Spectral narrowing and saturation-induced rebroadening in 3.51-μm xenon laser amplifiers

The spectral width of the amplified spontaneous emission emitted by a xenon 3.51-\ensuremath{\mu}m laser amplifier was measured as a function of discharge length using optical heterodyne techniques. Measurements were made for $^{136}\mathrm{Xe}$ at a pressure of 190 mTorr and also for mixtures of $^{136}\mathrm{Xe}$ at a partial pressure of 190 mTorr with helium at partial pressures of 1.3, 2.5, and 4 Torr. Spectral narrowing and, usually, saturation-induced rebroadening were observed for each set of pressures used. The spectral-narrowing data in the regime of unsaturated amplifier gain are in reasonable agreement with existing spectral-narrowing theories which assume a random phase among excited atoms. The observed saturation-induced spectral-rebroadening rates are lower than those predicted by such theories. This behavior indicates that a more complex theoretical treatment of the influence of saturation on a broadband radiation field may be required.