A SIMPLE-MODEL OF A LASER WITHOUT INVERSION

We consider a simple model for a four level atom as a candidate for laser action without population inversion. Our system has a degenerate or quasi-degenerate ground state doublet and two higher excited states; one of these higher levels is coupled to the ground states by a coherent field of variable strength, whose role is to create atomic coherence between the lowest levels; the other is also coupled to the ground states but only when laser action develops. This model includes an incoherent pump mechanism to produce some population in the top lasing state. We calculate the absorption profile of the laser transition and identify the conditions under which gain may develop. We find that gain is available over a suitable range of values of the driving field strength and incoherent pump rates. The shape of the gain profile and the frequency at which the profile is maximum change as one varies the operating conditions. We provide a physical interpretation of our results with the help of a convenient set of dressed atomic states.