FIBERTEK, INC.

BalloonWinds Laser Transmitter Update

Floyd Hovis, Fibertek, Inc.

Jinxue Wang, Raytheon Space and Airborne Systems

Michael Dehring, Michigan Aerospace Corp.

June 29, 2005

FIBERTEK, INC.

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

FIBERTEK, INC.Airborne vs. Space-Based Laser Doppler Wind Lidar Requirements

Airborne Space-based

Wavelength UV (355 nm) UV (355 nm)

Pulse energy 5 - 200 mJ 150 - 600 mJ

Repetition rate 50 – 2000 Hz 50 –200 Hz

Vibration environment Operate in 0.3 grms Survive 10 grms

Lifetime 2 x 108 shots 5 x 109 shots

Cooling Conductive to liquid or air Pure conductive coolingcooled heat exchanger

Thermal environment Spec energy in ±5°C band Spec energy in ± 5°C band

Survive 0° to 50°C cycling Survive –30° to70°C cycling

FIBERTEK, INC.

Laser Transmitter Overview

Summary of Approach

An all solid-state diode-pumped laser transmitter featuring:

Injection seeded ring laser Improves emission brightness (M2)

Diode-pumped zigzag slab amplifiers Robust 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 / boresight Stable and reliable operation over stable 1.0 m cavity and optical bench environment

Conduction cooled Eliminates circulating liquids w/in cavity

High efficiency third harmonic generation Reduces on orbit power requirements

Space-qualifiable electrical design Reduces cost and schedule risk for a future space-based mission

FIBERTEK, INC.

Laser Transmitter Overview

BalloonWinds Laser Transmitter Design Goals & Specifications

Spec Goal

1 µm pulse energy 230 mJ 300 mJ

355 nm pulse energy 70 mJ 150 mJ

Pulse Rate 50 Hz 70 Hz

THG efficiency >30 % > 50%

355 nm beam quality M2 ~ 2 M2 ~ 2

Frequency stability < 150 MHz/hr < 50 MHz/hr

Cooling Conductive Conductive

Lifetime 1 billion shots 1 billion shots

FIBERTEK, INC.

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

Expansiontelescope

LBO doubler

355 nm output

LBO tripler

Pump diodes

Amplifier

Isolator

FIBERTEK, INC.

Laser Transmitter Overview

1 m Ring Resonator Design

Nd:YAG Pump Head

Diode Pumped Increased efficiency / Reduced size - weight Brewster angle slab Eliminates need for end face coating, high fill factor Conduction cooled Elimination of circulating liquids / increased MTBF

1 m Resonator

Telescopic Ring Resonator Allows better control of the TEM00 like mode size 90˚ Image Rotation Homogenizes beam parameters in 2 axes RTP Based Q-Switch Thermally compensated design / high damage threshold

RTP Based Phase Modulator Provides reduced sensitivity to high frequency vibration Zerodur Optical Bench Boresight stable over environment

Performance Features

Design features address issues associated with stable operation in space

FIBERTEK, INC.

Ring Oscillator Design

Diode pumped Increased efficiency / Reduced size - weight Brewster angle design Simplifies optical alignment, high volume fill factor Conduction cooled Elimination of circulating liquids / increased MTBF

Brewster Angle Slab Design

Key to efficient operation is extracting beam profiletailored to slab geometry

Pump Diodes

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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 TEM00, M2 =1.2 at 50 Hz30 mJ TEM00, M2 =1.3 at 100 Hz50 mJ square supergaussian, M2 = 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 suppressor2. Cube polarizer3. Odd bounce slab4. Steering wedge5. /2 waveplate6. Mode limiting aperture7. RTP phase modulator8. 45° Dove prism9. Non-imaging telescope10. RTP q-switch

1 2 3 4 5 6 2

2 4 9 5 8 5 7 2

5

10

Seed

FIBERTEK PROPRIETARY

Final Zerodur Optical Bench (12cm x 32cm)

FIBERTEK, INC.Ring Oscillator Design TEM00 Results

100 Hz TEM00 Oscillator Beam Quality Measurements

Output energy 30 mJ/pulse

M2 was 1.2 in non-zigzag axis, 1.3 in zigzag axis

FIBERTEK, INC.Ring Oscillator Design Square Supergaussian Results

50 Hz Square Supergaussian Oscillator Beam Quality MeasurementsOutput energy was 50 mJ/pulse

M2 was 1.2

No hot spots in beam from near field to far field

M2 data Near field profile

FIBERTEK, INC.Amplifier DesignBalloon Winds Brewster Angle Slab

Diode pumped Increased efficiency / Reduced size & weight Brewster angle design Simplifies optical alignment Pump on bounce geometry Maximize overlap with high gain regions, high efficiency Conduction cooled Elimination of circulating liquids / increased MTBF Reduced tip pumping Minimizes thermal distortions at slab tips Mature technology Reduces 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

FIBERTEK, INC.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)

FIBERTEK, INC.Amplifier Design BalloonWinds Slab Amplifier Thermal Modeling

54.200

54.400

54.600

54.800

55.000

55.200

55.400

55.600

55.800

0 0.03 0.06 0.09 0.12 0.15 0.18 0.21 0.24 0.27 0.3

Beam Position (zig-zag direction) (cm)

Averge Temperature (C)

FEA Data Parabolic Curve Fit

Thermal lens curve fit - focal length ~ 4 m

Operational parameters used for thermal model - 8 arrays

- 16 bars per array - 75 W/bar (optical) - 150 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

FIBERTEK, INC.Amplifier Design BalloonWinds Slab Performance Modeling

Oscillator Configuration 100 µs pump pulse 75 W/bar 60 bars

Oscillator Output 50 mJ/pulse M2 = 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

Model of Dual BalloonWinds Amplifiers

808 nm pump pulse width (µs)

0 50 100 150 200 250

Amplifier outut energy (mJ)

0

100

200

300

400

500

600

700

Amplifier 2 outputAmplifier 1 output

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

FIBERTEK, INC.Oscillator/Amplifier IntegrationSquare Supergaussian Extraction Results

50 Hz NASA Ozone Amplifier Beam Quality Measurements• Input was 50 mJ, M2 = 1.2, supergaussian beam• Output was >340 mJ (17 W), Mx

2 = 1.6, My2 = 1.5,

M2 data Near field beam profile of amplifier#2 output

Beam quality vs. output energy and efficiency are a key lidar system level trades

FIBERTEK, INC.

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 355 nm) @ 50 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 nminput

/2 @ 1064nm

FIBERTEK, INC.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 deff = 0.835 pm/V Angular sens. = 5.31mrad-cm Walkoff = 6.24 mrad Temp. sens. = 6.6°C-cm

25 mm Type II LBO for THG deff = 0.521 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

FIBERTEK, INC.BalloonWinds Mechanical DesignFeatures

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

FIBERTEK, INC.BalloonWinds Laser Transmitter Optical Bench Layout

Bench design allows for second amplifier for power scaling

Oscillator Bench

MainBench

Oscillator BenchFlexure Mounts

Oscillator BenchFlexure Mount

Main BenchPivot Mount

Main BenchFlexure Mount

Oscillator Head

Resonance Detector

Seed LaserCollimator

LBO SHGOven

LBO THGOven

Amplifier Slab Pedestal

Energy Monitor

Amplifier Diode Pedestal

Optical Isolator

Expansion Scope

Down Scope

FIBERTEK, INC.BalloonWinds Mechanical DesignOptics Canister

Final Design

Canister

Optical Bench

SealedCover

MountingFeet (qty 3)

Transmit Beam Window

InternalElectronics

FIBERTEK, INC.BalloonWinds Mechanical DesignIntegrated Optics & Electronics Canisters

Final Design: Complete Assembly

LaserBeam

LaserCanister

LaserElectronicsUnit (LEU)

FIBERTEK, INC.BalloonWinds Mechanical DesignLaser Canister Thermal Control

Canister Thermal Control

Cooling FansCooling Fins

Shroud

LEU

Canister

FIBERTEK, INC.

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.)

OscillatorHead

AmplifierHead

OscillatorHead

AmplifierHead

AirFlow

AirFlow

Even with a relatively inefficient thermal interface material the amplifier temperature rise is manageable with conductive heat transfer to air cooled fins.

FIBERTEK, INC.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

FIBERTEK, INC.

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.