Multistage conversion of pump energy into low-frequency radiation

It is shown that the efficiency of conversion of pump to long-wavelength radiation can be increased by a multistage process in a single nonlinear element. The plane-wave approximation is used in a study of the characteristics of multistage conversion in an oscillator with low-frequency feedback. The dependences are obtained of the conversion efficiency on the number of conversion stages and on the excess of the pump power density over the self-excitation threshold. It is shown that the phase-matching conditions of the multistage process of parametric conversion are satisfied by a proustite crystal in the interaction of the e–eo waves in each stage and also by systems in which far infrared radiation is generated as a result of stimulated scattering by polaritons in an LiIO₃ crystal when the interaction of waves is of the e–ee type.