Measurement of birefringence of low-loss, high-reflectance coating of m-axis sapphire

The barefringence of a low-loss, high-reflectance coating applied to an 8-c m-diameter sapphire crystal grown in the m-axis direction has been mapped. By monitoring the transmission of a high-finesse Fabry.-Perot cavity as a f unction of the polarization of the input light, we find an upper limit for the magnitude of the birefringence of 2.5 x 10(-4) rad and an upper limit i n the variation in direction of the birefringence of 10 deg. These values a re sufficiently small to allow consideration of m-axis sapphire as a substr ate material for the optics of the advanced detector at the Laser Interfero meter Gravitational Wave Observatory. (C) 2001 Optical Society of America.