METHOD OF PRODUCING AN OPTICAL FIBER

The disclosed method of producing optical fiber comprises providing a silica-based glass rod of diameter D that comprises a core region of effective diameter d surrounded by cladding material (D/d typically > 2) and a silica-based porous jacketing tube, inserting the glass core rod into the porous tube, consolidating the porous material and sintering the jacketing tube onto the core rod such that a substantially pore-free optical fiber preform results. Fiber is then drawn from the preform in the conventional manner. Consolidation and sintering is carried out at a temperature below the softening temperature of the material, typically < 1600 DEG C. In a preferred embodiment, the porous tube is produced by a sol/gel process that comprises mixing colloidal silica (surface area 5-100 m /g) and water, adding quaternary ammonium hydroxide to the sol such that a pH in the range 11-14 results, lowering the pH into the range 4-11 by addition of methyl or ethyl formate, casting the sol into an appropriate mold and permitting it to gel. The relatively low processing temperature is an advantageous feature of the inventive process that, inter alia, makes it easier to maintain the fiber core concentric with the cladding.