Optical fiber fabrication

Claim 1

Process for fabrication of a glass fiber optical transmission line comprising a core section and a cladding wherein the cladding has an index of refraction of a value lower than the maximum index of the core for energy of the wavelength to be transmitted, which comprises introducing a moving stream of a vapor mixture including at least one compound glass forming precursor together with an oxidizing medium into a tube while heating the tube so as to react the said mixture to deposit reacted mixture so as to produce a smooth glassy deposit on the inner surface of the tube, in which heating of the tube and contents are by a hot zone which moves relative to the tube, the hot zone being produced by a corresponding heat source external to the tube, the said heat source comprising a plasma generator for producing a fire ball within the said vapor mixture within the tube, CHARACTERIZED IN THAT the said hot zone is a tandem hot zone including at least two zones; one designated Zone I and another designated Zone II in which Zone I is generated primarily by the said fire ball and has a central region at a fire ball temperature and an enveloping inner tube wall at a temperature below the glass transition temperature of the deposit and in which Zone II is defined by that zone within which the inner tube surface is at a temperature above the said glass transition temperature.