Optical fiber and method of manufacturing optical fiber

An optical fiber comprising:

• a central core portion that is made of silica based glass containing germanium (Ge);
• an intermediate layer that is made of silica based glass and formed on an outer periphery of the central core portion;
• a trench layer that is made of silica based glass and formed on an outer periphery of the intermediate layer;
• and
• a cladding portion that is made of silica based glass and formed on an outer periphery of the trench layer, wherein
• relationships Δ1>Δ2>Δ3 and 0>Δ3 are satisfied, where Δ1 is a relative refractive-index difference of the central core portion with respect to the cladding portion, Δ2 is a relative refractive-index difference of the intermediate layer with respect to the cladding portion, and Δ3 is a relative refractive-index difference of the trench layer with respect to the cladding portion,
• Δ1 is equal to or larger than 0.34% and equal to or smaller than 0.37%,
• |Δ3| is equal to or larger than 0.1% and equal to or smaller than 0.25%,
• Δ1×|Δ3| is equal to or smaller than 0.08%2,
• Δ2 is equal to or larger than −0.04% and equal to or smaller than 0.04%,
• when a core diameter of the central core portion is 2a, and an inner diameter and an outer diameter of the trench layer are 2b and 2c, respectively, b/a is equal to or larger than 1.8 and equal to or smaller than 3.6 and c/a is equal to or larger than 4.0 and equal to or smaller than 5.0,
• a mode field diameter at a wavelength of 1310 nm is equal to or larger than 8.8 μm, and
• a transmission loss at a wavelength of 1550 nm is equal to or smaller than 0.195 dB/km.