PEM photoelastic modulators change the polarization state of a light beam at frequencies between 20 and 100 kHz. The modulator optical element acts as a “dynamic waveplate”to produce an oscillating birefringence. This can convert linear polarized light into light which oscillates betweencircular, elliptical and linear states. PEM photoelastic modulators feature a “split-head” configuration; the electronic and optical components are housed in separate enclosures. This minimizes the size of the unit which is placed in the optical train. It also simplifies making the optical head magneticfield compatible or vacuum compatible when this is necessary.