Refractive Index Modulation in Optical Fibers Fabricated by the Vapor Oxidation Process

Abstract

One method used to fabricate preforms which are drawn into high-bandwidth optical fibers is the chemical vapor oxidation process. In this process, silicon, germanium, and boron chlorides are vaporized and made to pass through a rotating fused silica tube. A flame traversing the tube causes the gases to oxidize and deposit on the inside of the tube as a glass. By varying the concentrations of gases, the index of refraction of the glass layer can be controlled. With many deposition layers, a glass preform with the desired index profile can be fabricated to reduce multi-mode dispersion. The optimum index profile, n(r), can be expressed as:

$$\begin{array}{*{20}{c}} {{n^2}(r) = n_1^2(1 - 2\Delta {{(r/a)}^\alpha })}&,&{\Delta = ({n_1} - {n_2})/{n_1}} \end{array}$$

where n₁ is the on-axis index, n₂ is the cladding index, a is the core radius, and α is on the order of 2. Increases in dispersion arise from deviations of the index gradient, α, from the optimum and from local perturbations in the index profile. Olshansky¹ has shown that narrow bumps on dips in refractive index can adversely affect pulse broadening and that perturbations at large radial positions in the fiber cause the most significant effects. Therefore, the measurement and control of such perturbations is important in achieving very high bandwidth fibers.