Nicolas Cézard, Simon Le Méhauté, Philippe Benoit, Fiber-based coherent DIAL/Doppler Lidar at 1645 nm for remote sensing of methane plumes Nicolas Cézard, Simon Le Méhauté, Philippe Benoit, Didier Goular, Didier Fleury, Julien Le Gouët, Christophe Planchat, Matthieu Valla, Agnès Dolfi-Bouteyre, Xavier Watremez

Motivations CH4 gas blowouts Spectroscopy Laser design Lidar results CH4 gas blowouts Elgin platform (North sea) Blowout in March 2012 51 days gaz leak 238 workers evacuated ≈ 6000 tons CH4 released Aliso Canyon (California) Blowout in October 2015 118 days gaz leak 11.300 people evacuated ≈ 80.000 tons CH4 released N.Cézard, ONERA, France - CLRC 18th

Related risks in case of leaks Motivations Spectroscopy Laser design Lidar results Improving CH4 leak rate monitoring is a priority for better decision making CH4 properties Related risks in case of leaks Explosive gas Human injuries Greenhouse gas Environmental pollution Energy source (natural gas) Financial losses Passive cameras have assets but also weaknesses Thermal contrast between gas plume and background Camera performance High Good Low Poor The plume is invisible if thermal contrast is zero Camera only get column-integrated concentrations Quantification requires hyper/multi spectral cameras Aliso Canyon, Dec, 2015, Image/Environmental Defense Fund N.Cézard, ONERA, France - CLRC 18th

Opportunity for local flux measurement by lidar Motivations Spectroscopy Laser design Lidar results Assets of a Lidar measurement Independent from thermal contrast Range-resolved Quantification possible with DIAL Lidar Wind measurement with Doppler Lidar Opportunity for local flux measurement by lidar Wind speed m/s Scanned lidar surface Our goal To design and build a single Lidar with dual function CH4 & Doppler for local flux measurements N.Cézard, ONERA, France - CLRC 18th

Laser & Lidar design Specifications Motivations Motivations Spectroscopy Laser & Lidar design Laser design Results Lidar results Specifications Monitoring requirement System requirement Laser requirement Compact/Robust technology for field experiments (shipborne, airborne…) Fiber system Fiber laser Doppler measurement Coherent detection* Narrow linewidth Low M² Polarized CH4 measurement Laser wavelength on CH4 absorption band 1.6 µm laser Range-resolved <100m Pulsed Pulsed < 650 ns Long range 1km High Peak power Highest possible Peak power High rep. rate (limit 150 kHz) *Bonus: coherent detection also allows high solar rejection for day operations N.Cézard, ONERA, France - CLRC 18th

Silica fiber transparency Motivations Spectroscopy Laser design Lidar results CH4 atmospheric spectroscopy Cross section threshold 10-5 ppm-1.m-1 threshold Absorption bands Silica fiber transparency 3,3 µm 8 µm 2,4 µm 1,6 µm The 1.6 µm band is selected for its high cross section and compatibility for fiber operation N.Cézard, ONERA, France - CLRC 18th

Concentration Bias induced by 1°K error Motivations Spectroscopy Laser design Lidar results 1.6 µm CH4 absorption band Temperature sensitivity Line Concentration Bias induced by 1°K error 1 0.41% 2 0.44% 3 0.47% 4 Line at 1645.54 nm is selected for its high intensity and low temperature dependance N.Cézard, ONERA, France - CLRC 18th

1645.54 nm CH4 line spectroscopy Motivations Spectroscopy Laser design Lidar results 1645.54 nm CH4 line spectroscopy ON 30.000 ppm.m Optical Depth OFF Noticeable H2O absorption line on the left side of the CH4 line H2O contribution can be cancelled using the OFF-line wavelength N.Cézard, ONERA, France - CLRC 18th

The challenge is to amplify laser light at 1645 nm Motivations Spectroscopy Laser design Lidar results Master Oscillator Fiber Power Amplifier (MOFPA) A miniaturized laser seeder (DFB laser diode at 1645 nm / 1 nm tunability) Narrow linewidth (≈ 1 MHz) Polarized Continuous wave Low power (20 mW) Commercially available One or several fiber amplifiers Not commercially available at 1645 nm for high power pulse operation … 10 µJ 1 µJ The challenge is to amplify laser light at 1645 nm N.Cézard, ONERA, France - CLRC 18th

Motivations Spectroscopy Laser design Lidar results Amplifying light at 1645 nm Erbium-doped fiber amplifiers ? Silica fiber Raman gain 1645 nm Erbium cross section 1645 nm Consider Raman amplification Maximum Gain at 13 THz Raman shift Raman frequency shift (THz) Raman amp. needs high power pump  Optimal Raman amp. @1645 nm is obtained with a 1535 nm pump (EDFA efficiency !) No Erbium absorption at 1645 nm EDFA-based high power 1535 nm pump Optimal Raman Amplification of a 1645 nm seeder N.Cézard, ONERA, France - CLRC 18th

Stimulated Brillouin Scattering (SBS) Motivations Spectroscopy Laser design Lidar results Stimulated Brillouin Scattering (SBS) SBS limits apply both to 1535 nm pump wave AND 1645 nm signal wave wavelength Brillouin scattering (~ 10 GHz) Raman scattering (~ 13 THz) λI Signal 1645 nm λS Stokes Brillouin Signal λSP Stokes Brillouin Pump λP Pump 1535 nm Maximum peak power How to increase Pmax ? Consequence Increase effective area Aeff  Beam quality would decrease (multimode fiber) Decrease fiber length  Raman net Gain would decrease Decrease Brillouin gain For the 1535 nm pump : using laser spectral broadening For the 1645 nm signal : using Brillouin spectrum broadening (mechanical strain applied to the fiber - poster yesterday by L.Lombard) N.Cézard, ONERA, France - CLRC 18th

Two-stage Raman power amplifier architecture Motivations Spectroscopy Laser design Lidar results Two-stages MOFPA architecture Two-stage Raman power amplifier architecture P.Benoit & al, Proc. SPIE 9728, Fiber Lasers XIII (Photonics West 2016) Pulse profile Spectral profile N.Cézard, ONERA, France - CLRC 18th

The designed fiber lidar is compliant with our objectives Motivations Spectroscopy Laser design Lidar results Laser performance Properties Requirement Value Wavelength 1,6 µm band 1645,5 nm (ON-line) Pulse duration Pulsed <650 ns 100 ns Pulse Energy Highest possible 15 µJ (≈150 W peak power) Pulse Repetition rate High but < 150 kHz 20 kHz (0,3 W cw) Pulse Spectral width Narrowest possible (Fourier limited) 10-12 MHz Polarized Yes Yes (PM fiber) Beam quality Low M² Monomode fiber M²=1 The designed fiber lidar is compliant with our objectives N.Cézard, ONERA, France - CLRC 18th

Combined DIAL-Doppler coherent architecture Motivations Spectroscopy Laser design Lidar results Combined DIAL-Doppler coherent architecture A classical coherent lidar architecture for Doppler function Two alternated seeders for CH4 line (ON-OFF or step-scan mode) N.Cézard, ONERA, France - CLRC 18th

Motivations Spectroscopy Laser design Lidar results VEGA Lidar prototype * VEGA = Vent-Gaz (wind & Gas) Seeders Pump amplifier Raman amplifier Raman pre-amp. Controls and supply Lidar detection N.Cézard, ONERA, France - CLRC 18th

Dual-function demonstration Motivations Spectroscopy Laser design Lidar results Dual-function demonstration Range-resolved wind measurements (aerosols) Integrated Path CH4 measurements using a hard target at 2.25 km Wind measurement on aerosols IPDA CH4 / H2O measurement Hard target Multiple wavelength scan Aerosols Hard target CNR vs. distance 17-Wavelength sequence OFF line ON lines H2O CH4 N.Cézard, ONERA, France - CLRC 18th

(barycenter estimator) Motivations Spectroscopy Laser design Lidar results Wind results CH4 results Wind measurement range vary with aerosol load 0,2 km < Range < 1.7 km have been observed during past weeks We compute corrected CNRON/CNROFF ratios Mixing ratios are obtained with a non linear fit Signal spectrogram (80 MHz baseband) Estimated wind (barycenter estimator) N.Cézard, ONERA, France - CLRC 18th

(barycenter estimator) Motivations Spectroscopy Laser design Lidar results Wind results CH4 results Wind measurement range vary with aerosol load 0,2 km < Range < 1.7 km have been observed during past weeks We compute corrected CNRON/CNROFF ratios Mixing ratios are obtained with a non linear fit Signal spectrogram (80 MHz baseband) Estimated wind (barycenter estimator) N.Cézard, ONERA, France - CLRC 18th

(barycenter estimator) Motivations Spectroscopy Laser design Lidar results Wind results CH4 results Wind measurement range vary with aerosol load 0,2 km < Range < 1.7 km have been observed during past weeks We compute corrected CNRON/CNROFF ratios Mixing ratios are obtained with a non linear fit Relative error ≈10-15% Significant random error for each point because of speckle pattern time correlation Signal spectrogram (80 MHz baseband) Estimated wind (barycenter estimator) First demonstration of simultaneous Wind and multi-gas IPDA CH4-H2O measurements Further quality checks will be done using reference instruments for comparison N.Cézard, ONERA, France - CLRC 18th

Conclusion and Outlooks We have demonstrated: A new fiber laser at 1645 nm, emitting 15 µJ, 20kHz, 100 ns pulses, narrow-spectrum, polarized, and spatially monomode An innovative coherent lidar architecture allowing simultaneous wind and absorption spectroscopy (DIAL or IPDA, 2 wavelengths or more) First results clearly demonstrate the lidar double capacity Perspectives include: Making more lidar tests, measurement cross-checking, error evaluations Improving Lidar integration (to gain size, mass, mobility…) Testing Range-resolved CH4 in a future campaign (gas release tests) A new possible tool w.r.t cross-validation efforts of MERLIN satellite (same l) N.Cézard, ONERA, France - CLRC 18th

The Lidar and Laser group at Onera (2017) N.Cézard, ONERA, France - CLRC 18th