Saturable absorber properties of a laser dye; stimulated propagation of the pump flux.
Inefficiencies in a laser dye resulting from excited state absorption are examined analytically with respect to its pump saturation behavior under conditions of small (extracted) signal gain. The spectrum of the lowest excited singlet state for such a dye shows a crossover point from excited state absorption sigma* to gain sigma(e), and this point can occur blue-shifted from the onset of ground-state absorption sigma(o). At wavelengths where sigma(o). sigma*, and sigma(e) operate simultaneously and at flux densities used for laser pumped dye oscillators and amplifiers, stimulated propagation of the pump flux compensates, in part, for excited state absorption losses. Under certain conditions, higher transmissions, flux densities, and excited state population densities are possible than for even a pure saturable absorber. A laser dye pumped between the crossover point and the onset of ground-state absorption sacrifices high excited state densities at the pump surface in order to give a uniform distribution deep into the medium. The percentage photon loss from excited state absorption is a function of sigma* and the initial pump flux only and is independent of sigma(e) and concentration.
Hammond
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PR