Measurement around 650 nm of reflectivity of multilayers mirrors used for free electrons laser

Abstract

Despite a very low gain the Orsay free electron laser has been operated above threshold using TiO₂/SiO₂ multilayer dielectric mirrors of extremely high reflectivity. We report two methods for measuring this reflectivity both based on an optical cavity decay time measurement. The high signal over noise ratio allow measurement of any losses such as scattering, absorption or transmission down to 10^-7. Due to the high U.V. flux of synchrotron radiation, reflectivity degradation from 99.992 % to 99.9 % has been observed.