Amplifying properties of heavily erbium-doped active fibres

The relative concentration of erbium ions undergoing nonradiative relaxation from the metastable to the ground level is measured in aluminosilicate glass fibres doped with erbium ions at concentration between 3×10¹⁸ and 10²⁰ cm^-3. The dependence of the fraction of such ions on the Er³⁺ concentration is determined for fibres containing different amounts of aluminium oxide in a core. It is shown that the fraction of erbium ions not involved in amplification substantially decreases with increasing the Al₂O₃ concentration. It is found that clustering leads to a considerable decrease in the gain in heavily Er³⁺-doped active fibres. The dependence of the quantum efficiency of a fibre amplifier on the erbium ion concentration is obtained based on the measurements performed. This dependence can be used for optimising the parameters of erbium-doped fibre amplifiers.