PURPOSE: To enable the formation of an optical fiber to a truly circular cross section by constituting the double clad layers consisting of the 1st and the 2nd clad layers to be constituted between a core layer and the outermost layer in such a way that the m.p. thereof is lower on the outermost layer side and is increasingly higher the nearer the boundary with the core layer.
CONSTITUTION: A thick clad layer 42 which has a refractive index approximately equal to silica glass and is added with the impurity for decreasing the m.p. so as to be added increasingly less from the outer layer side toward the center and a core layer 41 having the higher refractive index are provided on the inside of the outermost layer 42 consisting of silica glass. Since the clad layer is thickly formed with such constitution, the diffusion of the OH group contained in a quartz tube used as a starting material into the core layer is prohibited and since the amt. of the B2O3 added in the clad layer is decreased toward the boundary with the core layer, the loss caused by the absorption of the transmitted light leaking from the core layer by B2O3 is reduced, and the single mode optical fiber of a low transmission loss having a truly circular sectional shape is obtd.
TSUKAMOTO MAKOTO
YUASA MITSUO
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