Direct measurement of refractive indices (n_e, n_o) of a linear biréfringent retardation plate

A novel method is proposed to measure the indices of refraction of ordinary rays n_o and extraordinary rays n_e of a quartz wave plate simultaneously. An optical heterodyne interferometer, in which a common path loop of P and S waves is designed to directly measure the phase retardation of the wave plate versus the azimuth angle, is set up. The azimuth angle of the wave plate at zero degree is essential to the success of the measurement. It is located at the point where the measured value of the phase retardation is maximized. A test sample which is composed of one wedged quartz plate and one wedged non-birefringence BK7 glass plate is designed and measured. The limitation of this proposed method in the measurements of strongly biréfringent and highly dispersive materials such as TiO₂ is discussed.