Effects of concentration on the performance of erbium-doped fiber amplifiers

The dependence of erbium-doped fiber amplifier (EDFA) performance on the erbium ion concentration is studied experimentally and theoretically. The quantum efficiency of the amplifier is found to he strongly dependent on the erbium ion concentration, the signal wavelength, and the relative propagation direction of the pump and signal beams. This dependence is fully explained by the presence of an upconversion mechanism between ions residing in pairs or larger clusters and suggests that other sources of amplifier performance degradation (back-ground loss, excited state absorption, homogenous upconversion) are negligible. The experimental data show that in the present EDFA designs with over 80% quantum/conversion efficiency, the aluminum co-doped fibers with erbium ion concentration less than 20/spl times/10/sup 24/ m/sup -3/ (900 molar ppm Er/sup 3+/) are most suitable.