Characterization of Torch-Deposited Silica for Light-Guide Preforms

Germanium-doped silica soot particles from the Vapor-phase Axial Deposition (VAD) process were taken from unconsolidated light-guide preform boules and characterized by infrared spectroscopy, light scattering, x-ray diffraction, ICP elemental analysis, and TEM examination. The results show that while GeO₂ concentration decreases with increasing radius as required for the light-guiding refractive index profile, the fraction of the GeO₂ in the crystalline form increases with increasing radial position in the boule. Infrared spectra are consistent with most of the soot particles containing separate phases of partially crystalline GeO₂ and amorphous SiO₂ as opposed to a compound glass. The light scattering by particles suspended in media of various refractive indices is that expected from inhomogeneous particles, as required for the two-phase model. Water solubility experiments followed by infrared or ICP analyses confirm this model and further support a detailed structure for the particles in which the separate GeO₂ phase is present as a shell or coating on a predominantly silica core. Particles have sizes ranging from 10 to more than 125 nm, and they are mostly spherical in shape. Near the center of the boule, however, where substrate temperatures are higher for longer times, partially fused chains of spherical particles are found, and the sizes and shapes of the particles in both regions are consistent with growth by Brownian coagulation.