Optical fiber

An optical fiber comprising:

• a core portion;
• a side core layer surrounding a circumference of the core portion;
• and
• a cladding portion surrounding a circumference of the side core layer, wherein
• relations, Δ1>ΔClad>Δ2 and 0>Δ2, hold, where Δ1 denotes an average maximum relative refractive-index difference of the core portion with respect to an average refractive index of the cladding portion, Δ2 denotes a relative refractive-index difference of an average refractive index of the side core layer with respect to the average relative refractive index of the cladding portion, and ΔClad denotes a relative refractive-index difference of the average refractive index of the cladding portion with respect to pure quartz glass,
• Δ1 is 0.24% or more and 0.30% or less,
• Δ2 is −0.27% or more and −0.08% or less,
• (Δ1−Δ2) is 0.36% or more and 0.57% or less,
• b/a is 2 or larger where a core diameter of the core portion is 2a and an outer diameter of the side core layer is 2b,
• the optical fiber has an effective core area of 107 μm2 and larger and 130 μm2 or smaller, at a wavelength of 1550 nm,
• b/a is 4 or smaller, and
• the cable cutoff wavelength is 1460 nm or shorter.