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.

They are used in Ring Lasers and in Faraday Isolators.

A Faraday Rotator rotates light using the Faraday Effect. The degree of rotation is controled by an induced magnetic field. The two main uses for a Faraday Rotator are putting a polarizer in front so that back scatter is rotated and blocked by the polarizer and using them with wave plates to make a ring laser.

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:

${\displaystyle \beta =VBd\!}$

Where

• ${\displaystyle \beta }$ is the angle of rotation (in radians),
• ${\displaystyle B}$ is the magnetic flux density in the direction of propagation (in teslas),
• ${\displaystyle d}$ is the length of the path (in meters) where the light and magnetic field interact, and
• ${\displaystyle V}$ is the Verdet constant for the material. This empirical proportionality constant (in units of radians per tesla per meter, rad/(T·m)) varies with wavelength and temperature and is tabulated for various materials.