Saturation splitting in the spectrum of resonant degenerate four-wave mixing

The spectrum of degenerate four-wave mixing on a homogeneously broadened one-photon transition is shown to exhibit line splitting when the pump-intensity difference exceeds the saturation intensity. The splitting arises from a saturation mechanism reinforced by the pump imbalance. Experimental evidence of this effect, using SF(6) as the nonlinear medium, is presented.