1. 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

2. 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
people evacuated
≈ tons CH4 released
N.Cézard, ONERA, France - CLRC 18th

3. 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

4. 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

5. 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

6. 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

7. 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 nm is selected for its high intensity and low temperature dependance
N.Cézard, ONERA, France - CLRC 18th

8. 1645.54 nm CH4 line spectroscopy
Motivations
Spectroscopy
Laser design
Lidar results
nm CH4 line spectroscopy ON
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

9. 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

10. 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 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. (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

18. (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

19. (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

20. 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

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