BalloonWinds Laser Transmitter Update Floyd Hovis, Fibertek, Inc. Jinxue Wang, Raytheon Space and Airborne Systems Michael Dehring, Michigan Aerospace Corp. June 29, 2005

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 for NOAA 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 BalloonWinds Laser Transmitter Design Goals & Specifications SpecGoal 1 µm pulse energy 230 mJ300 mJ 355 nm pulse energy70 mJ150 mJ Pulse Rate 50 Hz70 Hz THG efficiency>30 %> 50% 355 nm beam qualityM 2 ~ 2 M 2 ~ 2 Frequency stability< 150 MHz/hr< 50 MHz/hr CoolingConductive Conductive Lifetime1 billion shots1 billion shots

Laser Transmitter Overview The BalloonWinds laser transmitter will use a single Brewster angle slab amplifier Fiber-coupled 1  m Seed Laser Fiber port Ring resonator Expansion telescope LBO doubler 355 nm output LBO tripler Pump diodes Amplifier Isolator

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 Diode pumpedIncreased efficiency / Reduced size - weight Brewster angle designSimplifies optical alignment, high volume fill factor Conduction cooledElimination of circulating liquids / increased MTBF Brewster Angle Slab Design Key to efficient operation is extracting beam profile tailored to slab geometry Pump Diodes

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 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

Amplifier Design Balloon Winds Brewster Angle Slab Diode pumpedIncreased efficiency / Reduced size & weight Brewster angle designSimplifies optical alignment Pump on bounce geometryMaximize overlap with high gain regions, high efficiency Conduction cooledElimination of circulating liquids / increased MTBF Reduced tip pumpingMinimizes thermal distortions at slab tips Mature technologyReduces risk, based on synthesis of previously developed pump on bounce and Brewster angle designs Design Features Design is a synthesis of Brewster angle and pump on bounce approaches Pump Diodes

Amplifier Design Slab Amplifier Thermal Modeling General Modeling Approach Use finite element codes to develop a thermal model of the diode pumped slab Assumes uniform thermal distribution in non-zigzag axes Estimate the lensing due different optical path lengths for different entry positions in the zigzag plane - Calculate the average temperature for rays at different positions in the zigzag plane - Fit the resulting temperature distribution to estimate the lensing Estimate the lensing due to slab bending - One uncompensated bounce from the long face Near normal incidence pump on bounce (NASA Ozone) Brewster angle pump on bounce (BalloonWinds)

Amplifier Design BalloonWinds Slab Amplifier Thermal Modeling Thermal lens curve fit - focal length ~ 4 m Operational parameters used for thermal model - 8 arrays - 16 bars per array - 75 W/bar (optical) us per pulse - 50 Hz Brewster angle pump on bounce Modeling predicts less thermal lensing in the BalloonWinds amplifier design than in the NASA Ozone amplifier design

Amplifier Design BalloonWinds Slab Performance Modeling Oscillator Configuration 100 µs pump pulse 75 W/bar 60 bars Oscillator Output 50 mJ/pulse M 2 = 1.2 Amplifier Configuration Vary pump pulse width 75 W/bar 128 bars/amplifier Vary delay to vary pump power Amplifier Output for 204 µs 250 mJ/pulse for 1 amp 600 mJ/pulse for two amps Low Energy Telescopic Resonator A single Brewster angle amplifier can meet the needs of most airborne direct detection wind lidars. Dual amplifiers are sufficient for some currently proposed space-based systems

Oscillator/Amplifier Integration Square Supergaussian Extraction Results 50 Hz NASA Ozone 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 Approach for BalloonWinds Investigated Type I BBO or LBO doublers for higher damage threshold and linearly polarized residual 1064 nm - Damage was an issue in early testing with KTP - BBO damage threshold is ~2X that of KTP, LBO damage threshold is ~4X that of KTP - Low cost (relatively), high quality BBO and LBO crystals are now commercially available Investigated change to single 25 mm LBO tripler - High quality, low cost (relatively) has recently become available - Ion beam sputtered AR coatings have demonstrated high damage thresholds and low reflectivities for triple AR coatings (1064/532/355 nm) Initial tests demonstrated 7.7 W of 355 nm for 17 W of 1064 nm pump at 50 Hz (45% conversion efficiency for 1064 nm to Hz)with single10 mm BBO doubler and single 25 mm Type II LBO tripler - Further optimization was possible by since SHG efficiency was only 50% Change to doubling in Type I LBO was evaluated for reduced angular sensitivity and walk-off - Final configuration of 25 mm Type LBO for SHG and 25 mm Type II LBO achieved 54% conversion with a 16 W 1064 nm pump, meeting the goal of >50% Type I LBO doubler 355 nm output Type II LBO tripler 1064 nm input 1064nm

Third Harmonic Generation Tests & Modeling Of Final THG Configuration With In-House NASA Ozone Pump All modeling used SNLO from Sandia Labs Used measured input 1064 nm pulse energies Used measured 1064 nm beam diameters Supergaussian coefficient = 3 25 mm Type I LBO for SHG d eff = pm/V Angular sens. = 5.31mrad-cm Walkoff = 6.24 mrad Temp. sens. = 6.6°C-cm 25 mm Type II LBO for THG d eff = pm/V Angular sens. = 3.47 mrad-cm Walkoff = 9.49 mrad Temp. sens. = 3.43°C-cm Results easily exceed the BalloonWinds specification of 30% conversion and achieve the goal of >50% Low Energy Telescopic Resonator

BalloonWinds Mechanical Design Features Dual bench on bench design - Zerodur oscillator bench is mounted to a larger optical bench that hold the amplifier and nonlinear conversion optics - Low thermal expansion Zerodur optical bench improves stability of injection seeding - Common overall mechanical bench improves boresight stability Vented canister design - Eliminates pressure induced distortion - Sintered metal filter is used to filter vent holes and eliminate particulate contamination - Internal getters will be used to control moisture and organic contaminants Oscillator and amplifier heads are directly conductively coupled to the canister base - Minimizes thermal distortion of the optical benches

BalloonWinds Laser Transmitter Optical Bench Layout Bench design allows for second amplifier for power scaling Oscillator Bench Main Bench Oscillator Bench Flexure Mounts Oscillator Bench Flexure Mount Main Bench Pivot Mount Main Bench Flexure Mount Oscillator Head Resonance Detector Seed Laser Collimator LBO SHG Oven LBO THG Oven Amplifier Slab Pedestal Energy Monitor Amplifier Diode Pedestal Optical Isolator Expansion Scope Down Scope

BalloonWinds Mechanical Design Optics Canister Final Design Canister Optical Bench Sealed Cover Mounting Feet (qty 3) Transmit Beam Window Internal Electronics

BalloonWinds Mechanical Design Integrated Optics & Electronics Canisters Final Design: Complete Assembly Laser Beam Laser Canister Laser Electronics Unit (LEU)

BalloonWinds Mechanical Design Laser Canister Thermal Control Canister Thermal Control Cooling Fans Cooling Fins Shroud LEU Canister

Laser Canister Thermal Analysis Canister Base Thermal Profiles Temperature profile with tin-lead interface (4.4 W/sq.in.) Temperature profile with Nusil interface (0.36 W/sq.in.) Oscillator Head Amplifier Head Oscillator Head Amplifier Head Air Flow Air Flow Even with a relatively inefficient thermal interface material the amplifier temperature rise is manageable with conductive heat transfer to air cooled fins.

BalloonWinds Laser Transmitter Status Summary All optical components have been ordered and >95% are in-house All electrical components have been ordered and >80% are in-house 90% of the mechanical components have been ordered and >75% are in-house The oscillator head has been assembled and tested Assembly of the ring oscillator optical bench is underway Assembly of the amplifier head is underway Primary optical bench is being cleaned in preparation for final assembly

Acknowledgments We wish to acknowledge the NASA Office of Earth Science Advanced Technology Initiatives Program, the NASA GSFC SBIR Program, the Raytheon Space and Airborne Systems Internal Research and Development Program, the Air Force SBIR Program, and the National Oceanic and Atmospheric Administration for their support of this work.