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].