State of the art in silicon immersed gratings for space

Abstract

Abstract-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.

Vicarious Calibration of the TROPOMI-SWIR module over the Railroad Valley playa

“…S5p is part of the Copernicus program headed by the European Commission in partnership with the European Space Agency (ESA). TROPOMI is the first Copernicus instrument aimed at monitoring the chemical composition of Earth's atmosphere over time (for an overview of the instrument design and performance, including the immersed grating, see e.g., Veefkind et al, 2012;Hoogeveen et al, 2013;van Amerongen et al, 2017;Kleipool et al, 2018;van Kempen et al, 2019;Ludewig et al, 2020) The instrument consists of two modules: the UVN module covering ultraviolet (UV), visible (VIS), and near-infrared (NIR) wavelengths and the SWIR module 2 which contains an immersed grating 3 (van Amerongen et al, 2017) and a HgCdTe detector 4 measuring the short-wavelength infrared (SWIR) wavelengths between 2305 and 2385 nm. The SWIR module was specifically designed to measure the dry air columns of carbon monoxide (CO) and methane (CH 4 ).…”