Recent Results and Progress on the Development of a Laser Absorption Spectrometer for Carbon Dioxide

1
Recent Results and Progress on the Development of
a Laser Absorption Spectrometer for Carbon
Dioxide Sink and Source DetectionGary D.
Spiers1, Sven Geier1, Mark Phillips2, Stacey
Boland1, Ilya Poberezhskiy1, Patrick Meras1,
Robert T. Menzies1 1Jet Propulsion Laboratory,
California Institute of Technology, Pasadena,
CA2Lockheed Martin Coherent Technologies, La
Fayette, COAcknowledgementsNASA ESTO IIP
ProgramNASA CETPD ProgramJPL RTD
ProgramAVIRIS Team
2
Concept for Global CO2 Laser Absorption
Spectrometer (LAS)

• Transmit and receive near nadir-pointing laser
  beams with on and off-line wavelength channels
• Ground surface reflection (land and sea) provides
  return signal requires co-aligned beams to
  obtain equal backscatter coefficients and equal
  depolarization factors for both channels
• Measure difference in integrated path absorption
  at these two wavelengths
• Use additional sensor data (temperature, surface
  pressure, altimetry) to extract value of CO2
  concentration
• Goal of 1 ppmv precision with 50-100 km
  horizontal resolution (large scale measurements)
• Eventual plan is to perform global measurement
  from Low Earth Orbit Satellite (LEOS) platform
• Development plan includes interim measurement and
  technology demonstration from airborne platform

R.T. Menzies and M. T. Chahine, Remote
atmospheric sensing with an airborne laser
absorption spectrometer, Appl. Opt.,13, pp.
2840-2849, 1974. M.S. Shumate, R. T. Menzies, W.
B. Grant, and D. S. McDougal, Laser absorption
spectrometer remote measurement of tropospheric
ozone, Appl. Opt., 20, pp. 545-553, 1981.
3
Why 2 microns?
Weighting Functions at 1.5µm
Weighting Functions at 2.0µm

• LAS at 2050.9nm biased to PBL (lt2km from surface)
  where CO2 source and sink structures are most
  measurable against background
• Strong bias to PBL allows a single on-line
  wavelength to be used in the LAS measurement
  without ambiguity
• R.T. Menzies D.M. Tratt, Differential laser
  absorption spectrometry for global profiling of
  tropospheric carbon dioxide selection of optimum
  sounding frequencies for high-precision
  measurements, Appl. Opt., 42, pp 6569 - 6577
  (2003).

4
LAS Wavelengths
Reference 4875.74 cm-1
The instrument senses at the online and offline
wavelengths. Reference laser is locked to a CO2
cell and is stable to better than 200 kHz
3?. Online and offline lasers are frequency
offset locked from the reference laser to better
than 300 kHz 3??(online) and lt600 kHz
3??(offline).
Online 4875.88 cm-1
Offline 4875.22 cm-1
5
The LAS Instrument
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Functional Configuration of LAS Transceiver
Optical Bench (Surface 1)
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• Layout finalized using PM spectrometer for CO2
  lock, with I and Q signal detection for Residual
  Amplitude Modulation (RAM) suppression
• Lasers and ¼ wave plates physically located on
  Surface 2
• Frequency Offset Lock (FOL) beat detectors
  physically located remotely in Offset-Lock
  Controller chassis (fiber-coupled)

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7
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8
Calibration/ Validation Issues

• In order to meet our goal of a few ppmv
  sensitivity to CO2 fluctuations we must be
  certain that fluctuations we see in the intensity
  ratio of the two channels are due to fluctuations
  in CO2 and not other causes such as
• External temperature, pressure, turbulence and
  humidity fluctuations
• Internal fluctuations in laser power and receiver
  sensitivity
• To this end we have developed a number of
  calibration schemes to cross-check the stability
  and repeatability of the instrument
• Initially we use a short path length to a target
  with a CO2 cell in the path to permit convenient
  variation of the CO2 absorption. The target is a
  belt sander arranged to provide the 15MHz Doppler
  shift tracking signal.
• We use longer horizontal path lengths over which
  the variation in carbon dioxide with time can be
  seen. We can tune the online wavelength to
  increase the sensitivity to CO2 fluctuations over
  short path lengths. For these experiments we have
  a portable calibrator that can sit beneath the
  instrument and can be easily switched in and out
  of the beam path to check for drifts in the
  instrument as a function of time.
• The instrument is placed on an aircraft and
  tested.
• The final step is to compare the instrument
  performance against calibrated in-situ CO2
  sensors.
• In all cases we monitor those atmospheric
  parameters (pressure, temperature and humidity)
  that will significantly impact the intensity
  ratio and based on analyses conducted previously
  we can back out changes due to these atmospheric
  parameters to leave the change due to
  fluctuations of the CO2 concentration.

9
Laboratory Testing
Laboratory testing has been conducted to evaluate
noise performance of the instrument and to
evaluate the precision of the instrument in a
semi-controlled environment. Beams from the LAS
were directed down the laboratory to a belt
sander 17 m from the instrument. A gas cell of
length 87.5 cm was placed in the beam path 7m in
front of the belt sander. Measurements were
collected both with and without CO2 in the gas
cell.
LAS instrument behind screens
Turning Mirror
CO2 cell
Lab tests indicate differential transmittance
measurements agree with theoretical prediction to
1.
10
2006 Engineering Checkout Flight
During the initial aircraft flight test in late
June the data collection computer experienced a
power supply failure and the flight was
terminated. The data collection computer was
returned to the vendor but after hardware repair
the vendor was unable to get the software to work
correctly. An alternate data collection solution
was put together at JPL using existing hardware
and software from another project and used in
early August to collect data during a subsequent
flight one month after the initial flight. Data
collection was limited to a low duty cycle 8 by
the available software. Limitations of the data
collection system meant that the laser output
power could not be continuously monitored
limiting the ability to normalize the offline and
online return signals. Flights were conducted
over RMOTC, El Mirage Dry Lake Bed and the
Pacific Ocean. Results from El Mirage Dry Lake
Bed will be presented.
11
Instrument Configuration
Control Display
LAS control Electronics
KVM switch
Data Collection Computer
LAS instrument
Support Computer
Gigabit ethernet hub
Data Logger
Mech. I/F to Aircraft
LiCor
In-flight Calibrator
INS GPS
H2O
P
T
12
Aircraft Installation
13
Data Collection Location
El Mirage Dry Lake Bed 34o 38 40 N, 117 35 51
W, 2800 asl
96 km
8 km
14
First Data Collection

• Collected during transit flight to Van Nuys.
• Altitude 8kft agl
• Each periodogram represents 20.48 ms of data.
• Pilot sped up and slowed down aircraft

15
August 7th El Mirage Data Online and Offline
Signals
Key C- Calibration T Transit 3 3 kft agl 4
4 kft agl 5 5 kft agl 6 6 kft agl 7 7 kft
agl 8 8 kft agl
16
Ln(offline/online) from calibrator signal
Calibrator only Ln(offline/online) ratio from
calibrator is independent of altitude (once
instrument stabilizes)
instrument stabilized
17
Signal spectral linewidth from calibrator
Signal spectral linewidth from calibrator
independent of altitude (once instrument
stabilizes)
Offline
Online
Instrument stabilized
18
Signal spectral linewidths from ground
Return signal only Altitude dependent minimum
linewidth of return signal from ground
Offline
Online
19
Doppler Return Frequencies
Offline
Online
20
Modeling
21
Engineering Checkout Flight Results
22
HC-PCF based locking cell

• Replace conventional gas cell with gas filled
  hollow core photonic crystal fiber (HC-PCF)
• Use of gas filled HC-PCF has been documented in
  the literature however the issues that need to be
  addressed are filling and sealing the fiber in a
  manner consistent with space based operation and
  connectorization of the fiber such that it can be
  integrated into a system.
• We have developed methods of filling, sealing and
  connectorizing the HC-PCF and demonstrated our
  ability to measure CO2 absorption spectra in the
  gas filled HC-PCF.

D. Chang, Method for rugged fiber-pigtailing of
hollow-core photonic crystal fiber and filling
with gas of arbitrary pressure, JPL NTR 41272,
2004. F. Benabid, F. Couny, J. C. Knight, T. A.
Birks and P. Russell, Compact, stable and
efficient all-fibre gas cells using hollow-core
photonic crystal fibres, Nature, vol. 434, pp.
488-491, 2005. J. Tuominen, T. Ritari, H.
Ludvigsen, Gas sensing using air-guiding
photonic bandgap fibers, Optics Express, 4080,
Vol. 12 No. 17, 23 August 2004. J. Tuominen, T.
Ritari, H. Ludvigsen, and J.C. Petersen, Gas
filled photonic bandgap fibers as wavelength
references, Optics Communications, vol. 255, pp.
272-277, 2005.
23
Plans

• Over the next year we will
• Overfly the Great Plains ARM site for comparison
  with in-situ mast measurements and aircraft flask
  collections
• Data collection duty cycle has been increased
  from 8 to gt40 by development of custom rather
  than off the shelf software
• Improved data collection hardware subsystem with
  redundant power supplies, 12 TB of RAID 5
  capacity
• On board in-situ measurement of CO2 using a LiCor
  instrument
• Cross-comparisons with OCO FTIR
• Comparison flights with the LaRC/ITT teams 1.5
  micron system

24
Conclusion

• We have completed a successful engineering
  checkout flight of the Carbon Dioxide Laser
  Absorption Spectrometer.
• We detected a differential absorption equivalent
  to a model value of 410 /- 10 ppmv CO2 although
  there is no independent validation of this
  result.
• The magnitude of the error is dominated by data
  collection issues rather than instrument
  stability issues.
• During the next year we will fly comparison
  flights with in-situ and other airborne CO2
  sensors.
• We will continue to retire component technology
  risks.