Unveiling Earth's Climate: Manufacture and Test of Flight Models for CO2M's Dispersing PG+P Assemblies

We provide an overview of the Fraunhofer IOF activities regarding the PG+P assemblies, which are the central dispersing optical components in the CO2 instrument (CO2I) within the Copernicus Anthropogenic Carbon Dioxide Monitoring (CO2M) satellite mission. Our contribution presents design, qualification, manufacture, assembly, integration and tests (AIT) of the (up-to-now) three fully assembled proto-flight models (PFMs). Each assembly consists of a fused silica grism (PG, that is, prism and grating) and a fused silica counter-prism (P). The counter-prism additionally implements the aperture stop of the optical system. The diffraction grating is a surface relief grating whose manufacture process relies on electron-beam lithography, plasma-assisted reactive ion etching and atomic layer deposition. The bonding of the PG, that is, merging prism and grating into a single optical element, is realized by plasma-activated bonding. This is a direct bonding method which forms covalent molecular bonds between two fused silica surfaces. Each optical surface is functionalized by individually designed anti-reflection coatings. They operate at incident angles of up to 72°. The optical aperture stop is implemented directly to the entrance surface of the counter-prism using a black absorber coating which is patterned by photolithographic techniques and a lift-off process. Eventually, the PG+P is assembled in an iso-static configuration in a two-stage gluing process via three titanium bipods and mechanically mounted to a titanium baseplate for high alignment accuracy of only a few μm. Finally, environmental tests are conducted.