Optical fiber

An optical fiber comprising:

• a core portion;
• a side core layer circumferentially surrounding the core portion;
• a cladding portion circumferentially surrounding the side core layer;
• and
• a coating layer circumferentially surrounding the cladding portion, wherein
• when an average maximum relative refractive-index difference of the core portion to an average refractive index of the cladding portion is Δ1, a relative refractive-index difference of an average refractive index of the side core layer to the average refractive index of the cladding portion is Δ2, a relative refractive-index difference of the average refractive index of the cladding portion to pure silica glass is ΔClad, Δ1>ΔClad>Δ2 and 0>Δ2 hold,
• the Δ1 is 0.18% or more and 0.24% or less,
• the Δ2 is −0.27% or more and −0.12% or less,
• (Δ1−Δ2) is 0.36% or more and 0.45% or less,
• when a core diameter of the core portion is 2a and an outer diameter of the side core layer is 2b, b/a is 2.5 or more and 5 or less,
• an effective core area at a wavelength of 1550 nm is 105 μm2 or more and 130 μm2 or less,
• a cable cut-off wavelength is 1530 nm or less, and
• a macrobending loss at a wavelength of 1550 nm when bent at a diameter of 30 mm is 1 dB/m or less.