Functional integration and variational equations of nonlinear optics

The variational principle based on functional representation of evolution operator matrix element of optic system is proposed for the derivation of nonlinear optic equations. As an example of the one-mode field equations are derivated in the medium which consist of resonance and nonresonance two-level atoms. The relation among self-induced transparence, nonlinear scattering and self-focusing processes is discussed.