Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated b y a one-dimensional light pattern is studied when an alternating square-wav e electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge -field formation in the case of a Gaussian beam and interference light patt ern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulati on effects under two-beam coupling are estimated from the numerical calcula tions.