A number of Earth-observation missions, particularly those aimed at monitoring atmosphere composition, require hyperspectral instruments measuring Earth reflectance and emission at very fine spectral resolution. Examples include instruments for ESA’s Sentinel 5 mission, operating in the short-wave IR (SWIR) spectral region to quantify greenhouse gases, and also in the near-IR region (Oxygen-A band) to provide data on clouds, aerosols and atmosphere pressure. Conventional dispersive spectrometers, typically using diffraction gratings, are needed to provide data at moderate spatial resolution over large swath widths. Fine spectral resolution, together with the system aperture needed for radiometric resolution, tends to demand large beam diameters at gratings; this can lead to excessively large spectrometers, driven by the apertures of gratings and the associated collimators and camera lenses. This presents problems for accommodation of space-based spectrometers, particularly on small platforms.<p> </p>Apertures, and complete spectrometer sizes, are reduced if the gratings provide high angular spectral dispersion (radians/nm) – this is a property of “immersed” gratings <sup>1,2</sup>. An immersed grating is a reflecting grating formed on a prism of refracting material, in which the incident and diffracted beams are both in the refracting medium. Instruments will typically use immersed gratings in silicon for short-wave IR bands, and silica for near-IR bands. This paper describes example designs for spectrometers using immersed gratings, and early results from a current development of immersed gratings are outlined.
We present the status of our immersed diffraction grating technology, as developed at SRON and of their multilayer optical coatings as developed at CILAS. Immersion means that diffraction takes place inside the medium, in our case silicon. The high refractive index of the silicon medium boosts the resolution and the dispersion. Ultimate control over the groove geometry yields high efficiency and polarization control. Together, these aspects lead to a huge reduction in spectrometer volume. This has opened new avenues for the design of spectrometers operating in the short-wave-infrared wavelength band. Immersed grating technology for space application was initially developed by SRON and TNO for the short-wave-infrared channel of TROPOMI, built under the responsibility of SSTL. This space spectrometer will be launched on ESA's Sentinel 5 Precursor mission in 2015 to monitor pollution and climate gases in the Earth atmosphere. The TROPOMI immersed grating flight model has technology readiness level 8. In this program CILAS has qualified and implemented two optical coatings: first, an anti-reflection coating on the entrance and exit facet of the immersed grating prism, which reaches a very low value of reflectivity for a wide angular range of incidence of the transmitted light; second, a metal-dielectric absorbing coating for the passive facet of the prism to eliminate stray light inside the silicon prism. Dual Ion Beam Sputtering technology with in-situ visible and infrared optical monitoring guarantees the production of coatings which are nearly insensitive to temperature and atmospheric conditions. Spectral measurements taken at extreme temperature and humidity conditions show the reliability of these multi-dielectric and metal-dielectric functions for space environment. As part of our continuous improvement program we are presently developing new grating technology for future missions, hereby expanding the spectral range, the blaze angles and grating size, while optimizing performance parameters like stray light and wavefront error. The program aims to reach a technology readiness level of 5 for the newly developed technologies by the end of 2012. An outlook will be presented.
New immersed grating technology is needed particularly for use in imaging spectrometers that will be used in sensing the atmosphere O<sub>2</sub>A spectral band (750nm - 775 nm) at spectral resolution in the order of 0.1 nm whilst ensuring a high efficiency and maintaining low stray light. In this work, the efficiency, dispersion and stray light performance of an immersed grating are tested and compared to analytical models. The grating consists of an ion-beam etched grating in a fused-silica substrate of 120 mm x 120mm immersed on to a prism of the same material. It is designed to obtain dispersions > 0.30°/nm<sup>-1</sup> in air and >70% efficiency. The optical performance of the immersed grating is modelled and methods to measure its wavefront, efficiency, dispersion and scattered radiance are described. The optical setup allows the measurement of an 80mm beam diameter to derive the bidirectional scatter distribution function (BSDF) from the immersed grating from a minimum angle of 0.1° from the diffracted beam with angular resolution of 0.05°. Different configurations of the setup allow the efficiency and dispersion measurements using a tuneable laser in the 750nm-775nm range. The results from the tests are discussed with the suitability of the immersed gratings in mind for future space based instruments for atmospheric monitoring.
TROPOMI, the Tropospheric Monitoring Instrument, is a passive UV-VIS-NIR-SWIR trace gas spectrograph in the line of SCIAMACHY (2002) and OMI (2004), instruments with the Netherlands in a leading role. Both instruments are very successful and remained operational long after their nominal life time.<p> </p> TROPOMI is the next step, scheduled for launch in 2015. It combines the broad wavelength range from SCIAMACHY from UV to SWIR and the broad viewing angle push-broom concept from OMI, which makes daily global coverage in combination with good spatial resolution possible. Using spectral bands from 270-500nm (UV-VIS) 675-775nm (NIR) and 2305-2385nm (SWIR) at moderate resolution (0.25 to 0.6nm) TROPOMI will measure O3, NO2, SO2, BrO, HCHO and H2O tropospheric columns from the UV-VIS-NIR wavelength range and CO and CH4 tropospheric columns from the SWIR wavelength range. Cloud information will be derived primarily from the O2A band in the NIR. This will help, together with the aerosol information, in constraining the light path of backscattered solar radiation. Methane (CH4), CO2 and Carbon monoxide (CO) are the key gases of the global carbon cycle. Of these, Methane is by far the least understood in terms of its sources and is most difficult to predict its future trend. Global space observations are needed to inform atmospheric models. The SWIR channel of TROPOMI is designed to achieve the spectral, spatial and SNR resolution required for this task. <p> </p>TROPOMI will yield an improved accuracy of the tropospheric products compared to the instruments currently in orbit. TROPOMI will take a major step forward in spatial resolution and sensitivity. The nominal observations are at 7 x 7 km2 at nadir and the signal-to-noises are sufficient for trace gas retrieval even at very low albedos (down to 2%). This spatial resolution allows observation of air quality at sub-city level and the high signal-to-noises means that the instrument can perform useful measurements in the darkest conditions.<p> </p> TROPOMI is currently in its detailed design phase. This paper gives an overview of the challenges and current performances. From unit level engineering models first results are becoming available. Early results are promising and this paper discusses some of these early H/W results.<p> </p> TROPOMI is the single payload on the Sentinel-5 precursor mission which is a joint initiative of the European Community (EC) and of the European Space Agency (ESA). The 2015 launch intends to bridge the data stream from OMI / SCIAMACHY and the upcoming Sentinel 5 mission. The instrument is funded jointly by the Netherlands Space Office and by ESA. Dutch Space is the instrument prime contractor. SSTL in the UK is developing the SWIR module with a significant contribution from SRON. Dutch Space and TNO are working as an integrated team for the UVN module. KNMI and SRON are responsible for ensuring the scientific capabilities of the instrument.
In order to measure atmospheric concentrations of carbon monoxide, methane, water and carbon dioxide from spaceborne
platforms, Short-Wave Infrared (SWIR) immersed grating spectrometers are employed. Due to the need to
minimise detector dark current and internal black body radiation from the spectrometer’s own structure, these
instruments are operated at cryogenic temperatures. ESA’s Sentinel 5-Precursor is a small satellite science mission; the
platform comprises the Tropospheric Monitoring Instrument (TROPOMI), which includes a SWIR module.
Optical mounts have been developed for the SWIR module which meet the requirements to cope with the differences in
thermal expansion between the optical elements and their structural mounts over cryogenic temperature ranges, be robust
against the mechanical environment during launch, and maintain optical alignment stability with a tight volume
Throughout the design of the SWIR spectrometer, flexures were deployed to control deformations due to thermal
expansion, the design of interfaces between materials of differing coefficient of thermal expansion was carefully
managed, and the geometry of adhesive pads was tightly controlled. Optical mounting concepts were evaluated using
finite element analysis (FEA). A breadboard programme was undertaken to verify these concepts. FEA and breadboard
results were correlated to provide confidence in the design.
The breadboard programme consisted of thermal cycling and pull-testing of adhesive joints, as well as environmental
and optical testing of representative subsystems.
Analysis and breadboarding demonstrated that the optical mounting design will survive the mechanical and thermal
environments, and verified the stability of the optical alignment requirements.
Novel optical mounting structures have been designed, analysed, assembled, tested and integrated into the optical
assemblies of the TROPOMI SWIR spectrometer, creating a compact and robust state of the art instrument. These
concepts are applicable to instruments for astronomical missions aiming to characterise exoplanets, as well as Earth
The Tropospheric Monitoring Instrument, TROPOMI, is a passive UV-VIS-NIR-SWIR spectrograph, which uses sun
backscattered radiation to study the Earth's atmosphere and to monitor air quality, on both global and local scale. It
follows in the line of SCIAMACHY (2002) and OMI (2004), both of which have been very successful. OMI is still
operational. TROPOMI is scheduled for launch in 2015. Compared with its predecessors, TROPOMI will take a major
step forward in spatial resolution and sensitivity. The nominal observations are at 7 x 7 km<sup>2</sup> at nadir and the signal-tonoises
are sufficient for trace gas retrieval even at very low albedos (2 to 5%). This allows observations of air quality at
sub-city level. TROPOMI has reached CDR status and production of flight model units has started. Flight detectors have
been produced and detector electronics is expected to be finished by mid-2013. The instrument control unit is undergoing
extensive tests, to ensure full instrument functionality. Early results are promising and this paper discusses these H/W
results, as well as some challenges encountered during the development of the instrument.
New immersed grating technology is needed particularly for use in imaging spectrometers that will be used in sensing
the atmosphere O2A spectral band (750nm - 775 nm) at spectral resolution in the order of 0.1 nm whilst keeping a high
efficiency and low stray light. In this work, an Ion-beam etched grating in a fused-silica substrate of 100 mm 100mm
immersed on a prism of the same material is designed to obtain dispersions > 0.30°/nm<sup>-1</sup> and 70% efficiency. The optical performance of the immersed grating is modelled and methods to measure its efficiency and scattered radiance are
described. The optical setup allows the measurement of an 80mm beam diameter to derive the bidirectional scatter
density function (BSDF) from the immersed grating from a minimum angle of 0.1° from the diffracted beam with
angular resolution of 0.05°.