We create an interferometric closed-loop by recording a set of three volume gratings inside a photorefractive BaTiO_3 crystal. These gratings divide a single input beam into two beams that interfere in the far-field. The diffraction efficiency of the gratings depends on the angle between the grating vector and the c-axis, the full angle between the writing beams and their intensity ratio. A PZT-driven mirror phase modulates one of the writing beams that result in a beam modulation rate of up to 700 Hz. This demonstrates the feasibility of a photorefractive crystal as a laser beam modulator. Laser beam modulation is also carried out in LiNbO_3 crystal. Two physical geometries have been considered with this crystal. We record only two gratings to form an interferometric path. In this case, applying an external ac electric field to the crystal along the path of one arm of the interferometer results in the desired modulated output. We also describe a theoretical model for an efficient photorefractive beam modulator.
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