Saturation of the lower transition in a general three-level atomic system. Sup- pose a signal that produces a stimulated transition probability W12 = W21 (no degeneracy) is applied to the 12 transition of a three-level system. Solve the complete steady-state rate equations to obtain the saturation behavior of the population difference AN12 without making any optical-frequency approxi- mation. In general terms, how does this saturation resemble or differ from the saturation of a simple two-level system?