Resonator-free amplification of spontaneous radiation under recombination pumping conditions

A one-dimensional radiative transfer model is used in a study of the amplification of spontaneous radiation as a result of a population-inverted ionic transition pumped by recombination. The problem of such transfer is solved analytically in the steady-state case and it is shown that the intensities of the radiation emitted in both directions are the same irrespective of the distribution of the unsaturated gain over the length of the active medium. Numerical calculations are reported of the stage of establishment of light fluxes. It is shown that during this transient stage a nonlinear interaction between opposite light fluxes is responsible for nonmonotonic behavior of the output radiation intensity. The problem of amplification in a traveling pump wave is treated analytically.