Mass density and the Brillouin spectroscopy of aluminosilicate optical fibers

Abstract

Provided herein is a detailed analysis of the Brillouin properties of alumina-doped silica optical fiber. The acoustic velocity of alumina in silica is shown to be a very strong function of its mass density, which can vary significantly from sample-to-sample and likely originates from the observed linear relationship between the longitudinal elastic modulus and the mass density. Further, the refractive index versus the alumina concentration provides a very sensitive probe of this mass density, and can be used to derive other structural details about the alumina. For example, for the first time to the best of our knowledge measurements of the thermo- and strain-acoustic coefficients (TAC and SAC, respectively) of the alumina dopant in silica-based fiber are presented and it is shown that these quantities are not strongly influenced by the density of alumina. Further, the material acoustic damping does not appear to be strongly influenced by the density. The TAC and SAC, or the dependence of the acoustic velocity on temperature or strain, respectively, are both found to be negative and large for alumina, in fact much larger than those for silica. Alumina thus represents a unique and potentially very useful material for the compositional tuning of the Brillouin scattering characteristics of optical fibers for distributed sensing and other applications. Conversely, these properties of alumina reduce the effectiveness of using applied temperature or strain gradients to fiber in order to suppress Brillouin scattering in fiber laser systems.