Patent ID: 7164835

Claim:
An optical fiber comprising: a core provided at a center; a first cladding layer provided on a periphery of the core; a second cladding layer provided on a periphery of the first cladding layer; and a third cladding layer provided on a periphery of the second cladding layer, wherein a maximum refractive index of the core is greater than any of maximum refractive indices of the first cladding layer, the second cladding layer, and the third cladding layer, and the maximum refractive index of the second cladding layer is smaller than any of the maximum refractive indices of the first cladding layer and the third cladding layer, a value of a 2 /a 1 is not less than about 2.5 and not more than about 4.5, where a 1 represents a radius of the core, and a 2 represents a radius of an outer periphery of the first cladding layer, a relative refractive index difference of the core with respect to a maximum refractive index of the third cladding layer is not less than about 0.20% and not more than about 0.70% and a refractive index volume V of the second cladding layer expressed by Formula (1) is 25% μm 2 or higher and is 110% μm 2 or lower, V = 2 ⁢ ∫ a 2 a 3 ⁢  Δ ⁢ n ⁢ ( r )  · r ⁢ ⅆ r ( 1 ) wherein “r” is a radius, Δ n (r) is a relative refractive index difference at the radius “r” with respect to a maximum refractive index of the third cladding layer, a 2 is a radius of an outer periphery of the first cladding layer, and a 3 is a radius of an outer periphery of the second cladding layer.