Origin of the Reaction of Probe Spectra on the Coupling Pump Laser Absorption

In this paper it is shown that the effect of the probe spectrum on the pump absorption results from a probe-frequency-dependent modulation of the populations of the states involved in the transitions interacting with the radiation. We distinguish between two elementary situations: 1. simple two-level systems and V-tvpe three-level systems and 2. Lambda-tvpe three-level systems. In a four-level V-type configuration, where the two vertical legs represent the transitions that interact with the pump(s), and the diagonal leg represents the transition that interacts with the probe. There are two contributions to the pump spectrum. The left-hand transition corresponds to a Lambda-tvpe situation and the right-hand transition to a V-type situation.