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 echelle-spectrometer 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 French-Belgium-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

100 мм

Диф. решетка

щель

Телескоп

АОПФ произвольно поляризованного света

КоллиматорДетектор (вертикально)

диафрагма поля

Солнце

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).

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.)

Typical evolution of atmospheric spectral transmittances through one solar occultation observed by SOIR spectrometer.

Optical studies of the Earth atmosphere with high spectral resolution

• A compact high-resolution system, consisting of an echelle-spectrometer, 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.

• 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.

Measuring of atmospheric greenhouse gases contents

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 cross-dispersion 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.

AOTF bandpass: 50 cm-1

Wavelength, µm

• 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 / Spectra-Physics / 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 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.

Treatment of experimental data

Optical layout of the compact echelle-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.

Key parameters and key components of the spectrometer

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. Examples of spectra, recorded during

the laboratory modeling of the spectrometer.

230 232 234 236 238 240 242 244 246 248 2500.0

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230 232 234 236 238 240 242 244 246 248 2500.0

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Early spectral records of the 1152 nm line of a He-Ne laser

for slightly different angular positions of the echelle-grating

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.

Absorption spectrum of H2O vapor in the laboratory air. (December, 2001)

0 64 128 192 256 320 384 448 512

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Measured spectrum

Intensity, transmittance

Pixel number of the linear detector array

7260 7270 7280 7290 7300 7310

Synthetic spectrum

Wavenumber, cm-1

= 1.38 мкм, / 3000056th order of diffraction

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0,00E+00

2,00E-02

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1,40E-01

1,60E-01

1,80E-01

1 28 55 82 109 136 163 190 217 244 271 298 325 352 379 406 433 460 487

Ряд1

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“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)

during study of ground-based 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 spectrometerat 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 echelle-grating 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.