Magneto-Optic Positioning

As part of a program for the production of accurate scales and grids it became necessary to develop a device which could locate the lines on such a scale or grid with a precision better than one microinch. The system described in this paper accomplishes this result by forming an image of the line with polarized light whose angle of polarization varies with the position of the line. The angle is zero on the optic axis and the rotation depends on both the sense and amount of the line displacement from that axis. The polarization of the light is modulated with a Faraday modulator. The combination of position sensitive polarization and Faraday modulation produces a signal whose phase depends on the sense and whose amplitude depends on the amount of displacement of the line image from the optic axis. Using this system sensitivities of one microampere per microinch displacement can be achieved with unit magnification.