SPEXone is a compact five–angle spectropolarimeter that is being developed as a contributed payload for the NASA Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) observatory, to be launched in 2022. SPEXone will provide accurate atmospheric aerosol characterization from space for climate research, as well as for light path correction in support of the main Ocean Color Instrument. SPEXone employs dual beam spectral polarization modulation, in which the state of linear polarization is encoded in a spectrum as a periodic variation of the intensity. This technique enables high polarimetric accuracies in operational environments, since it provides snapshot acquisition of both radiance and polarization without moving parts. This paper presents the polarimetric error analysis and budget for SPEXone in terms of polarimetric precision and polarimetric accuracy. We consider factors that contribute to instrumental polarization and modulation efficiency, which will be calibrated on-ground with high, but finite accuracy. The sensitivity to dynamic systematic effects in a space environment, such as degradation and ageing of components and small variations in the temperature and thermal gradients is addressed and quantified. Finally, the impact of scene dependent error sources, mainly resulting from stray light, are assessed and the total polarimetric error budget is presented. We show that SPEXone complies with the radiometric SNR requirement of 300, yielding a minimum polarimetric precision of 200 (fully polarized light) to 300 (unpolarized light) over the full spectral range for dark ocean scenes at high solar zenith angle. Assuming a stray light correction factor of 5 and considering a moderate contrast scene, the expected in-flight polarimetric accuracy of SPEXone is 1.5 · 10<sup>−3</sup> for unpolarized scenes and 2.9 · 10<sup>−3</sup> for highly polarized scenes, compliant with the polarimetric accuracy requirement. This performance should enable SPEXone to deliver the data quality that enables unprecedented aerosol characterization from space on the NASA PACE mission.
In this article the immersed gratings for the ESA Copernicus Sentinel-5 mission are presented. The manufacturing approach is shown and the optical performance of the SWIR-3 immersed gratings as well as the results of the environmental tests are discussed. The immersed gratings show an average efficiency of 60% and a wavefront error of 200 nm rms. The total integrated scatter over the complete stray-light hemisphere excluding ghosts from internal reflections is found to be 0.2% using a conservative estimate. A method for the derivation of the wavefront error from separate surface measurements is presented and the results are compared to measurements with an experimental Shack- Hartmann setup. The immersed gratings are produced by bonding a prism to a wafer with a grating. Environmental tests and testing at operational temperatures show the suitability of this approach for complex space optical components. The article concludes with possible improvements in the optical performance of future immersed gratings.
We have developed a 6 dm<sup>3</sup>-sized optical instrument to characterize the microphysical properties of fine particulate matter or aerosol in the Earth atmosphere from low Earth orbit. Our instrument can provide detailed and worldwide knowledge of aerosol amount, type and properties. This is important for climate and ecosystem science and human health [1, 2]. Therefore, NASA, ESA and the European Commission study the application of aerosol instruments for planned or future missions. We distinguish molecular Rayleigh scattering from aerosol Mie-type scattering by analyzing multi-angle observations of radiance and the polarization state of sun light that is scattered in the Earth atmosphere . We measure across the visible wavelength spectrum and in five distinct viewing angles between -50° and +50°. Such analysis has been traditionally done by rotating polarizers and band-filters in front of an Earth observing wide-angle imager. In contrast, we adopt a means to map the linear polarization state on the spectrum using passive optical components . Thereby we can characterize the full linear polarization state for a scene instantaneously. This improves the polarimetric accuracy, which is critical for aerosol characterization, enabling us to distinguish for example anthropogenic from natural aerosol types. Moreover, the absence of moving parts simplifies the instrument, and makes it more robust and reliable. We have demonstrated this method in an airborne instrument called SPEX airborne [5, 6] in the recent ACEPOL campaign together with a suite of state-of-the art and innovative active and passive aerosol sensors on the NASA ER-2 high-altitude research platform . An earlier report on the SPEX development roadmap was given in . In this contribution we introduce SPEXone, a compact space instrument that has a new telescope that projects the five viewing angles onto a single polarization modulation unit and the subsequent reflective spectrometer. The novel telescope allows the observation of five scenes with one spectrometer, hence the name. We describe the optical layout of the telescope, polarization modulation optics, and spectrometer and discuss the manufacturability and tolerances involved. We will also discuss the modelled instrument performance and show preliminary results from optical breadboards of the telescope and polarization modulation optics. With SPEXone we present a strong and new tool for climate research and air quality monitoring. It can be used to study the effect of atmospheric aerosol on the heating/cooling of the Earth and on air quality. Also, SPEXone can improve the accuracy of satellite measurements of greenhouse gas concentrations and ocean color that rely on molecular absorption of reflected sunlight by providing detailed knowledge of the aerosol properties, required to accurately trace the light path in presence of scattering.
SPEXone is developed in a partnership between SRON Netherlands Institute for Space Research and Airbus Defence and Space Netherlands with support from the Netherlands Organisation for Applied Scientific Research (TNO) as a Dutch contribution to the NASA PACE observatory launching in 2022.
CILAS is involved in a scientific project for SRON Netherlands Institute for Space Research, on the development of multilayer coatings for the silicon immersed grating prism, which is a key component of the short-wave-infrared (SWIR) channel of the TROPOMI imaging spectrometer. <p> </p>For the project, two specific coatings have been implemented and qualified by CILAS: first, an antireflection coating deposited on the entrance and exit facets of the immersed grating prism, which reaches a very low value of reflectivity in the infrared [2305nm; 2385nm] spectral range and for a wide angular range [0° to 47°] of incidence of the transmitted light, and second, a metal-dielectric absorbing coating for the third facet of the prism to eliminate parasitic light inside the silicon prism.
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.
Immersed gratings offer several advantages over conventional gratings: more compact spectrograph designs, and by using standard semiconductor industry techniques, higher diffraction-efficiency and lower stray-light can be achieved. We present the optical tests of the silicon immersed grating demonstrator for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We detail the interferometric tests that were done to measure the wavefront-error and present the results of the throughput and stray-light measurements. We also elaborate on the challenges encountered and lessons learned during the immersed grating demonstrator test campaign that helped us to improve the fabrication processes of the grating patterning on the wafer.
Immersed gratings offer several advantages over conventional gratings: more compact spectrograph designs, and by using standard semiconductor industry techniques, higher diffraction-efficiency and lower stray-light can be achieved. We present the optical tests of the silicon immersed grating demonstrator for the Mid-infrared E-ELT Imager and Spectrograph, METIS. We detail the interferometric tests that were done to measure the wavefront-error and present the results of the throughput and stray-light measurements. We also elaborate on the challenges encountered and lessons learnt during the immersed grating demonstrator test campaign that helped us to improve the fabrication processes of the grating patterning on the wafer.
The use of Immersed Gratings offers advantages for both space- and ground-based spectrographs. As diffraction takes place inside the high-index medium, the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and a smaller spectrometer size. Short-wave infrared (SWIR) spectroscopy is used in space-based monitoring of greenhouse and pollution gases in the Earth atmosphere. On the extremely large telescopes currently under development, mid-infrared high-resolution spectrographs will, among other things, be used to characterize exo-planet atmospheres. At infrared wavelengths, Silicon is transparent. This means that production methods used in the semiconductor industry can be applied to the fabrication of immersed gratings. Using such methods, we have designed and built immersed gratings for both space- and ground-based instruments, examples being the TROPOMI instrument for the European Space Agency Sentinel-5 precursor mission, Sentinel-5 (ESA) and the METIS (Mid-infrared E-ELT Imager and Spectrograph) instrument for the European Extremely Large Telescope. Three key parameters govern the performance of such gratings: The efficiency, the level of scattered light and the wavefront error induced. In this paper we describe how we can optimize these parameters during the design and manufacturing phase. We focus on the tools and methods used to measure the actual performance realized and present the results. In this paper, the bread-board model (BBM) immersed grating developed for the SWIR-1 channel of Sentinel-5 is used to illustrate this process. Stringent requirements were specified for this grating for the three performance criteria. We will show that –with some margin– the performance requirements have all been met.
We present results of our integrated approach to the development of novel diffraction gratings. At SRON we manufacture prism-shaped silicon immersed gratings. Diffraction takes place inside the high-refractive index medium, boosting the resolving power and the angular dispersion. This enables highly compact spectrometer designs. We are continuously improving the cycle of design, simulation and test to create custom gratings for space and ground-based spectroscopic applications in the short-wave infrared wavelength range. Applications are space-based monitoring of greenhouse and pollution gases in the Earth atmosphere and ground-based SWIR spectroscopy for, a.o., characterization of exo-planet atmospheres . We make gratings by etching V-shaped grooves in mono-crystalline silicon. The groove facets are aligned with the crystal lattice yielding a smooth and highly deterministic groove shape. This enables us to predict the polarized efficiency performance accurately by simulation. Feeding back manufacturing tolerances from our production process, we can also determine reliable error bars for the predicted performance. Combining the simulated values for polarized efficiency with ray-tracing, we can optimize the shape of the grating prism to eliminate unwanted internal reflections. In this contribution we present the architecture of our design and simulation platform as well as a description of test setups and typical results.
We present innovative, immersed grating based optical designs for the SMO (Spectrograph Main Optics) module of the
Mid-infrared E-ELT Imager and Spectrograph, METIS. The immersed grating allows a significant reduction of SMO
volume compared to conventional echelle grating designs, because the diffraction takes place in high refractive index
silicon. Additionally, using novel optimization techniques and technical solutions in silicon micromachining offered by
the semiconductor industry, further improvements can be achieved. We show optical architectures based on compact,
double-pass Three Mirror Anastigmat (TMA) designs, which appear advantageous in terms of one or several of the
following: optical performance, reduction of volume, ease of manufacturing and testing. We explore optical designs,
where the emphasis is put on manufacturability and we investigate optical solutions, where the ultimate goal is the
highest possible optical performance. These novel, silicon immersed grating based design concepts are applicable for
future earth and space based spectrometers.
We have developed the technology to manufacture an immersed grating in silicon for the Mid-infrared E-ELT Imager
and Spectrograph, METIS. We show that we can meet the required diffraction-limited performance at a resolution of
100000 for the L and M spectral bands. Compared to a conventional grating, the immersed grating drastically reduces the
beam diameter and thereby the size of the spectrometer optics. As diffraction takes place inside the high-index medium,
the optical path difference and angular dispersion are boosted proportionally, thereby allowing a smaller grating area and
a smaller spectrometer size. The METIS immersed grating is produced on a 150 mm industry standard for wafers and
replaces a classical 400 mm echelle. Our approach provides both a feasible path for the production of a grating with high
efficiency and low stray light and improves the feasibility of the surrounding spectrometer optics.
In this contribution we describe and compare the classical-grating solution for the spectrometer with our novel
immersed-grating based design. Furthermore, we discuss the production route for the immersed grating that is based on
our long-standing experience for space-based immersed gratings. We use standard techniques from the semiconductor
industry to define grating grooves with nanometer accuracy and sub-nanometer roughness. We then use optical
manufacturing techniques to combine the wafer and a prism into the final immersed grating. Results of development of
the critical technology steps will be discussed.
We have developed a novel diffraction grating based on lithographical techniques and anisotropic etching in silicon. The
grating is designed for the short-wave-infrared channel of the TROPOMI imaging spectrometer that will be launched on
ESA's Sentinel 5 Precursor mission to monitor trace gases in the earth atmosphere. Stringent requirements on both the
imaging properties and the quality of the spectra translate to a high-tech grating. In our design the dispersion and
resolution is increased with a factor 3.4 with respect to conventional gratings by using the grating in immersion, such
that diffraction takes place inside the silicon grating material. By lithographic patterning and anisotropic etching of the
mono-crystalline silicon we precisely control line spacing and blaze angle. The grating has a line spacing of 2.5 μm and
is operated in sixth order. We show that an efficiency of 60% is reached on a 50 x 60 mm<sup>2</sup> grating surface. We compare
our test results with numerical calculations for grating efficiency for both polarizations and find good agreement.