PURPOSE:To make possible low loss even a long wavelength band by forming the clad in a single mode type with the glass composition of SiO2 base and setting the refractive indices of the core, clad and support layers at the specific relations. CONSTITUTION:A core 1 is formed by P2O5-SiO2 or GeO2-SiO2, a clad 3 by P2O5- SiO2 or GeO2-SiO2 and a support layer 3 by quartz glass, respectively. If the refractive index of the core 1 is let n0, the refractive index of the clad 2 n1, and the refractive index of the support layer n2, then the conditions of n1n2, n0-n1 n1-n2 are so set that the region of the clad 2 including the core 1 becomes of single mode. As a result of this, losses are low even in a long wavelength band and the inprovement in transmission characteristics may be achieved.
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