Brillouin spectroscopy of Nd-Ge co-doped silica fibers

The effect of Nd-doping on both the longitudinal acoustic velocity and material damping coefficient (Brillouin spectral width) of silica fiber are measured. We find that Nd-doping results in anomalously large changes in the acoustic properties of SiO₂, such as that material damping is greatly exacerbated by the incorporation of Nd. We find that the spectral width increases by about 151% per wt% of Nd₂O₃ (for small [Nd]). This is very large compared to the effect of Ge-doping, as observed in this work. This may result from poor coordination in the SiO₂/GeO₂ matrix, and may be considerably different with the use of co-dopants such as Al that tend to improve the solubility of the rare earths. The acoustic velocity is found to decrease at a rate of about 6.1% per wt% of Nd₂O₃.