A theoretical investigation of collisional contributions to the gain line shape of coherently driven three-level systems is presented. The theory differs from previous work in that it includes, in addition to the usual thermal-bath mechanisms, direct collisional coupling between optically active states. It is shown that for stronger coupling between the stimulated levels than between the pump and two-quantum levels (i) the small signal gain g obtained with off-resonant pumping is greater than that obtained with resonant pumping, (ii) the unsaturated gain profile becomes asymmetric in proportion to the degree of pump detuning, and (iii) the unsaturated gain asymmetry is translated into the emission profiles gI obtained with nonzero values of stimulated intensities I. These results may be of particular importance in the case of optically pumped submillimeter lasers which exhibit large differences in the thermalization rates of the pump (V-V) and stimulated (R-R) transitions.
© 1978 Optical Society of AmericaFull Article | PDF Article
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