Ordinary solitons in photorefractive crystals

Using Fedorov's covariant coordinate-free methods, we study the solutions of the nonlinear wave equation that describes the self-focusing of a light beam with arbitrary wave normal directions in a crystal. The directions of the phase normals that allow the existence of bright ordinary screening solitons in non-centrosymmetrical crystals are determined. The results obtained are applied to calculate the existence directions of solitons in lithium niobate. (authors)