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Lidar for NASA Applications
T. Y. Fan (MIT LL) and Upendra Singh (NASA)
Sensors and Instrumentation Webinar
Aug. 18, 2020
This material is based upon work supported by the National Aeronautics and Space Administration under Air Force Contract No. FA8702-15-D-0001. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Aeronautics and Space Administration .
DISTRIBUTION STATEMENT A. Approved for public release. Distribution is unlimited.
S&I Webinar- 2TYF 08/02/20
Outline
• Introduction to active optical systems, primarily lidars, for NASA applications– Active optical systems characterized by use of laser source
• Example development – water-vapor lidar transmitter
Active Optical Remote Sensing Technologies
AORS Need: National need exists for reliable, efficient, space-capable AORS systems for civilian and defense applications in the area of Earth sciences, planetary exploration, aviation safety, chemical and biological detection, and tactical imaging. Core technology developments for these applications are not addressed by industry suppliers because of limited market.
Unique AORS Capabilities:- High resolution profiling capability for atmospheric trace species- High precision tropospheric wind measurements-Wavelength specificity for chemical and biological detection- Altimetry for surface mapping, Ocean mixed layers, ice topography
AORS Applications:-Weather and severe storm prediction (winds, humidity)- Atmospheric chemistry, climate and radiation (ozone, aerosols, clouds)- Carbon cycle (CO2, biomass)- Surface mapping (ocean, land, ice)- Space science (planetary exploration, space transport, communication) - Chemical and biological agent detection (Homeland Security, DoD)
AORS Strategies
ØLaser based instruments are applicable to a wide range of NASA’s Earth Science, Planetary Science, Aeronautics, and Human Explorations and Operation Mission Directorate needs
ØRisk in lidar missions can be significantly reduced by progress in a few key technologies
ØModest NASA investment towards proposed strategy will have significant impact on future space-based active remote sensing missions
ØStrategic alliance with other government organizations, industry, and academia for leveraging and accelerating advancement of key technologies
LIDAR - LIght Detection And Ranging
Lidar is analogues to Radar, where lightwaves, instead of radiowaves, are sent into the atmosphere and returns are collected which contains the information about the interacting atmospheric constituents, their microphysical properties and profile.
Lidar is an active optical remote sensing technique able to provide measurements with a very high resolution in time and altitude
6
LASER
TELESCOPE
PMT
Time
Atmosphere
MONFI
PMTMONFI
Time
AcquisitionDetectionSpectral selection
Pin-hole
Collimation lens
Lidar Techniques
Backscatter Lidar• Cloud • Aerosol• Ocean
Differential Absorption Lidar (DIAL)• O3, H2O (profile)• CO2, CH4 (column)
Doppler Lidar• Wind Fields
Altimetry Lidar• Ice Sheet Mass Balance • Vegetation Canopy• Land Topography
fDopplerFrequency
TransmitPulse
Return
Velocity = (l/2) fDoppler
TArrivalTime
TransmitPulse
Return
Range = (c/2)TArrival
TArrival Time
TransmitPulse
Return
Density = IS/ITRange = (c/2)Tarrival
IT
IS
loff lon
TransmitPulses
Returns
Concentration = log[ I(lon)/ I(loff)]
Wavelength
Active Optical Measurementsin the Earth Sciences
Atmospheric Water Vapor
River Stage Height
Water & Energy Cycle
Land Surface Topography
Surface Deformation
Terrestrial Reference Frame
Earth Surface & Interior
Biomass
Vegetation Canopy
Fuel Quality & Quantity
CO2 & Methane
Trace Gas Sources
Land Cover & Use
Terrestrial & Marine Productivity
Carbon Cycle & Ecosystems
Aerosol Properties
Total Aerosol Amount
Cloud Particle Properties
Cloud System Structure
Ozone Vertical Profile & Total Column Ozone
Surface Gas Concentrates
Atmospheric Composition
Tropospheric Winds
Atmospheric Temperature and Water Vapor
Cloud Particle Properties
Cloud System Structure
Storm Cell Properties
Weather
Ocean Surface Currents
Deep Ocean Circulation
Sea Ice Thickness
Ice Surface Topography
Climate Variability
Doppler
Active Optical Measurementsin the Earth Sciences
Atmospheric Water Vapor
River Stage Height
Water & Energy Cycle
Land Surface Topography
Surface Deformation
Terrestrial Reference Frame
Earth Surface & Interior
Biomass
Vegetation Canopy
Fuel Quality & Quantity
CO2 & Methane
Trace Gas Sources
Land Cover & Use
Terrestrial & Marine Productivity
Carbon Cycle & Ecosystems
Aerosol Properties
Total Aerosol Amount
Cloud Particle Properties
Cloud System Structure
Ozone Vertical Profile & Total Column Ozone
Surface Gas Concentrates
Atmospheric Composition
Tropospheric Winds
Atmospheric Temperature and Water Vapor
Cloud Particle Properties
Cloud System Structure
Storm Cell Properties
Weather
Ocean Surface Currents
Deep Ocean Circulation
Sea Ice Thickness
Ice Surface Topography
Climate Variability
Altimetry
Active Optical Measurementsin the Earth Sciences
Atmospheric Water Vapor
River Stage Height
Water & Energy Cycle
Land Surface Topography
Surface Deformation
Terrestrial Reference Frame
Earth Surface & Interior
Biomass
Vegetation Canopy
Fuel Quality & Quantity
CO2 & Methane
Trace Gas Sources
Land Cover & Use
Terrestrial & Marine Productivity
Carbon Cycle & Ecosystems
Aerosol Properties
Total Aerosol Amount
Cloud Particle Properties
Cloud System Structure
Ozone Vertical Profile & Total Column Ozone
Surface Gas Concentrates
Atmospheric Composition
Tropospheric Winds
Atmospheric Temperature and Water Vapor
Cloud Particle Properties
Cloud System Structure
Storm Cell Properties
Weather
Ocean Surface Currents
Deep Ocean Circulation
Sea Ice Thickness
Ice Surface Topography
Climate Variability
DIAL
Active Optical Measurementsin the Earth Sciences
Atmospheric Water Vapor
River Stage Height
Water & Energy Cycle
Land Surface Topography
Surface Deformation
Terrestrial Reference Frame
Earth Surface & Interior
Biomass
Vegetation Canopy
Fuel Quality & Quantity
CO2 & Methane
Trace Gas Sources
Land Cover & Use
Terrestrial & Marine Productivity
Carbon Cycle & Ecosystems
Aerosol Properties
Total Aerosol Amount
Cloud Particle Properties
Cloud System Structure
Ozone Vertical Profile & Total Column Ozone
Surface Gas Concentrates
Atmospheric Composition
Tropospheric Winds
Atmospheric Temperature and Water Vapor
Cloud Particle Properties
Cloud System Structure
Storm Cell Properties
Weather
Ocean Surface Currents
Deep Ocean Circulation
Sea Ice Thickness
Ice Surface Topography
Climate Variability
Backscatter
LIDAR is a Multi-Enterprise Need
Turbulence detection Wind shear detection
Wake vortices
Science
Aeronautics
Clouds/Aerosols
Tropospheric Winds
Ozone
Carbon Dioxide
Biomass Burning
Water Vapor
Surface Mapping
Laser Altimetry
Oceanography
Surface Topography
Molecular Species
Exploration Systems
Lander Guidance/ Control
Lander Hazardous Winds/Dust Avoidance
Mars Atmospheric Winds
Biological Elements (C, N, H, S, P)
Optical CommunicationSpacecraft Automatic
Rendezvous/Capture Wind Profiling for
Launch Vehicles
Enabling Technology ElementsLidar Technologies
Scanner
Receiver
AutoAlign
Pointing
Telescope
Detector
SMD ESMD
CO2 Profiling X
Global Winds X
Ozone Profiling X
Chem/Bio Sensing X
Landing/Rendezvous X
Water Vapor Profiling X
Laser Transmitter Technologies Measurements
XRanging/Altimetry X
Clouds/Aerosols X
Customers
X
2-Micron LidarTransmitter
FrequencyController
Amplifier
IRWavelength
Converter
UVWavelengthConverter
1-Micron LidarTransmitter
X
X
X
X
Surface Material/State XX
Ampl
S&I Webinar- 14TYF 08/02/20
TIM on Active Optical Systems
• Held on July 31 – Aug. 2, 2018• Purpose (from Proceedings intro)
“The TIM aimed at focusing NASA’s directions to attain the necessary TRLs to meet the Agency-level priority Active Optical measurements in Space and Aeronautics”
• Covered active optical systems for Earth science; planetary science; entry, descent, and landing; aeronautics; and optical communication
• Proceedings available from NASA Technical Report server– https://ntrs.nasa.gov/search.jsp– Search for 20200000065– Includes assessment and recommendations,
presentations, and written synopses of presentations
Technical Report Cover Page
S&I Webinar- 15TYF 08/02/20
Water Vapor is a Key Atmospheric Constituent
Cloud Formation Albedo Precipitation
Water vapor plays a key role in weather, radiative balance, atmospheric dynamics, surface fluxes, and soil moisture – 2018 NAS decadal survey recognizes global, high-resolution measurements will revolutionize understanding
S&I Webinar- 16TYF 08/02/20
Water Vapor Measurements for Weather and Climate Prediction
Satellite-based Passive SoundingSurface Measurements
Radiosondes Aircraft-Based Measurements
S&I Webinar- 17TYF 08/02/20
Water Vapor is a Key Atmospheric Constituent
• Passive IR and microwave sounders are the backbone of the numerical weather prediction and climate science communities
• They provide limited resolution and sensitivity in the lower troposphere (i. e., close to the Earth’s surface)
• Water vapor was identified as being synergistic and cross-cutting over 5 of the 6 NASA Decadal Survey science and applications priorities
• Water vapor differential absorption lidar (DIAL) was identified as a potential candidate for accurate and high-resolution water vapor profiles
IR Sounder
Airborne DIAL
NAS 2018 Decadal Survey
Water-Vapor Vertical Profile
Space-based water-vapor lidar has been of interest for over three decades
S&I Webinar- 18TYF 08/02/20
DIAL Concept
Ismail and Browell, in Encyclopedia of Atmospheric Sciences (2015)
S&I Webinar- 19TYF 08/02/20
Airborne Water-Vapor DIAL
LASE
HALO
• NASA’s Lidar Atmospheric Sensing Experiment (LASE) water vapor DIAL has been the community standard for high resolution measurements since the early 90s
• NASA LaRC has developed High Altitude Lidar Observatory (HALO), a replacement for LASE and serves as a testbed to vet technologies for future space-based missions
• Current laser transmitters are too inefficient and complex for space instrument
DIAL Instruments
Lidar data courtesy of A. Nehrir (NASA LaRC)
Water Vapor
Aerosol Backscatter
Water Vapor Dry UT/LS
Moist PBL
Clouds
Dry Tropospheric Folds
S&I Webinar- 20TYF 08/02/20
• Many water-vapor absorption bands – band around 820 nm is attractive for measurements– Approximately right strength for needed
measurement dynamic range– Efficient, low-noise Si detectors
• Laser transmitter is the challenge– High efficiency and low complexity for small SWaP– High power, good beam quality and pulsed operation
Wavelength and Transmitter
Laser Efficiency Power (W) ~PulseEnergy Complexity
Ti:Sapphire 2% 5 mJ 4 parts
Diode 50% 1 µJ 1 part
Tm:LiYF4 (YLF) 8% 50 mJ 2 parts
Er:YAG 4% 3-10 mJ 3 parts
Existing
Potential
820 nm Laser Survey
Tm:YLF
Ti:Sapphire
Schematic Setup
Diodes
Diodes
Diodes Nd 2x
Silicon Sensitivity and Laser Wavelengths
Er:YAGDiodes 2x
Wavelength (nm)600 800400 1000
Qua
ntum
Eff
icie
ncy
0.0
0.2
0.4
0.6
0.8
1.0
Tm Nd/Yb
2x Nd/Yb
2x Tm
Diodes
Ti: Sapphire
2XEr:YAG
Existing
A transmitter for space-based mission has been a challenge pursued for >3 decades
S&I Webinar- 21TYF 08/02/20
Small-Sat Water-Vapor Lidar
• NASA LaRC is developing a small-sat (ESPA class) water vapor lidar concept with baseline of doubled Er:YAG transmitter
• Relies on photon-counting receivers and high pulse repetition frequency (PRF) transmitters – Nominal goals are 3-4 mJ/pulse at 2–3 kHz PRF– Increasing pulse energy to 10-15 mJ would greatly
improve daytime performance
• Can Tm:YLF based transmitter provide relevant performance?– Potentially simpler and more efficient than Er:YAG– Er:YAG baseline demonstrated. Challenge is to scale
peak and average power. – Tm:YLF development funded under NASA Advanced
Components Technology (ACT) program
Water Vapor ProfilingPBL HeightsAerosol/Cloud ProfilingMethane Mapping
Atmospheric Boundary Layer Lidar PathfindEr (ABLE)
S&I Webinar- 22TYF 08/02/20
Tm3+ Laser Physics and Materials
• Many reports on efficient 1.9-µm lasers• Prior operation at ~820 nm largely limited to
Tm-doped fluorzirconate fibers• Intermediate states complicate dynamics
because Tm ions trapped in long-lived Level 2• Need to reduce Level 2 population
– Cryogenic operation reduces needed Level 3 population for operation, which reduces Level 2 population
– Choose host materials that have relatively large Level 3 lifetime
– Reduce Level 2 population by lasing on Level 2 –Level 1 transition at 1.9 µm
• Physics in part validated by low-power demonstrations in Tm:YAG and Tm:YLF
Tm3+ Energy LevelsLevel 3
Level 1
t~ 1 ms
t~ 10 ms
3H4
3H6
Level 23F4
Trapping
T. Y. Fan et. al., “Cryogenic Tm:YAG Laser in the Near Infrared,” IEEE J. Quant. Electron., vol. 51, 10 (2015)C. E. Aleshire et al., “Efficient cryogenic near-infrared Tm:YLF laser,” Opt. Express 25, 13408-13413 (2017)
S&I Webinar- 23TYF 08/02/20
Tm3+ Energy Levels
Co-lasing at 1.9 µm
Level 3
Level 1
t~ 1 ms
t~ 10 ms
3H4
3H6
Level 23F4
Trapping
• Many reports on efficient 1.9-µm lasers• Prior operation at ~820 nm largely limited to
Tm-doped fluorzirconate fibers• Intermediate states complicate dynamics
because Tm ions trapped in long-lived Level 2• Need to reduce Level 2 population
– Cryogenic operation reduces needed Level 3 population for operation, which reduces Level 2 population
– Choose host materials that have relatively large Level 3 lifetime
– Can reduce Level 2 population by lasing on Level 2 – Level 1 transition at 1.9 µm
• Physics in part validated by low-power demonstrations in Tm:YAG and Tm:YLF
T. Y. Fan et. al., “Cryogenic Tm:YAG Laser in the Near Infrared,” IEEE J. Quant. Electron., vol. 51, 10 (2015)C. E. Aleshire et al., “Efficient cryogenic near-infrared Tm:YLF laser,” Opt. Express 25, 13408-13413 (2017)
Tm3+ Laser Physics and Materials
S&I Webinar- 24TYF 08/02/20
• Laser resonator designed for simultaneous operation at 820 nm and 1.9 µm transitions– Force co-lasing to reduce population trapping
• CW slope efficiency is 46% (relative to incident power) using a Ti:S pump laser – approaching efficiency for Nd:YAG at 1.06 µm
• CW models predict output at 1.9 µm should be nearly constant with pump power, as observed
• Near-diffraction-limited output beam
Tm:YLF CW Results using Liquid Nitrogen Cooling
Tm:YLF Laser
0.2 0.4 0.6 0.8 1 1.20
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
Incident Power, W
Out
put P
ower
, W
1876 nm
816 nmTotal
Incident Power, W
Out
put P
ower
, W
Results validate cw models and confirm effects due to population trapping
Output Beam
Laser Performance
S&I Webinar- 25TYF 08/02/20
Thermo-optic Properties at Cryogenic Temperature
Temperature (K)
Properties of Undoped YAG
10
15
20
25
30
35
40
45
50
0
1
2
3
4
5
6
7
8
100 150 200 250 300
THER
MAL
CO
NDUC
TIVI
TY (W
/m K
)
CTE(ppm/K), dn/dT (ppm
/K)
TEMPERATURE (K)
UNDOPED YAG
Ther
mal
Con
duct
ivity
(W/m
K)
CTE (ppm/K), dn/dT (ppm
/K)
Favorable
Favorable
• Poor thermal conductivity k can inhibit heat removal, resulting in large temperature non-uniformity
• This changes the index, through dn/dT, and results in beam distortion
• Thermal expansion (CTE) creates stress which can cause depolarization and damage
Thermal effects limit average power and beam quality
dn/dT
beam distortion
CTE
stress induced birefringence
temperature non-uniformity
k
Cryogenic cooling significantly improves thermo-optic properties in crystalline dielectrics
S&I Webinar- 26TYF 08/02/20
Tm:YLF Spectra
Absorption matches efficient diode laser pumps and emission matches water-vapor absorption lines in 812-817 nm range
300 K
80 K
80 K
300 K
Pump
Absorption Emission
Laser
S&I Webinar- 27TYF 08/02/20
Tm:YLF Lidar Transmitter Attributes
• Pulsed operation – few kHz pulse repetition frequency (PRF) with pulse width in the < 1 µs range (sets vertical resolution)
• Pulse-to-pulse wavelength agility to move on/off water vapor absorption line• Narrow linewidth (~ <100 MHz) and high spectral purity compared with water-vapor absorption features• Relatively simple and efficient (including power required for cryocooler)
Tm:YLF20 W laser
output
Cryocooler
10 W thermal (80 K reservoir)
160 W electrical
Pump Laser 50 W
pump
100 W electrical
20 W fluorescence and pump (300 K reservoir)
170 W thermal (300 K reservoir)
50 W thermal (300 K reservoir)
Average Power Accounting Notional Performance Goals
Attribute GoalPulse Energy >10 mJPRF 2–3 kHz
Wavelength agility Switch between pulsesSpectral purity >99.9%Electrical efficiency >5%
Key is to demonstrate temporal waveforms, average power, and efficiency to serve as existence proof
S&I Webinar- 28TYF 08/02/20
Cryogenic Cooling
• Pros– Experimentally simple– Allows easy calorimetry by nitrogen boil-off rate
• Cons– Fixed temperature– LN2 consumable
• Pros– Only consumable is electricity– Allows easier temperature variation
• Cons– More challenging integration– Hard to do accurate calorimetry
Liquid Nitrogen Cryostat Stirling Cooler
Using LN2 cryostat for development but rely on Stirling cooler for space application
S&I Webinar- 29TYF 08/02/20
Laser Resonator Configuration
• Standard resonator configuration for electro-optic Q-switching
Output Coupler
Tm:YLF
High Reflector
Pump Source (781 nm)
PockelsCell
Dot Reflector
816 nmoutputl/2 plate
LN2 Dewar
QWPPolarizer
S&I Webinar- 30TYF 08/02/20
Tm:YLF Tuning Proof-of-Principle using an Etalon
0.00E+00
1.00E-05
2.00E-05
3.00E-05
4.00E-05
5.00E-05
6.00E-05
7.00E-05
8.00E-05
9.00E-05
814 814.5 815 815.5 816
Pow
er (a
u)
Wavelength (nm)
Output Spectra with Varying Etalon Angle
Deg 0
Deg .5Deg 1
Deg 2Deg 2.5Deg 3
Deg 4Deg 5
Deg 6
~2 nm of tuning demonstrated, limited by etalon thickness – shows ability to access multiple water-vapor absorption lines
S&I Webinar- 31TYF 08/02/20
Q-Switched Operation at High PRF
• Burst mode operation (4 ms pump pulses at 50 Hz), net 20% pump duty cycle
• 5.4 W average power with ~7.2 mJ/pulse (4 kHz Q-switched PRF, ~750 pulses per s)
• 23% optical and 31% slope efficiency – Appears to be some rolling over at higher output
Tm:YLF Output Waveform for a Single Pump Pulse
31% Slope
1.1 µs (FWHM)
Time (ms)
Output Power
S&I Webinar- 32TYF 08/02/20
Beam Characteristics
• Beam is sampled in near field through HR mirror– Average output power is 5.8 W –
maximum achieved in this demonstration
– 4 ms pump pulses at 50 Hz
• Near TEM00, Gaussian beam shape
• No evidence of thermo-optic effects –beam appears the same near threshold
Horizontal Vertical
Beam Image
Cryogenic operation enables excellent beam quality, as expected
S&I Webinar- 33TYF 08/02/20
Continuing Development
• Improve efficiency and power– Use lower doped Tm:YLF gain elements– Reduce intracavity losses – lower loss components and reduce water vapor inside resonator
• Implement more lidar transmitter functionality– Single-frequency operation– Stirling-cooler head implementation– Continuous 2 – 3 kHz PRF waveform– On-line, off-line operation with high spectral purity
• Brassboard laser for use in airborne platform to increase TRL• Other, non-transmitter challenges
– Single-photon detector arrays, particularly pushing photon detection efficiency– Narrowband filters, wavelength-agile filters of particular interest
• Space-based measurements starting a decade from now?
Results to date are promising but more needs to be done
S&I Webinar- 34TYF 08/02/20
Acknowledgements
Lincoln Laboratory
Chris AleshireSteve Augst
Merlin HoffmanLeo MissaggiaPeter O’BrienJuan Ochoa
Patricia ReedCharles Yu
NASA LaRC
Amin Nehrir
S&I Webinar- 35TYF 08/02/20
© 2020 Massachusetts Institute of Technology.
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