Compensated frequency biasing system for ring laser

Claim 1

Phase shifter apparatus for providing non-reciprocal differential phase shift in counter-flowing light beams passing therethrough comprising: first and second magnetic-field-controllable optical phase shifter means, said optical phase shifter means including non-reciprocal first and second means for shifting the phase of first and second circularly polarized light beams passing therethrough along respective first and second light beam transmission paths, each said means for shifting the phase of circularly polarized light beams having substantially parallel first and second ends, said first ends being bonded to a first face of first quarterwave plate means for reciprocal conversion of light between linearly and circularly polarized states, said second ends being bonded to a first face of second quarter-wave plate means for reciprocal conversion of light between linearly and circularly polarized states, means for orienting said first and second light beam transmission paths substantially perpendicular to said respective first ends, light beam directing means for providing a third light path joining said first and second light paths for the purpose of providing a continuous light path, and first and second magnetic field generator means respectively coupled to said first and second magnetic-field-controllable optical phase shifting means for providing equal oppositely directed magnetic fields and said respective first and second light bEam transmission paths and for the purpose of producing differential phase shift in said light beams traveling along said light paths in opposite directions while cancelling any phase shift effect thereon caused by the presence of an ambient magnetic field.