BalloonWinds Laser Transmitter Update

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

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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

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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
cooling cooled heat exchanger Thermal
environment Spec energy in 5C band Spec energy
in 5C band Survive 0 to 50C
cycling Survive 30 to70C cycling
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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 mm 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
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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 gt30 gt 50 355 nm beam
quality M2 2 M2 2 Frequency
stability lt 150 MHz/hr lt 50 MHz/hr
Cooling Conductive Conductive Lifetime
1 billion shots 1 billion shots
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Laser Transmitter Overview

• The BalloonWinds laser transmitter will use a
  single Brewster angle slab amplifier

Ring resonator
Fiber port
Fiber-coupled 1 mm Seed Laser
Isolator
Expansion telescope
Amplifier
Pump diodes
LBO tripler
LBO doubler
355 nm output
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Laser Transmitter Overview
1 mm Ring Resonator Design
Performance Features

• NdYAG 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 mm 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

Design features address issues associated with
stable operation in space
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Ring Oscillator Design
Brewster Angle Slab 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
Key to efficient operation is extracting beam
profile tailored to slab geometry
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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 Hz
• 30 mJ TEM00, M2 1.3 at 100 Hz
• 50 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

Final Zerodur Optical Bench (12cm x 32cm)
FIBERTEK PROPRIETARY
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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

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Ring Oscillator Design Square Supergaussian
Results

• 50 Hz Square Supergaussian Oscillator Beam
  Quality Measurements
• Output 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
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Amplifier DesignBalloon Winds Brewster Angle Slab
Design Features
? 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 Minimiz
es thermal distortions at slab tips ? Mature
technology Reduces risk, based on synthesis
of previously developed pump on
bounce and Brewster angle designs
Design is a synthesis of Brewster angle and pump
on bounce approaches
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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)
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Amplifier Design BalloonWinds Slab Amplifier
Thermal Modeling
Operational parameters used for thermal model
- 8 arrays - 16 bars per array - 75 W/bar
(optical) - 150 us per pulse - 50 Hz
Thermal lens curve fit - focal length 4 m
Brewster angle pump on bounce
Modeling predicts less thermal lensing in the
BalloonWinds amplifier design than in the NASA
Ozone amplifier design
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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
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
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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 gt340 mJ (17 W), Mx2 1.6, My2 1.5,

M2 data
Near field beam profile of amplifier2 output
Beam quality vs. output energy and efficiency are
a key lidar system level trades
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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) _at_ 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 gt50

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Third Harmonic GenerationTests 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. 6.6C-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.
3.43C-cm Results easily exceed the BalloonWinds
specification of 30 conversion and achieve the
goal of gt50
Low Energy Telescopic Resonator
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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

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BalloonWinds Laser Transmitter Optical Bench
Layout
Expansion Scope
Oscillator Bench Flexure Mounts
Oscillator Bench
Optical Isolator
Amplifier Diode Pedestal
Oscillator Head
Amplifier Slab Pedestal
Resonance Detector
Main Bench Pivot Mount
Energy Monitor
Main Bench
Oscillator Bench Flexure Mount
LBO SHG Oven
Down Scope
LBO THG Oven
Seed Laser Collimator
Main Bench Flexure Mount
Bench design allows for second amplifier for
power scaling
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BalloonWinds Mechanical DesignOptics Canister
Sealed Cover
Optical Bench
Internal Electronics
Canister
Transmit Beam Window
Mounting Feet (qty 3)
Final Design
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BalloonWinds Mechanical DesignIntegrated Optics
Electronics Canisters
Laser Electronics Unit (LEU)
Laser Beam
Laser Canister
Final Design Complete Assembly
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BalloonWinds Mechanical DesignLaser Canister
Thermal Control
LEU
Canister
Cooling Fins
Cooling Fans
Shroud
Canister Thermal Control
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Laser Canister Thermal Analysis
Canister Base Thermal Profiles
Air Flow
Amplifier Head
Air Flow
Amplifier Head
Oscillator Head
Oscillator Head
Temperature profile with Nusil interface (0.36
W/sq.in.)
Temperature profile with tin-lead interface (4.4
W/sq.in.)
Even with a relatively inefficient thermal
interface material the amplifier temperature rise
is manageable with conductive heat transfer to
air cooled fins.
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BalloonWinds Laser Transmitter Status
Summary
All optical components have been ordered and gt95
are in-house All electrical components have been
ordered and gt80 are in-house 90 of the
mechanical components have been ordered and gt75
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
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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.