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Optical Parametric Technology for Methane Measurements 

Martha Dawsey 1 * 

Kenji Numata 2 
Stewart Wu 1 
Haris Riris 1 

1 NASA- Goddard Space Flight Center, Greenbelt, MD 20771, USA 
2 Department of Astronomy, University of Maryland, College Park, MD 20742, USA 

Corresponding author: 


Atmospheric methane (CH 4 ) is the second most important anthropogenic greenhouse gas, with approximately 25 
times the radiative forcing of carbon dioxide (CO 2 ) per molecule. Yet, lack of understanding of the processes that 
control CH 4 sources and sinks and its potential release from stored carbon reservoirs contributes significant uncertainty 
to our knowledge of the interaction between carbon cycle and climate change. 

At Goddard Space Flight Center (GSFC) we have been developing the technology needed to remotely measure CH 4 
from orbit. Our concept for a CH4 lidar is a nadir viewing instrument that uses the strong laser echoes from the Earth’s 
surface to measure CH4. The instrument uses a tunable, narrow-frequency light source and photon-sensitive detector to 
make continuous measurements from orbit, in sunlight and darkness, at all latitudes and can be relatively immune to 
errors introduced by scattering from clouds and aerosols. 

Our measurement technique uses Integrated Path Differential Absorption (IPDA), which measures the absorption of 
laser pulses by a trace gas when tuned to a wavelength coincident with an absorption line. We have already 
demonstrated ground-based and airborne CH 4 detection using Optical Parametric Amplifiers (OPA) at 1651 nm using a 
laser with approximately 10 pj/pulse at 5kHz with a narrow linewidth. Next, we will upgrade our OPO system to add 
several more wavelengths in preparation for our September 2015 airborne campaign, and expect that these upgrades will 
enable CH4 measurements with 1% precision (10-20 ppb). 

Keywords: lidar, laser radar, methane, IPDA, optical parametric oscillator, optical parametric amplifier, OPO, OPA. 


The last report by the Intergovernmental Panel on Climate Change (IPCC ) 1 attributes the increase of the 
atmospheric concentrations of greenhouse gases above their preindustrial levels to the burning of fossil fuels and other 
anthropogenic sources. As the concentration of greenhouse gases steadily increases, the subsequent radiative forcing 
will likely have a significant impact on Earth’s climate. Presently our knowledge and understanding of the important 
processes controlling greenhouse gas concentrations is incomplete. Current observations of greenhouse gases are mostly 
from in situ sites (surface and tower), airborne platforms, and space-based passive spectrometers. Initial space 
measurements of methane and other greenhouse gases came from SCIAMACHY on European Space Agency’s (ESA’s) 
environmental satellite (ENVISAT) mission 2 ' 4 , the infrared atmospheric sounding interferometer (IASI) on the Centre 

National d’Etudes Spatiales’s (CNES) MetOp satellite 5 " 6 , and NASA’s atmospheric infrared sounder (AIRS) on that 
agency’s Aqua Mission 7 ' 8 . Additional space measurements are now available from Japan Aerospace Exploration Agency 
or Japan Space Exploration Agency’s (JAXA’s) greenhouse gases observing satellite (GOSAT) mission, which was 
launched in 2009 9 ' 12 . All four instruments are passive spectrometers, and their observations are limited. Measurements 
using surface reflected sunlight by GOSAT and SCIAMACHY are limited to the sunlit areas of earth, and their data 
products are significantly affected by atmospheric scattering and the presence of clouds 13 " 14 ; measurements in thermal 
infrared by AIRS and IASI have a measurement weighting function peaked in the mid-troposphere 15 and are not 
sensitive to the sources and sinks of greenhouse gases at the surface but are very sensitive to atmospheric temperature 

Methane is a very important greenhouse gas because its radiative forcing is approximately 23 times larger per 
molecule than CO 2 \ and the methane mixing ratio is increasing along with CO 2 15 " 16 . Anthropogenic sources of methane 
include fossil fuel production, rice farming, livestock, and landfills, while natural sources include wetlands, wild fires, 
and termites 17 . Oxidation by hydroxyl radicals in the atmosphere and oxidation by nonsaturated soils both serve as 
important sinks for methane. Additionally, there are large reservoirs of methane in the form of methane hydrates 18 that 
are contained in the continental shelf with large reservoirs of carbon in the permafrost regions of Siberia, North America, 
and Europe. These are of major concern, because as global temperatures rise and the permafrost thaws, some of this 
carbon will be converted to methane and released into the atmosphere 19 " 20 . In order to have a better understanding of the 
global impact of this methane, a better understanding of methane distribution and its sources and sinks is imperative 21 " 23 . 

To address this need for a better understanding of greenhouse gases, there is a call for NASA to implement 
active CO 2 sensing from a space-based platform, currently called Active Sensing of CO 2 Emissions over Nights, Days, 
and Seasons (ASCENDS). Included in this call is a statement that “If appropriate and cost-effective methane technology 
becomes available, methane capability should be added” to the ASCENDS mission 24 . To address this call at Goddard 
Space Flight Center (GSFC), we have developed lidar technology based on optical parametric generation (OPG) devices 
that can be used to measure methane remotely from a space-based platform. This technology has been demonstrated in 
ground measurements of methane, carbon monoxide, carbon dioxide, and water vapor 25 ' 26 . In 201 1 this technology was 
demonstrated on an airborne platform over California at altitudes from 3 to 1 1 km. Additional airborne demonstrations 
of the optical parametric techniques are planned for September 2015. 


The strongest absorption bands for methane are at 1.65, 2.2, 3.3, and 7.8 pm, with the line at 3.3pm being ideal 
for making high-sensitivity measurements of methane in low-pressure planetary atmospheres, such as that on Mars. The 
low pressure on Mars essentially Doppler broadens the absorption lines, allowing measurements by a laser spectrometer 
with sufficient signal-to-noise ratio (SNR) and high spectral resolution. This 3.3pm absorption line is also well suited 
for low altitude applications, such as pipeline leak detection 27 ' 30 , or in-situ detection 31 ' 33 . 

However, in the Earth’s atmosphere the absorption lines at 3.3pm are too strong for an active space-based 
sensing platform, because they would simply absorb all of the laser radiation before it reached the ground. Additionally, 
there are adjacent water vapor absorption lines that would cause significant interference in the signal, but are also highly 
variable, which would make an accurate measurement from space at 3.3pm very challenging. 

The methane absorption line at 1.65pm is well suited for an active space-based sensing platform, for several 
reasons. Partially because it is almost two orders of magnitude weaker than the line at 3.3pm, but also because this 
spectral region is relatively free of interference from other absorption lines, as seen in Figure 1. For these reasons, the 
methane absorption lidar at GSFC has chosen to focus on this spectral region. 

Figure 1. Atmospheric transmission for US standard atmosphere at 1650-1653nm from 400km. 


We have developed a direct-detection lidar to measure column methane abundance using the integrated path 
differential absorption (IPDA) technique. It uses a rapidly tuned pulsed laser, scanned across a selected methane 
absorption line, with a time-resolved receiver using a sensitive detector. Theoretically, only two wavelengths are 
necessary for this technique (“on” and “off’ the absorption line), but in practice, additional wavelengths in conjunction 
with the line shape has proven to be quite valuable. The additional data allows solving for instrumental and systematic 
errors, such as etalon fringes with various periods and baseline drifts. In 201 1, the system was scanned across 20 
wavelengths distributed evenly across the 165 lnm absorption line. A laser transmitter for this system must have high 
enough pulse energy and a narrow enough spectral line width at the spectral absorption line 34 " 36 . Because they have 
sufficient wavelength tuning range and energy, even at wavelengths where traditional laser gain media do not work, 
optical parametric generation (OPG) devices are ideal for this application. The IPDA lidar transmitter in this effort has 
three major components as shown in Figure 2: 1) a seed laser, 2) a pump laser, and 3) an OPG device that will 
generate/amplify the tunable laser radiation at -165 lnm. 

CH 4 Absorption 




To surface 




from surface 

Figure 2. High level functional block diagram of the IPDA Methane Lidar. 


In an OPG device, a nonlinear optical crystal is used to divide an incident laser pulse (or pump) into two 
photons: a signal and an idler. The wavelengths of the signal and the idler photons must satisfy the energy and the phase 
matching conditions in the crystal. Two OPG techniques are investigated: an optical parametric amplifier (OP A), and an 

optical parametric oscillator (OPO). The OPA was built and demonstrated in the airborne test flights in 201 1 over 
California. For the 2015 test flights, both the OPA and an OPO will be flown. 

4.1 OPA 

An OPA is a seeded version of optical parametric generation, and a simplified block diagram of the OPA based 
lidar is shown in Figure 3. A pulse single-frequency 1064nm Nd: YAG laser (pump) and a continuous-wave (CW) 

165 lnm distributed feedback (DFB) laser diode (seed) are used to pump the nonlinear periodically poled lithium niobate 
(PPLN) crystal. The laser diode can be used to adjust the wavelength of the seed laser; here it is used at 1578nm to 
generate an idler wavelength at 3.3pm and 165 lnm, which amplifies the seed wavelength. In principle, any diode laser 
between 1530 and 1660 nm can be used as a seed in order to target different trace gases in the 1.5—1.65 pm and 3—4 pm 

Diode lasers are ideal for this application, because they are small, rugged, and have desirable spectroscopic 
characteristics. Their side mode suppression ratio typically exceeds 40dB, and they tune smoothly over a few 
nanometers. Additional advantages are that their wavelength can be tightly controlled via the temperature and current to 
the diode, the instantaneous linewidth can be less than 1 MHz, and they can be frequency stabilized using an external 
reference cell 37 . 

1064 nm 





O A Seed 

U / 


% l Signal 

Residual Pump 



i— Detector 3.3 pm — 

*— Detector 1 .65 pm 







Figure 3. A simplified block diagram of the OPA based lidar flown in 2011. 

In this system, the pump and the seed lasers are co-aligned and focused through the PPLN crystal. In order to 
optimize the phase matching at the target wavelength, the temperature of the crystal is varied from 70° to 170° C. The 
beam that is output from the crystal is then separated into three beam paths using dichroic mirrors (the idler, the signal, 
and the residual pump) so that each path has a different wavelength. At this point, the idler or the signal or both, can be 
used for trace gas detection, depending on the application. In the system shown in Figure 3, backscattered photons from 
both 1.651pm and 3.3pm are collected by receiver telescopes and detected by InGaAs and HgCdZnTe detectors. 

4.2 OPO 

The OPO built at GSFC is a parametric oscillator that uses a pulsed single-frequency 1064 Nd:YAG laser 
(pump) with a magnesium oxide-doped PPLN nonlinear crystal in a ring cavity. The master laser is frequency locked to 
the center of the reference methane absorption line at 165 lnm, and the length of the OPO cavity is controlled to maintain 
the resonance of the master laser within the cavity. The slave laser is offset phase-locked to the master laser, and the 
beatnote between the two is detected and used to maintain a constant integer frequency offset of free spectral ranges 
(FSR) of the OPO cavity. These loops ensure that the OPO cavity length is stabilized against the target methane line, 
while both seed lasers transmit through the OPO cavity. The two lasers are fast switched by an electro -optical switch 
and then injected into the OPO cavity as seed lasers. The pump laser generates a 1.064pm pump pulse, whose timing is 
synchronized to the optical switching of the OPO seeds. An experimental setup is shown in Figure 4. 

Figure 4. Experimental setup of the Methane lidar transmitter based on an OPO. The inset on the lower right shows the concept of 

the setup. 

The OPO built at GSFC has a narrower linewidth than the OP A, because it is narrowed by the optical feedback 
within the cavity, which is an advantage for the IPDA lidar technique. Additionally, it differs from other OPO systems 
in its control scheme and its high repetition rate (5 kHz) 38 " 40 . The higher repetition rate has been adopted for the 
ASCENDS mission in order to ensure overlap in the beam spots. The OPO used in this system does not require dynamic 
pulse-to-pulse adjustment, which is accomplished by combining an absolute frequency-locking technique, an optical 
phase-lock loop (OPLL), a phase modulation technique to lock the cavity length, and a fast electro -optical switch. 

An open path demonstration of the OPO has been successfully completed, and additional specifications and 
results are detailed in Numata, et al 41 . Our current work includes incorporating 2 to 6 additional wavelengths to the 
existing laser transmitter, and scaling the laser transmitter energy to ~300pJ. This system will be flown in an airborne 
demonstrator in the fall of 2015. 


In August of 2011, the OPA laser (~10pJ at 1651nm) was used in the transmitter of the airborne methane 
absorption lidar demonstrated using NASA’s DC-8 aircraft, based at Dryden Airborne Operations Facility (DAOF) in 
Palmdale, California. Three flights occurred over California’s Central Valley, all of which traversed similar paths with 
similar flight times (2.5 hours) and similar stepped altitudes (3, 4.8, 6.2, 7.9, 9.5, and 11.1km). 

The receiver telescope is a 20cm diameter commercial telescope (Vixen VC200L), and the light is then coupled 
into an AR coated 600pm fiber (Fiberguide CB18167). The receiver field-of-view (FOV) is 300prad, as determined by 
the receiver fiber core size, its numerical aperture, and the effective focal length of the telescope. The receiver fiber 
output is collimated and directed through a narrow bandpass filter (0.8nm, from Barr Associates), an iris, and then onto 
the detector. The detector used in 201 1 is a photon counting Hamamatsu photomultiplier tube (PMT), with a quantum 
efficiency of 1% at 1651nm, along with a high-speed amplifier. A field -programmable gate array (FPGA) with two 
delay generators provide the necessary timing signals, wavelength scan waveforms, and appropriate system delays for 
data acquisition. All of the data was time tagged by a GPS unit that also provided position information. Additional 
major parameters for this flight configuration are summarized in Table 1. 

Table 1. Flight Lidar Parameters 



Seed wavelength 

1650.9 nm 

Pump wavelength 

1064 nm 

Laser pulse rate 

-6.3 kHz 

Laser pulse width 

3 ns 

Laser pump energy 

60 M J 

Seed power 

15 mW 

OPA energy 

10 pj 

Laser divergence 

~300 prad 

Receiver diameter 

20 cm 

Receiver field of view 

300 prad 

Receiver bandpass 

800 pm (FWHM) 

Scan rate 

-250 Hz 

Averaging period 

1 s 

Detector efficiency 


Overall the lidar results from these flight demonstrations were consistent with the in-situ data gathered by a 
CRDS instrument (Picarro) flown on the same aircraft. Perfect agreement was not expected, partially because the in-situ 
data is taken at one point in space, while the lidar collects information over the entire column from the aircraft to the 
surface. A comparison of the lidar methane mixing ratio measured with the lidar and the in-situ Picarro as a function of 
flight time is shown in Figure 5. Additional results from this flight are detailed in Riris et al n . 

Time (Decimal Hours) 

Figure 5. Comparison of the lidar methane mixing ratio with the in-situ CRDS instrument values as a function of flight time in decimal 
hours (UTC) for August 25th, 2011 flight. A 20 s averaging time was used. 


An updated version of the airborne methane absorption lidar will be flown on NASA’s DC-8 in the fall of 2015, 
also over California. The general concept of operation has been proven and will not change, but there are two substantial 
improvements to the technology. The first is the inclusion of an OPO transmitter, and the second is the use of a new 
photon-counting HgCdTe detector, developed in collaboration with DRS technologies. 

The OPA was used in 201 1 because it is theoretically easier to align and tune than an OPO (because of the 
cavity). For the earlier flight campaign, the OPA generated ~10pJ at 165 lnm, which was sufficient for an airborne 
campaign. However, for a space-based lidar, ~300pJ will be required for the desired methane absorption measurements. 
When the OPA was scaled to a ~170pJ energy, the linewidth was ~ 1 0GHz (~91pm), which is too broad for accurate 
methane IPDA measurements. After extensive testing, it was concluded that it will be extremely difficult to scale the 
OPA’s transmitter energy while maintaining the necessary line-widths without radical improvements in the seed laser 
technology. While this may be possible in the future, our current flight concept for the 2015 flight campaign 
incorporates an OPO transmitter in addition to the OPA. 

We also plan to leverage a detector originally developed for the C02 absorption lidar (ASCENDS) in 
collaboration with DRS Technologies. It is a HgCdTe electron Avalanche Photo Diode (e-APD) detector assembly that 

offers a wide spectral response, high dynamic range, and substantially improved sensitivity (low noise) and lifetime 43 . 
This will improve both the SNR and dynamic range of the methane absorption lidar. 


Atmospheric methane is the second most important anthropogenic greenhouse gas, and understanding current 
global methane trends is a difficult challenge that cannot be resolved by existing measurement networks or satellite 
observations. This work directly addresses the objectives of NASA’s Earth Science Decadal Survey which explicitly 
calls for cost-effective global methane measurement technology. It additionally enables methane and water 
measurements with sufficient coverage, sensitivity, and precision to address pressing science questions for the carbon 
cycle and climate change. 

Our team at GSFC has a demonstrated capability for the remote measurements of trace gases. Building on that 
experience and technology, we have developed OPG technology appropriate for the desired methane measurements, and 
have demonstrated an airborne methane IPDA absorption lidar. Our next airborne campaign, in September 2015, will 
incorporate several significant upgrades with the expectation that the methane measurements will now have 1% precision 
(10-20 ppb). 


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