Commercial Erbium doped silica fibers fabricated through the modified chemical vapor deposition and direct nanoparticle deposition processes where Er3+ concentration is varied in a wide range are characterized by means of analyses of Er3+ fluorescence decay kinetics and nonlinear absorption coefficient at 978-nm pumping. Through theoretical modeling of the results of the entire experiment, the values of key parameters for the concentration-related effects, i.e. homogeneous and inhomogeneous upconversion processes, are determined.