Optical fiber and optical transmission line

An optical fiber comprising:

• a center core portion, a side core portion and clad portion in an order from an inner side, which has a dispersion value of 14 ps/nm/km or higher and 20 ps/nm/km or less at a wavelength of 1550 nm, a dispersion slope of 0.05 ps/nm2/km or higher and 0.08 ps/nm2/km or less at a wavelength of 1550 nm and a transmission attenuation of 0.2 dB/km or less at a wavelength of 1550 nm,
• wherein a relative refractive index difference Δ1 between the center core portion and the clad portion is 0.25% or larger and 0.50% or less, a relative refractive index difference Δ2 between the side core portion and the clad portion is 0.05% or larger and 0.30% or less, an inequality Δ2<Δ1 is satisfied, a ratio a/b between an outer diameter a of the center core portion and an outer diameter b of the side core portion is 0.3 or higher and 0.7 or less, and an effective core area Aeff at a wavelength of 1550 nm is 90 μm2 or larger.