Secondary energy transfer and nonparticipatory Yb3+ ions in Er3+-Yb3+ high-power amplifier fibers

The energy exchange processes within Er3+-Yb3+-codopedsilica fibers manufactured for high-power optical amplifiers are investigated through luminescent decay measurements and rate-equation modeling. The Er3+ and Yb3+ luminescence, as functions of pump power, exhibit an unquenchable Yb3+ to Er3+ cooperative energy transfer (CET) that does not subside with saturation of the Er3+ inversion. Consistent with the experimental results is the occurrence of secondary energy transfer to excited E3+ ions in their metastable I-4(13/2) state. The transfer coefficient for this secondary I-4(13/2) CET is determined to be of the same order of magnitude as that for the initial Yb3+ to ground-state Er3+ CET. Additionally, the decay measurements and modeling indicate that a fraction of Yb3+ ions does not participate in energy exchange with the Er3+. These nonparticipatory Yb3+ ions amounted to similar to15% of the total Yb3+ concentration and could constitute Yb3+ clusters. Both secondary CET and nonparticipatory Yb3+. will lower Er3+-Yb3+ fiber amplifier efficiencies. (C) 2004 Optical Society of America.