Effective unidirectional pumping for steady-state amplification without   inversion
  We discuss an opportunity to achieve amplification without inversion in three-level cascade scheme using an effective unidirectional pumping via bidirectional incoherent pump. Analytical solution to the population and the coherence are obtained in the steady-state regime. With a proper choice of the parameters, obtained here, the possibility for amplification without inversion is presented. 
Pankaj K. Jha
physics.atom-ph quant-ph