The 2-micron thulium and holmium-based lasers being considered as the transmitter source for space-based coherent Doppler lidar require high power laser diode pump arrays operating in a long pulse regime of about 1 msec. Operating laser diode arrays over such long pulses drastically impact their useful lifetime due to the excessive localized heating and substantial pulse-to-pulse thermal cycling of their active regions. This paper describes the long pulse performance of laser diode arrays and their critical thermal characteristics. A viable approach is then offered that allows for determining the optimum operational parameters leading to the maximum attainable lifetime

Amzajerdian, Farzin

Baker, Nathaniel R.

Barnes, Bruce W.

Kavaya, Michael J.

Meadows, Byron L.

Singh, Upendra N.

NASA Technical Reports Server

 1 
 
Qualification testing of laser diode pump arrays for a space-based  
2-micron coherent Doppler lidar 
. 
 
Farzin Amzajerdiana, Byron L. Meadowsa, Nathaniel R. Bakerb, Bruce W. Barnesa, Upendra N. Singha, 
Michael J. Kavayaa 
 
aNASA Langley Research Center, MS 468, Hampton, Virginia 23681-2199 
bLockheed Martin Mission Services, Langley Research Center, MS 468, Hampton, Virginia, 23681 
 
 
 
ABSTRACT 
The 2-micron thulium and holmium-based lasers 
being considered as the transmitter source for 
space-based coherent Doppler lidar require high 
power laser diode pump arrays operating in a long 
pulse regime of about 1 msec. Operating laser 
diode arrays over such long pulses drastically 
impact their useful lifetime due to the excessive 
localized heating and substantial pulse-to-pulse 
thermal cycling of their active regions. This paper 
describes the long pulse performance of laser 
diode arrays and their critical thermal 
characteristics. A viable approach is then offered 
that allows for determining the optimum 
operational parameters leading to the maximum 
attainable lifetime.  
INTRODUCTION 
Development of a space-based 2-micron coherent 
Doppler wind lidar1 faces a serious concern in its 
ability to achieve a reasonable operational lifetime 
in Earth orbit. Unfortunately despite considerable 
advances in high power quasi-CW laser diode 
arrays, their lifetime and reliability are still very 
limited when operated in the long pulse mode of 1 
msec or greater2,3. Rapid degradation and 
premature failure of laser diode arrays, when 
operated over long pulse duration, are further 
amplified by the high pump peak power required 
by the 2-micron gain medium for overcoming the 
excessive ground-state absorption losses.  
A number of laser diode arrays (LDAs) from 
different suppliers have been tested and their 
lifetime measured to assess the impact of long 
pulsewidth operation on the laser lifetime and 
reliability. The measurements to date indicate a 
lifetime of close to two orders of magnitude shorter 
than reported lifetime for similar type of arrays 
operated at 200 microseconds pulsewidth. In 
addition to shorter lifetime, the arrays experience a 
high rate of catastrophic failure when operated in 
long pulse regime. The failure of the laser diode 
arrays is mainly due to the excessive localized 
heating and substantial pulse-to-pulse thermal 
cycling of their active regions4,5. Figure 1 provides 
the measured junction temperature and extent of 
the thermal cycling of a stacked array of 100 W 
bars running at 1 msec pulse duration compared 
with 200 mircosecond pulsewidth operation. For 
this measurement the array was operated at 50 A, 
almost half of its rated level, and 12 Hz pulse 
repetition rate. The increased pulsewidth from 0.2 
to 1.0 msec for this particular array results in about 
3.6 degrees higher junction temperature increase 
and 3 times larger thermal cycling amplitude.   
Figure 1. Thermal cycling of an LDA operating 
in long pulse mode (1.0 msec pulsewidth) 
compared with operation in conventional Q-CW 
mode (0.2 msec pulsewidth). 
 
 
 
 
 
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Time  (msec)
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1.0 msec pulsewidth
0.2 msec pulsewidth
Drive current  50A
PRF             12 Hz
https://ntrs.nasa.gov/search.jsp?R=20070031769 2019-08-30T01:39:13+00:00Z
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The impact of the higher junction temperature and 
larger pulse-to-pulse thermal cycling on the LDA 
lifetime may be roughly estimated by an Arrhenius 
relationship written as:  
Lifetime (τ) ∝ (Ta-Tb)-N Exp(Ea/kTa)  (1) (1) 
where lifetime (τ) is expressed as a function of 
junction temperatures Ta and Tb measured 
immediately after and before the generated pulse, 
the activation energy (Ea) and Boltzmann's 
constant (k). The leading term accounts for the 
thermal cycling fatigue due to mismatch of thermal 
expansion coefficients of different package 
materials and various layers of the laser bar. The 
power N in the expression above can have a value 
between 2 and 5 depending on the materials 
properties based on the Manson-Coffin law for 
thermal fatigue.  It is obvious from this Arrhenius 
equation that reducing the temperature difference 
before and after the pulse is the key for increasing 
the lifetime to an acceptable level. This may be 
achieved through careful selection of the LDA 
package type, specifications for the array 
considering the pumping requirements, and 
defining its operational parameters.  
 
THERMAL MEASUREMENTS 
Several experimental tests have been developed 
to investigate the thermal characteristics of high 
power LDAs in order to evaluate various package 
designs and define the best operating parameters. 
The thermal characterization of LDAs includes the 
thermal imaging of the facets3, spectral shift and 
broadening measurements6, and a measurement 
technique we refer to as the “Forward Voltage-
Short Pulse” (FV-SP) technique. The FV-SP 
measurement technique is particularly useful as it 
can provide accurate measurement of the junction 
temperature just before and immediately after the 
generated optical pulses used in the Arrhenius 
lifetime relationship of Eqn. (1).  
The FV-SP measurement utilizes the diode 
characteristics of the LDA to measure its junction 
temperature. In this measurement, a series of 
relatively short and low current pulses, compared 
with the actual drive pulses, are applied to the 
LDA and the resultant voltage is measured with a 
high degree of precision (Fig. 2). The measured 
voltage drop across the array is related to the 
junction temperature through the diode I-V 
equation.  
83.3 msec (12 Hz)
1 msec
30 to 120
amps
300 mA, 35 μsec
 
Figure 2. The Forward Voltage-Short Pulse 
technique for measuring the LDA junction 
temperature. 
 
Another advantage of this measurement technique 
is its ability to determine the junction temperature 
while running the LDA at any operational 
parameters without tedious post processing 
required by other techniques such as time-
resolved spectral measurements. Fig. 3 is an 
example of the FV-SP measurements showing the 
peak junction temperature before and after the 
generated pulses and the thermal impedance 
(ratio of junction temperature rise to dissipated 
heat) as a function of drive current. It can be seen 
from these plots that the temperature rise during 
the pulse is almost a linear function of applied 
current. Using the measured junction 
temperatures in the Arrhenius expression (Eqn. 1), 
the relative impact of current de-rating can be 
estimated.  
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Applied Curent  (A)
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2.50
3.00
3.50
4.00
4.50
5.00
5.50
Th
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m
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 Im
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da
nc
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 (o
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/W
)
before pulse
after pulse
thermal impedance
 
Figure 3. Before and after pulse junction 
temperatures measured using the Forward 
Voltage-Short Pulse (FV-SP) technique. 
 
 3 
 
Fig. 4 illustrates the lifetime improvement resulting 
from reducing applied current. It worth noting that 
high power quasi-CW laser diodes arrays are 
complex electro-optical components and thus their 
lifetimes do not follow well defined or known 
predictable models such as Arrhenius 
relationships unless considerable statistical data is 
available for accurately specifying the activation 
energy (Ea) and thermal fatigue constant (N). 
However, results such as shown in Fig. 4 can still 
provide useful information by enabling a 
determination of the magnitude of improvement 
that can result from various measures for reducing 
the junction temperature and thermal cycling. Fig. 
4 clearly shows that up to an order of magnitude 
improvement in lifetime may result from de-rating 
by about 30% (i.e., operating at 70 A).  
1
10
100
1000
10000
0102030405060708090100110
Current (A)
R
el
at
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Li
fe
tim
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N=3.0
N=4.0
N=2.0
Pulsewidth = 1 msec
N=5.0
 
Figure 4. Relative lifetime expectancy versus 
applied current for different values of thermal 
fatigue constants. 
 
It is worth noting that both the activation energy 
(Ea) and thermal fatigue constant (N) used in the 
lifetime model (Eqn. 1) can be substantially 
different for different LDAs having drastically 
different package architecture and materials, or 
using bars with different active cavity dimensions, 
fill factor, composition, or coating. In other words, 
the junction temperature measurements alone are 
not sufficient for comparative analyses between 
LDAs manufactured by different vendors using 
varying designs and processes. This point is 
illustrated in Figure 5 where the thermal 
characteristics of two 6-bar arrays with similar 
performance specifications but very different 
package and bar designs are provided. Both 
arrays are operated at 50A and 12 Hz and a heat 
sink temperature of 25C. While the array 
designated as LDA#2 runs at a substantially 
higher temperature and experiences larger 
thermal cycling, it has demonstrated a somewhat 
longer lifetime. In fact, the lifetime testing of a set 
of LDA#2 samples showed no sudden failures 
while LDA#1 samples experienced a number of 
bar drop-outs and complete failures when 
operated in the long pulse mode. Therefore, 
additional tests revealing the bar quality and 
assembly workmanship are also critical in 
evaluating different vendors. At the minimum, 
these tests should include thermo-mechanical 
stress measurement of the bars showing the 
impact of thermal mismatch between the bar and 
its heatsink, and infrared imaging measurement 
showing hot spots revealing bar imperfections 
(material impurities and coating defects) and 
improper bar/heatsink attachment (e.g., solder 
voids).  
Figure 5. Thermal cycling of two different 6-bar 
LDAs operating over identical conditions. 
 
CONCLUSION 
The 2-micron thulium and holmium-based lasers 
are the primary transmitter candidate for global 
measurement of atmospheric winds from the earth 
orbit using coherent Doppler lidar technique. 
These lasers require high power laser diode pump 
arrays operating in a long pulse regime of about 1 
msec. Unfortunately, operating LDAs over such a 
long pulse duration drastically accelerates their 
gradual degradation and increases the possibility 
of sudden failure. Measurements to date indicate a 
lifetime of close to two orders of magnitude shorter 
than reported lifetimes for similar type of arrays 
operated at more conventional pulsewidths of less 
than 200 microseconds. However, the LDA lifetime 
may be improved by careful selection of the 
supplier, specification of their package (number 
bars and pitch) and by determining its optimum 
operational parameters by considering the solid 
state laser pump requirements and the mission 
objectives. Improvement of LDA lifetime and 
reliability require accurate characterization of their 
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0 200 400 600 800 1000
Time  (msec)
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LDA # 2
Drive current   50A
PRF                12 Hz
Pulsewidth      1.0 msec
LDA # 1
 4 
 
critical thermal properties, primarily the junction 
temperature of their bars and thermal impedance 
of their package. Among various thermal 
measurements deployed, the technique referred to 
as “Forward Voltage-Short Pulse” (FV-SP) 
technique proved to be most useful. The FV-SP 
measurements have enabled reasonable lifetime 
analyses leading to specification of operational 
parameters of LDAs and establishing the criteria 
for their de-rating. This measurement also 
provides useful data for comparison and 
evaluation of different package types from various 
suppliers and specifying their design (ex., 
geometry, number of bars, and pitch).  
 
ACKNOWLEDGMENTS 
The authors would like to express appreciation to 
NASA’s Earth Science Technology Office for 
funding this effort through the Laser Risk 
Reduction Program. 
 
References 
1. M. J. Kavaya, G. D. Emmitt, R. G. Frehlich, and 
F. Amzajerdian, “A space-based point design 
for global coherent Doppler wind lidar profiling 
matched to the recent NASA/NOAA draft 
science requirements,” 21th International Laser 
Radar Conference, Quebec, Canada, July 8-
12, 2002. 
2. Farzin Amzajerdian, Byron L. Meadows, 
Nathaniel R. Baker, Renee S. Baggott, 
Upendra N. Singh, Michael J. Kavaya, 
“Advancement of High Power Quasi-CW Laser 
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Pacific Environmental Remote Sensing 
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4. H. Brugger and P. W. Epperlein, “Mapping of 
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1990.  
5. Anna Kozlowska, et al, “Thermal properties of 
high-power lasers investigated by micro-
thermography,” SPIE Proc, 2711, 2005. 
6. Michael Kreissl, Tran Quoc Tien, Jens W. 
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8. F. Dorsch and F. Daiminger, “Aging test of high 
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Qualification Testing of Laser Diode Pump Arrays for a Space-Based 2-micron Coherent Doppler Lidar

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