Modified Bloch Equations for Subsystems of Two-Level Atoms

Modified Bloch equations that describe the interaction of high-power optical radiation with a dense resonance medium consisting of two subsystems of different two-level atoms of high density are obtained. It is shown that, unlike the one-component resonance medium in which the effect of internal optical bistability manifests itself on the red wing of the absorption line, in the two-component system it appears on the blue wing of one component of the system. The switching of the two subsystems from one steady state to another occurs at the same points of rotation of the hysteresis dependence of the population differences of resonance atomic levels on the radiation intensity that varies adiabatically slowly. The influence of the processes of upconversion on the spectral-threshold characteristics of the effect of internal optical bistability is investigated.