We update the status of a diode-pumped, Nd:YAG oscillator that is the prototype laser for NASA's DESDynI mission. After completing TRL-6 testing, this laser has fired over 5.5 billion shots in lifetesting

Arnold, Ed

Blair, Bryan

Coyle, D. Barry

Frederickson, Robert

Kay, Richard B.

Poulios, Demetrios

Scott, Stan

Stysley, Paul R.

NASA Technical Reports Server

Lifetest of the High Output Maximum Efficiency Resonator
(HOMER) Laser for the SAFFIRE Instrument on NASA's
DESDynI Project
Paul R. Stysley*, D. Barry Coyle*, Richard B. Kay**, Robert Frederickson -I', Demetrios Poulios**, Bryan
Blair*, Stan Scott*, and Ed Arnold j-
*NASA/Goddard Space Flight Center, Greenbelt, MD 20771
pmul.st' sle^nasr^_ nnv
"Department of Physics, American University, Washington, DC 20016
tScience Systems Applications Inc., Lanham MD
Abstract: We update the status of a diode-pumped, Nd:YAG oscillator that is the prototype laser for NASA's
DESDynI mission. After completing TRL-6 testing, this laser has fired over 5.5 billion shots in lifetesting.
(c)2011 Optical Society of America
OCIS codes: 140.3480. 140.3540, 140.3580, 280.3640
The HOMER (High Output Maximum Efficiency Resonator) laser is to be deployed on the SAFFIRE (Spaceborne
Altimeter For Forest & Ice Research and Exploration) instrument on NASA's DESDynI (Deformation, Ecosystem
Structure and Dynamics of Ice) Earth science program. At the time of publication, DESDynI is designed to employ,
along with a SAR, a multi-beam LIDAR in a 374 km sun synchronous orbit, producing five 1064 nm beams with 25in
	 on the Earth. DESDynI's LIDARs will study the Earth's biomass and glacial ice for a minimum of 2
years [1]. The SAFFIRE instrument requires that each laser fire at 241 pulses per second, produce a 9-10 ns laser
pulsewidth, and project a 17 mJ pulse with a TEMoo beam quality. This mission needs a highly efficient and reliable
laser instrument that will be capable of in excess of 6 billion shots over the lifetime of the mission, with a goal of 10
billion shots for margin. HOMER has already successfully met the beam output requirements in previous studies
and experiments ; firing over 6 billion shots at DESDynI mission relevant settings [2]. To further increase
confidence in the design, HOMER was mechanically upgraded from its earlier EDU configurations so that it could
effectively complete space-worthy TRL-6 (Technology Readiness Level 6) testing, including thermal vacuum
(TVAQ tests, vibration tests, and laser life testing. This new version was designated HOMER-2, as shown in figure
1. It is important to note that through all of this testing and experimentation we found the fundamentals and
advantages of an oscillator-only desi gn over a MOPA system remain the same. HOMER remains a single actively
Q-switched cavity with an 808 nm side pumped Nd:YAG gain medium employing a positive branch unstable
resonator (PBUR) [3]. The TVAC testing confirmed HOMER's survival temperature range of 0-40 C specification
by performing eight thermal cycles ranging from 50 C to -10C with 4 hour dwell times. TRL 6 testing also involved
vibration testing at the General Environmental Verification Standard (GEVS) in the x, y, and z axises. Vibration
testing exposed subtle flaws in the mechanical interfaces between the gain module and optical bench, as well as the
m Flezu'
Sealed Enclosur7JM nitoring Windows
GRM	 Active Q-Switch 	
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Optical Bench Truss, Removable 	 for Full	 Flexurebonding LDA
	
Out-of-Box Alignment 	 pedestal to optical benchL
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Figure 1. The frilly assembled HOMER-2 laser with the enclosure 	 Figure 2. Optical layout of the HOMER-2 lifetest. A 1064mn fiber
open for pre-lifetest inspection. Note, the flexures are mounted to	 coupled laser diode is reflected off reference cubes mounted on the
the "lid” and the radiator panel interface for HOMER-2 is below. 	 enclosure and the base plate. These determine any movement of laser
cavity vs. the outside environment.
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housing's titanium flexures. Quick modifications were made to validate the team's theories, and repeated vibration
tests showed immediately that these were the correct areas to focus on in the corning HOMER-ETU effort [4].
HOMER's optics were then inspected to confirm that no optical damage or degradation occurred. Next we
proceeded to do a final torquing, enclosure seal, and system inte gration for lifetime testing. The lifetest is currently
underway in a non-cleanroom laser lab environment at DESDynI-relevant parameters. We plan on operating at > 15
mJ pulses and 241 Hz for over a year until 10 billion shots are achieved. The lifetime experimental setup can be
seen in figure 2. To maximize the lifetest operation and minimize the degradation rate, the LDA's are driven far
below the manufacturer's rated limits. Thus we have shown a very low measured degradation rate of-- 250 uJ/B
shots, which we readily attribute to the 51% current derating and low pump pulse width of 65 us, for a low duty
cycle of 1.5%. Furthermore, should the laser catastrophically drop bars in the mission, there is more than enough
range in the drive electronics and LDA specifications to turn up the current and/or pulsewidth to compensate. The
Nd:YAG slab is chromium co-doped for radiation hardening and, pumped by seven, 4-bar G-package laser diode
arrays, and rated at 100W/bar at 100A. At beginning of life (BOL), HOMER-2 emitted as high as 15.7 n-J, with a
far field divergence of 1.0 mR x 0.9 mR shown in figure 3. After 3 B shots, the pulse energy dropped to 15 mJ at
which point we turned up the current by 1 A giving an output energy of 16n,0. As of the date of publication, the
laser has achieved over 5.5 billion shots and is currently running at 15.5 mJ. It has experienced no more than a 0.2.5
mJB energy degradation over time as seen in figure 4.
FY2MOTiirm
Figure 3. The CCD image simultaneously displays all reference	 Figure 4. Suniinary of the over 5.5 billion shots fired by
beams and the HOMER-2 far field beam. The far field measures 	 HOMER during its 9 months of lifetesting. HOMER has
approximately 0.9 mR x 1.0 mR.	 experience no more than 0.25 mJ/B loss between milestones.
In the upcoming year we plan to build another mechanical upgrade to the laser system for the DESDynI
program. This HOMER-ETU will apply lessons learned from the above reported TRL-6 environmental and lifetime
testing. It will also incorporate a flight prototype beam expander, LDA drive electronics, and a flight-like laser
control unit. These upgrades combined with HOMER will be considered the SAFFIRE laser instrument and will go
through more rigorous DESDynI LIDAR relevant TRL-6 level testing.
References
[1] httv://desdyiri.jpl.nasa.gov
[2] D. Bart' Coyle and Paul Stysley, "The High Output Maximum Efficiency Resonator (HOMER) Develop for Long Life. Space Based
Vegetation and Surface Imaging" Aerospace conference, IEEE, 2006.
[3] D. Barry Coyle. Richard B. Kay, Paul R. Stysley, Demetrios Poulios, "Efficient, Reliable, Long-lifetime, Diode-pumped Nd:YAG Laser for
Space-based Vegetation Topographical Altirnetry," App. Opt. 43, 5236-5242 (2004).
[4] D. Barry Coyle, Paul R. Stysley, Peter Rossoni, Robert Frederickson, Cheryl Salerno, Richard B. Kay, Demetrios Poulios, Bryan Blau, and
Ken Cory, "Flight Qualification of the High Output Maximtun Efficiency Resonator (HOMER) Laser for Space-based Remote Sensing
Applications" CLEO Conference, 2009.


Lifetest of the High Output Maximum Efficiency Resonator (HOMER) Laser for the SAFFIRE Instrument on NASA's DESDynI Project

Lifetest of the High Output Maximum Efficiency Resonator (HOMER) Laser for the SAFFIRE Instrument on NASA's DESDynI Project

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