"[1] It was initially developed for physical measurements including optical rotary dispersion and Faraday rotation, polarimetry of astronomical objects, strain-induced birefringence, and ellipsometry.
Photoelastic modulators are resonant devices where the precise oscillation frequency is determined by the properties of the optical element/transducer assembly.
The transducer is tuned to the resonance frequency of the optical element along its long dimension, determined by its length and the speed of sound in the material.
The amplitude of oscillation is adjusted by an external applied voltage that is proportional to the wavelength of the light passing through the modulator.
The final detected intensities at the fundamental and second harmonic of PEM operating frequency depend on the ellipticity and rotation introduced by the sample.