Application of the functional integration method to the laser theory

An approach is proposed for the investigation of nonequilibrium processes based on the functional integration method. A laser type system is considered. The expression for the density matrix of the two-level system in the classical field is obtained as well as the effective action functional of the radiation field. The equation for the stationary point of the effective action characterizes the process of the laser generation. In the equilibrium limit this equation describes the Bose-condensation in Dicke type models.