Seeded Stimulated Photorefractive Scattering

Stable phase conjugate reflectivities as high as 60% have been achieved with back-seeded stimulated photorefractive scattering (SPS) in a BaTiO3 crystal having too small a gain-length product for unseeded SPS. The beam-bending and build-up dynamics of the process are demonstrated by the three top-view photographs of the crystal reproduced in Figure 1. In Figure la, there is no external seed and, in this case, no self-pumped phase conjugation; the beam refracts into an a-face of the crystal, fans sharply to an angle of about 20° with respect to the c-axis, then straightens out to exit a c-face of the crystal. In Figures 1b and 1c, the sequence of steps leading to SPS are shown in the presence of a backward seed. In Figure 1b, the fanned light exiting the crystal strikes a non-specular, diffusive-type surface, the scattering from which acts as a back-injected seed and dramatically lowers the gain-length product threshold condition for stimulated scattering. The phase-conjugate reflection in Figure 1b is in the process of building up; pump depletion is already evident in this photo. Figure 1c is a photo of the steady-state beam trajectory inside the crystal.