Modulation inducing retrodirecting optical system - diode mount for gallium arsenide lasers, and generation of alkaline line by stimulated Raman emissio

Chang, G. K.

Cholet, P. H.

English

NASA Technical Reports Server

. 
3 . 
b PHILCO CORPORATION 
A SUBSIDIARY OF 
APPLIED RESEARCH LABORATORY 
4 
. 
Blue Bell, Pennsylvania 
Mltchell 6-9100 
15 February 1965 
TO: National Aeronautics and Space Administration 
Goddard Space Flight Center 
Glenn Dale Road 
Greenbelt, Maryland 20771 
Attention: Contracting Officer, Code 248 
SUBJECT: Theoretical and Experimental Investigation on Modulation- 
Inducing Retrodirective Optical Sys terns (MIROS). 
Contract NAS 5- 9765 (Philco Boob), Monthly Contract 
P rogres s  Report No. 3 for  the period 20 December 1964 
to 20 January 1965. 
SUMMARY OF WORK ACCOMPLISHED DURING THE REPORT PERIOD 
To improve the Q-switched laser  beam, the existing experimental appa- 
ratus was modified. 
l a se r s  which lased a t  the resonance D l ines of the cesium vapor. Further ex- 
periments on the modulation of single- crystal  SbSI a t  higher frequencies were 
carr ied out during the report period. 
Continued effort w a s  devoted to fabricating injection 
1. Experimental Activities 
A. GaAs Lasers 
A special diode mount f o r  GaAs lasers w a s  fabricated and 
It consisted of two copper electrodes, one 
The electrodes were 3/8 inch in 
tested during the past month. 
fixed in a nylon cylinder, and the second f ree  to slide under spring tension 
in the upper half of the same cylinder. 
diameter, with 31 32-inch diameter faces that were lapped flat  and parallel, 
and plated with indium. Diodes with epitaxial-grown junctions and ohmic 
contacts of sintered nickel were used. 
resistance of 0. 2 to 0. 3 ohm. 
The complete unit had a se r i e s  
Hard copy (HC) $/ 
1 Microfiche (MF) , d-8 
https://ntrs.nasa.gov/search.jsp?R=19650011741 2020-03-16T23:49:23+00:00Z
.. 
At 77°K these units lased a t  around 8 3 4 0 i  with a threshold 
of 4 x l o 3  amp/cm , using 2-microsecond pulses a t  a low repetition rate. 
The spectral  peak a t  the threshold is approximately 200A lower than 
reported f o r  identical material, using conventional s older ing technique s. 
This is an  indication of a lower effective junction temperature with a corre-  
sponding wider band gap (* 0. 04 ev). However, the thresholds a r e  twice a s  
high as the best conventional units of the same material. When the diodes 
were removed, they were found to have sunk a few mils  into the electrode 
surface. The crystal  faces were probably damaged in the process,  which 
increased the cavity losses. 
and should provide a f i r m e r  base. 
2 
Electrodes of tellurium copper have been made 
In an attempt to construct a lasing diode from closely com- 
pensated GaAs, a wafer which was less closely compensated than those used 
previously was divided into three sections for  separate diffusions of zinc and 
cadmium; the third section was used as  a control. 
ND/NA, was 1.3 with ND = 8.4 x 1017 ~ m - ~ .  
The compensated ratio, 
A probe test of the diffused samples showed the zinc-diffused 
Angle lapping 
piece to  be strong p-type, the cadmium-diffused piece to be moderately p- 
type, and the control to be very weakly p-type a t  the surface. 
and staining with Cu showed the junctions to be 120 p, 8.5 p, and much less 
than 1. 0 p in the zinc-diffused, cadmium-diffused, and control samples, 
respectively. 
Diodes have been prepared from the diffused samples. 
Line narrowing to about 60 A was observed, but the l a s e r  action 
Elec- 
t r o l y i n e  scence in the cadmium- diffused diodes showed emission peaks a t  
845OA. 
has not yet been observed. 
0 
The zinc-diffused diodes a l so  radiated a t  8450A. 
It is interesting to note that negative resistance has been ob- 
A typical diode w i l l  be insulating to 2 volts before dropping back to 
This may be caused by the close compensation of 
The formation of an insulating layer may 
served in the forward current-voltage characteristic of the cadmium-diffused 
diodes. 
1. 5 volts and conducting. 
the mater ia l  and the formation of a semi-insulating region on the p-side of 
the junction a t  low temperatures. 
cause difficulty in constructing a lasing diode. 
1. Monthly Contract Progress  Report No. 1 (15 December 1964). 
B. Generation of Alkaline Line by Stimulated Raman Emission 
The efficiency of the optical pumping of a ruby laser depends 
much upon the reflectivity of the cavity. Thus, to improve the pumping 
efficiency of the ruby l a s e r ,  a special s i lver  plating was applied t o  the 
existing laser cavity by the American Electroplating Company, Boston, 
Massachusetts. The silver-plated cavity was observed to have higher 
pumping efficiency. 
The time delay of formation of the Q-switched laser in the 
previous setup was found to be unstable when the pulse generator (Tektronix 
Type 161) w a s  triggered by the output from the "Sawtooth A" of the Model 
555 oscilloscope. To resolve this difficulty, the experimental apparatus f o r  
investigating Raman lasers '  was modified. 
output of the 555 scope to tr igger the 161 pulse generator,  the "Delayed 
Trigger" output f rom the 555 scope w a s  fed into the tr igger input of a Model 
585 oscilloscope. The gate output from the 585 scope, in turn, t r iggers  the 
laser power supply. 
Instead of using the "Sawtooth A" 
Previously, the ruby rod was directly cooled by flowing liquid 
The direct  cooling method was known to be effective nitrogen into the cavity. 
in cooling the ruby rod and the flush lamp to near  the liquid-nitrogen tempera- 
ture; however, i t  was found difficult to control the temperature. 
operation of a Q-switched l a se r  requires a constant temperature. 
a constant temperature, an indirect cooling method was used, in which cold 
nitrogen gas (- 170°K) was directed into the cavity. 
Stable 
To obtain 
C. Band- Edge Modulator 
Fur ther  experiments with SbSI were carr ied out using the 
setup described las t  month, together with a light chopper between the light 
source and the interference filter. Comparison of the zero-field signal due 
to the chopper with the signal af ter  application of an electr ic  field to the 
crystal  permitted calculation of the modulation index, m: 
Emax - E o  
m =  x 1 0 0 ,  
Emax 
2. Monthly Contract P rogres s  Report No. 2 (15 January 1965). 
3 
where Eo i s  the signal due to the chopper, measured on the scope, and 
Emax is the signal with an applied electric field. 
volts dc; m 2 3.5 percent fo r  500 volts ac  (peak to peak). 
tion index is expected to be higher, since some light reaches the detector 
without going through the crystal. 
observed with the setup shown in Figure 1. This arrangement is capable 
of operating a t  frequencies up to 40 kc. 
m 2 7 percent for 250 
The true modula- 
Response a t  higher frequencies w a s  
Figure 2 shows the crystal response a t  a 400-cps signal of 
With the wave analyzer, response up to 20 kc was 500 volts peak to peak. 
observed. This corresponds to a 40-kc crystal-response frequency, which 
i s  the limit of the present system. The change of the modulation index with 
frequency has not yet been determined, because the high resistance of the 
GaAs photodetector made i ts  frequency response uncertain. An Si diode, 
a s  well a s  a small photomultiplier tube with S - 1  response, wi l l  be used to 
determine the change of m a s  a function of frequency. 
also be improved to permit measurements a t  higher frequencies. 
The system w i l l  
D. Cesium Bulb 
The cesium sources previously used for  optical pumping in 
the MIROS program have deteriorated. 
using argon as the buffer gas to avoid the strong 8 9 5 2 A  line of xenon. 
line is within the bandpass of the 8 9 4 3 A  fi l ter  used in the experiments. 
degree of R F  excitation w a s  strongly dependent upon the argon pressure.  
The best sources (25 percent pumping efficiency) had pressures  around 
1 mm Hg. 
Several new sources were prepared 
This 
The 
PRINCIPAL INVESTIGATORS' TIME DEVOTED TO WORK 
The principal investigators assigned to the project and the time de- 
voted to the work by these individuals f rom 20 December 1964 to 20 January 
1965 a r e  as follows: 
4 
. 
GENERAL RADIO 
R-C OSCILLATOR - 
1210-c 
, *  
LIGHT 
SOURCE 
w 
> 
OENERAL RADIO GENERAL RADIO 
1206-8 9 4 1 - A  
Sb SI U N I T  AMPLIFIER - TRANSFORMER - 
I 
FILTER 
SCOPE 
OUANT-TECH 
WAVE ANALYZER 
M I C RO S COPE r 
w PHOTODIODE 
TEKTRON I X  
PREAMP 
I 2 2  
Figure 1 .  Experimental Setup f o r  Observing Band-Edge Modulation 
5 
Figure 2. Oscillogram of the Crystal  Response 
Upper Beam Horizontal = 2 milliseconds / c m  
(modulating signal) Vertical = 100 vol ts /cm 
Lower Beam Horizontal = 2 milliseconds / cm 
(modulated light) Vertical = 5 millivolts/cm 
6 
Pe r s onnel 
G. K. Chang 
J. Powers 
W. Haas 
A. Varga 
C. Wang 
G. Racette 
D. Cornelius 
C. Doyle 
P. Cholet 
Man-Hours 
120 
152 
77. 6 
40 
18 
36 
52 
38. 6 
9. 2 
PLANS FOR THE NEXT INTERVAL 
Continued effort will be devoted to  generating Raman lasers and t o  
fabricating GaAs injection lasers that are capable of pumping alkaline 
metal  vapors. The modulation index, m ,  as a function of frequencywill be 
determined. 
9Kav G. K. Chang 
P. H. Cholet 
Research Section Manager 
Approved: 
Applied Research Laboratory 
Re sea r ch Specialist 
7 


Theoretical and experimental investigation on modulation-inducing retrodirective optical systems /miros/  monthly progress report, 20 dec. 1964 - 20 jan. 1965

Theoretical and experimental investigation on modulation-inducing retrodirective optical systems /miros/ monthly progress report, 20 dec. 1964 - 20 jan. 1965

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