Laser Transmitter for the

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

2
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

3
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
Top Level Laser Space-Based Transmitter
Performance Goals
1 µm pulse energy 1 J Required for
measurements from space Final wavelength 355
nm Required for direct detection wind
lidar Pulse Rate 50 100 Hz Improved data
collection rate THG efficiency gt
45 Maximizes 355 nm output Beam quality M2
lt 2 Reduces size of collection
optics Frequency drift lt 5 MHz/s Allows
Doppler shift measurements Cooling Conductive
Space compatible Lifetime 3
years Space-mission requirement
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Laser Transmitter Overview
BalloonWinds Laser Transmitter Design Goals
Specifications
1 µm pulse energy 230 mJ 355 nm pulse
energy 70 mJ Pulse Rate 50 THG
efficiency gt 30 355 nm beam quality M2
2 Frequency stability lt 150
MHz Cooling Conductive Lifetime 1
billion shots
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Laser Transmitter Overview

• The basis for the BalloonWinds laser transmitter
  design is a system that was developed for NASA
  Langley with ATIP funding

Ring resonator
Fiber port
Fiber-coupled 1 mm Seed Laser
Mirror
Pump diodes
Amp 1
Expansion telescope
Dove prism
Slabs
Amp 2
Pump diodes
Mirror
KTP doubler
LBO tripler
355 nm output
0.5x telescope
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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
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

• 50 Hz TEM00 Oscillator Beam Quality Measurements
• Output energy 30 mJ/pulse
• M2 was 1.2 in both axes

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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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NASA ATIP Amplifier Design
Single-Sided Pumped and Cooled Amplifier Design
Performance Features
? Diode Pumped Increased efficiency /
Reduced size - weight ? Near Normal
incidence Simplifies AR coatings ? Pump on
bounce geometry High gain fill factor, high
efficiency ? Conduction cooled Elimination
of circulating liquids / increased MTBF ? Dove
Prism Between Stages Reduced astigmatism
Basic design has been validated with NASA ATIP
funding
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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

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Oscillator/Amplifier IntegrationSquare
Supergaussian Extraction Results

• 50 Hz 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
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 - 100 mJ/pulse at 1064 converted to 50
mJ/pulse at 355 nm, 50 Hz operation
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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

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BalloonWinds Laser Transmitter Design
Baseline Approach

• Requires gt3.5 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, M2 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

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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
? M2 1.2 Amplifier Configuration ?
170 µs pump pulse ? 55 W/bar ? 112
bars ? Vary delay to vary pump
power Amplifier Output ? 175 mJ/pulse
? M2 1.4
Low Energy Telescopic Resonator
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BalloonWinds Laser Transmitter Design
Baseline Optical Layout
Ring oscillator section
Amplifier section
43 cm
Harmonic converters
66 cm
Bench design allows allows for second amplifier
for power scaling
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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
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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 gt 50 Hz
• ? Bending of 1-sided pumped and cooled slabs
  limits power
• scaling
• Multiple funding sources and deliverables for
  2005-2006
• ? Add two 2-sided pumped and cooled
  amplifiers to the
• existing NASA Langley ATIP laser to scale
  to gt1 J/pulse _at_
• 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

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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.