Measurement of mirror birefringence with laser heterodyne polarimetry

A laser heterodyne polarimeter (LHP) designed for the measurement of the birefringence of dielectric super-mirrors is described and initial results are reported. The LHP does not require an optical resonator and so promises unprecedented accuracy in the measurement of the birefringence of individual mirrors. The working principle of the LHP can be applied to the measurement of vacuum birefringence and potentially ALPS (Any Light Particle Search).