Laser Transmitter for the BalloonWinds Program Floyd Hovis, Fibertek, Inc. Jinxue Wang, Raytheon Space and Airborne Systems Michael Dehring, Michigan Aerospace Corp.

Program Overview Program Objectives Develop a robust, single frequency 355 nm laser for airborne and space-based direct detection wind lidar systems –All solid-state, diode pumped –Robust packaging –Tolerant of moderate vibration levels during operation –Space-qualifiable design Incorporate first generation laser transmitters into ground-based and airborne field systems to demonstrate and evaluate designs –Goddard Lidar Observatory for Winds (GLOW) –Balloon based Doppler wind lidar being developed by Michigan Aerospace and the University of New Hampshire Iterate designs for improved compatibility with a space-based mission –Lighter and smaller –Radiation hardened electronics

Airborne vs. Space-Based Laser Doppler Wind Lidar Requirements AirborneSpace-based Wavelength UV (355 nm)UV (355 nm) Pulse energy mJ mJ Repetition rate 50 – 2000 Hz50 –200 Hz Vibration environment Operate in 0.3 g rms Survive 10 g rms Lifetime 2 x 10 8 shots5 x 10 9 shots Cooling Conductive to liquid or air Pure conductive cooling cooled heat exchanger Thermal environment Spec energy in ±5°C bandSpec energy in ± 5°C band Survive 0° to 50°C cyclingSurvive –30° to70°C cycling

Laser Transmitter Overview Summary of Approach An all solid-state diode-pumped laser transmitter featuring: Injection seeded ring laserImproves emission brightness (M 2 ) Diode-pumped zigzag slab amplifiersRobust and efficient design for use in space Advanced E-O phase modulator material Allows high frequency cavity modulation for improved stability injection seeding Alignment insensitive / boresightStable and reliable operation over stable 1.0  m cavity and optical benchenvironment Conduction cooledEliminates circulating liquids w/in cavity High efficiency third harmonic generationReduces on orbit power requirements Space-qualifiable electrical designReduces cost and schedule risk for a future space-based mission

Laser Transmitter Overview Top Level Laser Space-Based Transmitter Performance Goals 1 µm pulse energy 1 JRequired for measurements from space Final wavelength355 nm Required for direct detection wind lidar Pulse Rate 50 –100 HzImproved data collection rate THG efficiency> 45%Maximizes 355 nm output Beam qualityM 2 < 2Reduces size of collection optics Frequency drift< 5 MHz/sAllows Doppler shift measurements CoolingConductiveSpace compatible Lifetime3 yearsSpace-mission requirement

Laser Transmitter Overview BalloonWinds Laser Transmitter Design Goals & Specifications 1 µm pulse energy 230 mJ 355 nm pulse energy70 mJ Pulse Rate 50 THG efficiency> 30% 355 nm beam qualityM 2 ~ 2 Frequency stability< 150 MHz CoolingConductive Lifetime1 billion shots

Laser Transmitter Overview The basis for the BalloonWinds laser transmitter design is a system that was developed for NASA Langley with ATIP funding Fiber-coupled 1  m Seed Laser Fiber port Ring resonator Expansion telescope KTP doubler 355 nm output LBO tripler 0.5x telescope Slabs Mirror Dove prism Pump diodes Amp #2 Amp #1

Laser Transmitter Overview 1  m Ring Resonator Design Nd:YAG Pump Head Diode PumpedIncreased efficiency / Reduced size - weight Brewster angle slabEliminates need for end face coating, high fill factor Conduction cooledElimination of circulating liquids / increased MTBF 1  m Resonator Telescopic Ring ResonatorAllows better control of the TEM 00 like mode size 90˚ Image RotationHomogenizes beam parameters in 2 axes RTP Based Q-SwitchThermally compensated design / high damage threshold RTP Based Phase ModulatorProvides reduced sensitivity to high frequency vibration Zerodur Optical BenchBoresight stable over environment Performance Features Design features address issues associated with stable operation in space

Ring Oscillator Design Optical Schematic Design Features Near stable operation allows trading beam quality against output energy by appropriate choice of mode limiting aperture  30 mJ TEM 00, M 2 =1.2 at 50 Hz  30 mJ TEM 00, M 2 =1.3 at 100 Hz  50 mJ square supergaussian, M 2 = 1.2 at 50 Hz Injection seeding using an RTP phase modulator provides reduced sensitivity to high frequency vibration Zerodur optical bench results in high alignment and boresight stability 1. Reverse wave suppressor 2. Cube polarizer 3. Odd bounce slab 4. Steering wedge 5. /2 waveplate 6. Mode limiting aperture 7. RTP phase modulator 8. 45° Dove prism 9. Non-imaging telescope 10. RTP q-switch Seed FIBERTEK PROPRIETARY Final Zerodur Optical Bench (12cm x 32cm)

Ring Oscillator Design TEM 00 Results 50 Hz TEM 00 Oscillator Beam Quality Measurements Output energy 30 mJ/pulse M 2 was 1.2 in both axes

Ring Oscillator Design TEM 00 Results 100 Hz TEM 00 Oscillator Beam Quality Measurements Output energy 30 mJ/pulse M 2 was 1.2 in non-zigzag axis, 1.3 in zigzag axis

Ring Oscillator Design Square Supergaussian Results 50 Hz Square Supergaussian Oscillator Beam Quality Measurements Output energy was 50 mJ/pulse M 2 was 1.2 No hot spots in beam from near field to far field M 2 data Near field profile

NASA ATIP Amplifier Design Single-Sided Pumped and Cooled Amplifier Design Diode PumpedIncreased efficiency / Reduced size - weight Near Normal incidenceSimplifies AR coatings Pump on bounce geometryHigh gain fill factor, high efficiency Conduction cooledElimination of circulating liquids / increased MTBF Dove Prism Between StagesReduced astigmatism Performance Features Slabs Mirror Input from oscillator Final output Dove prism Pump diodes Basic design has been validated with NASA ATIP funding

NASA ATIP Oscillator/Amplifier Integration The ring oscillator and dual stage amplifier have been successfully integrated onto a semi-hardened brass board configuration –All turning mirrors are lockable, no gimbal mounts –Position insensitive wedge prisms are used for fine steering

Oscillator/Amplifier Integration Square Supergaussian Extraction Results 50 Hz Amplifier Beam Quality Measurements Input was 50 mJ, M 2 = 1.2, supergaussian beam Output was >340 mJ (17 W), M x 2 = 1.6, M y 2 = 1.5, M 2 data Near field beam profile of amplifier#2 output Beam quality vs. output energy and efficiency are a key lidar system level trades

Third Harmonic Generation GSFC High Brightness Laser Transmitter Approach Type II Potassium titanyl phosphate (KTP) for second harmonic generation High efficiency Space-qualified for CALIPSO Type II Lithium triborate (LBO) for third harmonic generation 50% conversion efficiency demonstrated in High Brightness Laser built for Goddard Space Flight Center mJ/pulse at 1064 converted to 50 mJ/pulse at 355 nm, 50 Hz operation KTP doubler 355 nm output LBO triplers 0.5x reduction telescope 1064 nm input 532 nm

Third Harmonic Generation 355 nm Generation with Ring Oscillator/Single Amp Oscillator configured for square supergaussian output Initial testing with previous converter configuration gave low results due to excess SHG New layout resolved excess SHG conversion Moved KTP before beam reduction Achieved 61% SHG with unfocussed beam Went to single LBO THG Back conversion appears to also also decreased THG with 0.5x down scope Achieved 43% conversion with single LBO THG - 64 mJ/pulse (3.2 W) of 355 nm for 165 mJ/pulse (8.25 W) 1064 nm pump at 50 Hz Further optimization is possible by increasing SHG efficiency to 67% Dual crystal THG will be revisited with a reduced magnification down scope Could reduce damage potential by lowering fluence on LBO Change to SHG in Type I BBO or LBO is being investigated for higher damage thresholds needed for scaling to higher pulse energies KTP doubler 355 nm output LBO tripler 0.5x reduction telescope 1064 nm input 532 nm

BalloonWinds Laser Transmitter Design Baseline Approach Requires >3.5 W of high beam quality 355 nm output at 50 Hz Oscillator design same as NASA ATIP developed ring oscillator Mature ready to build technology Uses a scaled up Brewster angle amplifier with the thermal & mechanical design developed in the NASA ATIP program Mature ready to build technology On axis beam propagation simplifies optical layout Power goals have been met with 55 W peak diode pumping 8.8 W, M 2 = 1.4 demonstrated at 1064 nm Use of 100 W peak power bars operated at 75 W provide significant design margin Final optical layout developed Laser canister is 13 cm x 43 cm x 66 cm

BalloonWinds Laser Transmitter Design A Single Amplifier Meets the Balloon Wind Lidar Requirements Oscillator Configuration 90 µs pump pulse 55 W/bar 100 bars Oscillator Output 40 mJ/pulse M 2 = 1.2 Amplifier Configuration 170 µs pump pulse 55 W/bar 112 bars Vary delay to vary pump power Amplifier Output 175 mJ/pulse M 2 = 1.4 Low Energy Telescopic Resonator

BalloonWinds Laser Transmitter Design Baseline Optical Layout Ring oscillator section Amplifier section Harmonic converters Bench design allows allows for second amplifier for power scaling 43 cm 66 cm

BalloonWinds Laser Transmitter Status Key optics are on order and due for delivery in late February Final detailing of the optical bench an canister is nearly complete An ICD for integration of the laser transmitter into the balloon gondola has been developed and reviewed The program is on track for a July laser transmitter delivery

Future Development Work Third harmonic conversion tests with 20 W, 50 Hz 1064 nm pump Design and testing of 2-sided pumped and cooled amplifiers for scaling to 1 J/pulse 1064 nm output at > 50 Hz Bending of 1-sided pumped and cooled slabs limits power scaling Multiple funding sources and deliverables for Add two 2-sided pumped and cooled amplifiers to the existing NASA Langley ATIP laser to scale to >1 50 Hz and 1064 nm Deliver a fieldable 1 J, 50 Hz 1064 nm source frequency converted to 355 nm to Raytheon Space and Airborne Systems for risk reduction testing Deliver a fieldable 100 Hz, 1 J, 1064 nm transmitter to the Air Force Research Labs for test and evaluation

Acknowledgments We wish to acknowledge the NASA Office of Earth Science, NASA Goddard Space Flight Center, NASA Langley Research Center, the Raytheon Space and Airborne Systems, the Air Force SBIR Program, and the National Oceanic and Atmospheric Administration for their support of this work.