Upconversion from the 4I13/2 and 4I11/2 levels in Er:YAG

The efficiency of erbium three-micron laser (laser transition 4I11/2→4I13/2) depends essentially on the ratio of the parameters of active energy transfer upconversion (ETU) from the laser levels. The parameters of both ETU processes can be obtained from the analysis of the shape of the kinetics of the 4I11/2 level in concentrated Er:YAG crystals, under short pulse pumping. Mathematical modeling is used to evaluate the sensitivity of the method and to estimate the errors which can be introduced by the inhomogeneous pumping and accidental impurities. It was found that the ratio of the parameters corresponding to the ETU from the laser levels is less sensitive to the pumping inhomogeneities than that corresponding to the lower laser level. A reduction of this ratio with increasing erbium concentration is observed.