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.
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