Laser Transmitter for the Tropospheric Wind Lidar Technology Experiment (TWiLiTE) Floyd Hovis, Fibertek, Inc. Bruce Gentry, NASA Goddard Space Flight Center

Laser Transmitter Specifications Performance Specifications/Design Performance Summary Table Parameter Specification Design Performance Margin Wavelength 355 nm NA Laser Energy (UV) >30 mJ 40 mJ @ 1.3% duty cycle > 33% Pulse Rep Rate 200 Hz Average Power > 6 W 8 W @ 1.3% duty cycle Beam Quality, M^2 < 3 < 2.5 17% Energy in the Bucket > 86% encircled energy into 3x d. l. beam Meet specification TBD Frequency Stability < 5MHz RMS for 30 sec < 50 MHz RMS for 30 min < 30 MHz RMS for 30 min 40% Seeding Efficiency >99.9% >99.9 Meets spec Pulsewidth > 15 ns ~13-15 ns None Linewidth <120 MHz @ 355 nm ~120 MHz Pointing stability < 10% of beam divergence Electrical Power (excl. chiller) 550 W 470 W 14% Thermal Management Conductive or Liquid Cooled Conductive to Liquid Lifetime 1 billion shots (75% diode derating) @ 1064 nm 1 billion shots @ 1064 nm

Environmental Design Performance Environmental Design Parameters - Laser Optics Module Parameter Operating range Survival range Ambient temperature (°C) 10 to 40* -30 to +50 Ambient pressure (mbar) 0 to 1050 *Assumes thermal interface plate maintained at nominal operating temperature +/-2°C Environmental Design Parameters - Laser Electronics Unit Parameter Operating range Survival range Ambient temperature (°C) 0 to 50* -30 to +50 Ambient pressure (mbar) 35 to 1050 0 to 1050 *Assumes liquid cooled interface plate for low pressure operation Design performance exceeds all environmental performance specifications

BalloonWinds, Raytheon, and Air Force Lasers Provided Basis For Key Design Features Three amplifier design Autonomous operation controlled through RS232 serial interface Nominal 28 VDC primary power Space-qualifiable electrical design Thermal control through conductive cooling to liquid cooled plates bolted to bottom of laser module 355 nm single frequency output of >380 mJ/pulse @ 60 Hz (23 W) Deliverable system will undergo extended life testing at Raytheon Electronics module Laser module Space-Winds Lidar Laser Transmitter Final acceptance testing was completed in November 2006

Conceptual Optical Layout Laser Transmitter Conceptual Optical Layout Ring Resonator Fiber-coupled 1 mm seed laser Optical isolator LBO doubler LBO tripler Power amplifier 532/1064 nm output Fiber port 355 nm output

Laser Housing Baseline Design Full Assembly Coolant connection Dual compartment optical cavity Oscillator and amplifier on opposite sides I-beam like structure for increased stiffness No pressure induced distortion of primary mounting plate Conductively cooling to liquid cooled center plane Hermetic sealing for low pressure operation Purge port Signal connectors Power connectors Coolant connection

Laser Housing Baseline Design Oscillator Compartment Ring Resonator Purge port 355 nm nm output window Coolant connection 1064 & 532 nm output window

Laser Housing Baseline Design Amplifier Compartment SHG THG Purge port Coolant connection 1064/532 nm output port, external beam dump to be added 355 nm output port, external beam expander to be added

Laser Housing Baseline Design Oscillator Compartment Size Top View 31 cm An ~ 31 cm x 25 cm x 14 cm canister accommodates all required optical and electrical components I-beam like mounting structure provides high mechanical stability All optical components are mounted to a surface that to first order does not experience pressure induced deformation 25 cm Side View 14 cm 31 cm

Ring Oscillator Performance Overview 1 mm Resonator Design Parameters Diode Bars Eight 6-bar arrays, 100 W rated-QCW, operated at 75 W peak power per bar Pulsewidth 56 ms Repetition rate 200 Hz Pump Energy 0.202 J Heat Dissipation 250 watts Slab Size 4.2 x 4.2 x 94 mm3 Doping Level 1.1 % Nd3+ Angle of Incidence 57˚ TIR Bounces 12 per pass Cavity Length 40 cm (physical) Cavity Magnification 1.5 Out-Coupling 40 % Output Pulse Energy 25 mJ Output Pulsewidth 13-15 ns Output Beam Size ~3 mm super gaussian (variable)

Power Amplifier Design Brewster Angle Slab Design Features  Even bounce Brewster angle design reduces beam pointing change due to slab movement  Equal number of 10 bar arrays per string (5) simplifies diode driver electrical design  Modeling assuming 100 W/bar arrays are operated at 75 W/ bar predicts 100 mJ/pulse output for 25 mJ/pulse input for 63 µs pump pulses  Mechanical mounts will be scaled down version of NASA Ozone designs Modeling predicts that extracting a power amplifier with 25 mJ/pulse achieves 100 mJ/pulse output at 1.3 % duty cycle

Third Harmonic Generation Results Of Fibertek IR&D  Characterized Type I LBO doubler for higher damage threshold and linearly polarized residual 1064 nm - Damage was an issue in early testing with KTP - LBO damage threshold is ~4X that of KTP - Low cost (relatively), high quality LBO crystals are now commercially available  Characterized 25 mm Type II 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)  Space-qualifiable laser delivered to Raytheon achieved 23 W of 355 nm for 44 W of 1064 nm pump at 50 Hz (52% conversion efficiency) Type I LBO doubler 355 nm output Type II LBO tripler 1064 nm input /2 @ 1064nm

Opto-Mechanical Design and Procurement Status Optical design is complete Long lead optical components are on order 808 nm pump diodes Zigzag slabs for oscillator, preamplifier, and amplifier Mechanical designs of diode pumped laser heads are complete Parts have been ordered Design of laser canister is nearly complete Some detailing of amplifier optical train and external interfaces remains to be done Goal is to order canister in February 2007

Electronics Overview Laser Module electronics Laser Electronics Unit Q-Switch Driver (high-voltage converter, high-voltage switch) Photo-detector (detects cavity resonance) SHG/THG Heaters and temperature sensors Cavity Modulator Seed Laser & Electronics Laser Electronics Unit Power input, filtering, conversion and distribution Diode Drivers (voltage converter, high-current pulse switching) Cavity modulator driver (HV power amplifier) Laser Controller board (pulse timing, system interface, controls) Temperature Control Boards Safety Interlocks All electrical designs were previously developed for the BalloonWinds and Raytheon Wind Lidar laser transmitters The Laser Unit is an enclosed canister containing the optical assemblies and laser bench, the laser diode heads, and the indicated electrical elements. It will likely be sealed with a slight overpressure internally, and use hermetic circular connectors for electrical interconnections. For the prototype laser system, the Electronics Unit will be a rack-mounted unit, consisting of several electrical assemblies interconnected inside an overall enclosure. For a flight unit, this enclosure need not be sealed, but the packaging of unit needs to be optimized for the specific available volume and footprint, and also for the best transfer of thermal loads to the radiator surface.

Software Interface Is Complete COLD 1 WARMUP 2 FAULT 3 HPWR 6 LPWR 5 DIAG 7 ARMED 4 Power-up CNTRL INITIALIZE CNTRL LASERDISARM CNTRL HTRSON CNTRL CLRINT CNTRL LASERARM CNTRL LPWRMODE CNTRL HPWRMODE CNTRL DIAGMODE CNTRL STOP Any active fault “1” “4” “-” (hyphen) “D” “7” “2” “8” “C” “A” Blue text indicates alternative command characters when operating laser system from HyperTerminal serial interface

Electronics Design and Procurement Status Software design is complete Design upgrades to allow high altitude unsealed operation is well underway Original plan was for commercial power electronics Laser control board design complete Power supply design complete Diode driver design complete Logic power supply design complete Safety controller design in work Updated seeding circuitry in work Crystal oven controller in work Key long lead components are on order High power, high reliability DC/DC converters High reliability EMI filter modules (MIL-STD-461C & D) Hermetic capacitors Electronics are scheduled to be finished in April 2007

Laser Subsystem Summary Mass Laser Optics Module - 16 kg (based on current design) Laser Electronics Unit - ~22 kg (estimated from BalloonWinds, may decrease Volume Laser Optics Module - 31 cm x 25 cm x 14 cm = 10,850 cm3 (based on current design) Laser Electronics Unit - TBD, expected to be somewhat larger than laser Power Estimated total 28 VDC power into system is 470 W Thermal Estimated total power dissipation is 450 W Estimated power dissipation Laser Optics Module is 250 W Estimated power dissipation Laser Electronics Unit 200 W Laser subsystem delivery in July 2007 For testing purposes, most system parameters that are fixed during normal operation will be implemented as programmable parameters. These parameters will be optimized during testing, and then made fixed for the final deliverable system. Parameters that are adjustable will in general be accessible through the serial communications interface. A command protocol will be developed that will provide a set of commands for commanding the laser operating modes, gathering important measurement and performance data from the laser, and adjusting a selected set of operating parameters. Other command functions that can be included are: hardware serial numbers, software version ID, self-test functions, and ground-test functions (low-power mode, reprogrammability of FPGA or software, watchdog enable/disable, etc.).

Acknowledgements Funding for this program was provided by the NASA Earth Science Technology Office as part of the Instrument Incubator Program