We examine a model of a driven two-level system with injected coherent atoms. We find a set of lasing phase diagrams for the resonant excitations. We show that the laser field in the system is related to the initial preparation of atoms, and that when the degree of this coherence is changed, the system exhibits bistability in the inversionless region. We also discuss the crucial role of atomic coherence in achieving lasing action. © 1996 The American Physical Society.

• Received 8 April 1996

DOI:https://doi.org/10.1103/PhysRevA.54.1604