Characterization of torch-deposited silica for lightguide 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 GeO sub 2 concentration decreases with increasing radius as required for the light guiding refractive index profile, the fraction of the GeO sub 2 in the crystalline form increased with increasing radial position in the boule. Infrared spectra are consistent with most of the soot particles containing separate phases of partially crystalline GeO sub 2 and amorphous SiO sub 2 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.