摘要 |
PURPOSE:To uniformize the refractive index distribution of a clad by adding a refractive index controlling substance to the inner or outer layer of the clad to control the difference in the specific refractive index between the inner and outer clad layers at the time of producing the title preform having a core and the inner and outer layers of the clad. CONSTITUTION:A glass material and gaseous fuel are introduced into burners 10 and 11 form the porous preform 9 having a core 7 and an inner clad layer 8, and the preform is heated to obtain the transparent glass preform 14 having a core 12 and an inner clad layer 13. The preform 14 is inserted into a glass tube 15 to form an outer clad layer, heated, and integrated. In the production stage, a refractive index increasing substance (e.g., chlorine) and a refractive index decreasing substance (e.g., fluorine) are added to the inner clad layer 13 or the outer clad layer 15 to control the difference in the specific refractive index between the inner and outer clad layers to <=0.003%, and the preform for a single-mode optical fiber is obtained. |