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We have studied the concentration quenching of Yb/sup 3+/ ion fluorescence in Yb-doped silicate glasses containing up to 3.4/spl times/10/sup 21/ cm/sup -3/ Yb/sup 3+/ ions. The absorption and fluorescence spectra are similar to those obtained for the Yb/sup 3+/ ion in many different matrices, with a radiative lifetime of approximately 1400 /spl mu/s. The fluorescence decay curves were different among samples, with strong dependence on the Yb concentration. The decay curves could always be resolved into two exponential components, indicating that the ions reside in two different sites, each of a different characteristic nonradiative decay mechanism. The fast decay times ranged between 6 and 300 /spl mu/s, and the slow ones ranged between 190 and 1250 /spl mu/s in different samples. The sites where ions exhibit the fast decay most probably consist of pairs of Yb/sup 3+/ ions. The nonradiative decay probabilities for each site mere directly proportional to the Yb/sup 3+/ concentration in the same site. We propose that the fluorescence quenching occurs by multiphonon nonradiative transitions involving polar local phonon bands created by the presence of the Yb/sup 3+/ ion.