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

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

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 M 2 < 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

LWG Aug 2010 ICESat-2 Laser Requirements ParameterATLAS Laser Transmitter Wavelength532 ± 1 nm Pulse Energy1 mJ, adjustable from µJ Pulse Energy Stability10% RMS over 1 s Pulsewidth< 1.5 ns Repetition Rate10 ±0.3 kHz Linewidth/Wavelength Stability85% transmission through 30 pm filter Polarization Extinction Ratio> 100:1 Spatial ModeM 2 < 1.6, Gaussian Beam Diameter15 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 Lifetime5 years plus 60 days on orbit Mass20 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

LWG Aug 2010 Bulk Solid State Transmitter Design 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:YVO 4 oscillator –Nd:YVO 4 preamp –Nd:YVO 4 power amp –High brightness 880 nm fiber coupled pump diodes Better mode overlap Lower thermal loading Transmitter Optical Schematic 532 nm output

LWG Aug 2010 Short Pulse Oscillator  Nd:YVO 4 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 TEM oo mode –Lower thermal effects than 808 nm

LWG Aug 2010 Typical Short Pulse Oscillator Performance Beam profile at output coupler X diameter = 291 µm Y diameter = 295 µm ParameterLaser Performance Pulse Energy146 µJ Pulse Energy Stability2.7% RMS over 1 s Pulse Width.98 ns Repetition Rate10 kHz Pulse Interval Stability< 0.01 µs Center Wavelength (IR) nm Spatial ModeM 2 x - 1.2, M 2 y Pointing Stability (shot-to- shot) 0.43% of divergence Pointing Stability (1 hour)0.53% of divergence

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

LWG Aug 2010 Oscillator/Preamp Results M 2 = 1.3 Total output energy– 470 µJ Extracted energy– 357 µJ Pump 10kHz14.5 W Optical to optical efficiency 24.6%

LWG August 2010 Amplifier 1064 nm Performance Most sensitive parameter is pump/seed overlap Mode matching in amplifier is key to high efficiency

LWG August 2010 Bulk Solid State Output vs. Total Diode Pump Power

LWG Aug 2010 Bulk Solid-State Optical to Optical Efficiency vs. Total Diode Pump Power

LWG Aug 2010 Bulk Solid-State 532nm Beam Quality vs. Amp Pump Power Amp pump Power (W) 532 nm laser power Mx2Mx2 My2My Beam quality improves at lower amp pump powers

LWG Aug nm Laser Power (W) Mx2Mx2 My2My M 2 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

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

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 T o 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)

LWG Aug 2010 Yb Fiber-MOPA Architecture  Multi-stage 1-  m pulsed seeder– –Based on established architecture at Fibertek –Uses COTS fiber-optics only  Final stage amplification to uJ/pulse 1064nm Seed 2X 6/125  m YDFA 10/125  m YDFA 30/250  m YDFA end-cap 400uJ (4W) 10uJ (0.1W) 0.1nJ (1uW) 1 nsec/10kHz pulse-carving 500nJ (5mW) 100mw cw 1-  m Pulsed Seeder (1nsec/10kHz) M Z M AOM

LWG Aug 2010 Yb Fiber Temporal Waveforms 3 rd stage Final stage  3.07 W average power demonstrated from final stage  900 ps pulse

LWG Aug 2010 Yb Fiber Beam Quality Measurement  M 2 ~ 300 µJ, 0.9 ns –M 2 x = 1.10 –M 2 y = 1.35

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 –M 2 ~ 1.3  Final bulk amplifier demonstrated –19 W output for 5 W 10 kHz –M 2 ~ 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

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:YVO 4 or Nd:YAG Fiber coupled 880 nm pump 1064 nm output

LWG Aug 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 summary

LWG Aug cm Cavity Nd:YVO 4 Results  Nd:YVO 4 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)  M 2 = 1.1 Highest energy results for 120 W peak pumping 880 nm pumping 2500 Hz Near field output beam profile M 2 data M 2 = 1.1

LWG Aug 2010 Approach for LVIS-GH  Requirements –1.5 mJ –3-6 ns –2500 Hz  Approach –Nd:YVO 4 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 widt h 30 cm cavity optimization results for 120 W peak pumping

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

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.