عنوان مقاله [English]
The three-level (or multi-level) atomic systems interact with a couple of (or more) classical fields are investigated in the weak (probe) field approximation. But it is not a suitable approximation for the quantized fields, where all probe and coupling fields are few number of photons, nor especially where the coupling field is in vacuum state. In this article the dynamical equations for upper-coupling three-level cascade atoms interact with a couple of quantized fields are exactly solved in the steady state. The atomic population, absorption and dispersion are investigated. The exact results are compared with the corresponding ones in the weak field approximation method.