PURPOSE: To obtain the titled parent material having refractive index of clad part lower than that of core part and jacket part, by using two or more burners for clad, making SiCl4 containing a fluorine compound flow in all the burners.
CONSTITUTION: A raw material for glass, an inert gas carrier, a combustion gas, and O2 are made to flow in the burner 1 for core, to form a core part. In order to make a ratio of the outer diameter of the clad part to the core part of ≥6, the two or more burners 2 and 3 are provided with a glass raw material for clad such as SiCl4, etc. containing a fluorine compound in such a way that it satisfies the relationship shown by the formula VSi(core)/VSi(clad)<1/30, and VF/VSi(clad)>1/4 where the real flow rate VSi(core) of SiCl4 to be fed to the burner 1, the real flow rate VSi(clad) of total SiCl4 to be fed to the burners 2 and 3, and the flow rate VF of total fluorine to be fed to the burners 2 and 3, a combustion gas, and O2, the piled material 4 of porous glas fine particles is grown in the axis direction, and it is made into transparent glass by heating.
TANAKA GOUTAROU
KANAMORI HIROO
NIPPON TELEGRAPH & TELEPHONE