Method for production of glass preform for optical fibers

Claim 1

A method of producing a glass preform for optical fibers comprising the steps of: providing a glass particulate soot mass consisting essentially of quartz in the shape of a preform suitable for producing an optical fiber; first heating the glass soot particulate mass in a gas atmosphere containing at least a gas of a non-hydrogen containing fluorine-based compound and a gas of cholrine-based compound containing no fluorine to dope said glass particulate mass with fluorine to provide optical fiber preform cladding which has a predetermined lower refractive index than the optical fiber preform core said first heating being effected in a muffle tube made of pure quartz and having iron and copper impurities; second heating the fluorine-doped glass particulate mass to produce a transparent glass preform for optical fibers in a muffle tube made of pure quartz; and recovering said glass preform; wherein in said first heating step (i) said non-hydrogen containing fluorine-based compound is present in an amount sufficient to incorporate fluorine into said glass particulate mass, and (ii) said chlorine-based compound containing no fluorine is present in an amount sufficient to prevent HF formation and to prevent incorporation of an impurity of iron and copper in the glass particulate mass and to thereby suppress light absorption by an impurity of iron and copper in said transparent glass preform wherein, to provide a predetermined desired relative refractive index difference between said cladding and said core by fluorine doping, said glass soot particulate mass is controllably heated during said first heating at a rate of 2° to 10° C./min. until the temperature of said glass soot particulate mass reaches a temperature of 1,100° to 1,400° C.

Claim 2

A method of producing a glass preform for optical fibers comprising the steps of: providing a glass particulate soot mass consisting essentially of quartz in the shape of a preform suitable for producing an optical fiber; first heating the glass soot particulate mass in a gas atmosphere containing at least a gas of a non-hydrogen containing fluorine-based compound and a gas of cholrine-based compound containing no fluorine to dope said glass particulate mass with fluorine to provide optical fiber preform cladding which has a predetermined lower refractive index than the optical fiber preform core said first heating being effected in a muffle tube made of pure quartz and having iron and copper impurities; second heating the fluorine-doped glass particulate mass to produce a transparent glass preform for optical fibers in a muffle tube made of pure quartz; and recovering said glass preform; wherein in said first heating step (i) said non-hydrogen containing fluorine-based compound is present in an amount sufficient to incorporate fluorine into said glass particulate mass, and (ii) said chlorine-based compound containing no fluorine is present in an amount sufficient to prevent HF formation and to prevent incorporation of an impurity of iron and copper in the glass particulate mass and to thereby suppress light absorption by an impurity of iron and copper in said transparent glass preform wherein, to provide a predetermined desired relative refractive index difference between said cladding and said core by fluorine doping, said glass soot particulate mass is controllably heated during said first heating at a rate of 2° to 10° C./min. until the temperature of said glass soot particulate mass reaches a temperature of 1,100° to 1,400° C.