Sol-gel process for forming a germania-doped silica glass rod

Abstract

A sol-gel process for forming a germania-doped silica glass rod that is suitable for use as a preform in the manufacture of high numerical aperture optical fibers. The sol is produced by mixing together prescribed quantities of alkoxides including tetraethyl orthosilicate and tetraethyl orthogermanate, deionized water, ethanol and a precipitation catalyst such as hydrofluoric acid. Premature precipitation of germania is avoided by adding ethanol and deionized water to an alkoxide solution at a controlled, drop-wise rate and by then similarly adding the catalyst at a controlled, drop-wise rate. After drying the resulting gel, sintering is performed by chlorinating the dry gel to remove residual hydroxyl groups, then oxygenating the chlorinated gel to remove the chlorine, and finally heating the oxygenated gel in helium to a temperature of about 1300.degree. C. Chlorinating and oxygenating the gel for extended time durations, and maintaining the gel above 1200.degree. C. for less than 30 minutes, ensures that the final glass rod will not bubble when subsequently raised in temperature to 1800.degree. C. during the final fiber draw step.