A measuring system (40) for measuring a magnitude of a quantity in accordance with the invention includes a light source (52) for providing a light beam (58); a polarization controller (44), optically coupled to the light beam, for rotating a plane of polarization of the light beam through an angle in response to a control signal, which is proportional to the magnitude of the quantity being measured to provide a first rotated polarized beam of light; a sensor (46), coupled optically to the first rotated polarized beam of light and to the quantity being measured, which rotates the plane of polarization of the first rotated polarized beam through an angle proportional to the quantity being measured to provide a second rotated polarized beam of light; an analyzer (42), coupled optically to the second rotated polarized beam of light, having a constant plane of polarization for detecting the second rotated polarized beam of light produced by the quantity being measured; and a feedback and output circuit, coupled to an output of the analyzer, for providing an output proportional to magnitude of the quantity being measured and producing the control signal.