It is shown that the photorefractive grating produced by a pair of plane waves in periodically poled lithium niobate includes an additional set of spatial harmonics related to the periodic domain structure. This results in new schemes for photorefractive wave coupling. Using the modified phase matching conditions and the concept of optical oscillation we accurately describe the position of the diffraction peaks and explain the main characteristics of self‐organized photoinduced scattering reported recently. ©1996 American Institute of Physics.