Optical properties measurement of an Al2O3 mirror substrate for the Large-Scale Cryogenic Gravitational Wave Telescope (LCGT)

The optical property of a large mirror substrate of 250 mm in diameter was successfully measured at the required resolution to meet the target sensitivity of the Large-Scale Cryogenic Gravitational Wave Telescope (LCGT) project. Although the quality of the substrate piece was not sufficient to achieve the target sensitivity, we could present how to cope with the crystal orientation for reducing any negative effect of birefringence of the substrate, and we could also produce useful data for manufacturers to improve the quality.