Enhanced beam amplification in a photorefractive Bi(12)TiO(20) crystal by internal reflections.

We demonstrate experimentally that internal ref lections of a signal and (or) a pump beam allow one to increase beam amplification by two-beam coupling in a long Bi(12)TiO(20) crystal. When fanning is negligible, we achieve an enhancement of the amplification by adjustment of the spatial period of the transformation of the beam's polarization states with periodic reflections of the beams on the crystal boundaries. For the case of strong fanning the fanned beam is redirected by the reflections on the crystal surface, which allows one to use it as a pump beam, thus increasing net amplification gain.