Simultaneous Birefringence and Absorption Mapping in Large-Size Sapphire Substrates for Gravitational-Wave Interferometry

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, we are using large-size Sapphire crystals 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. 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 ∆n ≈ 10 −10 and a spatial resolution of 0.3 × 0.3 mm 2. In this paper, we will further show how we could draw information of internal stress distributions from the measured birefringence and 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.