Evaluation of the Performance of Photoelastic Modulator by Admittance Parameter Measurements

Abstract

An automatic system for admittance measurements has been built to evaluate the performance of a photoelastic modulator (PEM). It consists of an LCR meter, a frequency synthesizer, and a personal computer. With this system, the resonance frequency, the frequency dependence of an absolute value of impedance |Z|, and the phase angle θ of the PEM are measured automatically with a 0.1 Hz step. The value of |Z| at θ=0 is the resistance R of the equivalent electric circuit of the PEM. The quality factor Q is evaluated from the frequency dependence of admittance parameters. A PEM with a small value of R and high value of Q is found to be efficient and have a small residual birefringence in optical measurements. It is known that the residual birefringence of the PEM brings about errors and artifacts in polarization modulation instruments such as the modern circular dichroism spectrometer. Thus, it is worthwhile to estimate the performance of the PEM from admittance parameter measurements without installing the PEM into a spectrometer.