Method of fabricating high strength optical preforms

Abstract

A process is disclosed for finishing high-strength multilayer optical preforms capable of being drawn into long optical fibers useful for light-wave communications. The preforms are of the type prepared by sequential chemical-vapor-deposition of the various desired glass layers within a tubular glass substrate that is then collapsed into a solid cylindrical preform comprising a cylindrical light-transmitting core surrounded by a concentric cladding layer and one or more additional layers within the collapsed substrate. In order for the outermost deposited additional layer to constitute a thin high-compression layer on the surface of the completed optical structure, it is necessary first to remove substantially all of the substrate layer from the preform in such a manner as to leave the adjoining high-compression layer intact. This is accomplished by selecting a glass for the substrate layer that is capable of being etched away faster than the high-compression layer, and then using controlled preferential etching to remove the substrate layer without penetrating or damaging the high-compression layer. This method makes it possible to provide a thinner and more effective outer high-compression layer, on the surface of the completed preform, which is of uniform radial thickness regardless of any nonlinearity or non-concentricity of the core, enclosing layers, or substrate layer. An optical fiber drawn from the completed preform will then retain a uniform high-compressive stress in its outer layer that contributes materially to its tensile strength, durability, and fiber life.