Presentation Overview

1. High Efficiency Laser Designs for Airborne and Space-Based Lidar ApplicationsF. 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. WysockiFibertek, Inc

2. 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

3. 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

4. ICESat-2 Laser Requirements • 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

5. Bulk Solid State TransmitterDesign Overview Transmitter Optical Schematic • 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 532 nm output

6. Short Pulse Oscillator 1 µm polarizer 880 nm HR /4 • 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 Fiber Coupling Optics Output coupler Composite YVO4 rod with HR Conduction Cooled Diode Array Pump Source EO Q-Switch

7. Typical Short Pulse OscillatorPerformance Beam profile at output coupler X diameter = 291 µm Y diameter = 295 µm

8. Oscillator 1064nm Linewidth • Oscillator is linewidth narrowed • Analyzer etalon resolution is 4.9 pm • 8 mm etalon • Reflectivity finesse 14 • Linewidth = 5.9 pm

9. Oscillator/Preamp Results M2 = 1.3 Total output energy – 470 µJ Extracted energy – 357 µJ Pump power @ 10kHz 14.5 W Optical to optical efficiency 24.6%

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

11. Bulk Solid State Output vs. Total Diode Pump Power

12. Bulk Solid-State Optical to Optical Efficiency vs. Total Diode Pump Power

13. Bulk Solid-State 532nm Beam Quality vs. Amp Pump Power Beam quality improves at lower amp pump powers

14. Bulk Solid-State 532 nm Beam Quality vs. OutputPower Varied by Amp Delay M2 data at 532 nm with P=12.9W Beam at focus at 532nm with P=12.9W

15. 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

16. 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)

17. Yb Fiber-MOPA Architecture 1-mm Pulsed Seeder (1nsec/10kHz) 400uJ (4W) 10uJ (0.1W) 500nJ (5mW) 0.1nJ (1uW) • 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 100mw cw 1064nm Seed AOM M Z M end-cap 10/125mm YDFA 1 nsec/10kHz pulse-carving 2X 6/125mm YDFA 30/250mm YDFA

18. Yb Fiber Temporal Waveforms 3rd stage Final stage • 3.07 W average power demonstrated from final stage • 900 ps pulse

19. Yb Fiber Beam Quality Measurement • M2 ~ 1.25 @ 300 µJ, 0.9 ns • M2x = 1.10 • M2y = 1.35

20. 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

21. 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 Fiber coupled 880 nm pump End pumped Nd:YVO4 or Nd:YAG 1064 nm output 1064 nm output

22. 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 summary

23. 40 cm Cavity Nd:YVO4 Results M2 = 1.1 Near field output beam profile M2 data Highest energy results for 120 W peak pumping • 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 = 1.1 880 nm pumping results @ 2500 Hz

24. Approach for LVIS-GH 30 cm cavity optimization results for 120 W peak pumping • 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

25. 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

26. Acknowledgements Support for this work was provided by Goddard Space Flight Center through the Laser System Services Engineering contract and the NASA SBIR office.