Observation of strongly nonquadratic homogeneous upconversion in Er3+-doped silica fibers and reevaluation of the degree of clustering

From careful studies of the 1530-nm fluorescence decay, we obtain the rate of energy transfer upconversion as a function of the inverted population fo r a series of nine highly Er-doped silica fibers (with concentrations from 0.3 to 8.6.10(25) Er3+-ions per m(3)). The results demonstrate that the slo w component (microsecond to millisecond scale) of the upconversion, usually referred to as the homogeneous upconversion, is clearly nonquadratic in it s dependence on the inverted population, contrary to previous assumptions i n the literature. In a second part, we present a new detailed model for ene rgy transfer upconversion, permitting-to our knowledge for the first time-c alculation of the rate of migration accelerated upconversion for any given spatial distribution of Er3+-ions. We demonstrate that the results from the decay measurements may be explained with this model. Nest, we review the r esults from a CW green fluorescence detection experiment for the determinat ion of the degree of clustering, which was previously performed on five of the nine fibers. We find accordance between these results and our model, wi th parameters consistent with those needed to fit the results of the decay experiment, and we arrive at a new conclusion about the nature of clusterin g.