PURPOSE: To reduce transmission loss, by forming the first thin clad layer made of a glass having a m.p. lower than that of the outermost silica glass and the second clad low m.p. glass layer on the first clad layer in a thickness enough to prevent invasion of OH from the outermost layer.
CONSTITUTION: In producing an optical fiber especially by the CVD method form the inside, on the inside of the outermost silica glass layer 4, the second clad layer 3 is formed, which is made of low m.p. glass composed of SiO2 as well as B2O3 and P2O5, has the same refractive index as the layer 4 and has the thickness enough to prevent invasion of OH from the layer 4. On the inside of the layer 3, the first thin clad layer 32 is formed, which is made of SiO2 contg. a slight amt. of P2O5, has a m.p. lower than that of the layer 4 but higher than that of the layer 3, and has a refractive index lower than that of the core 1 made of SiO2 contg. GeO2 and higher than the layer 3. The transmitted light oozed out of the core 1 is enclosed in the layer 32 in such a constitution, and prevented from being absorbed by B2O3 in the layer 3 and by the OH groups, too, and the obtained optical fiber has a true circular section, and small transmission loss.
OKAMURA KOUJI
YUASA MITSUO
