Polarization modulator

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A polarization modulator is a technical device that periodically changes the polarization properties of a light beam . This is used in modulators of optical spectrometers . It is the optical equivalent of a phase modulator from electronics .

There are three types of polarization modulators, which essentially differ in the type of polarizer used:

  1. Modulator with linearly polarizing filters
  2. Active crystal modulator
  3. Modulator with liquid as the active medium

Modulator with linearly polarizing filters

A linear polarizer is mounted on a motor shaft with which the polarization direction of the light then rotates.

Alternatively, two mutually twisted, linear polarization filters are mounted on a swinging arm that moves them alternately into the beam path.

If a λ / 4 plate is firmly placed behind one of these arrangements , in the second case with an orientation of 45 degrees with respect to the two linear polarizers, the light beam is then alternately polarized to the right and left, in the first case with constant transitions along linear polarization the optical axes of the plate.

Active crystal modulator

There was a commercial product called "Phasqeezer" (sic) from Erdmann & Grün, which consisted of a crystal with the properties of a retardation plate, which was clamped mechanically between the poles of a powerful electromagnet, with some mechanical preload. If a sinusoidal alternating voltage was applied to the magnet , an alternating mechanical force was exerted on the crystal, which changed its optical properties, so that the modulation state of the light behind it changed accordingly. If a linear polarizer with a suitably aligned polarization direction was arranged beforehand and the mechanical bias and the amplitude of the alternating voltage for the magnet were correctly adjusted, then at the output a light wave varying steadily between left and right circular polarization was obtained.

The photoelastic modulator works similarly, only with piezoelectric instead of electromagnetic excitation .

The Pockels cell rotates the direction of polarization not mechanically, but directly via an electric field .

Modulator with liquid as the active medium

This type of polarization modulator uses the variability of the refractive index of certain liquids through an external electric or magnetic field, see Kerr cell or Faraday rotator .