Calibrations of phase modulation amplitude of photoelastic modulator

Abstract

A multiple harmonic intensity ratio technique is proposed for calibrating the modulation amplitude (Delta(0)) of a photoelastic modulator (PEM). A data acquisition system is utilized to investigate the frequency response of this technique. Delta(0) determined by this technique is independent of frequency, which is proved using the reflection and transmission setups. In addition to confirm our calibration using the digitized oscilloscope waveform, we also obtain a set of ellipsometric parameters under various values of Delta(0) before and after calibration. We also introduce a correction factor to correct the effect caused by the shift of the modulation amplitude in the process of etching. An optical thick film is used to calibrate Delta(0) using PEM ellipsometry, and its refractive index and extinction coefficient are also obtained in etching. Finally, we suggest the use of the traces of the ellipsometric parameters under various thicknesses for monitoring instead of calculating the thickness in real time.