lwg august 2010 high efficiency laser designs for airborne and space-based lidar applications f....
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FIBERTEK, INC. LWG August 2010
High Efficiency Laser Designs for Airborne and Space-Based Lidar Applications
F. Hovis, R. Burnham, M. Storm, R. Edwards, J. Edelman, K. Andes, P. Burns, B. Walters, Y. Chen, F. Kimpel, E. Sullivan, K. Li, C. Culpepper,
J. Rudd, X. Dang, J. Hwang, S. Gupta, T. Wysocki
Fibertek, Inc
FIBERTEK, INC. LWG Aug 2010
Presentation Overview
Approaches to high efficiency lasers
ICESat-2 class laser design overview– Bulk Nd solid-state– Hybrid bulk Nd solid-state/Yb fiber
High-efficiency, single-frequency ring laser development– NASA Phase 1 SBIR– Laser Vegetation Imaging System – Global Hawk
(LVIS-GH) transmitter
Future design updates
FIBERTEK, INC. LWG Aug 2010
Fibertek Design Approaches
Diode-pumped, bulk solid-state 1 µm lasers– Transverse pumped
• Well developed technology• Scaling to > 1 J/pulse, > 100 W demonstrated for fieldable systems
Maintaining M2 < 1.5 a challenge at higher powers• True wall plug efficiencies have been limited to ~8%
– End pumped• Well developed technology• Power scaling has been limited by pump sources• High brightness and power, fiber-coupled pump sources are a rapidly
developing and enabling technology COTS devices with > 100 W CW from 200 µm core fibers are readily available
• True wall plug efficiencies of 15%-20% are possible High efficiency is easier in low energy, high repetition rate systems
Fiber lasers– Ultimate high efficiency end pumped transmitters
• Kilowatts of high beam quality have been demonstrated in CW lasers• High brightness and power, fiber-coupled pump sources are a rapidly
developing and enabling technology• Energy scaling is key challenge
FIBERTEK, INC. LWG Aug 2010
ICESat-2 Laser Requirements
Parameter ATLAS Laser Transmitter
Wavelength 532 ± 1 nm
Pulse Energy 1 mJ, adjustable from 300-1000 µJ
Pulse Energy Stability 10% RMS over 1 s
Pulsewidth < 1.5 ns
Repetition Rate 10 ±0.3 kHz
Linewidth/Wavelength Stability 85% transmission through 30 pm filter
Polarization Extinction Ratio > 100:1
Spatial Mode M2 < 1.6, Gaussian
Beam Diameter 15 mm limiting aperture
Beam Divergence < 108 µrad
Pointing Stability (shot-to-shot) < 21.6 µrad (RMS) over 1 s
Pointing Stability (long-term) < 100 µrad
Lifetime 5 years plus 60 days on orbit
Mass 20 kg
Volume (cm) < 50(L) x 30(W) x 15(H)
Wall plug efficiency >5% for 800 µJ – 1000 µJ energies
Original Laser Support Engineering Services (LSES) contract was to support rebuild of original ICESat laser for ICESat-2– 1064 nm– 50 mJ/pulse– 50 Hz
After LSES award the ICESat-2 design transitioned to micro-pulse lidar approach updates
FIBERTEK, INC. LWG Aug 2010
Bulk Solid State TransmitterDesign Overview
Considered multiple design options– All bulk solid-state– All fiber– Hybrid
• Fiber front end• Final bulk solid state amp
Final choice was schedule driven– Need a TRL 6 laser by February
2011
Settled on all bulk solid-state approach– Short pulse Nd:YVO4 oscillator– Nd:YVO4 preamp– Nd:YVO4 power amp– High brightness 880 nm fiber
coupled pump diodes• Better mode overlap• Lower thermal loading
Transmitter Optical Schematic
532 nm output
FIBERTEK, INC. LWG Aug 2010
Short Pulse Oscillator
Nd:YVO4 gain medium– Nd:YVO4 is more efficient– 1 ns pulses can be achieved in Nd:YVO4 at fluences well
below optical damage thresholds– Relatively high absorption at 880 nm
Short linear cavity with electro-optic Q-switch– < 1.5 ns pulsewidth– Low timing jitter
High brightness 880 nm fiber coupled pump diodes– Better overlap with TEMoo mode– Lower thermal effects than 808 nm
EOQ-Switch
Conduction CooledDiode Array Pump Source
Composite YVO4 rod with HR
FiberCoupling
Optics
/4
Output coupler
1 µm polarizer880 nm HR
FIBERTEK, INC. LWG Aug 2010
Typical Short Pulse OscillatorPerformance
Beam profile at output coupler X diameter = 291 µm Y diameter = 295 µm
Laser #1 Beam Quality Data, 3/3/2010
Position (mm)
200 400 600 800
Bea
m d
iam
ete
rs (
mm
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
X dataY dataX fitY fit
M2x = 1.21
M2x = 1.24
ParameterLaser
PerformancePulse Energy 146 µJPulse Energy Stability 2.7% RMS over 1 sPulse Width .98 nsRepetition Rate 10 kHzPulse Interval Stability < 0.01 µsCenter Wavelength (IR) 1064.14 nmSpatial Mode M2
x - 1.2, M2y - 1.2
Pointing Stability (shot-to-shot)
0.43% of divergence
Pointing Stability (1 hour)
0.53% of divergence
FIBERTEK, INC. LWG Aug 2010
Oscillator 1064nm Linewidth
Oscillator is linewidth narrowed
Analyzer etalon resolution is 4.9 pm– 8 mm etalon– Reflectivity finesse 14
Linewidth = 5.9 pm
8
FIBERTEK, INC. LWG Aug 2010
Oscillator/Preamp Results
M2 = 1.3
Total output energy – 470 µJExtracted energy – 357 µJPump power @ 10kHz 14.5 WOptical to optical efficiency 24.6%
FIBERTEK, INC. LWG August 2010
Amplifier 1064 nm Performance
• Most sensitive parameter is pump/seed overlap• Mode matching in amplifier is key to high efficiency
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
5
10
15
20
25
Amp Output for 40W 880nm Pump, 10 kHz operation
Pout (measured) Model 86% Model 60%
Oscillator Input, W
Ampl
ifier
Out
put,
W
FIBERTEK, INC. LWG August 2010
20 30 40 50 60 7002468
101214161820
1064nm laser power
532nm laser power
Total 880 nm diode pump power (W)
Lase
r pow
er (W
)
Bulk Solid State Output vs. Total Diode Pump Power
FIBERTEK, INC. LWG Aug 2010
20 25 30 35 40 45 50 55 60 65 700
5
10
15
20
25
30
35
1064nm Efficiency
532nm efficiency
Total 880 nm diode pump power (W)
Effici
ency
(%)
Bulk Solid-State Optical to Optical Efficiency vs. Total Diode Pump
Power
FIBERTEK, INC. LWG Aug 2010
Bulk Solid-State 532nm Beam Quality vs. Amp Pump Power
Amp pump Power (W)
532 nm laser power
Mx2 My
2
40 12.6 1.184 1.272
40 12.6 1.142 1.179
32 10.5 1.09 1.1
24 7.6 1.19 1.1
16 4.5 1.03 1.04
8 2.2 1.015 1.032
Beam quality improves at lower amp pump powers
FIBERTEK, INC. LWG Aug 2010
532nm Laser Power (W)
Mx2 My
2
12.9 1.11 1.09
7.3 1.11 1.14
5.6 1.10 1.13
0 200 400 600 8000
0.5
1
X DataX FitY DataY Fit
Beam Diameter vs Position
Distance from Transform Lens (mm)
Bea
m D
iam
eter
(m
m)
M2 data at 532 nm with P=12.9W Beam at focus at 532nm with P=12.9W
Bulk Solid-State 532 nm Beam Quality vs. Output
Power Varied by Amp Delay
FIBERTEK, INC. LWG Aug 2010
Solid State Brassboard Full Transmitter Performance Summary
Laser meets specifications for – Energy: achieved 12.9W at 532nm
• 68% conversion efficiency from 1064nm to 532nm in LBO– 532nm Laser energy can be tuned with 2 methods:
• Adjust power amplifier pump power• Adjust timing between Q-sitch pulse and amplifiers.
Constant input power Data shows NO change in divergence or pointing.
– 532 nm beam quality: ~ 1.2– 532 nm pulsewidth: <1.3ns– 532 nm linewidth: <16 pm with etalon OC
• Instrument limited• Fully linewidth narrowed oscillator not yet incorporated
– Pointing stability at 1064nm: 2% of the divergence
FIBERTEK, INC. LWG Aug 2010
Bulk Nd Solid State vs. Hybrid
Hybrid– Advantages
• Single frequency with DFB/DBR stability• Pulse width selectable, 300 ps to 1.5 ns• High pulse format flexibility • Extremely stable To triggering• Fibertek environmental data looks very good• Use of bulk solid state amp allows easy energy scaling
– Challenges• Yb Parts supply chain is immature.
Very select vendors produce good parts in any reliable manner.• High parts count
Bulk solid state Nd Laser– Advantages
• Mature technology - supply chain, materials selections, cleaning & bake out procedures• Clear design margin identification and optical damage design rules• Simplest and lowest cost to produce.• Smaller and lower weight
– Challenges• Linewidth not single frequency BUT has substantial optical damage margin and can get high
transmission through 30 pm etalon (532 nm)
FIBERTEK, INC. LWG Aug 2010
Yb Fiber-MOPA Architecture
Multi-stage 1-mm pulsed seeder–– Based on established architecture at Fibertek– Uses COTS fiber-optics only
Final stage amplification to 300-400 uJ/pulse
1064nm
Seed
2X 6/125mm YDFA 10/125mm YDFA 30/250mm
YDFA
end-cap
400uJ(4W)
10uJ(0.1W)
0.1nJ (1uW)
1 nsec/10kHzpulse-carving
500nJ (5mW)100mw cw
1-mm Pulsed Seeder (1nsec/10kHz)
M Z M AOM
FIBERTEK, INC. LWG Aug 2010
Yb Fiber Temporal Waveforms
3rd stage
Final stage
3.07 W average power demonstrated from final stage
900 ps pulse
FIBERTEK, INC. LWG Aug 2010
Yb Fiber Beam Quality Measurement
M2 ~ 1.25 @ 300 µJ, 0.9 ns– M2
x = 1.10
– M2y = 1.35
FIBERTEK, INC. LWG Aug 2010
Hybrid Summary
Successfully demonstrated all fiber amplifier front end– All work done with residual in-house fibers– 300 µJ– 0.9 ns– M2 ~ 1.3
Final bulk amplifier demonstrated– 19 W output for 5 W input @ 10 kHz– M2 ~ 1.3
Need to increase fiber front end to 500 µJ – Achievable with new custom fiber– Not compatible with ICESat-2 schedule
Promising approach for future systems
FIBERTEK, INC. LWG Aug 2010
High-Efficiency, Single-Frequency Ring Laser
Development
Synthesis of other Fibertek development work– High efficiency bulk solid-state gain
media– Single- frequency ring lasers– Robust packing designs for field
applications
Appropriate design for longer pulsewidth applications– ≥ 3 ns– Lidar systems for winds, clouds,
aerosols, vegetation canopy, ozone, ……..
Initial work supported by NASA Phase 1 SBIR
Phase 1 SBIR led to contract for Laser Vegetation Imaging Sensor – Global Hawk (LVIS-GH) lidar transmitter
Brassboard short pulse ring oscillator
1064 nm output
End pumped Nd:YVO4 or
Nd:YAG
Fiber coupled 880 nm pump
1064 nm output
FIBERTEK, INC. LWG Aug 2010
40 cm Cavity Nd:YAG Results
Nd:YAG has better storage efficiency but lower gain
– 230 µs lifetime– Longer pulsewidths
Thermal effects limited initial repetition rate scaling tests
Pulse pumping improves efficiency
Highest energy results summaryPump
Wavelength (nm)
Pump Rep-rate (Hz)
Pump Pulsewidth
(us)Pump Power (W)
1064 nm Power (W)
1064 nm Energy (mJ)
Optical to Optical eff.
(%)
Pulsewidth (ns)
885 3000 150 24.21 W (53.8 Wpeak) 4.86 1.62 19.09 ~ 13-15885 2000 150 16.14 (53.8 W peak) 3.22 1.61 19.90 13-15885 1500 200 16.14 (53.8 W peak) 3.09 2.06 19.14 ~ 13-15808 2000 150 15 (50 W peak) 3.3 1.65 22.00 15808 1500 200 15 (50 W peak) 2.98 1.99 19.87 13
FIBERTEK, INC. LWG Aug 2010
40 cm Cavity Nd:YVO4 Results
Nd:YVO4 has lower storage efficiency but higher gain
– 100 µs lifetime– Higher absorption– Shorter pulsewidths
Reduced thermal effects relative to Nd:YAG
1% doping gave slightly higher efficiencies
35% optical to optical efficiency
– 1 mJ/pulse– Scalable to at least 8 kHz (8 W
average power)
M2 = 1.1
Highest energy results for 120 W peak pumping
880 nm pumping results @ 2500 Hz
Pump Wavelength (nm)
Pump Rep-rate (Hz)
Pump Pulsewidth
(us)
Pump Power (W)
1064 nm Power (W)
1064 nm Energy (mJ)
Optical to Optical eff.
(%)
Pulsewidth (ns)
880 2500 110 16.14 3.6 1.44 22.30 7.4880 2500 58 17.55 5.02 2.01 28.60 6.09
Near field output beam profile M2 data
M2 = 1.1
FIBERTEK, INC. LWG Aug 2010
Approach for LVIS-GH
Requirements– 1.5 mJ– 3-6 ns– 2500 Hz
Approach– Nd:YVO4
• Higher efficiency• Shorter pulse width
– 30 cm cavity • LVIS-GH requires 3-6 ns
pulsewidth– Dual compartment sealed
canister• Low distortion in high altitude
environment• Derived from TWiLiTE design
Brassboard results– 2500 Hz– 1.7 mJ– 4.3 ns pulse width
30 cm cavity optimization results for 120 W peak pumping
Pump Wavelength (nm)
Pump Rep-rate (Hz)
Pump Pulsewidth
(us)O.C %
Pump Power (W)
1064 nm Power (W)
1064 nm Energy (mJ)
Optical to Optical eff.
(%)
Pulsewidth (ns)
880 2500 58 30 17.55 4.3 1.72 24.50 5.15880 2500 63 30 19.06 4.46 1.78 23.40 5.16880 2500 58 40 17.55 4.32 1.73 24.62 4.54880 2500 63 40 19.06 4.46 1.78 23.40 4.56880 2500 68 40 20.58 4.6 1.84 22.35 4.57880 2500 70 40 21.18 4.65 1.86 21.95 4.50880 2500 58 45 17.55 4.20 1.68 23.93 4.26880 2500 58 50 17.55 3.96 1.58 22.56 4.36
FIBERTEK, INC. LWG Aug 2010
Future Work
Proposed as a NASA Phase 2 SBIR Injection seeding
– Modified ramp & fire approach– Scale to > 2 kHz
Power scaling– End pumped amplifier– Derived from ICESat-2 and Phase
1 designs Field hardened packaging
– Sealed for high altitude use– Dual compartment– Separate electronics module
Suitable for multiple near and longer term applications
– HSRL 1 transmitter replacement– Hurricane & Severe Storm
Sentinel transmitter– Next generation aerosol lidars– Pump for methane lidar– Pump for ozone lidar
FIBERTEK, INC. LWG Aug 2010
Acknowledgements
Support for this work was provided by Goddard Space Flight Center through the Laser System Services Engineering contract and the NASA SBIR office.