ABSTRACT
In high-sensitive laser interferometers, such as the gravitational-wave detector KAGRA, ultra-high-quality mirrors are essential. In the case of KAGRA, where cavity mirrors are cooled down to 20 K, large-size Sapphire crystals are used as the substrate for the main mirrors to achieve both a good optical quality (i.e., low absorption and uniform refractive index) and optimized thermal behavior under cryogenic temperatures. To implement the very tight optical specifications required by this demanding application, it is mandatory to test the optical homogeneity of different substrates. In order to characterize refractive-index inhomogeneities of large-size uniaxial samples such as the KAGRA Sapphire test masses, we developed a dedicated setup, allowing to resolve birefringence changes with a sensitivity in the order of [Formula: see text] and a spatial resolution of [Formula: see text]. Moreover, the same setup allows us to simultaneously record residual absorption maps, thus allowing for a comparison between birefringence and absorption features. In this paper, we will present for the first time measurements on a KAGRA-sized Sapphire substrate which has been characterized in terms of absorption already in an earlier work. Both birefringence inhomogeneities and absorption distributions will be compared and correlations discussed.
ABSTRACT
In order to establish scattering measurements in material investigations for gravitational-wave detectors, we have built-up devices for measuring the hemispherical scattering distribution of materials which are planned to be used in those detectors as suppressors of scattered light. The measurement benches we have built, a hemispherical goniometer and a direct back-scatterometer, have a maximum background noise of â¼10 -4sr -1 BRDF at 1.064 µm wavelength which is the wavelength of the laser-light for our large interferometer for detecting gravitational waves, KAGRA. With these instruments, we have characterized the surface scattering of, e.g., NiP platings, metals, and different carbonaceous coatings, which are supposed to minimize the amount of scattered light in interferometers. The three most important materials for KAGRA's construction (SiC, "Solblack", and "VantaBlack") are presented in this paper. Furthermore, we will try to explain the scattering distributions with the generalized Harvey-Shack model (smooth-surface approximation) which is a common method for surface-scattering calculations. At the end, we give also some valuations about the vacuum compatibility of the materials, which is important for instruments like KAGRA that work under ultra-high vacuum conditions.
ABSTRACT
Scattered light in inteferometric gravitational wave detectors needs to be reduced so that it will not harm the actual signals coming from a gravitational wave. In this paper, we report on the application of the theory of light scattering from mirrors in interferometric detectors having multilayer coatings on their surfaces and compared the results with single-surface scattering theories, which are traditionally used in the field of gravitational wave detectors. For the first time in this field, we have calculated the scattering distributions of the power-recycling, the signal-recycling, and the beam-splitter mirrors in KAGRA (a cryogenic interferometric gravitational wave detector currently under construction in the Kamioka mine in Japan) by using models of their multilayer coatings. Furthermore, we have performed simulations to show the differences between multilayer scattering and single-surface scattering models in the back-scattering of mechanical structures close to the mirrors and the impact on the sensitivity of the KAGRA detector. We show that the back-scattering by using those coatings can be larger by up to almost two orders of magnitude and they also give rise to additional scattering features that should be taken into account for all optical applications in gravitational wave detectors.
