A hydrocolloid-based photoelastic modulator

Birefringent gelatin and other hydrocolloids can serve as the optical elements of simple, inexpensive photoelastic modulators. Driven harmonically by a speaker coil, a small block of gelatin acts as a variable linear retarder and can thus be used to sinusoidally vary the polarization of a laser beam passing through it. We model this effect with Mueller matrices and show that our gelatin modulator behaves as predicted. This uncomplicated yet versatile device is well suited for several types of polarimetry experiments that do not require high precision, and it makes an excellent pedagogical tool for students in advanced undergraduate optics course.