Optical scatter of quantum noise filter cavity optics

We report on measurements of light scattering from two two-inch super-polished fused silica substrates before and after applying (ATFilms) ion-beam sputtered highly-reflective dielectric coatings. We used an imaging scatterometer, that illuminates the sample with a linearly polarized 1064 nm wavelength laser at a fixed angle of incidence and records images of back scatter for azimuthal angles in the plane of the laser beam, to measure the Bidirectional Reflectance Distribution Function (BRDF) and estimate the total integrated scatter for both samples, before and after coating. We find application of these highly reflective coatings leads to an increase of the integrated scatter of the primary surface by more than 50%. In addition, the BRDF function of the coated optics takes on a pattern of maxima and zeroes versus azimuthal angle that is qualitatively consistent with bulk scattering from the coating layers. These results are part of a broader study to understand optical loss in quantum noise filter cavities for interferometric gravitational-wave detectors. For the Advanced LIGO detector, which is envisioning 16 m cavities, the measured level of scattering loss of 5.00 ±.30 ppm and 3.38 ±.20 ppm to wide angles, is acceptable, though reducing the loss further would improve LIGO’s quantum-noise limited performance. a