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.