| 摘要 |
PURPOSE:To obtain totally-synthesized optical fiber parent material without deteriorating the transmission characteristics and at high yield by dehydrating and vitrifying in two stages a porous parent material comprising a core soot layer and a clad soot layer both formed by the VAD process in the atmosphere contg. gaseous chlorine with each specified flow rate of gaseous chlorine. CONSTITUTION:Gaseous chlorine added at a flow rate l1 defined by the formulas I-III is used for the formation of a glass rod having purity comprising a core layer 31 and a clad layer 32 in accordance with the VAD process. During the addition of the gaseous chlorine, a porous glass layer is deposited to the external periphery of said glass rod to vitrify the deposited porous glass layer. The value l1 is set at a larger value than l2 which is a flow rate of gaseous chlorine used for the formation of an outside clad layer 33. By this constitution, a difference DELTAn of the specific refractive index is generated between the clad layer 32 and the outside clad layer 33 of the glass rod to high purity as illustrated by the Figure B. Since a boundary between the glass rod of high purity and the outside layer 33 formed at the external periphery thereof becomes distinct, the thickness of the total clad layer is calculated exactly by determining a ratio between the diameters r, i.e. (outside diameter of the outside clad layer)/ (D: outside diameter of the glass rod of high purity), determining also the thickness of the outside layer 33. |
