The effect of fourth-order coherence on ultrafast modulation spectroscopy (
UMS) with phase-conjugation geometry (PCUMS) in a cascade three-level syste
m is investigated using chaotic and phase-diffusion models. It has been fou
nd that the modulation terms of the beat signal depend on the second-order
coherence function, and different stochastic models of the laser held affec
t only the fourth-order coherence function. The difference between the PCUM
S and UMS is discussed from a physical viewpoint.