Amplification without inversion in a medium with collisional dephasing

Amplification without inversion in the presence of collisions in V and Lamb da systems is studied within the framework of density-matrix formalism. We show that while it is possible to achieve amplification without inversion c lose to Raman resonance in both systems, it is affected by collisions in a different way. In the V system, collisions enhance quantum interference, wh ereas in the Lambda system they quench it. This effect depends crucially on the collision characteristics, essentially on the relative values of the c ollisional relaxation rates for the Raman coherence and for the optical coh erences. For the Lambda system, different incoherent pumping processes are studied: pumping via the upper level and direct pumping between the two low -energy levels. The direct pumping process is interesting for the frequency up-conversion of the coherent and incoherent pump fields. Furthermore, wit h direct pumping the gain always occurs without inversion. [S1050-2947(99)0 9101-5].