  Population inversion on the 5D-6P transition in Rb atoms produced by cw
excitation at different wavelengths has been analysed by comparing the
generated mid-IR radiation at 5.23 um originated from amplified spontaneous
emission and isotropic blue fluorescence at 420 nm. A novel method of detecting
two-photon excitation in atomic vapours using ASE is suggested. We have
observed directional co- and counter-propagating emission at 5.23 um. We find
that the power dependencies of the backward- and forward-directed emission can
be very close, however their spectral dependencies are not identical. The
mid-IR emission in Rb vapours excited by nearly counter-propagating beams at
780 and 776 nm does not exactly coincide spatially with the applied laser
beams. The presented observations could be useful for enhancing efficiency of
frequency mixing processes and new field generation in atomic media.
