Fiber optic technology was tentatively selected in the SPS baseline design to transmit a stable phase reference throughout microwave array. Over a hundred thousand microwave modules are electronically steered by the phase reference signal to form the power beam at the ground receiving station. The initially selected IF distribution frequency of the phase reference signal was at 980 MHz or a submultiple of it. Fiber optics offers some significant advantages in view of the SPS application. Optical transmission is highly immune to EMI/RFI, which is expected to be severe when considering the low distribution power. In addition, there are savings in both mass, physical size, and potentially in cost

Lindsay, T. O.

Nalos, E. J.

English

NASA Technical Reports Server

SPS FIBER OPTIC LINK ASSESSMENT 
T, 0. Lindsay, E. J. Nalos . 
Boei ng Aerospace Company 
1. INTRODUCTION 
Fiber opt ic  technology has been tenta t ively  selected in the SPS baseline design 
to  transmit a s table  phase reference throughout the microwave array. Over a hundred 
thousand microwave modules will be e lect ronical ly  steered by the phase reference signal 
t o  form the power beam a t  the ground receiving s ta t ion .  The i n i t i a l l y  selected IF d i s -  
t r ibut ion frequency of the phase reference signal has been s e t  a t  980 MHz or  a submultiple 
of i t .  
Fiber optics offers  some s ignif icant  advantages i n  view 07 the SPS application. 
Optical transmission i s  highly immune t o  EMI/RFI, which i s  expected t o  be severe when 
considering the low dis t r ibut ion power (<lmW). In addit ion,  there will be savings i n  
both mass, physical s i ze ,  and potentialiy in cost .  
2. FIBER OPTIC LINK VERIFICATION PROGRAM 
2.1 TASK DESCRIPTION 
The purpose of the present program i s  to  demonstrate f e a s ib i l i t y  of a f i be r  opt ic  
l ink a t  980 MHz fo r  SPS application.  The specif ic  tasks are:  1 )  Analyze exist ing optical  
f ibe rs  f o r  use in the phase dis t r ibut ion f iber /opt ic  link w i t h  emphasis on phase change 
e f fec t s  and a b i l i t y  to  transmit high frequency IF s ignals ;  i .e . ,  low attenuation and ade- 
quate bandwidth; 2) Analyze sui table  optical emitters and detectors to  determine feas i -  
b i l i t y  of operation and usage a t  980 MHz; 3 )  Select  and purchase optical emitters,  
detectors,  and f ibers  fo r  1 ink development; 4) Design and construct impedance matching 
systems for  matching the optical emitter and detector to  laboratory equipment; and 5 )  
Assemble and t e s t  a two-way l ink a t  980 FlHz consisting of matched detectors,  emitters,  and 
a two-fiber cable of minimum length of 200 meters. 
In the  present phase control system fo r  SPS, a two-way l ink i s  required in the 
phase dis t r ibut ion system a t  each level t o  achieve phase compensation fo r  phase changes 
induced by temperature changes and other property changes in the electronic c i r cu i t .  
2.2 FIBER OPTIC LINK DESIGN 
The resul ts  of the component selection fo r  the f i be r  opt ic  l ink are  sumnarized 
i n  Table 1 below. 
-- TABLE 1: COMPONENT -- .- SELECTION FOR FIBER - OPTIC TEST -- LINK --- 
Component Type Features - - - - 
Emitter GaAl As Mu1 ti -Mode 1 .  
Injection Laser Diode 2 .  
3. 
GaAlAs Sin~7?-Mode 1. 
Injection Laser Diode* 2. 
3. 
4. 
Light Emitting Diode 1. 
(LED) 2. 
3. 
4. 
Moderate cost  
High power 
High modulation bandwidth 
High power 5. Low threshold 
High coupling e f f .  6 .  High r e l i ab i l i t y  
High bandwidth 7 .  Narrow spectral width 
Low dis tor t ion 
No thresh01 d current 
Low dis tor t ion 
Low cost  
Stable operating' point 
https://ntrs.nasa.gov/search.jsp?R=19810008031 2020-03-21T15:12:09+00:00Z
TABLE 1: COMPOllENT SELECTION FOR FIBER OPTIC TEST LIiJK (Continued) 
C 
------- -- .-- 
Component Type Features 
Detector  S i  1  i con  Avalanche 1. Gain - BW product  = 80 GHz 
Photodi ode* 2. High RCVR S I N  
3. Moderate cos t  
S i l i c o n  P I N  Photo- 1 . Low b ias  vo l tage  *Selected f o r  1 i n k  
d iode 2 .  Stab le  opera t ing  p o i n t  development 
3. Low c o s t  
F i  ber Step-index g lass  1. Low c o s t  
Mu1 ti -mode 2. Low a t t enua t i on  
Graded-index g lass  1. Moderate cos t  
Mu1 ti -mode* 2. High bandwidth 
3. Low a t t enua t i on  
Step-index g lass  1. Extremely h i gh  bandwidth 
Si  n g l  e-mode 2. Low a t t enua t i on  
3. Poor coup l ing  e f f i c i e n c y  
As a r e s u l t  o f  the  i nves t i ga t i ons ,  multi-mode graded index f i b e r  was chosen due 
t o  i t s  h i gh  bandwidth, low a t tenuat ion ,  a v a i l a b i l i t y ,  and h igh  coupl i n g  e f f i c i e n c y  w i t h  
i n j e c t i o n  l a s e r  diodes; single-mode i n j e c t i o n  l a s e r  diode was se lec ted  f o r  i t s  h i gh  band- 
width, h i gh  output ,  and e x c e l l e n t  l i n e a r i t y ;  and an avalanche photodiode was se lec ted  
because o f  i t s  h i gh  bandwidth and supe r i o r  s e n s i t i v i t y .  
The l i n k  w i l l  operate a t  a  wavelength o f  820 nm where p resent  l a s e r  diodes and 
avalanche photodiodes a re  r e a d i l y  a v a i l a b l e  and o f f e r  good re1  i a b i l  i ty .  F ibe r  at tenuat ion,  
al though n o t  minimum, reaches an acceptable va lue a t  820 nm also.  
The i n j e c t i o n  l a s e r  diodes were purchased from Nippon-E lec t r i c  i n  Japan; t h e  
two- f iber  cable was obta ined from S iecor  ( f i b e r s  manufactured by Corning Glass Works); 
and the avalanche photodiodes from RCA. 
One o f  t h e  problems t o  be solved f o r  the  980 MHz f e a s i b i l i t y  l i n k  was t o  develop 
simple, b u t  e f f e c t i v e ,  s i gna l  coupl i n g  techniques f o r  t he  e m i t t e r  and de tec to r .  The 
approach chosen i s  i l l u s t r a t e d  schemat ica l l y  i n  F igure  2. The use o f  t he  47n r e s i s t o r  
i n  se r i es  w i t h  t he  i n j e c t i o n  l ase r  d iode causes approximately 500 t o  be seen by t he  
d r i v e r  a m p l i f i e r  and i t  a l so  a ids  i n  conver t ing  the  d r i v e r  ou tpu t  -LO a c u r r e n t  source 
which i s  needed by t he  d iode f o r  l i n e a r i t y .  The ou tpu t  s i gna l  c u r r e n t  from the  avalanche 
photodiode f lows d i r e c t l y  i n t o  the  50n i n p u t  impedance o f  t h e  l abo ra to ry  a m p l i f i e r .  I n  
both cases, the  dc b i a s i n g  networks a re  i s o l a t e d  from the  s i gna l  paths by shorted quar te r -  
wave mi c r o s t r i  p  techniques. 
2.3 EXPERINENTAL RESULTS 
The r e s u l t s  o f  an i n i t i a l  t e s t  t o  couple 980 MHz through a sample l i n k  a r e  shown 
by Figure 2. The f i b e r  l eng th  was 300 meters and the  type i s  s i m i l a r  t o  t h a t  t o  be used 
i n  t he  two-way l i n k  development. Resul ts  a r e  l i s t e d  f o r  two values o f  de tec to r  b ias ing .  
The ou tpu t  vo l tage  waveforms were monitored us ing  a sampling osc i l l oscope  and, i n  both 
cases, the  t r a c e  was s t a b l e  and noise- f ree.  
The t e s t  setup was s i m i l a r  t o  t h a t  shown i n  F igure  3. The e m i t t e r  and de tec to r  
r n o d ~ l ~ . .  a r e  tcwards the  r i g h t  foreground shown w i t h  a l eng th  o f  c o i l e d  f i b e r  o p t i c  cable. 
Laboratory equipment inc ludes  a 980 MHz frequency syn thes izer ,  a  vec to r  vo l tmeter ,  
o s c i l  loscope, preamp1 i f i e r ,  and b ias ing  and mon i to r ing  equipment. 
The emitter and detector modules used i n  the i n i t i a l  t e s t  are  shown i n  Figure 4a 
and 4b. The thermal environment aboard the SPS i s  expected to be widely variable with 
values anticipated between -50°C and +150°C. Therefore, a major subject of i n t e r e s t  
involves the variation in propagation time through a f ibe r  as temperature i s  changed. 
Propagation t i ne  i s  d i rec t ly  related t o  the transmitted phase and i s  known t o  .)e affected 
by thermal expansion and refract ive  index variation. Data was a l so  taken to determine 
the magnitude of the phase variation versus temperature as  i l lus t ra ted  in Figure 6. 
The phase sens i t iv i ty  i s  not low enough t o  obviate the rleed fo r  phase compensation except 
possibly fo r  the  shor tes t  ( l a s t )  level of phase dis t r ibut ion.  
For a one-way l ink length of 200 meters, the transmitted phase would vary 
approximate7y 2.5 degrees fo r  every O C  of temperature change a t  980 MHz. This r a t e  i s  
acceptable with the present phase control system because of the two-way link length 
compensation. The two lengths of f ibe r  will be adjacent fo r  the to ta l  l ink ,  providing 
accurate tracking and matching. 
A t  the outer levels  of the phase reference dis t r ibut ion network, the l ink lengths 
averageclo meters and comprise over 90% of a l l  of the elements. I t  may be possible to  
eliminate the return l ink i n  such cases as the phase s h i f t  will be great ly  reduced for  the 
short  runs, averagin3 0.125 degrees of s h i f t  per "C. 
As f i be r  opt ic  technology progresses, longer wavelengths should be investigated 
where bandwidth and attenuation character is t ics  are superior fo r  fused s i  1 ica f ibers .  I t  
i s  anticipated tha t  phase s h i f t  sens i t iv i ty  may be reduced a t  longer wavelengths becsuse 
of dispersive changes in the refract ive  index. Fiber optics represent a promising 
approach fo r  the phase dis t r ibut ion system for  the SPS and merit fur ther  development t o  
real ize  t he i r  f u l l  potential .  
FIGURE 1 TEST CONFIGURATION FOR 2 WAY FIBER OPTIC LINK, 
OCTECTOR MODULE: 
V 8 ~ ~  + 
- 
Bur  4 
FIBER: CORfiING IVPO 
Length = 303 tteters 
Atten = 3.9 db/km @ 900 m 
BW 870 Biz-ka 
W.A = 0.218 
FIGURE 2 FIBER OPTIC LINK DESIGN SPECIFICATIONS 
FIGURE 3 INITIAL 980 MHz LINK TEST SETUP 
1 5 7  
FIGURE 4 n EMITTER MODULE BOARD. FIGURE 4~ DETECTOR MODULE BOARD 
TEMPERATURE, OC 
FIGURE 5 PHASE CHANGE OF GRA3ED INDEX FIBER vs, TEMPERATURE 
@ C O R N I N G  F l B E R  303 METERS LONG 
e FREQUENCY 980 MHz 


SPS fiber optic link assessment

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