Accurate global measurement of carbon dioxide column with the aim of discovering and quantifying unknown sources and sinks has been a high priority for the last decade. In order to uncover the "missing sink" that is responsible for the large discrepancies in the budget the critical precision for a measurement from space needs to be on the order of 1 ppm. To better understand the CO2 budget and to evaluate its impact on global warming the National Research Council (NRC) in its recent decadal survey report (NACP) to NASA recommended a laser based total CO2 mapping mission in the near future. That's the goal of Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission - to significantly enhance the understanding of the role of CO2 in the global carbon cycle. Our current goal is to develop an ultra precise, inexpensive new lidar system for column measurements of CO2 changes in the lower atmosphere that uses a Fabry-Perot interferometer based system as the detector portion of the instrument and replaces the narrow band laser commonly used in lidars with a high power broadband source. This approach reduces the number of individual lasers used in the system and considerably reduces the risk of failure. It also tremendously reduces the requirement for wavelength stability in the source putting this responsibility instead on the Fabry- Perot subsystem

Georgieva, E. M.

Heaps, W. S.

Huang, W.

NASA Technical Reports Server

A BROAD BAND LIDAR FOR PRECISE ATMOSPHERIC CO2
COLUMN ABSORPTION MEASUREMENT FROM SPACE
Georgieva, E.M I , Heaps, W.S.2 , Huang, W.3
'Goddard Earth Sciences and Technology Center, UMBC, Baltimore, MD 21228, USA
e-mail: Flena.M.0 t tip <<t. -ttt z:t
2 NASA, Goddard Space flight Center, Greenbelt, MD 20771, USA
3 Science Systems and Applications, Inc., Lanham, MD 20706
Abstract Accurate global measurement of carbon dioxide
column with the aim of discovering and quantifying unknown
sources and sinks has been a high priority for the last decade.
In order to uncover the "missing sink" that is responsible for
the large discrepancies in the budget the critical precision for a
measurement from space needs to be on the order of Ippm [I].
To better understand the CO 2 budget and to evaluate its
impact on global warming the National Research Council
(NRC) in its recent decadal survey report (NACP) to NASA
recommended a laser based total CO2 mapping mission in the
near future [2]. That's the goal of Active Sensing of CO2
Emissions over Nights, Days, and Seasons (ASCENDS) mission
-to significantly enhance the understanding of the role of CO2
in the global carbon cycle. Our current goal is to develop an
ultra precise, inexpensive new lidar system for column
measurements of CO2 changes in the lower atmosphere that
uses a Fabry-Perot interferometer based system as the detector
portion of the instrument and replaces the narrow band laser
commonly used in lidars with a high power broadband source.
This approach reduces the number of individual lasers used in
the system and considerably reduces the risk of failure. It also
tremendously reduces the requirement for wavelength stability
in the source putting this responsibility instead on the Fabry-
Perot subsystem.
Instrumentation; measurement; metrology; remote sensing;
atmospheric composition; optical instruments; absorption;
interferometry; Fabry-Perot.
I.	 INTRODUCTION
This paper describes the development, initial
testing and preliminary experimental results for our novel
CO, lidar sensor for 1.57 microns under development. In
our initial setup we are using superluminescent light
emitting diode (SLED) as a source and our previously
developed Fabry-Perot interferometer subsystem as a
detector part [3], [4], [5].
The detector part (Figure 1) has been developed
over the last few years at Goddard as a passive sensor to
measure CO2 column using scattered solar flux and was
tested at two flight campaigns [3], [6]. This system employs
Fabry-Perot etalons to create a differential response to the
absorption of sunlight by carbon dioxide absorption lines
near 1.57 microns. The sensor was further improved by
Figurel. This shows the mechanical setup of the
improved Fabry-Perot detector for the broad band lidar.
Light enters the system at the upper left and passes
through the narrow band filter. Some of it is then split
off and passes through a polarization compensator and
onto a detector (Reference). The unsplit component
continues toward the right through a Fabry-Perot etalon
and is detected (Fabry-Perot signal).
changing the detectors, downsizing and advancing the
design in order to permit prompt and easy alignment. The
new smaller design for the receiver makes more efficient
use of light from a collimated source than the original
passive system did which was optimized for sunlight. By
modifying the composition and geometry of the SLED its
output can be tailored to some extent to meet specific
requirements for operating wavelength and bandwidth. The
output of a commercially available off the shelf SLED
manufactured by EXALOS has —70% of the output power
between 1540nm and 1600 nm and is almost centered over
the 1567 nm to 1574 nm region used by the Fabry-Perot
detector. The total output power for the device can be on the
order of 20 mW. Figure 2 shows the measured SLED output
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Figure 2. The output of the SLED increases with
increasing the drive current. The spectral output is quite
broad covering almost 100 nm. We only use a narrow
region 2-3 nm wide centered on 1571 nm. The OPA is
designed to amplify this region in particular.
measurement from space, but it is satisfying for initial
laboratory work on the feasibility of the technique.
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Figure 3 Change in ratio (FP signal/Reference signal)
as a function of CO2 pressure in an absorption cell.
11. EXPERIMENTAL
A. Observation of CO 2 absorption in the lab using SLED
source
The Fabry—Perot etalon in the detector part of the sensor
is solid fused silica with a free spectral range (FSR) of 0.306
nm and a refractive index of 1.443 at k = 1571 nm. The
spacing between etalon fringes is matched to the spacing of
CO2 lines through etalon thickness. Similarly, the width of
fringes is approximately matched to CO, line width by
appropriate choice of the reflecting coatings on the etalon
surfaces. Alignment between etalon fringes and CO2
absorption lines can be further adjusted by temperature
tuning the Fabry—Perot [3], [4], [5], [6]. To validate in the
Figure 4. This shows the layout of the Optical Parametric
Amplifier (OPA) set-up in the lab.
laboratory that the broadband technique is feasible the
changes in the ratio of Fabry—Perot to reference signals were
monitored for different CO2 pressures using input SLED
light (Figure 3) and miltipass gas cell filled with carbon
dioxide at different pressures. The pressure experiment was
done with modulated SLED output using a pulse generator.
B. Coupling a SLED source to an amplifeer
It is necessary to boost the output power up to the tens or
hundreds of Watts for longer range measurements from
space platforms. There are several approaches to
accomplishing this. The most common one is to use a fiber
amplifier device known as an EDFA (Erbium Doped Fiber
Amplifier) for this purpose [7],[8], [9], [10], [11]. EDFA's
have been developed for use in fiber optical communications
system so they are rugged and reliable.
OPA for SLED
Figure 5. This is a diagram of the operation of the
Optical Parametric Amplifier (OPA).
Our more recent efforts have indicated that the pulse width
obtainable with an EDFA is not optimal for the broadband
lidar approach. An Optical Parametric Amplifier (OPA)
offers higher conversion efficiency, broad tuning range and
shorter pulse width. We have therefore decided to use an
Optical Parametric Amplifier (OPA) instead of the EDFA for
this amplification process.
In the OPA ew light from the SLED at 1570 nm is focused
into a crystal of KTP and pulses from the Nd:YAG are
overlapped in the same crystal (Figure 4, Figure 5). Non-
linear processes in the KTP crystal split the Nd:YAG
photons whose combined energy is equal to that of the
single Nd:YAG photon. One of these two photons will be at
the SLED wavelength so the light coming out of the KTP
crystal contains an amplified signal at 1570 nm, The residual
photons at 1.06 from the Nd:YAG and the "idler"photons at
3.3 microns are separated and the latter are discarded. The
amplified 1570 is transmitted for the broadband lidar. The
spectral region 1571-1573 nm for the COz band is well suited
for this measurement because there is no interference from
other trace gases. Also there are better detectors than for 2
micron region.
Figure 6 shows a laser scan of one of our bandpass
filters. One of these will be used in the lidar receiver and
another will be used between the SLED output and the OPA
to restrict the range of amplified wavelengths to those that
our receiver can actually detect. The prefilters and Fabry-
Perot interferometer optics are mounted in ovens and
temperature controlled.
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These filters are used in the receiver to confine the
range of wavelengths observed to those over which a
good alignment between the COz absorption lines and
the Fabry-Perot transmission peaks can be maintained.
We had reported an amplified output of 2 mJ per pulse from
the OPA from the Nd:YAG laser with power of 42 mJ per
pulse. Improvements in the beam quality of the Nd:YAG and
in the alignment of this system have resulted in an increase
in the OPA energy to 14.6 mJ per pulse.
The output of the OPA was measured using an ORIEL
grating monochromator. As the power of the energy output
increase the difference between OPO/OPA decreases due to
the saturation effect. For this measurement the KTP crystal
was at room temperature. By placing the KTP crystal in an
oven we were able to study its temperature dependence
which is important for the system performance. If we
increase the temperature of the crystal from 25 to 55 degrees
C, the peak of the OPOIOPA output shifts to shorter
wavelengths and the shift is on the order of one nm which
agrees with reports in the literature [12].
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Figure 7. OPA output
The spectral width has been reduced to about 2 nm. All of
the photons coming out of the OPA are within the bandpass
of our lidar receiver.
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Figure 8. Schematic of the broadband lidar system using
SLED. Nd:YAG and OPA.
lln Figure 8 we show a schematic of the field lidar
system. There are CO, absorption lines located at 1571.7,
1572.0, 1572.3, 1572.65, and 1573.0 nm that will absorb
light from this transmitter.
The output of the SLED alone is a continuous laser beam
with an average power on the order of 20 mW. The output of
the OPA are14mJ pulses at 15 Hz.
C. Broadband two micron lidar for CO 2 column
measurements
We are interested to develop a similar lidar for the 2
micron spectral region. The strong CO 2 band with
wavelengths in the vicinity of 2 pin provides a second and
totally independent measure of the CO, abundance. The 2
pm band measurements are also sensitive to variations in
atmospheric pressure and humidity along the optical path.
The 2 µm band spectra are very sensitive to the presence of
aerosols which will tremendously enhance the measurement
accuracy.
The performance of a lidar for measuring CO, at 2 pm
has been simulated assuming a 500 km orbit with a sampling
scale footprint 100m. The telescope diameter is 1.5 in
the Field of View (FOV) of the sensor is 200 Arad. The laser
output of 100nsec pulses at 10 kHz will have an average
power of 22 Watts. The laser beam will be passing through a
prefilter (FWHM = 5nm) with the same design as the
prefilter that will be used in the receiver so the output light is
at 2. 054 µm.
We did an etalon study and the Fabry-Perot etalon for the
receiver will be a 2.5 mm fused silica etalon with finesse =
Figure 9. Generated synthetic spectrum for the two
micron laser
17. For the narrow band prefilter the specifications are:
thickness 14.2 pm and 25finesse. In the simulations we
assume an Earth albedo of 10%. The generated synthetic
spectrum with the prefilter shape, the Fabry-Perot passbands
and the CO, absorption lines at that spectral region is shown
in the Figure 9.
I1I. CONCLUSIONS
We presented here a prototype active Fabry-Perot based
sensor for absorption measurements of total column carbon
dioxide using broadband laser sources for 1.58µm and 2
gm. The sensitivity and feasibility of the technique has been
demonstrated in the laboratory when used with an
absorption cell filled with carbon dioxide. Additionally,
more laboratory experiments, field measurements and
airborne tests will validate the instrument's sensitivity to
measure atmospheric CO2.
The sensor described here has the potential for providing
relatively simple optical detection for carbon dioxide. The
future technology to study the Earth system and climate will
more and more shift toward spaceborne lidars instead of
passive sensors. The advantage of lasers is that they can
determine the optical path length for the measurement
process very precisely eliminating a serious source of error
that may affect passive systems. Lasers can also operate
without the need for sunlight and so can make
measurements of the full diurnal cycle of CO2 around the
whole earth. Therefore the broadband approach for accurate
CO, measurement from space is very important to be fully
developed and implemented.
REFERENCES
[1] O'Brien D. M., Rayner P. J., "Global observations of the carbon
budget 2. CO, column from differential absorption of reflected
sunlight in the 1.61 mm band of COz", Journal of Geophysical
Research 107 (D18), Art.No 4354, 1-16 (2002).
[2] Earth Science and Applications from Space: National Imperatives for
the Next Decade and Beyond, Committee on Earth Science and
Applications from Space: A Community Assessment and Strategy for
the Future, National Research Council, pp.9, Table 2, 4-14 (2007).
[3] Georgieva E.M., Wilson E.L., Miodek M., Heaps W.S., "Total
Column Oxygen Detection Using Fabry-Perot Interferometer`,
Optical Engineering 45 (11), Paper No 115001, 1-1](2006).
[4] E.L. Wilson, E. M. Georgieva, W. S. Heaps, "Development of a
Fabry-Perot interferometer for ultra-precise measurements of column
CO,", Measurement Science and Technology 18, 1495-1502 (2007).
[5] W.S. Heaps, E.L. Wilson, Georgieva E.M, Precision Measurement of
Atmospheric Trace Constituents Using a Compact Fabry-Perot
Radiometer", International Journal of High Speed Electronics and
Systems (UHSES), 18, 3, September, 2008.
[6] Georgieva E.M., W. S. Heaps, E. L. Wilson.: "Differential
Radiometers using Fabry-Perot Interferometrc Technique for Remote
Sensing of Greenhouse Gases", IEEE Transactions on Geoscience
and Remote Sensing (TGARS), 46, 10, October special issue, 2008.
[7] W.S.Heaps, `Broadband lidar technique for precision CO,
measurement", Proc. SPIE, Vol. 7111, 711102 (2008).
[8] Pruitt J, Dobbs ME, Gypson M, Neff BR, Sharp WE, "High-speed
CW lidar retrieval using spectral lock-in algorithm", SPIE, Vol.
5154,138-145, 2003.
[9] Abshire JB, Riris H, Hasselbraek, Allan G„ Weaver C, Mao JP,
"Airborne Measurements of CO2 Column absorption using a Pulsed
Wavelength-scanned Laser Sounder Instrument", CLEOlQELS 2009,
Vol.S 1-5, 255-256, 2009.
[10] Parkes AM, Keen KA, McNaghten ED, "Trace gas detection using a
novel cantilever-based photoacoustic spectrometer with multiplexed
optical fiber-coupled diode lasers and fiber-amplification ",SPIE
Vol.6770, C7701-C7701, 2007.
[11] Neff BR, Dobbs ME, Gypson M, Pruitt J, Sharp WE, "CO2
monitoring with a field deployable NIR standoff environmental
sensor", SPIE, Vol. 5154, 118-125, 2003
[12] Stultz R.D. and Ehritz M.E, "Temperature (-32degC to+90degC)
Performance of a 20 Hz Potassium Titanyl Phosphate Optical
Parametric Oscillator", OSA TOPS on Advanced Solid-State Lasers,
Vol.l, Stephen A.Payne and Clifford Pollock (eds.), 1996.


A Broad Bank Lidar for Precise Atmospheric CO2 Column Absorption Measurement from Space

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