Optimization of Production Process of Optical Fiber Preform by Vapor-Phase Axial Deposition Method

点击翻译按钮获取中文摘要

The production process of silica fine particles by the vapor-phase axial deposition (VAD) method was simulated considering the whole process as a sequence of the following three stages: (1) combustion reaction in hydrogen diffusion flames; (2) production of silica particles; and (3) movement of silica particles in the flame. In the stage (1), the combustion reaction was simulated using a chemical model composed of twenty-one pairs of elementary reactions. In the stage (2), the relation between the above combustion reaction and the production reaction of silica particles was discussed by adding a reaction of silicon tetrachloride with oxygen. In the last stage, the movement of the silica particles in the flow of the combustion gas and their deposition on the preform surface were simulated on the basis of the fluid dynamics so that the necessary conditions for the efficient growth of the preform were looked for.The following results were obtained: (1) The regions of the combustion and the production of silica particles are completely separated, and the combustion reactions take place mainly at the base of the flame. The oxygen in excess flows around the combustion-reaction region and them approaches the central part of the flame. (2) Silicon tetrachloride flows along the center through the combustion region without reaction. There it is heated by the combustion reactions and finally reacts with oxygen producing fine particles of silica. (3) Near the preform surface, the flow velocity decreases and then the movement of the silica particles becomes very sensible to change with thermophoresis phenomena. The temperature gradient must be, therefore, made larger to raise the silica deposition yield on the preform. (4) The combustion gas must envelope laminarly the preform all around its surface in order to get uniform deposition.