Abstract: An optical fiber includes: a central core portion; an intermediate layer configured to surround an outer periphery of the central core portion; a trench layer configured to surround an outer periphery of the intermediate layer; and a cladding portion configured to surround an outer periphery of the trench layer. The central core portion is made of silica based glass that does not contain germanium (Ge). ?1>?2>?3 and ?Clad>?3, where ?1 represents an average maximum relative refractive-index difference of the central core portion relative to pure quartz glass, ?2 represents an average relative refractive-index difference of the intermediate layer relative to the pure quartz glass, ?3 represents an average relative refractive-index difference of the trench layer relative to the pure quartz glass, and ?Clad represents an average relative refractive-index difference of the cladding portion relative to the pure quartz glass. ?1 is equal to or larger than 0.05%.

Abstract: An optical fiber includes: a central core portion; an intermediate layer configured to surround an outer periphery of the central core portion; a trench layer configured to surround an outer periphery of the intermediate layer; and a cladding portion configured to surround an outer periphery of the trench layer. The central core portion is made of silica based glass that does not contain germanium (Ge). ?1>?2>?3 and ?Clad>?3, where ?1 represents an average maximum relative refractive-index difference of the central core portion relative to pure quartz glass, ?2 represents an average relative refractive-index difference of the intermediate layer relative to the pure quartz glass, ?3 represents an average relative refractive-index difference of the trench layer relative to the pure quartz glass, and ?Clad represents an average relative refractive-index difference of the cladding portion relative to the pure quartz glass. ?1 is equal to or larger than 0.05%.