Faraday Rotator Mirror (FRM) is a passive device rotating a signal's polarization by 90° and reflecting the signal back into the fiber. Need more info? Contact our expert. An important. A polarization rotator is an optical device that rotates the polarization axis of a linearly polarized light beam by an angle of choice. Such devices can be based on the Faraday effect, on birefringence, or on total internal reflection.