Phase-shifting (Sagnac) interferometer with external phase control

A novel ring configuration for phase-shifting interferometry with external phase-shifting control is presented. The device is a polarization (ring) interferometer, in which the reference and test arms are parts of the same collimated beam. The key point is to manage the polarization of the light such that orthogonal linear polarizations describe counterpropagating paths in the ring interferometer. The phase shift between the two waves is externally controlled with a Pockels cell, which permits fast phase modulation without the need for moving parts inside the interferometer.