s .  
descr ibe  here how f i b e r  bundles o f  t h e  n o n - l i n e a r  m a t e r i a l  can s u b s t a n t i a l l y  
e l i m i n a t e  t h e  a v a i l a b i l i t y  problem. Such " p i x e l i z e d "  c r y s t a l s ,  i.e., t h e  
f i b e r  bundles,  can cons iderab ly  inc rease t h e  convers ion quantun e f f i c i e n c y  
a v a i l a b l e  a t  low pump power d e n s i t i e s .  
t h r u  t h e  use o f  f i b e r  o p t i c s  can cons iderab ly  add t o  t h e  ruggedness and use- 
f u l n e s s  o f  ins t ruments  based on t h e i r  i n c o r p o r a t i o n  when compared w i t h  those 
u t i  1 i z i  ng d i  s c r e t e  components. 
I 
The i n t e g r a t i o n  o f  o p t i c a l  components 
DISCUSSION 
L i n e a r  F i b e r  Opt ics  
I n f r a r e d  f o c a l  p lanes a r e  now i n  a phase o f  extreme i n t e g r a t i o n  pressure.  
Users a r e  n o t  s a t i s f i e d  w i t h  a few good d e t e c t o r s  which scan a scene t o  t h e r e -  
by c o n s t r u c t  a p i c t u r e .  
has c rea ted  a l a r g e  demand f o r  CCD o r  C I D  f o c a l  p lane ar rays  which " p i x e l i z e "  
a scene w i t h  cons iderab le  f a c i l i t y  and accuracy. The cos t  investment t o  pro-  
duce custom a r r a y s  i s  cons iderab le  t o  t h e  p o i n t  o f  b e i n g  p r o h i b i t i v e  i n  many 
instances.  Thus o n l y  a few standard a r r a y  fo rmats  have become a v a i l a b l e ,  and 
those a r e  i n  s i l i c o n  f o r  t h e  v i s i b l e  and HgCdTe f o r  t h e  i n f r a r e d .  
Rather t h e  a v a i l a b i l i t y  o f  m i c r o c i r c u i t  technology 
I n  c e r t a i n  imaging a p p l i c a t i o n s  t h e  d e t e c t o r  p i x e l  s i z e  i s  n o t  matched 
c o n v e n i e n t l y  t o  t h e  s p a t i a l  r e s o l u t i o n  o f  t h e  te lescope.  Several f a c t o r s  
a f f e c t  t h e  s p a c i a l  r e s o l  u t i o n ;  most notably they are "seeing", diffraction, 
o p t i c a l  q u a l i t y .  
severa l  d e t e c t o r s  measure severa l  s p a t i a l  r e s o l  u t i o n  elements or vice versa 
This  r e s u l t s  i n  a l o s s  o f  in fo rmat ion .  
Any o f  these can s e v e r e l y  degrade t h e  r e s o l u t i o n  so t h a t  
and 
I n  F igure  1 we show a proposal  f o r  c o u p l i n g  d i s s i m i l a r  s p a t i a l  r e s o l u t i o n  
scales so t h a t  i n  e f f e c t  t h e y  a r e  equ iva len t .  A c o n i c a l  f i b e r  bundle o f  
a p p r o p r i a t e  l e n g t h  i s  matched a t  one end t o  t h e  s p a t i a l  r e s o l u t i o n  s c a l e  o f  
t h e  f o c a l  p lane and a t  t h e  o t h e r  end t o  t h e  d e t e c t o r  array scale.  Note t h a t  i f  
t h e  d e t e c t o r  s i z e  i s  much smal le r  than t h e  r e s o l u t i o n  s i z e ,  care  must be exer-  
c ised  i n  t h e  cone des ign so as n o t  t o  t o t a l l y  r e t r o r e f l e c t  t h e  l i g h t  i n  t h e  
cone b e f o r e  i t  e x i t s  o n t o  t h e  de tec tors .  
The scheme o u t l i n e d  i n  F igure  1 can be used advantageously i n  t h e  
v i s i b l e ,  near IR, and midd le  I R ,  where i n t e g r a t e d  c i r c u i t  technology produces 
s u i t a b l e  d e t e c t o r  ar rays.  I n  t h e  f a r  IR, t h e  s i l i c o n  and HgCdTe d e t e c t o r s  do 
n o t  f u n c t i o n .  There are,  so f a r ,  o n l y  germanium based d e t e c t o r s  which a r e  
p h y s i c a l l y  s i z e a b l e  as well as discrete. I n  t h i s  case a s o l u t i o n  i s  a l s o  
p o s s i b l e  us ing f i b e r  o p t i c s  o f  a p p r o p r i a t e  m a t e r i a l s  and i s  shown i n  F i g u r e  2. 
S u i t a b l e  f i b e r  m a t e r i a l s  a r e  KRS5 and AgBr. The cone i s  t i e d  a t  t h e  f o c a l  
p lane end, w h i l e  t h e  d e t e c t o r  end i s  f r e e  as i n  a broom c o n s t r u c t i o n .  I n d i -  
302 
v i d u a l  d e t e c t o r s  a r e  then f r e e  t o  be p laced over  an a p p r o p r i a t e  s p a t i a l  s c a l e  
w i t h  due c o n s i d e r a t i o n  t o  such a n c i l l a r y  components as i n t e g r a t i n g  
spheres, preamps, l o a d  r e s i s t o r s ,  and s t r e s s i n g  clamp. It should be noted 
t h a t  f i b e r  bundles a r e  advantageous i n  space a p p l i c a t i o n s  f o r  r o u t i n g  f o c a l  
p lane images i n t o  compartments s h i e l d e d  from cosmic rays.  
Non l inear  F i b e r  Opt ics  
One o f  t h e  r e a l l y  promis ing developments i n  o p t i c a l  m a t e r i a l s  i n  t h e  l a s t  
decade i s  t h e  exp los ion  o f  our  knowledge about t h e  p r o p e r t i e s  and a p p l i c a t i o n s  
o f  n o n - l i n e a r  o p t i c a l  c r y s t a l s .  Since n o n - l i n e a r  p r o p e r t i e s  were f i r s t  
i n v e s t i g a t e d ,  i t  has been r e a l i z e d  t h a t  one o f  t h e i r  t r u l y  o r i g i n a l  
a p p l i c a t i o n s  i s  i n  t h e  convers ion o f  i n f r a r e d  r a d i a t i o n  i n t o  v i s i b l e  where 
ext remely s e n s i t i v e  d e t e c t o r s  a r e  a v a i l a b l e .  There have been severa l  success- 
f u l  ins t ruments  b u i l t  f o r  t h i s  purpose, b u t  a l l  s u f f e r  from one o r  another 
l i m i t a t i o n s  imposed by t h e  a v a i l a b i l i t y  o f  pmp sources o r  t h a t  o f  t h e  r e q u i -  
s i t e  n o n - l i n e a r  m a t e r i a l .  It appears t h a t  some o f  these major  problems can be 
a l l e v i a t e d  th rough t h e  i n t r o d u c t i o n  o f  non-1 i n e a r  f i b e r  bundles as proposed 
here.  
F i r s t  I'll l i s t  t h e  b a s i c  t h e o r e t i c a l  d e s c r i p t i o n  o f  n o n - l i n e a r  upcon- 
vers ion .  
p ropogat ing  t h r u  a u n i a x i a l  g r v s t a l  w i t h  an o r d i n a r y  index o f  r e f r a c t i o n  no 
and an e x t r a o r d i n a r y  index n , then t h e  c r y s t a l  can be r o t a t e d  t o  a s p e c i f i c  
angle 8 , t h e  m i x i n g  angle,  such t h a t  t h e  laws o f  conserva t ion  o f  energy and 
rnomentd f o r  t h e  waves can be s i m p l y  r e l a t e d  as 
I f  photons o f  two d i f f e r e n t  f requencies,  v1 and v2 a r e  s imu l taneous ly  
v1 + v2 = v3 energy conserva t ion  
kl + k 2  = k3 momentum conserva t ion  
and 9 h 
i.e., a t h i r d  wave a t  v i s  produced. P o l a r i z a t i o n  must a l s o  be con- 
s i d e r e d  and t h i s  leads  t o  t w s  types o f  momentum conserva t ion  o r  phase matching. 
For Type 1 m i x i n g  b o t h  waves have t h e  same o r d i n a r y  p o l a r i z a t i o n  
nolvl + no2v2 = n e 3v3 Type 1 
For  Type 2 phasematching, or thogonal  p o l a r i z a t i o n s  a r e  mixed 
e e n I v1 + nozv2 = R 3v3 
For  u n i a x i a l  c r y s t a l s ,  t h e  p o l a r i z a t i o n  can be expanded i n  terms o f  t h e  
e l e c t r i c  f i e l d  s t r e n g t h  E 
303 
The n o n - l i n e a r  i n t e r a c t i o n  i s  p r o p o r t i o n a l  t o  t h e  product  o f  f i e l d  o f  
s t r e n g t h  o f  t h e  two waves and t h e  c r y s t a l ' s  e f f e c t i v e n e s s  as a n o n - l i n e a r  
medium i s  measured by diJk. Under c e r t a i n  assumptions about t h e  symmetry o f  
t h e  p o l a r i z a t i o n  m a t r i x  , .g., Kleinman symmetry, t h e  exper imenta l  e q u i v a l e n t  
i j k  o f  di .k i s  def f .  t h e o r e j i c a l l y .  
I n  p r a c t i c e  i t  i s  a lmost  imposs ib le  t o  eva lua te  d 
For an ins t rument ,  t h e  p r a c t i c a l  parameter o f  i n t e r e s t  i s  t h e  quantum 
convers ion e f f i c i e n c y  T) 
n1n2n3 '2'3 L x  
- 2 -  
where x = - (Akl) and Ak = I k3-kl-k2 
2 
I i s  t h e  l a s e r  pump i n t e n s i t y  usua 
i n t e r a t t i o n  l e n q t h  o f  t h e  c r v s t a l .  The 
l y  chosen a t  v and 1 i s  t h e  
erm i n  b r a c k e t $  i s  r e l a t e d  t o  t l e  
coherence l e n g t h  o f  t h e  i n t e r a c t i o n  X which i s  dimensionless.  O f  immediate 
p r a c t i c a l  i n t e r e s t  a r e  t h e  impor tan t  Manley-Rowe r e l a t i o n s h i p s  which govern 
t h e  power f l o w  from wave t o  wave 
v1 v3 
which e s s e n t i a l l y  s t a t e  t h a t  100% power convers ion o f  i n f r a r e d  i n t o  
v i s i b l e  photons i s  poss ib le .  
1 aboratory .  
Indeed t h a t  has been shown t o  be p o s s i b l e  i n  t h e  
Inst ruments have been demonstrated based upon d i f f e r e n t  n o n - l i n e a r  
c r y s t a l s  t o  produce i n f r a r e d  upconversion w i t h  respec tab le  NEP's and n ' s .  
and Townes (1977) have u s e f o p r o u s t i t e  (Ag3As S ) to7make an ast ronomical  
imager w i t h  an NEP = 3x10- 
t h e  Sun, Moon, Mercury and V Y  CMa. Voronin e t  a1 (19751 used AgGaS t o  
achieve 4 0 % ~  w i t h  a h i g h  p u r i t y  (4  (1.06 m )  < 0.1 cm- ) c r y s t a l  a 2 l e  t o  
n o t  t h e  s i g n a l  source power. Hence t h e  g r e a t  i n t e r e s t  i n  upconversion f o r  low 
l e v e l  s i g n a l  d e t e c t i o n .  Gurski  (1973) used a LiIO c r y s t a l  t o  cpgver 
r a d i a t i o n  w i t h  100% e f f i c i e n c y .  W/ 
Both Voronin and Boyd were a b l e  t o  achieve imaging, as have o t h e r  workers.  
Boyd 
W/m and an q = $xlO- . Images were ob ta ined o f  
u t i l i z e  h i g h  l a s e r  energy l e v e l s .  Note t R a t  depends o n l y  on t h e  pump power, 
He a l s o  achieved dn NEP = 1x10- 
The requirements o f  h i g h  f i e l d  s t r e n g t h s  f rom t h e  punp necessary t o  d r i v e  
t h e  n o n - l i n e a r  i n t e r a c t i o n  r e q u i r e s  focus ing  t h e  punp and s j g n a l  source i n  t h e  
c r y s t a l  t o  t h e  t h r e s h o l d  o f  damage, usualy a t  1 0 ' s  o f  MW/cm i n  p r e s e n t l y  
a v a i l a b l e  m a t e r i a l s .  Such h i g h  energ ies a r e  u s u a l l y  a v a i l a b l e  o n l y  from 
pu lsed l a s e r s  o f  low d u t y  cyc le .  This  s i t u a t i o n  i s  a disadvantage i n  low 
l e v e l  s i g n a l  d e t e c t i o n  and leads  t o  t h e  a p p a r e n t l y  l o w  n and NEP o f  Boyd and 
Townes 
3 04 
From t h e  express ion  f o r  , t h e  e a s i l y  c o n t r o l l e d  parameters o f  an up- 
conver te r  a r e  d ff, IL, and While 1\ grows l i n e a r l y  w i t h  
square o f  d The search f o r  m a t e r i a l s  w i t h  h ighe 
b u t  d r a m a t i f f g r d e r  o f  magnitude lgaps  a r e  very  u n l i k e l y .  
d are  l i m i t e d  t o  about 300x10- 
u!$ful def f  t o  Val ues l e s s  than 100x10 esu. 
Refinement o f  m a t e r i a l  q u a l i t y  i s  more e a s i l y  achieved than i n v e n t i n g  nfw 
m a t e r i a l s .  Present absorp t ion  c o e f f i c i e n t s  a r e  u s u a l l y  g r e a t e r  t h a n  0.1 cm- , 
l i m i t i n g  p r a c t i c a l  c r y s t a l  usage t o  10 cm lengths .  However, because o f  t h e  
need t o  focus i n  c r y s t a l s  t o  up t h e  f i e l d  s t r e n g t h ,  t h e  usefu l  p o r t i o n  o f  
c r y s t a l  i s  i n  p r a c t i c e  much l e s s  than t h i s  f i g u r e .  The s p e c t r a l  bandpass d v  
o f  an upfonvertef  i s  determined by t h e  c r y s t a l  l e n g t h  and i s  approx imate ly  
, i t  grows as t h e  
h d  1. 
esu19 S p e c i f i c  a p p l i c a t i o n  may l i m i t  t h e  
bP 
Avbp = 1- i n  cm- . 
I n  F igure  3, I propose t h a t  f i b e r  bundles o f  n o n - l i n e a r  m a t e r i a l  can be 
The f i b e r s  a r e  a b l e  t o  m a i n t a i n  a g iven punp i n t e n s i t y  I L  
used t o  a l l e v i a t e  sane o f  t h e  above r e s t r i c t i o n s  on t h e  performance o f  i n f r a -  
r e d  upconverters.  
over  g r e a t e r  l e n g t h s  i n  a focused beam p r o f i l e .  The f u l l  l e n g t h  o f  a f i b e r  
can be employed u s e f u l l y  t o  inc rease n. I f  i n  t h e  f i g u r e  the collimated 
beams o f  v 
preserved khroughout t h e  d e t e c t i o n  system. 
mean increased s p e c t r a l  r e s o l u t i o n .  F i b e r  bundles can a l s o  be formed f rom 
s e l e c t e d  h i g h  q u a l i t y  l e n g t h s  o f  f i b e r s .  
and v2 a r e  focused on t h e  c r y s t a l  f i b e r  bundle,  then t h e  image i s  
Longer lengths  o f  f i b e r s  employed 
A t  t h e  moment, t h e  most promis ing m a t e r i a l  a v a i l a b l e  f o r  p r o d u c t i o n  o f  
upconversion i n  f i b e r s  i s  AgGaS . Most a l l  o f  i t s  p r o p e r t i e s  a r e  known which 
a r e  e s s e n t i a l  t o  p r o d u c t i o n  o f  Zuch f i b e r s .  I n  a c o l l a b o r a t i v e  endeavor w i t h  
R. Byer and R. Feige lson o f  S tan ford  U n i v e r s i t y ,  D. Dimiduk, and myse l f ,  w i t h  
t h e  suppor t  o f  t h e  Ames Research Center 's  D i r e c t o r s  D i s c r e t i o n a r y  Fund, a r e  
work ing t o  t h e  r e a l i z a t i o n  o f  a f i b e r  o p t i c a l  n o n - l i n e a r  upconverter,  w i t h  t h e  
goal i n  mind o f  ach iev ing  s i n g l e  photon c o u n t i n g  i n  t h e  5-m region o f  the 
i n f r a r e d  w i t h  imaging c a p a b i l i t y .  
305 
REFERENCES 
Boyd, R. W .  and Townes, C.H,. Appl ied Physics L e t t e r s  - 31, 440 (1977) 
Gurski ,  T. R . ,  Appl ied Physics L e t t e r s  - 23, 273 (1973) 
Voronin, E.S. e t  a l ,  Soviet  Journal  o f  Quantun E l e c t r o n i c s  - 5, 597 (1975) 
306 
IMAGE PLANE 
DETECTOR ARRAY 
SUITABLE FOR 
GLASS FORMING MATERIALS 
Figure 1.- Mid near IR imaging focal plane disperser. 
IMAGE PLANE 
/ 
SUITABLE FOR 
e POLYMER MATERIALS 
* CRY TASTAL L I RI E MATER I ALS TO MUX 
Figure 2.- Far IR imaging fiber optic focal plane disperser. 
307 
RAD I AT I ON 
SOURCE 
I I 
I I 
BEAN SPLITTER 
EXPANDER 
FREQUENCY NONLINEAR 
UP CONVERSION MATERIAL PHASE 
OCCURS HERE MATCHED 
LINEAR 
FILTER CONE 
MICROCHANNEL 
PLATE INTENSl Fl ER 
DETECTOR ARRAY 
Figure 3.-  )lid IR focal plane disperser 
used for  imaging frequency upconversion. 
308 


Infrared fiber optic focal plane dispersers

Abstract

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