At NASA Goddard Space Flight Center, we have been developing a laser-based technology needed to remotely measuremethane (CH4) from orbit. Our lidar transmitter is based on an optical parametric process to generate near infrared laserradiation at 1651 nm, coincident with a CH4 absorption. In an airborne flight campaign in the fall of 2015, we tested two kindsof laser transmitters --- an optical parametric amplifier (OPA) and an optical parametric oscillator (OPO). The two lasertransmitters were successfully operated in the NASA’s DC-8 aircraft, measuring methane from 3 to 13 km with high precision.

Kenji Numata1, Haris Riris1, Stewart Wu1, Brayler Gonzalez1, Michael Rodriguez2,William Hasselbrack2, Molly Fahey1, Anthony Yu1, Mark Stephen1, Jianping Mao3, Stephan Kawa1

1NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA2Sigma Space, Inc., 4600 Forbes Blvd, Lanham-Seabrook, Maryland 20706, USA

3University of Maryland, College Park, Maryland 20742, USA

Airborne measurements of atmospheric methaneusing pulsed laser transmitters

�Background

�Burst-mode OPA �Seeded OPO

Overview

Laser sourceA

irborne results

�Pump laser (1064nm)o Yb-fiber amplifier, LMA fiber, built by Fiberteko Burst mode, 20 micro pulses, 3ns micro pulse widtho Works with low power (~20mW) seedo Minimizes output linewidth broadening

�Nonlinear crystalo 50mm MgO:PPLN

�Scanning seed lasero Beat against master laser for wavelength monitor

�2015 CH4 airborne campaign �Aircraft: NASA DC-8 (NA817)

o 1 engineering & 2 science flights, total ~12 hourso From Armstrong Flight Research Center, CA

�Telescope: 20cm, 300µrad field of viewo Transmitter divergence: ~150µrad

�Detector: DRS eAPD, 90% QE, ~109V/W�Compare OPA-OPO performance

�Output energyoReduced to ~40µJ per burst (not enough for space)oDue to several simplifications for the airborne demonstration

�Linewidth: ~500MHz Estimated from CH4 reference cell�Number of wavelength: 20 Step scanned across the line @ 10kHz

�Methane measurements for earth scienceo Strong greenhouse gas (>x20 radiative forcing than CO2)o Closing the carbon budget, global coverageo Methane hydrate in the Arctic (where passive spectrometer won’t work)

�Requirements for space instrumento Wavelength: ~1.65µm (outside fiber amplifier band)o Energy: >~250µJ (for 1% error, 10kHz rep. rate) Science Flight 1

Science Flight 2

Eng. Flight

OPA

OPO

Tx & Rx Optics

OPA/OPO

Transmit Optics

Electronics

O ���1.65 µm

To surfaceReflection from surface

O1…ON

Trace Gas (CH4 )Absorption

Detector & Filters

Receiver Optics

Pump Laser 1.06 µm

Seed Laser 1.65 µm

Transmitter Receiver

Flight demonstration on DC-8

~0.4%

�Pump laser (1064nm)o Seeded, active Q-switch, Nd:YAG laser built by NASA/GSFCo Single pulse, ~1.5mJ, ~60ns pulse widtho Works with low power (~20mW) seed

�Nonlinear crystalo 35mm MgO:PPLN

�4 slave seed laserso Optical PLLo Fast optical switch

Beam expander

Filter OPO optics

Pump laser

�Output energyo~240µJ (satisfying requirement for space) @ 10kHzoToo much energy for the airborne demonstration

�Linewidth: <~100MHz�Number of wavelength: 5�OPO cavity control

o Phase modulationo Mirror on PZTo Temperature control

~0.5%

�Analysis overviewo 1s averaging, uniform 1900ppb modelo No DRS non-linearity correction yeto ~0.5% error for the best ~9 min sectiono Stable output energyo Detector gain minimized at low altitude

�Problems identifiedo Detector saturation (too high energy)o Cavity unlock (to be improved)

�Analysis overviewo 1s averaging, uniform 1900ppb modelo ~0.4% error for the best ~20min sectiono Stable signal up to the highest altitude (~13km)

�Problems identifiedo Power stability (unstable LMA fiber mode)o Low output energy, wide linewidtho Retrieval with cloud return

Measurement conceptMonthly Global Map of the CH4 column-averaged volume

mixing ratios from GOSATGlobal average methane concentration in Earth’s

atmosphere GSFC methane sounder team Lidar instrument in DC-8’s cabin Flight paths

Methane lidar instrument

Burst-mode OPA concept

Burst pump generation

Linewidth broadening by OPA

Pump & signal burst pulsesConceptual design of our seeded OPO 5 phase-locked seed lasers

OPO flight breadboard

Output energy of our OPO

Linewidth estimation with CH4 cellOPA flight breadboard Linewidth estimation with CH4 cell Output energy of our OPA (per burst)

Output signal beam shape

Measured CH4 mixing ratio & in-situ monitor

Lock status during flight (0 = all locked)

Output energy during flight

Typical Tx/Rx pulse shape

OD vs range plot

Best OPO flight section (~13km altitude)

Typical Tx/Rx pulse shape OD vs range plot

Output energy during flight

Output wavelength during flight

Measured CH4 mixing ratio & in-situ monitor

Best OPA flight section (~13km altitude)