A simple, inexpensive photoelastic modulator

Abstract

A block of birefringent gelatin acts as a variable retarder when driven harmonically by a speaker coil and can be used to vary the polarization of a laser beam sinusoidally. We model this effect with Mueller matrices and show that the gelatin behaves much like a commercial photoelastic modulator and is suitable for a variety of polarimetry experiments in an advanced undergraduate optics course.