Molecular homogeneity in erbium-doped sol-gel waveguide amplifiers

High net gain levels have recently been reported in silica-on-silicon waveguide amplifiers, using the sol-gel method of glass deposition. Spectroscopic measurements indicate that high inversion levels are achieved at high erbium concentrations, suggesting a high uniformity in the erbium distribution. Here we show that high inversion and low scattering losses are only achieved in sol-gel waveguides which have had restricted heat treatment and that both degrade upon additional annealing. Evolution of scattering loss spectra are presented, and these are included in a model of the ytterbium-sensitized waveguide amplifiers to show that the gain reduction with heat treatment is not wholly due to pump and signal excess losses, but also to changes in the spectroscopic properties of the erbium ions. In particular, the formation of clusters in which inversion is rapidly quenched is indicated. Thus, we show that the superior performance of the sol-gel method for rare-earth doping of silica waveguides, for amplifier and laser applications, results from its ability to form homogenous molecular structures which are meta-stable and which consequently cannot be formed in glasses that require high-temperature consolidation or annealing.