Cavity field spectrum of a cascade three-level atom interacting with a single-mode field

The cavity field spectrum of a cascade three-level atom interacting with a single-mode field in an ideal cavity is investigated. When the atom is in the upper level and the field is vacuum initially, the number of Rabi peaks changes according to 2-->6-->4-->2-->4 with the increase of R = g(2)/g(1) and all Rabi peaks disappear for R much greater than 1. Three-peak,five-peak or seven-peak structure appear for weak initial fields, and single resonance peak appears for a strong field. When the atom is in the middle level initially. the cavity field spectrum is of a two-peak structure with R = 1 as the spectrum of Jaynes-Cummings model.