<title>Theoretical and experimental study of migration-assisted upconversion in high-concentration erbium doped silica fibers</title>

We report a new experimental and theoretical characterization of high concentration erbium doped silica fibers. Applying a pair-correlation function of erbium ions that is known from x-ray absorption fine structure spectroscopy, we develop a new statistical model of migration-assisted upconversion in erbium doped fibers. We find that the new model provides accurate fitting to our experimental data for the upconversion coefficient as a function of the metastable level population.