| 摘要 |
PROBLEM TO BE SOLVED: To increase the possession ratio of the core in an image guide and to improve the picture quality. SOLUTION: The temporal preform 6 of an optical fiber constituting the image guide is composed of a core 5, a clad layer 4a and a support layer 3, with the clad layer 4a composed of two layers of an inner clad 42a and an outer clad 41a. The support layer 3a can be removed by flame polishing by preliminarily controlling the coefficient of thermal expansion of the outer clad 41a to be lower than that of the inner clad 42a and controlling the softening temperature of the outer clad 41a to be higher than that of the inner clad 42a.
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