Abstract: One of embodiments relates to an optical fiber in which an alkali metal element is efficiently doped to its core to suppress transmission loss from increasing. A mean concentration or a concentration distribution of the alkali metal element is adjusted such that 0.48 or less is obtained as an weighted value obtained by weighting a distribution of field intensity of guided light at a wavelength of 1550 nm, with respect to a radial direction distribution of a ratio ID2/I?3 of an intensity ID2 of Raman scattering light by a silica three-membered ring structure and an intensity I?3 of Raman scattering light by a Si—O stretching vibration, in a cross-sectional region having a diameter of 20 ?m.

Abstract: One of embodiments relates to an optical fiber in which an alkali metal element is efficiently doped to its core to suppress transmission loss from increasing. A mean concentration or a concentration distribution of the alkali metal element is adjusted such that 0.48 or less is obtained as an weighted value obtained by weighting a distribution of field intensity of guided light at a wavelength of 1550 nm, with respect to a radial direction distribution of a ratio ID2/I?3 of an intensity ID2 of Raman scattering light by a silica three-membered ring structure and an intensity I?3 of Raman scattering light by a Si—O stretching vibration, in a cross-sectional region having a diameter of 20 ?m.