The VIRGO large mirrors: a challenge for low loss coatings

Large optical components (diameter 350 mm), having extremely low loss at 1064 nm, are needed for the large-scale Michelson-type interferometer VIRGO. Today, ion beam sputtering is the only deposition technique able to produce optical components having such performances. Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm, in diameter: two large substrates may also be coated at the same time for interferometer arm symmetry problem. The performances of this coater are described in terms of layer uniformity on a large scale (wavefront) and optical losses (absorption and scattering). The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and they have been installed on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the RMS wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performances, especially the absorption level and the mechanical quality factor Q of the mirrors and the substrate wavefront before coating.