LASING WITHOUT INVERSION .1. INITIAL ATOMIC COHERENCE

We present a nonlinear theory for lasing without inversion in three-level atomic systems by utilizing initial atomic coherence between two closely spaced lower levels. We discuss nondegenerate and degenerate cases for closed and open atomic systems. The equations of motion for the laser fields, and the steady state intensities of the fields are obtained. The realization of lasing without inversion is due to the absorption cancellation by the atomic coherence, i.e., the quantum interference. The quantum interference can bring about gain enhancement in the nondegenerate cases.