Process for preparing glass preform for optical fiber

A process for producing a glass preform for optical transmission fibers comprised of a substantially fluorine-free nitrogen-doped silica glass core and a surrounding fluorine-doped or undoped silica glass cladding layer, which comprises forming the cladding layer of the preform by depositing said fluorine-doped or undoped silica glass on said core in the molten state or depositing said fluorine-doped or undoped silica glass on said core in finely divided form and then sintering the same to yield said cladding layer, and which comprises forming the core of the preform by supplying a gaseous silicon compound, ammonia or a gaseous nitrogen compound, and an oxygen-containing gas as starting gases into a high temperature zone to produce fine particles of SiOxNy glass and depositing said fine particles in the form of a soot or a transparent glass on a starting member to produce nitrogen-doped silica glass.