We theoretically examine generation of sum-frequency radiation in a regular domain structure, taking into account phase changes for all the interacting waves in a dissipative medium. We show that in contrast to the constant-field approximation, in the constant-intensity approximation there is an optimal value for the longitudinal dimension (length) of the domains at which maximum conversion of the signal wave energy to the sum-frequency wave energy occurs. As the pump intensity increases, the optimal domain length decreases. As the losses of the interacting waves increase, both the conversion efficiency and the optimal domain dimensions decrease.Introduction. Studies of quasi-phase matched interactions are important in applied nonlinear optics today. This is because of technological developments in such multilayer media. For three-wave mixing, efficient energy exchange between interacting waves with frequencies ω 1 , ω 2 , and ω 3 , where ω 1 + ω 2 = ω 3 , can be realized under quasiphase matching conditions, when the wave mismatch is compensated by the reciprocal lattice vector for modulation of the nonlinear susceptibility 2π ⁄ Λ (Λ = 2π ⁄ Δ is the period of the modulation of the nonlinear susceptibility; Δ is the phase mismatch for the interacting waves) [1, 2].Frequency conversion processes in a regular domain structure with quasi-phase matched interactions have been studied in [3][4][5][6], where mainly the constant-field approximation was used. Analysis in the constant-intensity approximation [7] for nonlinear quasi-phase matched interaction of waves in the case of harmonic generation has shown that selecting optimal parameters for the problem can significantly improve the conversion efficiency compared with the traditional case of a homogenous nonlinear medium [8].The theory of quasi-phase matching has not been thoroughly studied for the process of second harmonic generation [9]. Processes of higher harmonic generation and parametric conversion are still in the study stage [1]. Secondorder frequency mixing processes have been successfully used for frequency conversion of laser emission, for example from the IR range to the visible region of the spectrum.In this paper, we present the results of a study of high-efficiency generation of sum-frequency radiation with quasi-phase matched interaction, using the constant-intensity approximation.Theory. The process of three-wave interaction at frequencies ω 1 , ω 2 , ω 3 (ω 3 = ω 1 + ω 2 ) in a nonlinear dissipative medium is described by the equations [10]
