Controlling the polarization transmitted through an electro-optic modulator in a laser feedback interferometer

High precision optical interferometry demands single polarization laser beams. This requires the implementation of polarization elements with high extinction ratios. For many applictions, commerially available Glan Thompson polarizers with extinctions ratios greater than 100000:1 are sufficient. Additionally, it is often critical to characterize the orientation of the polarization of the light incident on a sample and to control the orientation as the beam transmits through optical elements. The phase-shifted laser feedback interferometer implements a broadband electro-optic modulator to reliably introduce controlled phase shifts so as to determine changes in optical path length and fringe visibility. In order to produce a controller phase modulation without a concominant amplitude modulation, it is critical to control the orientation of the linear polarization as it enters the modulator. The implementation of the phase-shifted laser feedback interferometer uses a HeNe laser (with extinction 500:1) and an electro-optic modulator (New Focus, 4002) modulated by a high-voltage operational amplifier (New Focus, 3211). In order to calibrate the polarization, a Berek compensator (New Focus, 5540) is used to introduce a controlled retardance. When the Berek compensator is used in conjunction with wave plates, it is possible to implement a series of measurements to determine all four components of the Stokes vector. This approach can be used to completely characterize how an optical element alters the incident polarization.