Calibration of Two Difrerent Types of Modulators for an Application in Spectroscopic Phase Modulated Ellipsometry

Abstract

In order to choose a photoelastic modulator for an application in spectroscopic phase modulated ellipsometry a complete calibration of two different types of photoelastic modulators is achieved. First the model that describes the behavior of each of them is accurately determined since the calibration procedure depends on it. Both modulators behave as a static strain model with a phase shift of the form $delta = delta_0 + {cal A}sin omega t$. Using the suitable procedures, the driving voltage Vmod and the static strain δ0 of both devices are accurately determined as a function of the wavelength from 0.25 μm to 0.75 μm. A method to reduce the errors due to incorrect settings of the modulation voltage and the static birefringence is also proposed. An accuracy check of the calibration done by comparing the indexes of refraction of two silica prisms obtained with our ellipsometer and with the high precision goniometer method shows a very good agreement