A high-purity circular polarization modulator: Application to birefringence and circular dichroism measurements on multidielectric mirrors

We present a method for producing a modulated circular polarization of high purity, i.e. whose modulation frequency allows specific discrimination against linear polarization and unpolarized intensity. We apply it to measure the circular dischroism and birefringence of high reflectivity multidielectric mirrors: no circular dichroism is observed at a level of 10^?6, but birefringence values from 3 x 10^?4 to 2 x 10^?3 radian per reflexion under normal incidence are obtained.