Characterization of Gratings for space applications

Control stray light is very important in space industry. Indeed, it is one of the main factors in the decreasing of the signal-to-noise and the contrast. Stray light is generally caused by scattered light and ghost images. The Bidirectional Scatter Distribution function (BSDF) measurements allow the quantification of the scattered light by an optical element and becomes an important data to take into consideration when designing telescopes. Standard BSDF measurement goniophotometers often have a resolution of about 0.1 degrees and are mainly working in or close to the visible spectrum. This resolution is far too loose to characterize ultra-polished surfaces. Besides, wavelength range of BSDF measurements for space projects needs to be done far from visible range. How can we measure BSDF of ultra-polished surfaces and diffraction gratings in the UV and IR range with high resolution? We worked on developing a new goniophometer bench in order to be able to characterize scattering of ultra-polished surfaces and diffraction gratings used in everyday space applications. This ten meters long bench was developed using a collimated beam approach as opposed to goniophotometer using focused beam. Sources used for IR characterization were CO2 (10.6 mu m) and Helium Neon (3.39 mu m) lasers. Regarding UV sources, a collimated and spatially filtered UV LED was used. The detection was ensure by a photomultiplier coupled with synchronous detection as well as a MCT InSb detector. The so-built BSDF measurement instrument allowed us to measure BSDF of ultra-polished surfaces as well as diffraction gratings with an angular resolution of 0.02 degrees and a dynamic of 10(13) in the visible range. In IR as well as in UV we manage to get 10(9) with same angular resolution of 0.02 degrees. The 1m arm and translation stages allows us to measure samples up to 200mm. Thanks to such a device allowing ultra-polished materials as well as diffraction gratings scattering characterization, it is possible to implement those BSDF measurements into simulation software and predict stray light issues. This is a big help for space industry engineers to apprehend stray light due to surface finishes and to delete those effects before the whole project is done. We are now thinking of possible improvement on our optical bench to try to get dynamic in IR and UV similar to what we have in visible range (e.g. 10(13)).