Precision mapping of a silicon test mass birefringence

Excellent mechanical and thermal properties of silicon make it a promising material for the test masses in future gravitational wave detectors. However, the birefringence of silicon test masses, due to impurity and residual stress during crystal growth or external stress, can reduce the interference contrast in an interferometer. Using the polarization-modulation approach and a scanning system, we mapped the birefringence of a float zone silicon test mass in the «100»crystal orientation to assess the suitability of such material for future gravitational wave detectors. Apart from the stress-induced birefringence at the supporting area due to the weight of the test mass, the high resolution birefringence map of the silicon test mass revealed a high birefringence feature in the test mass. At the central 40 mm area, birefringence is in the range of mid 10 - 9 to low 10 - 8, which satisfy the requirement for future gravitational wave detectors.