Four-photon resonance parametric up-conversion of a weak infrared signal in a nonmonochromatic pump field

Four-photon parametric up-conversion of a weak infrared signal under conditions of two-photon resonance of a constant nonmonochromatic pump field is analyzed allowing for population migration, transient response of the medium, Stark shift of the levels, and width of the pump frequency spectrum. It is found that under steady-state conversion conditions in a nonmonochromatic pump field, it is possible to achieve an average conversion efficiency differing only negligibly from the maximum conversion efficiency in a monochromatic pump field. An analysis is made of the influence of optically induced relaxation on the average conversion efficiency in the transient regime. Numerical estimates are made of the transition parameters for an alkali metal vapor in a frequency-degenerate pump field exciting a two-photon transition.