Amplification without inversion in the presence of collisions in V and ⌳ systems is studied within the framework of density-matrix formalism. We show that while it is possible to achieve amplification without inversion close to Raman resonance in both systems, it is affected by collisions in a different way. In the V system, collisions enhance quantum interference, whereas in the ⌳ system they quench it. This effect depends crucially on the collision characteristics, essentially on the relative

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... values of the collisional relaxation rates for the Raman coherence and for the optical coherences. For the ⌳ 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, with direct pumping the gain always occurs without inversion.