Abstract

Excited-state absorption (ESA) may have considerable influence on the lasing properties of rare-earth doped solid-state lasers. ESA is a two step excitation process which occurs in several rare-earth ions. Both absorption steps take place within the same single ion. A similar process is up-conversion (UC) requiring two excited ions. ESA and UC can be utilized for the generation of visible fluorescence by the absorption of infrared pump photons, see e. g.^[l,2,3] However, in most cases they also produce undesirable losses. Especially the ESA process either reduces the pump efficiency of solid-state lasers or competes with stimulated emission (SE), decreasing or preventing laser output. This depends on whether the ESA wavelength is in the spectral region of the pump light or the fluorescence.^[4]

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