Abstract
The effect of fourth-order coherence on ultrafast modulation spectroscopy (UMS) with phase-conjugation geometry (PCUMS) in a cascade three-level system is investigated using chaotic and phase-diffusion models. It has been found that the modulation terms of the beat signal depend on the second-order coherence function, and different stochastic models of the laser field affect only the fourth-order coherence function. The difference between the PCUMS and UMS is discussed from a physical viewpoint.