Calibration of photoelastic modulator using direct current component

A method of calibrating the photoelastic modulator (PEM) using direct current component is proposed. The laser beam passes through a polarizer, the PEM to be calibrated and is then split into two sub-beams by a Wollaston prism. The two sub-beams are then detected by two detectors. The small peak retardation is independent of the fluctuation of the initial intensity. The calibration setup is compact. In experiments, a PEM was calibrated and the calibration error was less than 0.008 rad when the peak retardation changed in the range of 0.1λ-0.5λ.