High Efficiency Laser Designs for Airborne and Space-Based - PowerPoint Presentation

Slide1

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

Slide2

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 updatesSlide3

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 M

2

< 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 challengeSlide4

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 sPulsewidth< 1.5 nsRepetition Rate10 ±0.3 kHzLinewidth/Wavelength Stability85% transmission through 30 pm filterPolarization Extinction Ratio> 100:1Spatial ModeM2 < 1.6, GaussianBeam Diameter15 mm limiting apertureBeam Divergence< 108 µradPointing Stability (shot-to-shot)< 21.6 µrad (RMS) over 1 sPointing Stability (long-term)< 100 µradLifetime5 years plus 60 days on orbitMass20 kgVolume (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 updatesSlide5

Bulk Solid State Transmitter

Design Overview

Considered multiple design options

All bulk solid-state

All fiber

Hybrid

Fiber front end

Final bulk solid state amp

Final choice was schedule drivenNeed a TRL 6 laser by February 2011Settled on all bulk solid-state approachShort pulse Nd:YVO4 oscillatorNd:YVO4 preampNd:YVO4 power ampHigh brightness 880 nm fiber coupled pump diodesBetter mode overlapLower thermal loadingTransmitter Optical Schematic532 nm outputSlide6

Short Pulse Oscillator

Nd:YVO

4

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 pulsewidthLow timing jitterHigh brightness 880 nm fiber coupled pump diodesBetter overlap with TEMoo modeLower thermal effects than 808 nmEOQ-Switch

Conduction Cooled

Diode Array

Pump Source

Composite YVO

4

rod with HR

Fiber

Coupling

Optics

/4

Output coupler

1 µm polarizer

880 nm HRSlide7

Typical Short Pulse Oscillator

Performance

Beam profile at output coupler

X diameter = 291 µm

Y diameter = 295 µm

Parameter

Laser Performance

Pulse Energy

146 µJPulse Energy Stability2.7% RMS over 1 sPulse Width.98 nsRepetition Rate10 kHzPulse Interval Stability< 0.01 µsCenter Wavelength (IR)1064.14 nmSpatial ModeM2x - 1.2, M2y - 1.2Pointing Stability (shot-to-shot)0.43% of divergence Pointing Stability (1 hour)0.53% of divergenceSlide8

Oscillator 1064nm Linewidth

Oscillator is linewidth narrowed

Analyzer etalon resolution is 4.9 pm8 mm etalon

Reflectivity finesse 14

Linewidth = 5.9 pm

8Slide9

Oscillator/Preamp Results

M

2

= 1.3

Total output energy – 470 µJ

Extracted energy – 357 µJ

Pump power @ 10kHz 14.5 W

Optical to optical efficiency 24.6%Slide10

Amplifier 1064 nm Performance

Most sensitive parameter is pump/seed overlap

Mode matching in amplifier is key to high efficiencySlide11

Bulk Solid State Output vs. Total Diode Pump PowerSlide12

Bulk Solid-State Optical to Optical Efficiency vs. Total Diode Pump PowerSlide13

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

Amp pump

Power

(W)

532 nm laser power

M

x

2

My24012.61.1841.2724012.61.1421.1793210.51.091.1247.61.191.1164.51.031.0482.21.0151.032Beam quality improves at lower amp pump powersSlide14

532nm Laser Power (W)

M

x2

M

y

2

12.9

1.11

1.097.31.111.145.61.101.13M2 data at 532 nm with P=12.9WBeam at focus at 532nm with P=12.9WBulk Solid-State 532 nm Beam Quality vs. OutputPower Varied by Amp DelaySlide15

Solid State Brassboard Full Transmitter

Performance Summary

Laser meets specifications for Energy: achieved 12.9W at 532nm68% 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: ~ 1.2532 nm pulsewidth: <1.3ns532 nm linewidth: <16 pm with etalon OCInstrument limitedFully linewidth narrowed oscillator not yet incorporatedPointing stability at 1064nm: 2% of the divergenceSlide16

Bulk Nd Solid State vs. Hybrid

Hybrid

AdvantagesSingle frequency with DFB/DBR stability

Pulse width selectable, 300 ps to 1.5 ns

High pulse format flexibility

Extremely stable T

o

triggering

Fibertek environmental data looks very goodUse of bulk solid state amp allows easy energy scalingChallengesYb Parts supply chain is immature.Very select vendors produce good parts in any reliable manner.High parts countBulk solid state Nd LaserAdvantagesMature technology - supply chain, materials selections, cleaning & bake out proceduresClear design margin identification and optical damage design rulesSimplest and lowest cost to produce.Smaller and lower weightChallengesLinewidth not single frequency BUT has substantial optical damage margin and can get high transmission through 30 pm etalon (532 nm)Slide17

Yb Fiber-MOPA Architecture

Multi-stage 1-

mm pulsed seeder

–

B

ased on established architecture at Fibertek

Uses COTS fiber-optics only

Final stage amplification to 300-400 uJ/pulse

1064nm Seed2X 6/125mm YDFA10/125mm YDFA30/250mm YDFAend-cap400uJ(4W)10uJ(0.1W)0.1nJ (1uW)1 nsec/10kHzpulse-carving500nJ (5mW)100mw cw1-mm Pulsed Seeder (1nsec/10kHz)M Z MAOMSlide18

Yb Fiber Temporal Waveforms

3

rd

stage

Final stage

3.07 W average power demonstrated from final stage

900 ps pulseSlide19

Yb Fiber Beam Quality Measurement

M

2

~ 1.25 @ 300 µJ, 0.9 ns

M

2

x

= 1.10

M2y = 1.35Slide20

Hybrid Summary

Successfully demonstrated all fiber amplifier front end

All work done with residual in-house fibers300 µJ0.9 ns

M

2

~ 1.3

Final bulk amplifier demonstrated

19 W output for 5 W input @ 10 kHz

M2 ~ 1.3Need to increase fiber front end to 500 µJ Achievable with new custom fiberNot compatible with ICESat-2 schedulePromising approach for future systemsSlide21

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 SBIRPhase 1 SBIR led to contract for Laser Vegetation Imaging Sensor – Global Hawk (LVIS-GH) lidar transmitterBrassboard short pulse ring oscillator1064 nm output End pumped Nd:YVO4 or Nd:YAGFiber coupled 880 nm pump1064 nm output Slide22

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 summarySlide23

40 cm Cavity Nd:YVO

4

ResultsNd:YVO

4

has lower storage efficiency but higher gain

100 µs lifetime

Higher absorption

Shorter pulsewidths

Reduced thermal effects relative to Nd:YAG1% doping gave slightly higher efficiencies35% optical to optical efficiency1 mJ/pulseScalable to at least 8 kHz (8 W average power)M2 = 1.1Highest energy results for 120 W peak pumping880 nm pumping results @ 2500 HzNear field output beam profile M2 dataM2 = 1.1Slide24

Approach for LVIS-GH

Requirements

1.5 mJ

3-6 ns

2500 Hz

Approach

Nd:YVO

4

Higher efficiencyShorter pulse width30 cm cavity LVIS-GH requires 3-6 ns pulsewidthDual compartment sealed canisterLow distortion in high altitude environmentDerived from TWiLiTE designBrassboard results2500 Hz1.7 mJ4.3 ns pulse width30 cm cavity optimization results for 120 W peak pumpingSlide25

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 packagingSealed for high altitude useDual compartmentSeparate electronics moduleSuitable for multiple near and longer term applicationsHSRL 1 transmitter replacementHurricane & Severe Storm Sentinel transmitterNext generation aerosol lidarsPump for methane lidarPump for ozone lidarSlide26

Acknowledgements

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