Photoluminescence, amplification and generation of optical media in   without inversion and of induced radiation
  The observation of photoluminescence (PL) or achieve lasing modes in alcohol solutions of dyes or other media (silica glass fibers), doped in small quantities by certain active working elements, which are dipped in the environment, do not requires start the inversion, since the main role in time of the optical pumping process is given four-photon interaction and the associated amplification of those frequency components, that result from this interaction, and participating in the PL or generation. 
Valeriy E. Ogluzdin
physics.optics