LWG August 2010 High Efficiency Laser Designs for Airborne and Space-Based Lidar Applications F. Hovis, R. Burnham, M. Storm, R. Edwards, J. Edelman, K

FIBERTEK, INC. LWG August 2010

High Efficiency Laser Designs for Airborne and Space-Based Lidar Applications

F. Hovis, R. Burnham, M. Storm, R. Edwards, J. Edelman, K. Andes, P. Burns, B. Walters, Y. Chen, F. Kimpel, E. Sullivan, K. Li, C. Culpepper,

J. Rudd, X. Dang, J. Hwang, S. Gupta, T. Wysocki

Fibertek, Inc

FIBERTEK, INC. LWG Aug 2010

Presentation Overview

Approaches to high efficiency lasers

ICESat-2 class laser design overview– Bulk Nd solid-state– Hybrid bulk Nd solid-state/Yb fiber

High-efficiency, single-frequency ring laser development– NASA Phase 1 SBIR– Laser Vegetation Imaging System – Global Hawk

(LVIS-GH) transmitter

Future design updates


Fibertek Design Approaches

Diode-pumped, bulk solid-state 1 µm lasers– Transverse pumped

• Well developed technology• Scaling to > 1 J/pulse, > 100 W demonstrated for fieldable systems

Maintaining M2 < 1.5 a challenge at higher powers• True wall plug efficiencies have been limited to ~8%

– End pumped• Well developed technology• Power scaling has been limited by pump sources• High brightness and power, fiber-coupled pump sources are a rapidly

developing and enabling technology COTS devices with > 100 W CW from 200 µm core fibers are readily available

• True wall plug efficiencies of 15%-20% are possible High efficiency is easier in low energy, high repetition rate systems

Fiber lasers– Ultimate high efficiency end pumped transmitters

• Kilowatts of high beam quality have been demonstrated in CW lasers• High brightness and power, fiber-coupled pump sources are a rapidly

developing and enabling technology• Energy scaling is key challenge


ICESat-2 Laser Requirements

Parameter ATLAS Laser Transmitter

Wavelength 532 ± 1 nm

Pulse Energy 1 mJ, adjustable from 300-1000 µJ

Pulse Energy Stability 10% RMS over 1 s

Pulsewidth < 1.5 ns

Repetition Rate 10 ±0.3 kHz

Linewidth/Wavelength Stability 85% transmission through 30 pm filter

Polarization Extinction Ratio > 100:1

Spatial Mode M2 < 1.6, Gaussian

Beam Diameter 15 mm limiting aperture

Beam Divergence < 108 µrad

Pointing Stability (shot-to-shot) < 21.6 µrad (RMS) over 1 s

Pointing Stability (long-term) < 100 µrad

Lifetime 5 years plus 60 days on orbit

Mass 20 kg

Volume (cm) < 50(L) x 30(W) x 15(H)

Wall plug efficiency >5% for 800 µJ – 1000 µJ energies

Original Laser Support Engineering Services (LSES) contract was to support rebuild of original ICESat laser for ICESat-2– 1064 nm– 50 mJ/pulse– 50 Hz

After LSES award the ICESat-2 design transitioned to micro-pulse lidar approach updates


Bulk Solid State TransmitterDesign Overview

Considered multiple design options– All bulk solid-state– All fiber– Hybrid

• Fiber front end• Final bulk solid state amp

Final choice was schedule driven– Need a TRL 6 laser by February

2011

Settled on all bulk solid-state approach– Short pulse Nd:YVO4 oscillator– Nd:YVO4 preamp– Nd:YVO4 power amp– High brightness 880 nm fiber

coupled pump diodes• Better mode overlap• Lower thermal loading

Transmitter Optical Schematic

532 nm output


Short Pulse Oscillator

Nd:YVO4 gain medium– Nd:YVO4 is more efficient– 1 ns pulses can be achieved in Nd:YVO4 at fluences well

below optical damage thresholds– Relatively high absorption at 880 nm

Short linear cavity with electro-optic Q-switch– < 1.5 ns pulsewidth– Low timing jitter

High brightness 880 nm fiber coupled pump diodes– Better overlap with TEMoo mode– Lower thermal effects than 808 nm

EOQ-Switch

Conduction CooledDiode Array Pump Source

Composite YVO4 rod with HR

FiberCoupling

Optics

/4

Output coupler

1 µm polarizer880 nm HR


Typical Short Pulse OscillatorPerformance

Beam profile at output coupler X diameter = 291 µm Y diameter = 295 µm

Laser #1 Beam Quality Data, 3/3/2010

Position (mm)

200 400 600 800

Bea

m d

iam

ete

rs (

mm

)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

X dataY dataX fitY fit

M2x = 1.21

M2x = 1.24

ParameterLaser

PerformancePulse Energy 146 µJPulse Energy Stability 2.7% RMS over 1 sPulse Width .98 nsRepetition Rate 10 kHzPulse Interval Stability < 0.01 µsCenter Wavelength (IR) 1064.14 nmSpatial Mode M2

x - 1.2, M2y - 1.2

Pointing Stability (shot-to-shot)

0.43% of divergence

Pointing Stability (1 hour)

0.53% of divergence


Oscillator 1064nm Linewidth

Oscillator is linewidth narrowed

Analyzer etalon resolution is 4.9 pm– 8 mm etalon– Reflectivity finesse 14

Linewidth = 5.9 pm

8


Oscillator/Preamp Results

M2 = 1.3

Total output energy – 470 µJExtracted energy – 357 µJPump power @ 10kHz 14.5 WOptical to optical efficiency 24.6%


Amplifier 1064 nm Performance

• Most sensitive parameter is pump/seed overlap• Mode matching in amplifier is key to high efficiency

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

5

10

15

20

25

Amp Output for 40W 880nm Pump, 10 kHz operation

Pout (measured) Model 86% Model 60%

Oscillator Input, W

Ampl

ifier

Out

put,

W


20 30 40 50 60 7002468

101214161820

1064nm laser power

532nm laser power

Total 880 nm diode pump power (W)

Lase

r pow

er (W

)

Bulk Solid State Output vs. Total Diode Pump Power


20 25 30 35 40 45 50 55 60 65 700

5

10

15

20

25

30

35

1064nm Efficiency

532nm efficiency

Total 880 nm diode pump power (W)

Effici

ency

(%)

Bulk Solid-State Optical to Optical Efficiency vs. Total Diode Pump

Power


Bulk Solid-State 532nm Beam Quality vs. Amp Pump Power

Amp pump Power (W)

532 nm laser power

Mx2 My

2

40 12.6 1.184 1.272

40 12.6 1.142 1.179

32 10.5 1.09 1.1

24 7.6 1.19 1.1

16 4.5 1.03 1.04

8 2.2 1.015 1.032

Beam quality improves at lower amp pump powers


532nm Laser Power (W)

Mx2 My

2

12.9 1.11 1.09

7.3 1.11 1.14

5.6 1.10 1.13

0 200 400 600 8000

0.5

1

X DataX FitY DataY Fit

Beam Diameter vs Position

Distance from Transform Lens (mm)

Bea

m D

iam

eter

(m

m)

M2 data at 532 nm with P=12.9W Beam at focus at 532nm with P=12.9W

Bulk Solid-State 532 nm Beam Quality vs. Output

Power Varied by Amp Delay


Solid State Brassboard Full Transmitter Performance Summary

Laser meets specifications for – Energy: achieved 12.9W at 532nm

• 68% conversion efficiency from 1064nm to 532nm in LBO– 532nm Laser energy can be tuned with 2 methods:

• Adjust power amplifier pump power• Adjust timing between Q-sitch pulse and amplifiers.

Constant input power Data shows NO change in divergence or pointing.

– 532 nm beam quality: ~ 1.2– 532 nm pulsewidth: <1.3ns– 532 nm linewidth: <16 pm with etalon OC

• Instrument limited• Fully linewidth narrowed oscillator not yet incorporated

– Pointing stability at 1064nm: 2% of the divergence


Bulk Nd Solid State vs. Hybrid

Hybrid– Advantages

• Single frequency with DFB/DBR stability• Pulse width selectable, 300 ps to 1.5 ns• High pulse format flexibility • Extremely stable To triggering• Fibertek environmental data looks very good• Use of bulk solid state amp allows easy energy scaling

– Challenges• Yb Parts supply chain is immature.

Very select vendors produce good parts in any reliable manner.• High parts count

Bulk solid state Nd Laser– Advantages

• Mature technology - supply chain, materials selections, cleaning & bake out procedures• Clear design margin identification and optical damage design rules• Simplest and lowest cost to produce.• Smaller and lower weight

– Challenges• Linewidth not single frequency BUT has substantial optical damage margin and can get high

transmission through 30 pm etalon (532 nm)


Yb Fiber-MOPA Architecture

Multi-stage 1-mm pulsed seeder–– Based on established architecture at Fibertek– Uses COTS fiber-optics only

Final stage amplification to 300-400 uJ/pulse

1064nm

Seed

2X 6/125mm YDFA 10/125mm YDFA 30/250mm

YDFA

end-cap

400uJ(4W)

10uJ(0.1W)

0.1nJ (1uW)

1 nsec/10kHzpulse-carving

500nJ (5mW)100mw cw

1-mm Pulsed Seeder (1nsec/10kHz)

M Z M AOM


Yb Fiber Temporal Waveforms

3rd stage

Final stage

3.07 W average power demonstrated from final stage

900 ps pulse


Yb Fiber Beam Quality Measurement

M2 ~ 1.25 @ 300 µJ, 0.9 ns– M2

x = 1.10

– M2y = 1.35


Hybrid Summary

Successfully demonstrated all fiber amplifier front end– All work done with residual in-house fibers– 300 µJ– 0.9 ns– M2 ~ 1.3

Final bulk amplifier demonstrated– 19 W output for 5 W input @ 10 kHz– M2 ~ 1.3

Need to increase fiber front end to 500 µJ – Achievable with new custom fiber– Not compatible with ICESat-2 schedule

Promising approach for future systems


High-Efficiency, Single-Frequency Ring Laser

Development

Synthesis of other Fibertek development work– High efficiency bulk solid-state gain

media– Single- frequency ring lasers– Robust packing designs for field

applications

Appropriate design for longer pulsewidth applications– ≥ 3 ns– Lidar systems for winds, clouds,

aerosols, vegetation canopy, ozone, ……..

Initial work supported by NASA Phase 1 SBIR

Phase 1 SBIR led to contract for Laser Vegetation Imaging Sensor – Global Hawk (LVIS-GH) lidar transmitter

Brassboard short pulse ring oscillator

1064 nm output

End pumped Nd:YVO4 or

Nd:YAG

Fiber coupled 880 nm pump

1064 nm output


40 cm Cavity Nd:YAG Results

Nd:YAG has better storage efficiency but lower gain

– 230 µs lifetime– Longer pulsewidths

Thermal effects limited initial repetition rate scaling tests

Pulse pumping improves efficiency

Highest energy results summaryPump

Wavelength (nm)

Pump Rep-rate (Hz)

Pump Pulsewidth

(us)Pump Power (W)

1064 nm Power (W)

1064 nm Energy (mJ)

Optical to Optical eff.

(%)

Pulsewidth (ns)

885 3000 150 24.21 W (53.8 Wpeak) 4.86 1.62 19.09 ~ 13-15885 2000 150 16.14 (53.8 W peak) 3.22 1.61 19.90 13-15885 1500 200 16.14 (53.8 W peak) 3.09 2.06 19.14 ~ 13-15808 2000 150 15 (50 W peak) 3.3 1.65 22.00 15808 1500 200 15 (50 W peak) 2.98 1.99 19.87 13


40 cm Cavity Nd:YVO4 Results

Nd:YVO4 has lower storage efficiency but higher gain

– 100 µs lifetime– Higher absorption– Shorter pulsewidths

Reduced thermal effects relative to Nd:YAG

1% doping gave slightly higher efficiencies

35% optical to optical efficiency

– 1 mJ/pulse– Scalable to at least 8 kHz (8 W

average power)

M2 = 1.1

Highest energy results for 120 W peak pumping

880 nm pumping results @ 2500 Hz

Pump Wavelength (nm)

Pump Rep-rate (Hz)

Pump Pulsewidth

(us)

Pump Power (W)

1064 nm Power (W)

1064 nm Energy (mJ)


(%)

Pulsewidth (ns)

880 2500 110 16.14 3.6 1.44 22.30 7.4880 2500 58 17.55 5.02 2.01 28.60 6.09

Near field output beam profile M2 data

M2 = 1.1


Approach for LVIS-GH

Requirements– 1.5 mJ– 3-6 ns– 2500 Hz

Approach– Nd:YVO4

• Higher efficiency• Shorter pulse width

– 30 cm cavity • LVIS-GH requires 3-6 ns

pulsewidth– Dual compartment sealed

canister• Low distortion in high altitude

environment• Derived from TWiLiTE design

Brassboard results– 2500 Hz– 1.7 mJ– 4.3 ns pulse width

30 cm cavity optimization results for 120 W peak pumping

Pump Wavelength (nm)

Pump Rep-rate (Hz)

Pump Pulsewidth

(us)O.C %

Pump Power (W)

1064 nm Power (W)

1064 nm Energy (mJ)


(%)

Pulsewidth (ns)

880 2500 58 30 17.55 4.3 1.72 24.50 5.15880 2500 63 30 19.06 4.46 1.78 23.40 5.16880 2500 58 40 17.55 4.32 1.73 24.62 4.54880 2500 63 40 19.06 4.46 1.78 23.40 4.56880 2500 68 40 20.58 4.6 1.84 22.35 4.57880 2500 70 40 21.18 4.65 1.86 21.95 4.50880 2500 58 45 17.55 4.20 1.68 23.93 4.26880 2500 58 50 17.55 3.96 1.58 22.56 4.36


Future Work

Proposed as a NASA Phase 2 SBIR Injection seeding

– Modified ramp & fire approach– Scale to > 2 kHz

Power scaling– End pumped amplifier– Derived from ICESat-2 and Phase

1 designs Field hardened packaging

– Sealed for high altitude use– Dual compartment– Separate electronics module

Suitable for multiple near and longer term applications

– HSRL 1 transmitter replacement– Hurricane & Severe Storm

Sentinel transmitter– Next generation aerosol lidars– Pump for methane lidar– Pump for ozone lidar


Acknowledgements

Support for this work was provided by Goddard Space Flight Center through the Laser System Services Engineering contract and the NASA SBIR office.

Documents

LWG August 2010 High Efficiency Laser Designs for Airborne and Space-Based Lidar Applications F. Hovis, R. Burnham, M. Storm, R. Edwards, J. Edelman, K