Accurate efficiency evaluation of energy-transfer processes in phosphosilicate Er^3+-Yb^3+-codoped fibers

A new approach for the prediction of energy-transfer efficiencies in codoped Er-Yb double-clad fiber (EYDCF) is presented. Ab initio calculations have been performed on the basis of migration-assisted energy-transfer models. The two main Yb3+-to-Er3+ energy transfers as well as the Er3+ cooperative upconversion were theoretically estimated, and the results indicate that the secondary energy-transfer parameter is more efficient than the upconversion process. Theoretical calculations were found to be consistent with experimental energy-transfer parameters recorded in various fiber samples. We show that the first Yb3+-to-Er3+ energy-transfer parameter that is needed in all modelings based on rate equations can be calculated by use of a simple relation. Our approach also permits the prediction of the laser efficiency of EYDCFs, and a comparison of measured and calculated maximum laser slope efficiency shows a good agreement.