Tabletop experimental demonstration of a heterodyne polarimetry method for in situ substrate birefringence...

High-quality test mass substrates play essential roles in laser interferometric gravitational wave detectors. Inhomogeneous birefringence distribution in test mass substrates, however, can degrade the sensitivity of the detector by introducing optical loss and disturbing the interferometer controls. In this paper, we present a heterodyne polarimetry method that has the potential for in situ birefringence characterizations, hence diagnosing the gravitational wave interferometer. We experimentally demonstrate the proposed method with a tabletop setup. We also discuss its applicability to current and future gravitational wave detectors and the detectable limit.