| 摘要 |
PURPOSE:To obtain a high-characteristic optical fiber from a large-sized porous glass base material by heating the base material under reduced pressure to convert only the surface layer to transparent glass and to form a preform having an internal porous glass layer and then drawing the preform. CONSTITUTION:A porous glass layer is formed on a porous glass base material consisting of a core and a clad low in refractive index with respect to the core or on the vitrified core and a starting rod constituting a part of the clad to obtain a composite base material 1. The base material 1 is heated (furnace core tube 2, heater 3 and vacuum pump 4) under reduced pressure to convert only the surface layer to transparent glass 5a, and a preform 5 having an internal porous glass layer 5b is formed. While the initial porous glass layer 5b in the preform 5 is converted to transparent glass in a high-temp. heating furnace (drawing heater 6), the preform 5 is drawn to form a fiber 7. The heating under reduced pressure is preferably carried out at <=20Pa pressure. |
