Strong interest in heavily doped erbium-doped waveguides1 and in the applications of heavily doped erbium-doped fibers (EDFs) in lasers has resulted in several recent studies of collective processes in erbium ions. Rapid cross relaxation within a fraction of clustered ions, called pair-induced quenching (PIQ), has been identified as the main cause of EDF performance degradation.2-4 To date, studies on the dependence of EDF performance on ion concentration have been focused on pair/cluster-induced nonsaturable absorption of EDFs,2 which leads to the reduced quantum efficiency (QE) of EDF lasers.4 Recently, detailed theoretical analysis of the effect of PIQ on the performance of erbium-doped fiber amplifiers (EDFAs) has also been reported.4