Molecular dynamics study on the co-doping effect of Al₂O₃ and fluorine to reduce Rayleigh scattering of silica glass

Further attenuation of Rayleigh scattering of silica glass is required to extend the capacity of long-distance optical fiber communication. To theoretically examine the effect of aluminum and fluorine co-doping on the density fluctuations of silica glass, which is related to Rayleigh scattering, a set of force-matching potentials (FMP) for simulating F-doped aluminosilicates was developed using Bayesian optimization based on density functional theory calculations. Molecular dynamics (MD) simulations with the new FMP could evaluate the densities of silica glasses to which a small amount of fluorine was doped and those of aluminosilicate glasses with a wide range of aluminum contents within reasonable accuracy. The FMP successfully represents the changing role of aluminum from a network former without a compensating cation (threefold coordination) to that with a compensating cation and a charge compensator in the aluminosilicates. Indeed, relative concentrations of four, five, and sixfold-coordinated aluminum observed by NMR measurements were reproduced better than the original Teter potential at a high aluminum content. At an aluminum content lower than 1 mol%, threefold-coordinated aluminum was observed, which is consistent with ESR measurements. After careful validations of the FMP, the effect of the co-doping of alumina and fluorine on the density fluctuations of silica glass was computationally examined. Consequently, it was expected that the co-doping might not sufficiently attenuate the Rayleigh scattering, even though 1 wt% fluorine would be able to reduce the density fluctuations of aluminosilicate glasses for some extent. This is because more alumina-doping increases density fluctuations of silica glass if the drawing temperature and procedure are the same with those for silica glass fiber. Thereby, a possible fabrication process to sufficiently attenuate the Rayleigh scattering of the F-doped aluminosilicate glass was proposed, according to the density fluctuation analysis at high temperature.