Drawing condition dependence of pure‐silica‐core single‐mode fibers

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Abstract
The conditions have been investigated for drawing the SiO2 core/F‐SiO2 cladding single‐mode optical fiber which is expected to have the lowest optical loss among the silica optical fibers. In this type of optical fiber, the softening temperature of the core is higher than that of the cladding, and the softening temperature distribution is different from that of the GeO2‐SiO2 core/SiO2 cladding‐type optical fibers. In this study, the residual stress in the fiber during drawing was investigated theoretically and experimentally, considering the softening temperature difference between the core and cladding. The following results were obtained. In the SiO2 core single‐mode optical fiber, the tensile stress remains in the core where it is proportional to the drawing tension and inversely proportional to the area of the cross section. The residual stress influences the refractive index of the core and the cut‐off wavelength, but degrades the propagation loss characteristic. Finally, this paper will show that: (1) this optical fiber belongs to the F‐SiO2 core/F‐SiO2 cladding type from the viewpoint of the material, and (2) the depressed‐type structure is the most suitable.