  Specific features of nonlinear interference processes at quantum transitions
in near- and fully-resonant optically-dense Doppler-broadened medium are
studied. The feasibility of overcoming of the fundamental limitation on a
velocity-interval of resonantly coupled molecules imposed by the Doppler effect
is shown based on quantum coherence. This increases the efficiency of
nonlinear-optical processes in atomic and molecular gases that possess the most
narrow and strongest resonances. The possibility of all-optical switching of
the medium to opaque or, alternatively, to absolutely transparent, or even to
strongly-amplifying states is explored, which is controlled by a small
variation of two driving radiations. The required intensities of the control
fields are shown to be typical for cw lasers. These effects are associated with
four-wave mixing accompanied by Stokes gain and by their interference in fully-
and near-resonant optically-dense far-from-degenerate double-Lambda medium.
Optimum conditions for inversionless amplification of short-wavelength
radiation above the oscillation threshold at the expense of the
longer-wavelength control fields, as well as for Raman gain of the generated
idle infrared radiation, are investigated. The outcomes are illustrated with
numerical simulations applied to sodium dimer vapor. Similar schemes can be
realized in doped solids and in fiber optics.
