Fresnel drag technique for determining the spin-axis orientation of a spherical rotor

This paper describes a new method of angle measurement by optical means that may prove useful in gyrocompass applications. In this device, the relative orientation of the spin axis of a transparent spherical rotor is determined by measuring the Fresnal drag effect associated with the motion. This is done by placing the spinning sphere in the path of the optical beam inside the cavity of a ring laser and measuring the beat frequency between the oppositely traveling waves as a function of the spin-axis orientation. Differential sensitivities greater than 10⁵Hz/rad are easily obtained with this technique. In this paper, experiments with this device are described and sources of error are pointed out. By carefully constructing such a device, it should be possible to determine the direction cosines of the rotor axis angle to an accuracy of a few parts per million.