Photoelastic modulator

A photoelastic modulator (abbreviated to PEM) is an optical device which modulates the polarization state of visible light or infrared radiation.

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

To achieve a sufficiently high amplitude of the modulation, resonance is used: it is formed from a standing wave with each half wavelength of the quartz plate as well as in the piezoelectric element mounted on it. At the splice between the piezoelectric element and quartz glass plate is therefore (as at the ends ) a maximum of the oscillation amplitude, but a node of the distortion; Therefore, the splice is subjected to mechanical stress is relatively low. The propagation speed in the material of the PEM plate (usually: quartz glass) determined the length of the plate, and the frequency at which the piezoelectric element excites the vibration plate to the density.

Use only the central portion of the plate in which the retardation effect can be assumed to be spatially constant. Time varies the delay here sinusoidal. As amplitude typically λ / 2 or λ / 4 is selected.

When on the same physical principle ( photoelastic effect) based acousto-optical modulator (AOM) is used in contrast to the PEM no region of constant delay, but a three-dimensional lattice of the density and resulting refractive index variations.

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

• Optical component