Laser instruments have great potential in enabling a new generation of remote-sensing scientific instruments. NASA s desire to employ laser instruments aboard satellites, imposes stringent reliability requirements under severe conditions. As a result of these requirements, NASA has a research program to understand, quantify and reduce the risk of failure to these instruments when deployed on satellites. Most of NASA s proposed laser missions have base-lined diode-pumped Nd:YAG lasers that generally use quasi-constant wave (QCW), 808 nm Laser Diode Arrays (LDAs). Our group has an on-going test program to measure the performance of these LDAs when operated in conditions replicating launch and orbit. In this paper, we report on the results of tests designed to measure the effect of vibration loads simulating launch into space and the radiation environment encountered on orbit. Our primary objective is to quantify the performance of the LDAs in conditions replicating those of a satellite instrument, determine their limitations and strengths which will enable better and more robust designs. To this end we have developed a systematic testing strategy to quantify the effect of environmental stresses on the optical and electrical properties of the LDA

Allan, Graham R.

Kashem, Nasir B.

Stephen, Mark

Troupaki, Elisavet

Vasilyev, Aleksey

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Package - Nr of bars I Output Power @ Max Current 
Space Qualification of Laser Diode Arrays 
Elisavet Troupaki, Nasir B. Kashem, Graham R. Allan, Aleksey Vasilyev & Mark Stephen. 
NASA Goddard Space Flight Center 
Laser and Electro-optics Branch, Code 554, 
Greenbelt, MD 20771 
Introduction 
Laser instruments have great potential in enabling a new generation of remote-sensing scientific 
instruments. NASA’s desire to employ laser instruments aboard satellites, imposes stringent reliability requirements 
under severe conditions. As a result of these requirements, NASA has a research program to understand, quantify 
and reduce the risk of failure to these instruments when deployed on satellites. Most of NASA’s proposed laser 
missions have base-lined diode-pumped Nd:YAG lasers that generally use quasi-constant wave (QCW), 808 nm 
Laser Diode Arrays (LDAs). Our group has an on-going test program to measure the performance of these LDAs 
when operated in conditions replicating launch and orbit. In this paper, we report on the results of tests designed to 
measure the effect of vibration loads simulating launch into space and the radiation environment encountered on 
orbit. Our primary objective is to quantify the performance of the LDAs in conditions replicating those of a satellite 
instrument, determine their limitations and strengths which will enable better and more robust designs. To this end 
we have developed a systematic testing strategy to quantify the effect of environmental stresses on the optical and 
electrical properties of the LDA. 
Experimental Procedure 
We first characterize each LDA to establish a baseline for individual array performance and status 
(characterizing mode). The LDAs are then operated under controlled conditions for a specified number of pulses 
(operating mode) and remeasured to establishes an initial degradation slope. To help discriminate between normal 
aging effects and those induced by the stress the LDAs were divided into a control group and test group. The 
control samples were subjected to procedures identical to the test samples except for the applied stress. Great care 
was taken to ensure repeatability in our test processes so any observed change may be attributed to the applied 
stressor rather than measurement inconsistency. The test group of LDAs were subjected to a controlled stress: either 
vibration or radiation and recharacterized to detennine any effects of the stressor. The LDAs were again operated 
and remeasured to determine the degradation rate and detect longer term effects that may be attributed to the 
stressor. 
and uniformity, thermography, and a microscopic facet inspection [ 11. A lifetime test station is also used which 
allows for long term power monitoring as the device accumulates pulses. From the power measurements lasing 
threshold was also determined. The devices were characterized (characterization mode) under the following QCW 
conditions: 200 ps current pulses of 70 A or 100 A peak at 30Hz repetition rate with a duty cycle of 0.6% whle 
maintained at a bulk temperature of 25°C. The operating mode uses a higher pulse repetition rate of lOOHz, at a 2% 
duty cycle, same current and temperature, 25°C. This shortens the time to accumulate the desired 50 million pulses 
and is within the specified operating parameters. 
We monitored a total of 12, 808nm, QCW LDAs, fiom different vendors, labeled (Vl) and (V2). In order 
to minimize variations between test devices we chose LDAs fiom within a vendors same production run. Typical 
specifications of the LDAs are shown in Table 1. 
The characterization consists of four measurements: power and spectral content, relative power per emitter 
v 2  G - 2  200 W @ lOOA 
Table 1. Typical optical power and current of the test LDAs. 
Four LDAs formed the control group, the remaining LDAs, the test group. The test group was further sub-divided 
into a vibration-test group and a radiation-test group; two devices for vibration stress, two devices for y-irradiation 
and the remaining four LDAs for proton irradiation. Table 2 summarizes the tests performed on each group. In the 
next sections we more fully describe the vibrational tests followed by the results, then the radiation tests and results. 
DIODE 1 - 1 .Troupaki.doc 
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Troupaki - SSDLTR 2005 
y - irradiation 
Proton irradiation 
Vibration Tests 
facilities. We used two random-vibration profiles to simulate the vibration experienced by the payload during the 
launch phase. The first random-vibration profile was developed for the Space Transportation System (STS) and the 
Expendable Launch Vehicle (ELV), program and is used for random-vibration qualification testing of subsystems 
The vibrational tests were conducted in-house at one of Goddard Space Flight Center's vibrational-test 
Stressor I #ofLDAinTest&Control I Diodes I Tests 
2 + l  control V2 Radiation doses: Shad & 200laad. 
4+2 control V1&V2 Radiation doses: 30 laad & 60krad. 
I Vibration I 2 +1 control I V1 I Vibration levels: 14 grms & 20 grms. I 
Frequency (E) 
20 
20-50 
50-800 
800-2000 
2000 
Overall 
Profile 1 Profile 2 
.026 g2/Hz .052 g2/Hz 
+6 dB/octave +6 &/octave 
.16 g2/Hz .32 gz/Hz 
-6 &/octave -6 &/octave 
.026 g2/Hz .052 g2/Hz 
14.1 grms 20.0 grms 
50 
45 
14@*8' 140 
1 20 * 
100 i 
B B 
d 
X v i *  
B E &  
Q R  
4 20 .$ 4 15 2 
0 10 20 30 40 50 60 70 80 
Current (A) 
Figure 2. Optical Power (A) and Conversion Efficiency (B) as a fbnction of laser drive current are shown for one of 
the devices. This graph shows that the devices are not susceptible to vibrationally induced damage up to 20 gnns. 
The data are; 0 - initial characterization, A- after 50 million pulses, 0- post 14 grms vibration, 0 - post 14 grms & 
a total of 100 million pulses, X- post 100 million pulses and post vibration to 20 grms, +- post vibration to 20 grms 
and 150 million pulses. 
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Troupaki - SSDLTR 2005 
~ ~~~ 
Initial characterization 
Postlst operation (5.10~ pulses) 
The optical power and electrical to optical conversion efficiency as function of dnve current for one device through 
all phases of the Vibrational test is shown in Figure 2. Each curve consist of six sets of data; initial characterization, 
characterization after 5 x lo7 shots (50 million), characterization post 14 grms vibration, characterization post 14 
grms vibration and another 5 x lo7 shots (100 million total), characterization post 20 grms vibration, character- 
ization post 20 grms vibration and another 5 x lo7 shots (150 million shots total). Similar graphs can be produced 
for all the devices tested and in each case all curves overlap. Table 4 lists the peak optical power, the center 
wavelength of the temporally-integrated, spectral output[ 11, conversion efficiency and threshold current pre- and 
post-vibration for the device graphed in figure 2. These devices are insensitive to random vibration along 3 axes up 
to 20 grms, as expected. 
[wl [=I [AI P o l  
132.6 805.68 14.3 49.5 
132.4 805.70 14.4 49.6 
Time 
~ ~~ 
Post vibration to 14 grms 
Post 2nd operation (5.10’ pulses) 
Conversion I I Efficiency Max Optical Power I 
~ ~~ ~- 
132.3 805.70 14.3 49.3 
131.3 805.69 14.3 49.0 
~~ 
Post vibration to 20 grms 
Post 3rd operation (5.107 pulses) 
131.5 805.69 14.2 49.2 
132.4 805.74 14.4 49.2 
Table 4. Typical values for maximum optical power, peak wavelength, threshold current and conversion efficiency 
before and after vibrations for a device. 
Radiation Tests 
flux experienced at any time depends on such parameters as transit time - for interplanetary travel, orbit type, 
spacecraft shielding, mission duration and solar activity. Total radiation doses are typically between 15 lcrads and 
100 krads. For example, the radiation dose for the Ice Cloud and Elevation Satellite (ICESat) spacecraft in low, 
polar Earth orbit, is expected to be around 100 krads accumulated over 5 years. The MErcury Surface, Space 
ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft is expected to accumulate 30 lcrads during its 
8 year exploratory mission. 
The total radiation dose for space missions is predictable and vary depending on mission. The radiation 
To simulate the radiation expected during a typical mission we expose the LDAs to either proton or y- 
radiation. Controlled exposure to high-energy protons can be used to predict the response of a device to heavy ion 
exposure in a wide range of Earth orbits[3]. Typical radiation dose for a space flight test are -60 krads(Si) for high- 
energy protons and -200 krads(Si) for y-radiation. The y-irradiation work was performed at the Johns Hopk~ns 
University Applied Physics Laboratory using a Cobalt-60 source producing y-rays at 1.17 MeV and 1.33 MeV with 
nearly 100% frequency of occurrence and a dose rate equal to 744 rads (Si)/min. The samples were exposed to 
5 lcrads(Si) and 200 krad(Si). The proton irradiation testing was performed at Indiana University Cyclotron Facility 
[2]. The samples were placed in the 200 MeV proton beam until they were exposed to a total dose of 30 lcrads or 60 
krads. 
We report both the threshold level and the electrical to optical power conversion efficiency pre- and post- 
radiation. The lasing threshold [7] is a sensitive test for radiation induced damage in LDAs. Radiation can cause 
internal defects within the semiconductor structure resulting in increased optical loss and decreased electrical 
efficiency. 
Results of Proton Irradiation. 
Four LDAs were irradiated with protons to the specified dose. The devices were stored until the residual 
activation had decayed to background levels before they were then recharacterized. The devices were essentially 
unaffected by the accumulated dose at the given proton flux. A slight increase in threshold-current was observed. 
This is consistent with defect formation within the material. Again changes were observed in the thermal profile 
consistent with normal use and evident on both the control and test samples. Figure 3 is a plot of the threshold- 
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~~ ~ 
Troupaki - SSDLTR 2005 
current for each device, control group and proton-irradiated test group as a function of event. Figure 3 Left, is an 
expanded plot of the optical power around threshold for one device pre- and post 60 krads of proton irradiation. 
After After After 
Initial Operation 30krads 60krads 
Event 
12 
0 
13 15 17 19 
Current (A) 
Figure 3. Threshold current for lasing pre and post-irradiation for control and test group (Left). 0 - V2 at 30 krad, 
- V2 at 60 had, A - V1 at 30 had, A - V1 at 60 had, W - V1 control sample, 0 - V2 control sample. The 
lines are guides for the eye. The change in threshold current is small and increases with exposure to radiation. 
Right is a plot of the optical power around threshold pre- and post proton irradiation. 
initial threshold values for the LDAs V1 and V2 respectively. The threshold for the LDAs exposed to the lower 
dose also increased 1.4% (Vl) and 3.1% (V2). The increase in threshold for both control samples was less than 1%. 
This results in a slight decrease in the conversion efficiency. These devices are currently accumulating 50 million 
shots and so the degradation slope, post -irradiation, was unavailable. However fiom observation of the optical 
power the degradation slope appears to be unchanged. The average optical spectra of the test LDAs also remained 
unchanged. 
The maximum shifts in threshold: after exposure to the highest dose, were equal to 3.5% and 4.7% of the 
1 
.- 
200 
L 
B 
F 
& .Y 
7 0.4 
E 
2 
5 80 
40 
g 160 
Y I 30 .6  5 120 
8 
v 
I 
0.2 
n 0 
802 804 806 808 810 
Wavelength (nm) 
0 20 40 60 80 100 
Current (A) 
Figure 4. Left, is a plot of the optical power (A) and Conversion Efficiency (B) as a function of drive current for 
the LDA pre and post exposure to 60krad of proton-radiation. Right is a typical result and plots the normalized 
average optical spectrum pre and post exposure to 60krad of y-radiation. A- initial characterization, 0 - after 
operation, 0 - after irradiation (60 krad). As can be clearly seen that y-radiation to this level and no effect on the 
spectra. 
Results of 7-Irradiation 
exposure rate of 744 rads (Si)/&. One device was irradiated to 5 krads(Si) and recharacterized. A second device 
was then added to the test group and both devices were irradiated for a second exposure of 200 krads(Si) at and 
recharacterized. The devices were essentially unaffected by the accumulated dose of the y-radiation. However 
We y-irradiated two LDAs with one control. This y-irradiation was performed in two steps with an 
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Troupaki - SSDLTR 2005 
changes were seen in the thermal profile consistent with normal use and evident on the control samples. The 
degradation slope has not been measured as we are waiting for the proton irradiated LDA "operation" to finish. 
Conclusions - Future Work 
Our main objective was to quantify the performance of the laser diode arrays in the conditions they are 
likely to encounter during a space flight. Our test protocol was designed to separate the changes due to 
manufacturing defects or aging from those induced by the stress parameters of vibration and radiation. Our results 
indicate that LDA performance is not affected by the vibration levels of 14.1 and 20 grms. Such vibration levels are 
higher than the vibration loads during a typical launch into space. We also observed that LDA are insensitive to y- 
radiation up to 205 krad(Si). We did observed small changes in LDA performance after exposure to proton 
radiation. LDA threshold current was affected by proton radiation with an average increase in the threshold current 
of 3.2% compared to 1% for the control samples. The maximum measured increase was 4.7% for one sample. We 
conclude that these LDAs are robust enough to survive the vibration and radiation stresses of most space flight 
missions. 
Future work will expand the proton irradiation test to include testing to higher doses and with different proton 
energies. 
Acknowledgements The authors would like to acknowledge the assistance of Matt Bevan at the Johns Hopluns 
University Applied Physics Lab and helpful discussions with Melanie Ott in the Parts, Packaging and Assembly 
Technologies Office at Goddard Space Flight Center. 
References 
[ 11 Alex Vasilyev, Graham R. Allan, John Schafer, mark A. Stephen and Stefan0 Young, "Optical & 
Thermal Analyses of High-Power Laser Diode Arrays", Technical Digest, Seventeenth Solid State and 
Diode Laser Technology Review, p-38, 8-10 June, 2004 Albuquerque, New Mexico. 
[2] General Environmental Verification Specification (GEVS) for STS and ELV Payloads, Subsystem and 
Components, http://arioch.gsfc.na.gov1302/gevs-seltoc.htm 
[3] Payload Vibroacoustic Test Criteria, NASA-STD-7001, June 21,1996 
[4] Dynamic Environmental Criteria, NASA-HDBK-7005, March 13,2001 
[5] Petersen E.L., "The SEU Figure of Merit and Proton Upset Rate Calculations", IEEE Trans. Nucl. Sc., 
[6] B. von Przewoski, T. Rinckel, W. Manwaring, G. Broxton, M. Chipara,T. Ellis, E.R. Hall, A. Kinser, 
"Beam Properties of the new Radiation Effects Research Stations at Indiana University Cyclotron Facility", 
Nuclear Space Radiation Efiects Conference, Data Workshop, Atlanta, Ga, 19-23 July 2004. 
[7] Johnston A.H., "Radiation Degradation Mechanisms in Laser Diodes", IEEE Trans. Nucl. Sc., Vol. 5 1, 
No 6, Dec. 2004. 
V O ~ .  4516, pp. 2550-2562, 1998. 
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