Аннотация:One of the possible mathematical models of Fourier filtration in a nonlinear optical system with feedback is proposed and investigated. As a base of the model is treated a quasi-linear parabolic equation for an additional phase modulation arising when a field is passing a layer of a nonlinear medium. The questions of existence, uniqueness and regularity of generalized solutions of the parabolic equation are investigated. Analysis of the linearized system leads to some recommendations how to choose filters reducing an input phase distortion. A computational analysis of filter possibility in a nonlinear system is given.