Simultaneous measurement of small birefringence magnitude and direction in real time

• We demonstrate a method to simultaneously measure the birefringence magnitude and direction in real time.
• A photoelastic modulator modulated at 40 kHz was measured. Its birefringence magnitude and direction are reappeared in real time.
• The maximum deviation of the birefringence magnitude is less than 1.68 nm, and the standard deviation of the birefringence direction is less than 0.47°.

A method to simultaneously measure the small birefringence magnitude and direction in real time is proposed. The laser light passes through a circular polarizer and the sample successively. Then the beam is split into two sub-beams by a beam splitter. Each sub beam is then split and analyzed by a Wollaston prism. Four intensities are simultaneously detected by four detectors to resolve the birefringence magnitude and direction in real time. The measurement result is immune to fluctuation of the initial intensity. In experiments, a photoelastic modulator with small peak retardation was measured at different birefringence magnitudes and directions. The maximum deviation of the birefringence magnitude is less than 1.68 nm, and the standard deviation of the birefringence direction is less than 0.47°. The validity of the method is verified.