New grating concepts in the NIR and SWIR spectral band for high resolution earth-observation spectrometers

We report about our latest achievements to realize the diffraction gratings during the development activities for a future Earth observation high resolution spectrometer studied by ESA. The gratings are manufactured by electron beam lithography on fused silica substrates. The optical performance is considerably increased by applying a dedicated high refractive index coating to the grating structure using atomic layer deposition (ALD). Thus, we were able to achieve diffraction efficiencies larger than 75% averaged over both linear polarizations states, i.e. TE and TM. At the same time, the polarization sensitivity is well below 10% in both cases. Finally, the diffraction gratings for the SWIR-1 spectral channel were bonded on a massive prism substrate in order to realize a GRISM element. This process was achieved by direct fused silica bonding performed under atmospheric pressure within special mechanical equipment designed and constructed particularly for this purpose.