Atmospheric CO<sub>2</sub> column measurements with an airborne intensity-modulated continuous wave 1.57&#xA0;&#x3BC;m fiber laser lidar

Jeremy T. Dobler; F. Wallace Harrison; Edward V. Browell; Bing Lin; Doug McGregor; Susan Kooi; Yonghoon Choi; Syed Ismail

doi:10.1364/AO.52.002874

Applied Optics
Vol. 52,
Issue 12,
pp. 2874-2892
(2013)
•https://doi.org/10.1364/AO.52.002874

Atmospheric CO₂ column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar

Jeremy T. Dobler, F. Wallace Harrison, Edward V. Browell, Bing Lin, Doug McGregor, Susan Kooi, Yonghoon Choi, and Syed Ismail

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Jeremy T. Dobler, F. Wallace Harrison, Edward V. Browell, Bing Lin, Doug McGregor, Susan Kooi, Yonghoon Choi, and Syed Ismail, "Atmospheric CO₂ column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar," Appl. Opt. 52, 2874-2892 (2013)

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Abstract

The 2007 National Research Council (NRC) Decadal Survey on Earth Science and Applications from Space recommended Active Sensing of ${CO}_{2}$ Emissions over Nights, Days, and Seasons (ASCENDS) as a midterm, Tier II, NASA space mission. ITT Exelis, formerly ITT Corp., and NASA Langley Research Center have been working together since 2004 to develop and demonstrate a prototype laser absorption spectrometer for making high-precision, column ${CO}_{2}$ mixing ratio measurements needed for the ASCENDS mission. This instrument, called the multifunctional fiber laser lidar (MFLL), operates in an intensity-modulated, continuous wave mode in the 1.57 μm ${CO}_{2}$ absorption band. Flight experiments have been conducted with the MFLL on a Lear-25, UC-12, and DC-8 aircraft over a variety of different surfaces and under a wide range of atmospheric conditions. Very high-precision ${CO}_{2}$ column measurements resulting from high signal-to-noise ratio ( $> 1300$ ) column optical depth (OD) measurements for a 10 s ( $\sim 1 km$ ) averaging interval have been achieved. In situ measurements of atmospheric ${CO}_{2}$ profiles were used to derive the expected ${CO}_{2}$ column values, and when compared to the MFLL measurements over desert and vegetated surfaces, the MFLL measurements were found to agree with the in situ-derived ${CO}_{2}$ columns to within an average of 0.17% or $\sim 0.65 ppmv$ with a standard deviation of 0.44% or $\sim 1.7 ppmv$ . Initial results demonstrating ranging capability using a swept modulation technique are also presented.

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Fig. 1. MFLL conceptual architecture.

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Fig. 2. Basic block diagram of MFLL instrument as flown in 2011.

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Fig. 3. Summary of MFLL flight test campaigns.

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Fig. 4. MFLL flight test on 22 October 2007 showing water-to-land transition. It shows the flight track in solid red line; and energy normalized on- and off-signals and OD measurements.

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Fig. 5. MFLL measurements made over the Department of Energy Atmospheric Radiation Measurement (DOE/ARM) Central Facility from an altitude of 4.6 km on 31 July 2009; (a) shows the flight track; (b) shows the in situ measured

{CO}_{2}

profile over the CF in red along with the

{CO}_{2}

profile measured earlier on a DOE Cessna aircraft shown in blue and a tower

{CO}_{2}

measurement shown in green; (c) shows the off-line surface reflectance signals with 1 s averaging from A to B; and (d) shows the variation in

{CO}_{2}

OD measurements in terms of

{XCO}_{2}

. The leg-averaged column

{XCO}_{2}

is shown in (d) by the solid line, and dashed lines are

\pm 1.5 ppmv

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Fig. 6. Comparison of MFLL measured and modeled

{CO}_{2}

OD’s on DC-8 flights over California’s Central Valley (top) and Rocky Mountains (bottom) in route to Railroad Valley (RRV), Nevada.

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Fig. 7. Comparison of measured and modeled

{CO}_{2}

OD’s expressed in terms of equivalent

{XCO}_{2}

from six flights during the 2011 ASCENDS DC-8 Campaign. The data for each of the flights is shown in the table along with range to flight levels, and measured—model

{XCO}_{2}

differences (

! {CO}_{2}

). The location and dates of the flights are: Flt. 0: Central Valley, California, 26 July; Flt. 1: Central Valley, California, 28 July; Flt. 3: Railroad Valley, Nevada, 3 August; Flt. 5: Four Corners, New Mexico, 9 August; Flt. 6: NOAA WBI Tall Tower at West Branch, Iowa, 10 August; Flt. 7: NOAA LEF Tall Tower at Park Falls, Wisconsin, 11 August.

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Fig. 8. Sampled output of the lock-in processor is illustrated in red and Eq. (14) fitted to the lock-in output in blue. Only at integer multiples of the sample period do the amplitude of the lock-in output and the estimate of the amplitude (C) provided by the fit of Eq. (14) match. At all other correlation delays, the amplitude of the sampled correlation is less than the true amplitude. Range is a function of the delay to the peak of the fit.

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Fig. 9. Range estimates obtained from the off-line

{CO}_{2}

return and time coincident returns from the on-board PN altimeter over the four corners region from the DC-8 on 7 August 2011. Sum square error (SSE) over this interval is 1.35 meters.

Mean difference = 0.2302 m

with an STD of 1.4418 m.

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Fig. 10. Examples of flight data showing the range discrimination of cloud returns from ground returns using the swept frequency IM-CW approach from the DC-8 on 4 August 2011. Panel (a) shows a 3-D representation of a large cloud return above a small ground return and panel (b) is the projection of the information in (a) onto 2-D image of signal profiles along the flight track illustrating the strong cloud and weak ground returns; panel (c) slows the distribution of signals from weak clouds and strong ground return signals and panel (d) is a superposition of data from (c) on a signal versus path length grid.

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Tables (2)

Table 1. Key MFLL Instrument Parameters as Flown in 2011 DC-8 Flights

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Table 2. Surface Reflectance and ${CO}_{2}$ Measurement Precision During 2010 ASCENDS DC-8 Campaign

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Equations (15)

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P_{rec} (λ_{0}, R) = P_{L} \frac{A_{0}}{R^{2}} * η (λ_{0}) * ξ (R_{T}) * β_{π} (λ_{0}, R) * Δ R * \exp [- 2 \int_{0}^{R} κ (λ_{0}, r) d r],

κ (λ_{0}, R) = Σ α_{i} (λ_{0}, R) + β_{θ \neq π} (λ_{0}, R),

P_{rec} (λ_{0}, R) = P_{L} * C * β_{π} (λ_{0}, R) * Δ R * e^{- 2 \int_{0}^{R} β_{θ \neq π} (λ_{0}, r) d r} * e^{- 2 \int_{0}^{R} \sum_{i \neq 0} α_{i} (λ_{0}, r) d r} * e^{- 2 \int_{0}^{R} α_{i = 0} (λ_{0}, r) d r},

\frac{P_{rec} (λ_{off}, R)}{P_{rec} (λ_{on}, R)} = \frac{P_{L_{off}}}{P_{L_{on}}} e^{2 (\int_{0}^{R} α_{i = 0} (λ_{on}, r) d r - \int_{0}^{R} α_{i = 0} (λ_{off}, r) d r)},

\int_{0}^{R} N (r) d r = \frac{1}{2} \int_{0}^{R} \frac{i}{Δ σ (λ_{on}, λ_{off}, r)} d r * [\int_{0}^{R} α_{i = 0} (λ_{on}, r) d r - \int_{0}^{R} α_{i = 0} (λ_{off}, r) d r],

V_{psd} = V_{sig} * \sin (ω_{sig} * t + θ_{sig}) * V_{LO} * \sin (ω_{LO} * t + θ_{LO}) = \frac{1}{2} * V_{sig} * V_{LO} * \cos ([ω_{sig} - ω_{LO}] * t + θ_{sig} - θ_{LO}) - V_{sig} * V_{LO} * \cos ([ω_{sig} + ω_{LO}] * t + θ_{sig} + θ_{LO}) .

V_{psd} = \frac{1}{2} * V_{sig} * V_{LO} * \cos (θ_{sig} - θ_{LO}) .

V_{psd 2} = \frac{1}{2} * V_{sig} * V_{LO (90)} * \sin (θ_{sig} - θ_{LO (90)}) .

R = {(I^{2} + Q^{2})}^{\frac{1}{2}} = V_{sig} .

θ = atan (Q / I),

ω_{sig} (t) = 2 * π * A * [(\frac{t}{a} - floor (\frac{t}{a} + 0.5)) + 0.5],

Δ τ = 2 \int_{z = b}^{z = t} Δ σ (λ_{on}, λ_{off}, T, P, z) \cdot η (T, P, RH, z) \cdot δ z

| P (t) | = C | \frac{\sin [π Δ f t (1 - \frac{| t |}{τ})]}{2 π Δ f t (1 - \frac{| t |}{τ})} |, 0 < t < τ \dots

| P (t) | = C | \frac{\sin [\frac{π Δ f}{a} (t - b) (1 - \frac{| t - b |}{τ})]}{2 \frac{π Δ f}{a} (t - b) (1 - \frac{| t - b |}{τ})} |,

| P (t) | = C | \frac{\sin [\frac{π (t - b)}{a^{'}}]}{[\frac{π (t - b)}{a^{'}}]} |,

Seed Laser Type	DFB Diode Laser FITEL FOL15DCWD
Line width	$< 6 MHz$ each wavelength
Side mode suppression ratio	$> 45 dB$
${CO}_{2}$ lines (vacuum)	1571.112 nm (On), 1571.061 nm (Off 1), 1571.161 nm (Off 2)
Altimetry line (vacuum)	1596 nm
Modulator	semiconductor optical amplifiers, Covega BOA1080
Modulation type	IM-CW
Optical amplifier	IPG EAD-5L EDFA ${CO}_{2}$ , IPG EAD-5L EDFA Altimetry
Output power	5 W (Total for ${CO}_{2}$ wavelengths $3 ∶ 2 ∶ 1$ ratio On, Off 1, Off 2) and 5 W for the altimeter wavelength (simultaneous transmission and reception of all wavelengths)
Bandpass filter width	2.4 nm ${CO}_{2}$
Telescope (shared with altimeter)	simple 8 in. diam. parabola, $F / 3$ , fiber coupled with $\sim 1.5 in.$ . center obscuration, 365 μm fiber core with 0.22 NA
Detectors	HgCdTe APD, $0.5 mm \times 0.5 mm$ ${CO}_{2}$ DRS—A3051 44 of 64 good diodes in parallel, Newport 818-BB InGaAs PIN ${CO}_{2}$ reference channel, Hamamatsu H10330 PMT altimeter
Detector collection efficiency	0.50 ${CO}_{2}$ accounting for bad diodes, 0.04 altimeter
Detector gain	$\sim 940$ with excess noise factor $\sim 1.3$ , 77 K as operated, altimeter (adjustable)
Transimpedance amplifier	Femto HCA-2M-1M gain $10^{6}$ ${CO}_{2}$ science channel and $10^{3}$ reference
Analog filter	1P-HP 3.5 kHz, 3P-LP 2 MHz
Analog IO	NI PXIe-6368
Sample rate	$2 Ms / s$
Encoding scheme	swept-frequency; $\sim 350 KHz \pm 250 KHz$ ( ${CO}_{2}$ ) 2011rolling tones; $50 KHz \pm \sim 3 KHz$ ( ${CO}_{2}$ ) 2009–2010 pseudo-noise (PN) (Altimeter)
Maximum unambiguous range	15 km 200 samples (Selectable) ${CO}_{2}$
Laser divergence angle	190 μrad (half angle)
Receiver FOV	240 μrad (half angle)
Receiver recording duty cycle	100%
Reporting interval	100 msec (10 Hz)

Surfaces	Desert	Desert	Vegetation	Vegetation	Oceanc
Location	Railroad Valley, Nevada	Needles, California	Central Valley, California	DOE ARM, Lamont, Oklahoma	Pacific off Baja
Median surface reflectancea [ ${sr}^{- 1}$ ]	0.143	0.118	0.098	0.080	0.019 $(0.03 - 0.06)$ d
1 s ${CO}_{2}$ SNRb ( ${CO}_{2}$ [ppmv])	630 (0.59)	612 (0.59)	545 (0.68)	560 (0.65)	$\sim 186$ (2.07)
10 s ${CO}_{2}$ SNRb ( ${CO}_{2}$ [ppmv])	1347 (0.27)	1443 (0.25)	1236 (0.30)	1460 (0.25)	$\sim 531$ (0.72)

Seed Laser Type	DFB Diode Laser FITEL FOL15DCWD
Line width	$< 6 MHz$ each wavelength
Side mode suppression ratio	$> 45 dB$
${CO}_{2}$ lines (vacuum)	1571.112 nm (On), 1571.061 nm (Off 1), 1571.161 nm (Off 2)
Altimetry line (vacuum)	1596 nm
Modulator	semiconductor optical amplifiers, Covega BOA1080
Modulation type	IM-CW
Optical amplifier	IPG EAD-5L EDFA ${CO}_{2}$ , IPG EAD-5L EDFA Altimetry
Output power	5 W (Total for ${CO}_{2}$ wavelengths $3 ∶ 2 ∶ 1$ ratio On, Off 1, Off 2) and 5 W for the altimeter wavelength (simultaneous transmission and reception of all wavelengths)
Bandpass filter width	2.4 nm ${CO}_{2}$
Telescope (shared with altimeter)	simple 8 in. diam. parabola, $F / 3$ , fiber coupled with $\sim 1.5 in.$ . center obscuration, 365 μm fiber core with 0.22 NA
Detectors	HgCdTe APD, $0.5 mm \times 0.5 mm$ ${CO}_{2}$ DRS—A3051 44 of 64 good diodes in parallel, Newport 818-BB InGaAs PIN ${CO}_{2}$ reference channel, Hamamatsu H10330 PMT altimeter
Detector collection efficiency	0.50 ${CO}_{2}$ accounting for bad diodes, 0.04 altimeter
Detector gain	$\sim 940$ with excess noise factor $\sim 1.3$ , 77 K as operated, altimeter (adjustable)
Transimpedance amplifier	Femto HCA-2M-1M gain $10^{6}$ ${CO}_{2}$ science channel and $10^{3}$ reference
Analog filter	1P-HP 3.5 kHz, 3P-LP 2 MHz
Analog IO	NI PXIe-6368
Sample rate	$2 Ms / s$
Encoding scheme	swept-frequency; $\sim 350 KHz \pm 250 KHz$ ( ${CO}_{2}$ ) 2011rolling tones; $50 KHz \pm \sim 3 KHz$ ( ${CO}_{2}$ ) 2009–2010 pseudo-noise (PN) (Altimeter)
Maximum unambiguous range	15 km 200 samples (Selectable) ${CO}_{2}$
Laser divergence angle	190 μrad (half angle)
Receiver FOV	240 μrad (half angle)
Receiver recording duty cycle	100%
Reporting interval	100 msec (10 Hz)

Surfaces	Desert	Desert	Vegetation	Vegetation	Oceanc
Location	Railroad Valley, Nevada	Needles, California	Central Valley, California	DOE ARM, Lamont, Oklahoma	Pacific off Baja
Median surface reflectancea [ ${sr}^{- 1}$ ]	0.143	0.118	0.098	0.080	0.019 $(0.03 - 0.06)$ d
1 s ${CO}_{2}$ SNRb ( ${CO}_{2}$ [ppmv])	630 (0.59)	612 (0.59)	545 (0.68)	560 (0.65)	$\sim 186$ (2.07)
10 s ${CO}_{2}$ SNRb ( ${CO}_{2}$ [ppmv])	1347 (0.27)	1443 (0.25)	1236 (0.30)	1460 (0.25)	$\sim 531$ (0.72)

Abstract

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Equations (15)

Applied Optics