Experimental and theoretical study of bulk light scattering in CaF_2 monocrystals

摘要

A variety of different types of light scattering (Rayleigh, Brillouin, Raman, and Mie scatterings), which may contribute to the transmission losses in the bulk of high-quality CaF_2 monocrystals, have been investigated experimentally and theoretically. The angular distribution of Rayleigh and Mie scatterings in the visible spectral range has been measured using an imaging technique. An effective technique for scattering measurements in the uv spectral range has also been implemented. Measurement of Raman scattering has shown that its contribution to the total scattering loss is negligible. In order to estimate the scattering from thermal fluctuations versus that from crystalline defects, a measurement of the temperature dependence of the scattering at the scattering angle of 90° has been performed. It has been shown that scatterings from thermal fluctuations and defects are of the same order of magnitude. Taking into account the dependence of the Brillouin scattering on the direction of the incident beam relative to crystalline axes, an upper limit of scattering loss due to thermal fluctuation in the perfect crystal has been evaluated theoretically. At 193 nm this limit appeared to be 2.6 X 10~(-5) cm~(-1), which is about two orders of magnitude less than the scattering loss in fused silica. Measured values of Rayleigh and Mie scatterings are in reasonable agreement with theoretical evaluation. Scattering measured in high-quality samples are comparable with the scattering calculated from thermodynamic fluctuations, indicating that the transmission of these samples is approaching the theoretical limit.