Photoelastic modulator

A photoelastic modulator (abbreviated: PEM) is an optical component that modulates the polarization state of visible light or infrared radiation.

The function is based on the fact that an optical medium (usually a quartz glass plate) is periodically compressed and expanded in one direction by a piezoelectric transducer . Because of the photoelastic effect , the refractive index is modulated and the optical medium behaves like a retardation plate with periodically modulated delay.

In order to achieve a sufficiently high amplitude of the modulation, resonance is used: A standing wave is formed, each with half a wavelength in the quartz glass plate as well as in the piezoelectric element mounted on it. At the glue point between the piezo element and the quartz glass plate there is therefore (as at the ends) a maximum of the oscillation amplitude, but a knot of the distortion; therefore, the joint is subjected to relatively little mechanical stress. The speed of propagation in the material of the PEM plate (usually: quartz glass) determines the length of this plate and the frequency with which the piezo element excites the plate to vibrate in density.

Only the central part of the plate is used, in which the delay effect can be assumed to be spatially constant. The delay here fluctuates sinusoidally over time. Usually λ / 2 or λ / 4 is chosen as the amplitude.

In the case of the acousto-optical modulator (AOM ) based on the same physical principle (photoelastic effect) , in contrast to the PEM, no area of ​​constant delay is used, but a spatial grid of density and the resulting refractive index variations.

Photoelastic modulators are used e.g. B. used in ellipsometers , the most common modulation frequency is 50 kHz .