Birefringence compensation method of test-mass substrates for gravitational wave detectors with arbitrary polarization states.

GW detectors are ultimately limited by thermal noise in their most sensitive region. Cryogenic operation combined with crystalline substrates and coatings is a promising approach to reduce this noise, thereby increasing their sensitivity and detection rate. However, crystalline materials can exhibit birefringent behaviors which will degrade the detector's sensitivity. Here, we demonstrate the use of a pair of identical electropolarization retarders to generate arbitrary polarization states and compensate birefringence of a KAGRA test-mass substrate.