Faraday rotator

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Polarization mechanism due to the Faraday effect. The field lines are usually closed through a permanent magnet around the rotator.

A Faraday rotator is an optical device that rotates the polarization of light due to the Faraday effect, which in turn is based on a magneto-optic effect.

The Faraday rotator works because one polarization of the input light is in ferromagnetic resonance with the material which causes its phase velocity to be higher than the other.

The plane of linearly polarized light is rotated when a magnetic field is applied parallel to the propagation direction. The empirical angle of rotation is given by:

$\beta = VBd \!$

Where $β$ is the angle of rotation (in radians).
$B$ is the magnetic flux density in the direction of propagation (in teslas).
$d$ is the length of the path (in metres) where the light and magnetic field interact.
Then $V$ is the Verdet constant for the material. This empirical proportionality constant (in units of radians per tesla per metre, rad/(T·m)) varies with wavelength and temperature and is tabulated for various materials.