This paper is devoted to the description and characterization of a new electro-optic sensor based on a polarimetric device operating in reflection mode, and on a closed-loop controller. The main features of the proposed sensor are versatility, insensitivity to the laser beam optical power fluctuation, and fully linear working mode. The resolution of the sensor, in terms of phase shift, is shown to be better than +/-20 mrad and its accuracy to +/-50 mrad.