Inversionless gain in a three-level system driven by a strong field and   collisions
  Inversionless gain in a three-level system driven by a strong external field and by collisions with a buffer gas is investigated. The mechanism of populating of the upper laser level contributed by the collision transfer as well as by relaxation caused by a buffer gas is discussed in detail. Explicit formulae for analysis of optimal conditions are derived. The mechanism developed here for the incoherent pump could be generalized to other systems. 
A.K.Popov, S.A.Myslivets, Gao Jin-yue, Zhang Han-zhuang,   B.Wellegehausen