Cooperative upconversion effects on the performance of Er³⁺-doped phosphate glass waveguide amplifiers

The cooperative upconversion processes of erbium ions in silver film ion-exchanged waveguides in a phosphate glass were studied from the pump intensity dependence of luminescence decay curves. Cooperative upconversion coefficients of the ⁴I_13/2 level, 7.7 ± 0.7 × 10^-19 and 9.3 ± 0.7 × 10^-19 cm³/s, were obtained for an Er³⁺ concentration of 1 × 10²⁰ cm^-3 in bulk and waveguide samples, respectively. These values are 1 order of magnitude smaller than the ones reported for silica glass. The increase in the cooperative upconversion coefficient with the increase in Er³⁺ concentration was found to be small. The effects of cooperative upconversion on the gain performance were analyzed for different Er³⁺ concentrations by a theoretical model based on experimentally measured gain with a 1.48-μm pump wavelength. Given the small cooperative upconversion coefficients in this glass, Er³⁺ concentrations potentially as high as 3.7 × 10²⁰ cm^-3 were shown to be feasible. Such high concentrations would result in a 12-dB gain at 150-mW pump power with a 4-cm-long waveguide.