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.