Process for producing doped vitreous silica for preparing a preform for an optical fibre

Claim 1

A process for the production of doped vitreous silica and the preparation of a preform for an optical fiber, which process comprises: (a) melting a dopant-containing batch mixture of oxides and carbonates, which is capable of forming a glass, having a composition such that a separation of phases in the resultant glass is induced by annealing the resultant glass, and such that said separated phases have an interconnected structure due to spinodal decomposition, and refining the molten glass resulting from the melting to obtain a homogeneous, bubble-free glass melt; (b) forming or shaping said melt and hardening it, to provide a homogeneous glass; (c) annealing said homogeneous glass, thereby producing a spinodal structure by phase separation, and wherein there is formed a hard phase of silica and at least one doping oxide, and a soft phase comprising the oxides of at least boron and sodium, said phases being interconnected; (d) etching said glass with one or more mineral acids, thereby dissolving said soft phase to obtain a porous glass consisting essentially of said hard phase; (e) rinsing said porous glass with distilled water to eliminate elements from said soft phase; (f) drying said glass by slowly raising the temperature, such that an open porosity is obtained which is 60% higher than the initial porosity; (g) producing a powder from said porous glass; and (h) forming a fiber preform from the powder, by deposition of said powder, on a surface as a glass.