A few years ago in photorefractive crystals of LiTaO3:Cu (1) and LiNbO 3:Cu (2) a new nonlinear optic phenomenon, called the parametric scattering of 'holographic type', has been observed. As in the case of well-known (3) wide-angle species of photoinduced scattering, this process was degenerate. But it occurred under condition of phase-matching for four coupling waves: 2K sub L(omega)=K sub s1(omega) + K sub s2(omega) and hence had narrow spatial ...