Optical fiber

An optical fiber comprising:

• 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, wherein
• the central core portion is made of silica based glass that does not contain germanium (Ge),
• Δ1>Δ2>Δ3 and ΔClad>Δ3, where Δ1 represents an average maximum relative refractive-index difference of the central core portion relative to pure quartz glass, Δ2 represents an average relative refractive-index difference of the intermediate layer relative to the pure quartz glass, Δ3 represents an average relative refractive-index difference of the trench layer relative to the pure quartz glass, and ΔClad represents an average relative refractive-index difference of the cladding portion relative to the pure quartz glass,
• Δ1 is equal to or larger than 0.05%,
• the intermediate layer, the trench layer, and the cladding portion are made of silica based glass that is doped with only fluorine (F) and chlorine (Cl),
• a core diameter of the central core portion is equal to or larger than 7.7 μm and equal to or smaller than 8.7 μm,
• a mode field diameter at a wavelength of 1310 nm is equal to or larger than 8.6 μm and equal to or smaller than 9.2 μm,
• a cable cutoff wavelength is equal to or smaller than 1260 nm, and
• a transmission loss at a wavelength of 1550 nm is equal to or smaller than 0.18 dB/km.