Doubly resonant four-photon interactions in cesium vapor

Four-photon interactions involving both a two-photon resonance with the 9d2 D3/2 level and a one-photon resonance with an np2P level (n = 7-10) are investigated in detail. Gain, gain saturation, gain threshold, and forward-to-backward generation ratios are measured. Comparison with numerical estimates indicates that 9d2 D3/2 to np2P lasing is the dominant gain mechanism but that a doubly resonant four-photon parametric oscillation makes a significant contribution. Gain saturation is dominated by population of np2P levels. Ionization is important, especially at higher pump powers.