Novel Technique for Improvement in Calibration of the Photoelastic Modulator in Circular and Linear Dichroism Spectroscopy

Both the voltage-induced and the inherent (static) birefringence of a photoelastic modulator (PEM) affect the phase difference between orthogonal components of light passing through the PEM. This phase difference determines the polarization state of the light and is essential for determining true circular and linear dichroism (CD and LD, respectively) spectroscopy. Presented here are a more complete theoretical model of CD and LD and a new technique to determine the phase difference and static birefringence of a PEM in CD and LD. The intensity of the light for various configurations of the analyzer is interpreted (by using Mueller matrices and Stokes parameters) to calibrate the voltage-induced phase difference and to characterize the static birefringence in the photoelastic modulator. The effects of the static birefringence as well as intermediate polarization states (between left- and right-circularly polarized light) on LD and CD are characterized. Appropriate adjustments to the voltage applied to the PEM in order to mitigate these effects are discussed. It is expected that the techniques presented here will have a broad impact on the calibration of CD and LD spectrometers.