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 2 grating surface. We compare our test results with numerical calculations for grating efficiency for both polarizations and find good agreement.

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.

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.

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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 [1].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.