Abstract: An optical detector for magnetic fields detects rotations in the polarization of a beam of polarized light caused by the magnetic fields. A modulator responsive to a fixed frequency signal introduces a fixed frequency of oscillation in the polarization of the light beam. The output signal of a polarization detector is filtered by a band pass filter and is fed, along with the fixed frequency signal, to a phase detector. The output of the phase detector is inverted and used as a feed-back signal to the modulator to cancel the rotation in polarization caused by the magnetic field. This feedback signal is also the final output signal of the detector.

Abstract: A method and apparatus are disclosed for retrieving magnetically recorded information by optical means. The magnetic recording is irradiated by a beam of coherent light whose plane of polarization is periodically varied. The light which is transmitted or reflected from the recording medium is analyzed in order to retrieve the magnetically recorded information.