Transcript компактный эшелле-спектрометр высокого разрешения с

Compact echelle - spectrometer of high resolution with the
sequential selection of orders for satellite studies of the
Earth's atmosphere
Компактный эшелле-спектрометр высокого
разрешения с последовательной селекцией порядков
для спутниковых исследований земной атмосферы
Vinogradov I., and work team
of experiments: RUSALKA and ORAKUL
Space Research Institute (IKI)
Planetary Exploration Division
Laboratory of Experimental Spectroscopy
IKI, Tarusa, 04 September 2007
Contents
• Background
• Optical studies of the Earth atmosphere
with high spectral resolution. Measuring of
atmospheric greenhouse gases contents.
• Optical principles of the compact echellespectrometer layout. Key parameters and
key components of the spectrometer.
• Preliminary results.
• Future trends and perspectives.
Background
• Laboratory demonstration of the proposed spectral method for
the space-borne atmospheric studies: -- Korablev O.I., Bertaux J.-L.,
Vinogradov I.I., Compact high-resolution IR spectrometer for atmospheric
studies, Proc. SPIE , 4818, 272-281, 2002.
• Earlier demonstration of the AOTF-echelle spectrometer for
laboratory studies, industry and environment monitoring by
American team: -- Baldwin D.P. et al., Proc. SPIE, 3534, 478-486, 1998.
• Acceptation of the SPICAV/SOIR experiment for the Venus
Express mission by ESA. Successful joint work of the FrenchBelgium-Russian SPICAV/SOIR team: -- Korablev O., Bertaux J.-L.,
Nevejans D., et al., SPICAV: A suite of Three Spectometers to Study the
Global Structure and Composition of the Venus Atmosphere, American
Geophysical Union, Fall Meeting 2005, abstract #P23E-02, 12/2005.
-- Nevejans D., et al., Compact high-resolution spaceborne echelle grating
spectrometer with acousto-optical tunable filter based order sorting for the
infrared domain from 2.2 to 4.3 μm, Appl. Opt. 45, 5191-5206 (2006).
• OCO (USA, Crisp D., et al., 2002), and Minicarb (France, Breon F.-M., et
al., 2001) projects and future spacecraft missions.
• Starting research work in IKI, focused on development of
effective optical spectral methods for space-borne studies of
greenhouse gases in the Earth’s atmosphere – end of 2003.
Optical prototype
(spring 2002)
Optical prototype
(2003)
Optical prototype
(2003)
SOIR : эшелле-спектрометр высокого
разрешения с разделением порядков дифракции
при помощи акустооптического фильтра
щель
Детектор
(вертикально)
Коллиматор
АОПФ произвольно
поляризованного света
диафрагма поля
Солнце
Телескоп
100 мм
3-D representation of main SOIR optics elements and ray tracing:
the entrance optics (1), the diaphragm (2), the AOTF (3), the AOTF exit
optics (4), the spectrometer slit (5), the off-axis parabolic mirror (6), the echelle
grating (7), the folding mirror (8), the detector optics (9), and the detector (10).
Nevejans D., et al., Appl. Opt. 45, 5191-5206 (2006).
Typical evolution of atmospheric spectral transmittances through one solar occultation
observed by SOIR spectrometer.
This solar occultation was collected on November 26th, 2006 during a sunset. It is obtained by making the ratio of
the solar spectrum seen through the Venus atmosphere to the unattenuated solar spectrum measured above the
atmosphere. At the beginning of the series, the light path does not cross the atmosphere, and transmittances are
equal to unity. As the sun sets, the light path goes deeper into the atmosphere, and two absorption processes take
place: the overall signal decreases due to extinction by aerosols, and gaseous absorption signatures appear. At
the end, the light path crosses the cloud layer located at an altitude around 60 km above the Venus surface (at
6051.5 km radius) and no light is transmitted anymore. The selection of a spectral interval is achieved through the
AOTF filter. In this particular range, the main absorption lines are from HDO (a trio of lines indicated by arrows),
and other features are from weak CO2 spectral lines. (Bertaux J.-L., Nature, 2007.)
Optical studies of the Earth atmosphere
with high spectral resolution
• A compact high-resolution system, consisting of an echellespectrometer, combined with an acousto-optical tunable filter
(AOTF) for separation and sequential selection of diffraction
orders, is being developed for space-borne studies of the
Earth’s atmosphere in the near IR range (0.7-1.7 m).
• This design allows to achieve high resolving power, λ/Δλ ~
20000-30000, making it possible clear resolution of individual
non-saturated lines within weak absorption bands of
atmospheric gases. By measuring value of atmospheric
absorption for sunlight, which is being reflected or scattered by
the Earth’s surface, this development can be used for accurate
measurements of important atmospheric gases contents, of
isotopic ratios and minor gases.
• By combining of high operational speed (~1 s for recording of a
single spectrum), and narrow field of view (FOV < 1°), the
system will be capable for highly localized gas concentration
measurements with spatial resolution of a few km near the
Earth’s surface.
Measuring of atmospheric greenhouse gases contents
• Monitoring of carbon dioxide (CO2) contents in terrestrial
atmosphere is an actual problem at present, because of its
severe influence on climatic conditions and changes.
Precise, and highly localized measurements of CO2
concentration are needed for adequate analysis of sharing
natural and anthropogenic processes in carbon dioxide
atmospheric balance. Similar global measurements from
ground-based stations with Fourier-spectrometers are
mostly widespread, whereas space-based investigations in
this field have not yet been adequately developed.
• Measuring of methane (CH4) atmospheric contents is
important for our detailed knowledge about ecosystems,
as well as for detecting of contaminating emissions at gas
pipelines at the Russian territory.
Two modes of observation from the orbit
• Nadir orientation of the spectrometer field of view. The
spectrometer detects emission from the Sun, after its double
pass through the Earth’s atmosphere: reaching to the Earth’s
surface, and being scattered by it to the spacecraft direction,
i.e. to the zenith.
• Observation of the bright
sun glint at the water
surface. This mode will
provide for much more
signal-to-noise ratio and
precision due to low
contribution from scattering
at aerosol particles and from
other disturbances.
Spacecraft
Basic spectral parameters
• The principle of the instrument is based at a scheme
of echelle-spectrometer, without any classical crossdispersion elements (which are, typically, passive and
bulky).
• Pre-selection of echelle-grating high diffraction orders
is carried out with the help of an acousto-optical
tunable filter (AOTF), placed inside the entrance
telescope, forming FOV of the instrument.
• AOTF, made by domestic industry, is a crystal of
paratellurite (TeO2), which have been cut and oriented
in a special way, providing for the so-called wide
aperture, wide passband configuration of the AOTF
device.
Separating of echelle-grating orders
•
•
•
•
For performing of each particular measurement, incoming radiation should be
filtered, and being passed to the spectrometer microslit only within a narrow spectral
interval, corresponding to a desired echelle-grating diffraction order.
The selection is carried out by digital synthesizing of the appropriate frequency and
power of the ultrasound acoustical wave, applied to the AOTF crystal by an
integrated piezotransducer.
This appears to be possible to match the AOTF bandpass and the echelle-grating
free spectral range for the complete spectral interval, covered by InGaAs linear array
detector used (by Hamamatsu, 512 pixels 25*500 µm, cut off 1,7 µm).
However, the instant spectral interval, covered by the detector, can be matched to
the FSR only for a “tradeoff” wavelength, given for reasons of data adequacy.
Wavelength, µm
AOTF bandpass:
50 cm-1
• For measurements of greenhouse gases atmospheric content,
there will be resolved individual non-saturated lines within weak
absorption bands of CO2 (1580 nm) and CH4 (1640 nm),
correspondingly, at 49th and 47th diffraction orders of the echelle
grating (24,355 gr/mm, blaze angle 70° by Newport / SpectraPhysics / Richardson Gratings).
• For increasing of accuracy of CO2 and CH4 concentration
measurements, there will be performed additional
measurements of O2 (at absorption bands 760 and 1270 nm,
corresponding to 101th and 61th diffraction orders), which
atmospheric content is well known.
• Selection of the given spectral intervals for measurements
allows coverage of largest parts of the absorption bands, as
well as measuring of the continuum – spectral region without
absorption, keeping information about the Earth’s surface
albedo and effect of aerosol particles.
Transmittance spectra: O2 (0.76 µm and 1.27 µm), CO2 (1.58 µm), CH4 (1.65 µm),
simulated for a standard model of atmosphere (summer in middle latitudes),
for nadir observations and the Sun zenith angle 0º (airmass 2).
Instrumental function have been considered (spectral resolution ~23000).
Treatment of experimental data
• Treatment of experimental data is based on the model of solar radiation
transport in atmosphere within spectral bands of molecular absorption of the
gases of interest.
• There will be considered direct simulation of atmospheric gases properties,
as well as the reciprocal problem of restoring atmospheric gas content,
corresponding to the measured integral (along the line of sight) molecular
absorption value.
• There should be considered uncertainty factors, which may strongly affect
accuracy of the final result:
- pressure variations near the surface,
- albedo variations,
- optical path uncertainty, which is complicated by photon scattering in the
atmosphere,
- temperature profiles,
- location, concentration and optical parameters of aerosol particles and
clouds,
- water vapor variations, affecting accuracy of CO2 concentration
measurements due to more effective broadening of CO2 absorption lines,
compared with O2 and N2.
Optical layout of the compact echelle-spectrometer
Key parameters and key components of the spectrometer
• Entrance FOV telescope – double lens F=120 mm, ø30 mm.
• Acousto-optical tunable filter (AOTF) – a crystal of paratellurite
(TeO2), passband 50 см-1 (FWHM), work range 0,7-1,7 µm
(87-36 MHz), spatial field 5*4 mm, angular aperture +/-2,5,
diffraction angle ~ 5.
• AOTF collimator – double lenses F=25 mm, ø10 mm.
• Etrance microslit – 0,05*0,7 mm.
• Spectrometer collimator optics – off-axis (10) parabolic mirror
F = 200 mm, 50*50*15 mm (F/D=6).
• Echelle-grating – 24,355 gr/mm, 70°, plane substrate
50*100*16 mm, by Newport / Spectra-Physics / Richardson
Gratings.
• Linear InGaAs detector, 512 pixels 25*500 µm, cut off 1,7 µm,
by Hamamatsu.
• Additional flash-memory.
• Supplementary digital photocamera for synchronous monitoring
of the observed area.
Pictures of the spectrometer version, assembled
and aligned at the Industrial division of IKI
3D-model of the spectrometer with the AOTF
AOTF module with embedded electronics
View from the AOTF input window
View from the AOTF output window
AOTF inside the entrance telescope of the spectrometer
A 3D-model of possible
placement of the RUSALKA
apparatus at a viewing port
of the RS ISS.
Looking for the Earth’s views!
3D-model of the CHIBIS microsatellite with a
microspectrometer onboard
There are shown fields of view of the ORAKUL spectrometer and of a photocamera.
Preliminary results.
1.0
1.0
0.8
0.8
Normalised Output
Normalised Output
Examples of spectra, recorded during
the laboratory modeling of the spectrometer.
0.6
0.4
0.2
Early spectral records of the
1152 nm line of a He-Ne laser
for slightly different angular
positions of the echelle-grating
0.6
0.4
0.2
0.0
0.0
230 232 234 236 238 240 242 244 246 248 250
230 232 234 236 238 240 242 244 246 248 250
Pixel Number
AOTF transmittance in terms of the
acoustical wave frequency,
recorded for the stable 1224 nm
line of a diode laser. AOTF
passband = 48 cm-1 FWHM.
Pixel Number
Absorption spectrum of H2O vapor in the laboratory air.
(December, 2001)
Wavenumber, cm-1
7260
1,0
1.0
7270
7280
7290
7300
7310
Synthetic spectrum
Measured spectrum
Intensity, transmittance
0,8
0.8
0,6
0.6
0.4
 = 1.38 мкм,
/  30000
56th order of diffraction
0,2
0.2
0
64
128
192
256
320
384
Pixel number of the linear detector array
448
512
“Raw” atmospheric
absorption spectra at the
CO2 band 1.58 µm,
independently recorded:
at the University of Reims
(France, 13:04, 16.02.2004
– upper picture),
and at Tarusa town
(Industrial division of IKI,
10:30, 10.02.2006 – bottom
picture)
1,80E-01
1,60E-01
1,40E-01
1,20E-01
(264)
1,00E-01
Ряд1
8,00E-02
6,00E-02
4,00E-02
2,00E-02
0,00E+00
1
28
55 82 109 136 163 190 217 244 271 298 325 352 379 406 433 460 487
during study of groundbased methods for solar
observations.
“Raw” atmospheric
absorption spectra at the
O2 band 1.27 µm,
recorded at the University
of Reims (France, 12:19,
16.02.2004 – bottom picture).
Laboratory calibrations of the spectrometer early model at
the University of Reims (France, 30.05-03.06.2005)
• Laboratory measurements were carried out for the pure
CO2, filling an optical multipass cell (optical path L=20
m), under pressure values of 75, 93.8, 283 millibar, at
the spectral range 6200-6250 cm-1 (near 1.60 µm),
temperature T=294 K. Light source – a tungsten filament
lamp.
• Detailed data treatment have been carried out, including
estimation of “continuum”, and calculation of the
spectrometer instrumental function.
• There have been carried out preliminary treatment of
solar spectra test recordings at the 1.60 µm band of
atmospheric CO2 molecular absorption.
Pictures of the experimental
layout for the laboratory
calibrations of the spectrometer
at the University of Reims
(France, 30.05-03.06.2005)
Recorded laboratory spectra treatment sequence:
- a “raw” record, the envelope is being shaped by angular diagram of the echellegrating blaze, by the AOTF transmission spectral contour, and other factors,
- comparison to the synthetic spectrum,
- estimation of continuum, non-absorbed intensity level,
- final comparison of reconstructed data and the model data.
Solar spectra test records at the 1.60 µm band of atmospheric CO2 molecular absorption
and its preliminary treatment:
- a “raw” record of the spectrum,
- approximate estimation and “removing” of the continuum,
- comparison to the model spectrum, correction of the spectral scale of the instrument,
- estimation of the spectrometer instrumental function.
Absorption lines of molecular oxygen O2 at 0.76 µm band,
recorded during ground- based solar observations
(Tarusa town, Industrial division of IKI, 11:39, 24.06.2005)
Future trends and perspectives
• The development have became possible, thanks to the
previous successful work of the IKI team at the joint preparation
of the SPICAV/SOIR experiment for the Venus Express mission
of ESA, and thanks to the RAS Presidium support (Program
P16..P13, and future experiment ORAKUL for the microsatellite
CHIBIS), and thanks to the ENERGIA Corporation (future
experiment RUSALKA for the ISS Russian segment).
• The development will be continued with:
- preparation of the experiment onboard the ISS Russian
segment,
- preparation of the experiment onboard the microsatellite
CHIBIS,
- ground-based studies, and ground-based validation of the
satellite data,
- optimizing optical and spectral parameters of the spectrometer
for other scientific targets, and for various satellite platforms.
• Unique parameters and versatility of the spectrometer are
favorable to its applications in many areas of science and
technology.