Manufacture of optical waveguide preforms by modified chemical vapor deposition (MCVD) is theoretically investigated. For the first time, a model accounting for the concurrent heat transfer, chemical kinetics, and silica aerosol dynamics during lightguide preform fabrication by MCVD is presented. Silica particles are formed by high-temperature oxidation of SiCl4, grow by coagulation, and deposit to the preform walls by thermophoresis and Brownian diffusion. Assuming first-order SiCl4 oxidation and approximating the aerosol size distribution by a log normal function throughout the process, five partial differential equations describe this process. The emphasis of this study is on the achievement of high process yield (deposition efficiency of MCVD).
Process conditions for operation in the particle transport-limited and reaction-limited regimes are quantitatively identified. Operation in the former regime results in complete oxidation of the inlet SiCl4 but only about half of the product SiO2 particles deposit to the tube wall. Operation in the latter regime results in limited oxidation of inlet SiCl4 (about 40%) but almost all product SiO2 particles deposit to the tube wall. This study shows that high process yields and deposition rate can be achieved in lightguide preform manufacture by MCVD by combining operation in the reaction-limited regime with recycling of the exit gases from the preform tube.