Abstract: The optical fiber includes a core, the first cladding, and second cladding. The core is made of silica based glass containing Cl. The first cladding and the second cladding are made of silica based glass containing fluorine. The refractive index of the first cladding is lower than that of the core. The refractive index of the second cladding is lower than that of the core and higher than that of the first cladding. The second cladding is divided into an outer region having a uniform refractive index and an inner region having a refractive index higher than that of the outer region. The difference ?P between the maximum refractive index of the inner region and the refractive index of the outer region is 0.02% to 0.10% in terms of relative refractive index with respect to pure silica based glass. The radial thickness R of the inner region is 10 ?m to 25 ?m.

Abstract: The optical fiber includes a core, the first cladding, and second cladding. The core is made of silica based glass containing Cl. The first cladding and the second cladding are made of silica based glass containing fluorine. The refractive index of the first cladding is lower than that of the core. The refractive index of the second cladding is lower than that of the core and higher than that of the first cladding. The second cladding is divided into an outer region having a uniform refractive index and an inner region having a refractive index higher than that of the outer region. The difference ?P between the maximum refractive index of the inner region and the refractive index of the outer region is 0.02% to 0.10% in terms of relative refractive index with respect to pure silica based glass. The radial thickness R of the inner region is 10 ?m to 25 ?m.

Abstract: Provided is an optical fiber having a large relative refractive index difference and a reduced transmission loss, as well as a manufacturing method therefor. An optical fiber preform 100, which is made of silica glass as the main element and which includes a core region having a relative refractive index difference of 2.0% or more and less than 3.0% on the basis of the refractive index of pure silica glass and a first cladding region provided around the core region and having a relative refractive index difference of ?0.8% or more and less than ?0.3% on the basis of the refractive index of pure silica glass, is drawn into a glass fiber. The glass fiber thus drawn is passed through an annealing furnace 21 installed below a drawing furnace 11, whereby the cooling rate of the glass fiber is restrained as compared with the case where it is cooled by air.