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Faraday rotator

(Science: radiobiology) a device which rotates the plane of polarization of an optical-light pulse, typically by using a glass disc suitably doped with a magnetic ion and placed in a magnetic field.

These devices are used to isolate (protect) a laser amplifier chain against back-reflection from the laser target, the faraday rotator in this case gives a 90-degree phase change on the round trip, so that the returning light is rejected by a polarizer which transmits the outgoing light. Magnetised plasmas also display the faraday rotation effect of light waves propagating along the magnetic field lines.