A Method to Calibrate Photoelastic Modulators with Surface Reflection
Y-B. Zhang, A-J. Zeng, L-H. Liu, J. Wang, L-L. Zhu, L-X. Zheng and H-J. Huang
A flexible method to calibrate the retardation amplitude of a photoelastic modulator (PEM) with surface reflection is proposed. A laser beam is split by a polarizing beam splitter to generate a calibration light beam. The calibration beam firstly passes through a PEM and then is reflected by the rear surface of the PEM. The reflected light beam is analysed by the polarizing beam splitter and detected by a photodetector. The second harmonic component of the detected signal is extracted and used to calibrate the retardation amplitude at 2.568000 radians. Because the second Bessel function J2(5.136) of the detected signal is zero, the calibration process is immune to the light intensity fluctuation which may be caused by laser source or ambient light. A PEM has been successfully calibrated with an uncertainty of 0.006280 radians (0.0001 of the wavelength, λ) by the proposed method. The calibration light passes through the PEM twice, so the calibration accuracy is two times higher than the typical second Bessel function zero method. It is a big advantage that this calibration method is independent of the light intensity fluctuation.
Keywords: He-Ne laser, photoelastic modulator (PEM), retardation amplitude, calibration, surface reflection