The Geoscience Laser Altimeter System (GLAS), launched in January 2003, is a laser altimeter and lidar for the Earth Observing System's (EOS) ICESat mission. The laser transmitter requirements, design and qualification test results and in-flight performance for this space-based remote sensing instrument is summarized and presented

Afzal, Robert S.

Dallas, Joseph L.

Mamakos, William

Melak, Anthony

Yu, Anthony W.

NASA Technical Reports Server

PERFORMANCE OF THE GLAS LASER TRANSMITTER IN SPACE 
Anthony W. Yu (I), Robert S. Afzal('), Joseph L. Dallas (3), Anthony Melak ( I ) ,  Alan Lukemire (4), 
Luis Ramos-Izquierdo (') and William Mamakos (6)  
( I )  NASA-GSFC, Laser & Electro-Optics Branch, MC554, Greenbelt, MD 20771, USA, E-mail: anthony.w.yu@nasa.gov 
(21 Aculight Corp., 11805 North Creek Parkway S., Ste 113, Bothell, WA 98011, Email: robert.ajial@aculight.com 
")Avo Photonics, 700 Business Center Drive, Suite #125, Horsham, PA 19044, USA, E-mail: jdallas@avophotonics.com 
(4) Space Power Electronics Inc., 108 Audrey Way, Kathleen, GA 31047, USA, Email: alan@spacepower.com 
(') NASA-GSFC, Optics Branch, MC551, Greenbelt, MD 20771, USA, Email: lramos@pop900.gsfc.nasa.gov 
"Design Interface Inc., 3451 Gamber Road, Finbburg, MD 21048, USA, Email: DI@transsys.com 
ABSTRACT beam quality, improved efficiency, and other 
technological advances. 
The Geoscience Laser Altimeter System (GLAS), 
launched in January 2003, is a laser altimeter and lidar U T T J ~ ~ U V T  (UIM1er O*I~C~OIU $2) 
for the Earth Observing System's (EOS) ICESat 
mission. The laser transmitter requirements, design 
and qualification test results and in-flight performance 
for this space-based remote sensing instrument is 
summarized and presented. 
1. INTRODUCTION 
The Geoscience Laser Altimeter System (GLAS) [I]  
launched January 12, 2003 at 4:45 PST on board a 
Boeing Delta I1 expendable launch vehicle from 
Vandenberg Air Force Base, CA, is the sole instrument 
for the ICESat [2] (Ice, Cloud and Land Elevation 
Satellite) mission. GLAS is a satellite laser altimeter 
and atmospheric lidar whose primary mission is the 
global monitoring of the Earth's ice sheet mass balance. 
GLAS also provides high precision land topography 
and global monitoring of aerosols and cirrus cloud 
heights. Combining a 1-m beryllium telescope, 1 GHz 
digitizer, Analog and Photon counting Si APD's, an 
on-board laser beam pointing measurement system [3], 
variable conductance heat pipes for thermal 
management, and a 2-color diode-pumped solid-state 
laser, the GLAS instrument is providing an 
unprecedented high precision and accuracy data set (5 
cm vertical accuracy, 2.4 cm precision) on the vertical 
structure of the Earth surface and Atmosphere. GLAS 
is designed to accommodate 3 transmitters intended to 
be operated sequentially on a common optical bench 
opposite the laser beam point measurement system 
known as the stellar reference system. Fig. 1 shows the 
location of the lasers on the GLAS instrument. The 
GLAS lasers [4,5] represent the next generation of 
space-based remote sensing laser transmitters. The 
previous state-of-the-art in space based solid-state 
lasers is the Mars Orbiting Laser Altimeter (MOLA) 
[6,7],  on the Mars Global Surveyor spacecraft 
collecting topography data of Mars [8]. 
The GLAS lasers generally have an order-of- 
magnitude higher performance than MOLA in power, 
Fig. 1. Graphic showing the location of the lasers in relation 
to the other elements of the instrument optical path. 
2. LASER DESIGN 
The top level performance requirements for the lasers 
are summarized in Table 1. 
Table I .  Top level optical requirements and resources. 
[ Pulse width 
Prouertv 
Pulse Energy (total) 
1064 nm 
532 nrn 
Repetition Rate 
Wavelength (Vacuum) 
[ Divergence 1 110 (+23, -10) prad 1 
Suecification 
ll0m.I 
75 mJ 
35 mJ 
40 Hz 
A1=1064.5 nm *lo0 pm 
Prime Power I llOW 
Mass 1 15.2Kg 
Far Field Circularity 
Pointing Jitter 
Boresight Reference 
Boresight Stability over 
I Volume I 54x 15x25 cm' 1 
> 0.67 
k 1 I prad (1 a) 
< 1000 pad (*50 pad) 
* 50 prad 
Additionally the laser needs to operate from 10°C to 
35°C and survive non-operating from 0°C to 50°C. 
The expected launch loads were up to 8 grms for 1 
https://ntrs.nasa.gov/search.jsp?R=20080039318 2019-08-30T05:23:28+00:00Z
minute. The mission life is for 3 years with a 5 year 
goal. Under continuous operation at 40 Hz the lasers 
will accumulate 1.26 billion shots per year. 
Particularly challenging in simultaneously meeting 
these requirements is the combination of short pulse 
width, high pulse energy and excellent beam quality. 
Short pulses are typically generated by short cavities 
with high gain. Shorter cavities tend to allow for 
higher order transverse modes in high gain lasers 
unless the pulse energies are low. To keep the 
transmitters optics small and to avoid the use of large 
beam expanders to meet the divergence requirements, 
the final beam quality needed to be in the range of M2 
= 2. Although ruggedized lasers tend to be crossed- 
porro power oscillators, a master-oscillator, power- 
amplifier (MOPA) design was the most promising 
architecture for meeting the transmitter performance 
objectives [9] .  In this particular case, a short pulse 
TEMoo oscillator is followed by 2 amplification stages 
to meet the final energy level. Optical aberrations were 
minimized by utilizing zig-zab slabs, and beam image 
inversions from prisms between the first and the 
second amplifier passes. 
The oscillator, pumped by two 100 W Q-cw diode- 
bars, is passively Q-switched, and generates 2 mJ, 5 ns 
near diffraction limited (M2 < 1 . l )  pulses at 40 Hz. 
The output pulses are expanded by a 2x telescope, and 
amplified by a double-pass preamplifier stage pumped 
by 8, 100 W bars resulting in 15 mJ pulses with an M2 
z 1.4. This stage utilizes a polarization coupled double 
pass, zig-zag slab with a porro-prism for beam 
symmetrization. After another 2 . 2 ~  expansion, the 
beam enters a power amplifier pumped by 44, 100 W 
bars. The pulses are amplified to 110 mJ after a double 
pass with an M2 = 1.8. The peak laser fluence in the 
final amplifier is 4 ~ / c m ~ .  The full power beam is then 
directed to a Lithium Triborate (LBO) doubler 
designed to convert 30% of the power into the green, 
followed by an achromatic, 6x final beam expander. 
The far-field beam divergence is "Gaussian-like" with 
a 110 p a d  full angle divergence and an M2 - 2. Fig. 2 
shows the output energy ofthe 2 colors as a function of 
diode drive current. To improve laser lifetime [lo], 
oscillator diodes are derated to 65 Whar (85 A) and 
amplifier diodes derated to 85 Whar (1 00 A) [ l  1,121. 
Since the laser is not actively Q-switched, the timing of 
the pulse emission from the oscillator is actively 
controlled through the diode drive current. The Laser 
Diode Power Electronics (LDPE) delivers 100 A of 
loop then adjusts the by-pass current to allow for more 
or less current to pass through the oscillator diodes to 
maintain a diode pump time of 200 ps. This loop is 
necessary to maintain the synchronization of the 
oscillator pulse emission to the maximum stored 
energy and gain of the amplifiers. 
Laser Energy va. Pump Current 
Pump Current [A] 
Fig. 2. Laser Pulse Energy as a fitnction of peak diode drive 
current. The operational design point was a drive current of 
100 A. 
The laser is all conductively cooled with the thermal 
interface to the spacecraft being a heat pipe mounted to 
a side-wall of the laser housing where the largest heat 
sources, the amplifier heads are also mounted. 
Thermal control of the oscillator diodes and the 
doubling crystal must be accommodated and accounted 
for internal to the laser, leading to a final prime power 
draw of 110 W @ 30V input. The mechanical design 
consists of a single block of aluminum milled out on 
two sides. One side contains the laser optics, pump 
heads, doubler and final beam expander. The opposite 
side contains all the electronics. Power to the diodes 
and other signals are fed though a minimum number of 
connectors and feed-throughs in the floor of the box 
from the electronics to optics side. This design is 
inherently rigid, and allows the separation of the 
electronics from the optics to maintain cleanliness and 
facilitate the fabrication process. Fig. 3 shows a photo 
of the flight laser ready for delivery. 
current to diodes that are electrically in series. A by- 
pass FET diverts nominally 15 A of current around the Fig. 3. Externalphotograph of the GLAS laser serial number 
oscillator diodes thereby further derating these diodes 2. Visible is the output port ofthe laser beam, the mounting 
in current. When current is delivered to the diodes a flexures, quick disconnects (remove before flight) for venting 
and purging with air, and a reference mirror used to align 
counter counts the time until an internal photodetector the pointing of the laser beam referenced to the mounting 
detects the emission of the laser pulse. A feedback feet. 
All materials and construction techniques were 
carefully considered to ensure radiation tolerance, 
vacuum compatibility and opto-mechanical stability. 
All piece parts followed a rigorous cleaning and 
vacuum baking procedure. Final laser bakeouts were 
conducted at 10.' torr and monitored by a quadrupole 
mass spectrometer. 
3. TESTING SUMMARY 
Pre-launch testing consisted of 3-axis vibrational 
survivability tests, EMCIEMI compatibility tests, 
magnetic, and thermal-vacuum temperature cycling. 
The flight lasers were vibrated to 8 grms for 1 minute 
each axis. The design had previously been verified 
with an as-built engineering test unit vibrated to 10.6 
grms each axis for 1 minute. Thermal vacuum testing 
(TVAC) consisted of 2 modes. First, the laser 
underwent 4 temperature cycles in vacuum over the 
operation and survival temperature ranges. The laser 
optics cavity was then sealed and back-filled with 16 
psia dry air. The laser then under went one full 
survival cycle to characterize the laser in this 
configuration. To minimize the risk of contamination 
being introduced to the laser optical compartment after 
full vacuum testing was completed by the laser 
subsystem, the optics cavity was sealed for delivery to 
the instrument. During instrument and spacecraft 
TVAC testing the laser optics cavity remained sealed. 
Upon launch a burst-disc installed on the laser ruptured 
and vented the lasers to space. The lasers operate in- 
flight, completely evacuated on both the electronics 
and optics side of the laser. All 3 lasers went though 
the full qualification process. At the time of launch, the 
lasers had each accumulated 142 million shots on 
average throughout all testing, which is 11% of the 3- 
year mission total. 
4. IN-FLIGHT OPERATION 
On February 20, 2002 Laser 1 was activated and the 
GLAS instrument began continuous date collection. 35 
days and 120 million shots later, Laser 1 unexpectedly 
ceased operation. Fig. 4 and Fig. 5 show the 1064 nm 
and 532 nm pulse energies for all 3 lasers accumulated 
so far on-orbit. An Independent GLAS Anomaly 
Review Board (IGARB) was formed to investigate the 
cause. The IGARB determined the most likely cause 
was an unexpected catastrophic failure of a diode pump 
array [13]. The root cause of the pump array failure 
was probably related to "manufacturing of the laser 
diode arrays introduced excessive indium solder that 
resulted in a metallurgical reaction that progressively 
eroded the gold conductors through the formation of a 
non-conducting gold-indium intermetallic, gold-indide, 
at a rate dependent on temperature [13]." Based on this 
hypothesis it was recommended to reduce the operating 
temperatures of the lasers from an initial set point of 
29"C, to slow down the potential Au-In growth rate. 
Additionally, the mission operation scenario was 
modified to operate the lasers in 30 or 45 day 
campaigns to ensure seasonal mapping of polar ice 
while preserving laser lifetime. This reduced the laser 
operation duty cycle from 100% to 27% on-time. 
OMS 1064nm Laser Energy Hlstory 
through end of Campaign L3d 
too 1 
Laser Operamn Tlma (days) 
Fig. 4. 1064 nm energy history accumulated over 8 
campaigns. Some energy step down's or ups are associated 
with laser temperature changes. Others are hypothesized to 
be diode bar drop outs. 
GLAS 532nm Laser Energy Hislory 
through end of Campaign L3d 
Laser Operation Duration (Days) 
Fig. 5. 532 nm energy history accumulated over 8 
campaigns. Some energy step down's or ups are associated 
with laser temperature changes. Others are hypothesized to 
be due to diode bar drop outs. 
Laser 2 was operated for 3 campaigns accumulating 
over 420 million shots on-orbit at temperatures from 
25°C to 17OC. Although there was no catastrophic 
failure of Laser 2, the degradation rate was faster than 
pre-launch predictions. It is hypothesized this decline 
is due to the interaction of the intense 532 nm beam 
with trace outgassing materials inside the laser. Based 
on this hypothesis it was recommended to further 
reduce the temperature of Laser 3 to 13OC. Laser 3 has 
now completed 5 campaigns and accumulated over 600 
million shots on-orbit. With the reduced operating 
temperature and lower 532 nm conversion efficiency, 
Laser 3 is demonstrating a significantly reduced 
degradation rate. A linear projection on the lifetime of 
Laser 3 would put it out beyond the 1 billion shot mark 
which was in-line with prelaunch projections. As an 
instrument, ICESat has now accumulated over 1 
Billion shots in space. 
5. SUMMARY 
The Geoscience Laser Altimeter System on NASA's 
ICESat mission has been operating in space for over 3 
years. The lasers so far have emitted a cumulative 
number of shots exceeding 1 billion. For an overview 
of the scientific results form ICESat see the special 
issues of Geophysical Research Letters [14]. (also see 
the ICESat web site: httv://icesat.gsfc.nasa.nov/ ) 
GLAS is a pioneering instrument and the instrument 
performance is truly extraordinary. The lasers' 
pulsewidth, energy, beam quality, and stability, have 
helped enable 2.4 cm precision, 5 cm accuracy 
altimetry measurements from a 600 km orbit, in a size, 
weight and efficiency within mission constraints. 
Despite the earlier unexpected loss of Laser 1, a larger 
than anticipate degradation rate in Laser 2, the GLAS 
lasers continue to collect high resolution vertical 
profiles of the Earth's atmosphere and land surface. 
With the current projections on the lifetime of Laser 3, 
it is still possible the GLAS instrument will meet or 
exceed prelaunch expectations. 
Acknowledgements: The authors would like to thank 
J. Abshire and the GLAS instrument team and 
especially the staffs at the SLTC for their hard work 
and dedication in developing the GLAS lasers. We 
also would like to thank P. Liiva and Haris Riris for in 
space operation data processing. 
6. REFERENCES 
1. Abshire, J.B, et. al, "Geoscience Laser Altimeter 
System (GLAS) on the ICESat Mission: On-orbit 
measurement performance," Geophysical Research 
Letters, Vol. 32, L21 S02,2005. 
3. Sirota, J. M., et. al., "The transmitter pointing 
determination in the Geoscience Laser Altimeter 
System," Geophys. Res. Lett., Vol. 32, L22S11,2005. 
4. Afzal, R. S., et. al., "The Geoscience Laser 
Altimeter System Laser Transmitter," OSA Conference 
on Lasers and Electro-Optics Digest, CMJ2, 2000. 
5. Afzal, R. S., et. al., "Space Qualification of the 
Geoscience Laser Altimeter System," OSA Conference 
on Lasers and Electro-Optics Digest, CWN4,2002. 
6. Afzal, R. S., "Mars Observer Laser Altimeter: 
Laser Transmitter," Applied Optics, ( 33), 15, 20 May, 
1994, p. 3 184. 
7. Afzal, R. S., "Space Operation of the MOLA 
Laser," OSA Advanced Solid State Laser Topical 
Meeting, Davos, CH, 2000. 
8. Smith D.E., et. al., "Mars Orbiter Laser Altimeter: 
Experiment summary after the first year of global 
mapping of Mars," Journal of Geophysical Research- 
Planets, Vol. 10,2001. 
9. Afzal, R. S., et. al., "The GLAS Laser Transmitter 
Breadboard," OSA TOPS Vol. 10, Advanced Solid 
State Lasers, p. 102 Orlando, FL, Jan. 27 - Jan. 29, 
1997. 
10. Stephen, M. A, et. al., "Multi-billion shot, High- 
Fluence Exposure of Cr4+:YAG Passive Q-switch," 
SPIE Proceedings from 29" Annual Boulder Damage 
Symposium, Vol. 3244, p. 517, 1997. 
11. Stephen, M. A., et, al., "Quasi-CW Laser Diode 
Bar Life Tests," SPIE Proceedings from 29Ih Annual 
Boulder Damage Symposium, Vol. 3244, Boulder, CO, 
p. 598, 1997. 
12. Krainak, M., et. al., "Performance Tests of Quasi- 
CW Diode Pump Arrays Conducted or Sponsored by 
NASA Goddard Space Flight Center," 1997 Digest of 
the LEOS Summer Topical Meetings: Advanced 
Semiconductor Lasers and Applications, 1997, p. 78. 
13. Kichak, R.A., IGARB Executive Summary, 
h~://icesat.gsfc.nasa.gov/~ublications.htm. 
14. Results from the Ice, Cloud, and land Elevation 
Satellite (ICESat) Mission, Geophysical Research 
Letters, Vol. 32, No. 21 and 22,2005. 
2. Zwally, H. J., et. al., "ICESat's laser 
measurements of polar ice, atmosphere, ocean, and 
land," Journal of Geodynamics, Vol. 34 405-445, 
2002. 


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