FIBERTEK, INC.

Laser Transmitter for the

BalloonWinds Program

Floyd Hovis, Fibertek, Inc.

Jinxue Wang, Raytheon Space and Airborne Systems

Michael Dehring, Michigan Aerospace Corp.

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

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

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 > 45% Maximizes 355 nm output

Beam quality M2 < 2 Reduces size of collection optics

Frequency drift < 5 MHz/s Allows Doppler shift measurements

Cooling Conductive Space compatible

Lifetime 3 years Space-mission requirement

FIBERTEK, INC.

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 > 30%

355 nm beam quality M2 ~ 2

Frequency stability < 150 MHz

Cooling Conductive

Lifetime 1 billion shots

FIBERTEK, INC.

Laser Transmitter Overview

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

Fiber-coupled 1 m Seed

Laser

Fiber port

Ring resonator

Expansiontelescope

KTP doubler

355 nm output

LBO tripler

0.5xtelescope

Slabs

Mirror

Mirror

Doveprism

Pump diodes

Pump diodes

Amp #2

Amp #1

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

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

50 Hz TEM00 Oscillator Beam Quality Measurements

Output energy 30 mJ/pulse

M2 was 1.2 in both axes

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.

NASA ATIP Amplifier Design

Single-Sided Pumped and Cooled Amplifier Design

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

Performance Features

Slabs

Mirror

Mirror

Input from oscillator

Final output

Doveprism

Pump diodes

Pump diodes

Basic design has been validated with NASA ATIP funding

FIBERTEK, INC.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

FIBERTEK, INC.Oscillator/Amplifier IntegrationSquare Supergaussian Extraction Results

50 Hz 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

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

KTP doubler

355 nm output

LBO triplers0.5x reductiontelescope

1064 nminput

/2 @ 1064nm /2 @ 1064nm @ 532 nm

FIBERTEK, INC.

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 beamWent 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 LBOChange to SHG in Type I BBO or LBO is being investigated for higher damage thresholds needed for scaling to higher pulse energies

KTP doubler

355 nm output

LBO tripler0.5x reductiontelescope

1064 nminput

/2 @ 1064nm /2 @ 1064nm @ 532 nm

FIBERTEK, INC.BalloonWinds Laser Transmitter Design

Baseline Approach

Requires >3.5 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

FIBERTEK, INC.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

Amplfier 1 Output vs. 808 nm Pump Pulse Width

808 nm pump pulse width (µs)

0 20 40 60 80 100 120 140 160 180

Amplifier 1 outut energy (mJ)

20

40

60

80

100

120

140

160

180

200

FIBERTEK, INC.BalloonWinds Laser Transmitter Design

Baseline Optical Layout

Ring oscillator section Amplifier section

Harmonic converters

Bench design allows allows for second amplifier for power scaling

43 cm

66 cm

FIBERTEK, INC.

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

FIBERTEK, INC.

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 > 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 >1 J/pulse @

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

FIBERTEK, INC.

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.