An optical fiber includes: a central core portion; an intermediate layer; a trench layer; and a cladding portion. Further, relationships Δ1>Δ2>Δ3 and 0>Δ3 are satisfied, where Δ1, Δ2, and Δ3 are a relative refractive-index difference of the central core portion, the intermediate layer, and the trench layer, respectively, 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%.sup.2, 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.

An optical fiber includes: a central core portion; an intermediate layer; a trench layer; and a cladding portion. Further, relationships Δ1>Δ2>Δ3 and 0>Δ3 are satisfied, where Δ1, Δ2, and Δ3 are a relative refractive-index difference of the central core portion, the intermediate layer, and the trench layer, respectively, 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%.sup.2, 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.

An optical fiber includes: a core portion made of glass; a cladding portion that is located on an outer periphery of the core portion and that is made of glass having a lower refractive index than a refractive index of the core portion; and a coating portion that covers an outer periphery of the cladding portion. Further, an average value of a relative refractive-index difference of a center core of the core portion is 0.1 to 0.5%, a fiber diameter including the coating portion is equal to or smaller than 220 μm, an effective cutoff wavelength is longer than 1260 nm and smaller than 1530 nm, and a mode field diameter of light at 1550 nm is equal to or larger than 9 μm.

An optical fiber includes: a core portion made of glass; a cladding portion that is located on an outer periphery of the core portion and that is made of glass having a lower refractive index than a refractive index of the core portion; and a coating portion that covers an outer periphery of the cladding portion. Further, an average value of a relative refractive-index difference of a center core of the core portion is 0.1 to 0.5%, a fiber diameter including the coating portion is equal to or smaller than 220 μm, an effective cutoff wavelength is longer than 1260 nm and smaller than 1530 nm, and a mode field diameter of light at 1550 nm is equal to or larger than 9 μm.

An optical fiber includes a glass portion, a primary coating layer, and a secondary coating layer. In the optical fiber, a value of microbend loss characteristic factor F.sub.μBL_GO is 2.6 ([GPa.sup.−1.Math.μm.sup.−10.5.Math.dB/turn].Math.10.sup.−27) or less, when represented by

F.sub.μBL_GO=F.sub.μBL_G×F.sub.μBL_O

by using geometry microbend loss characteristic F.sub.μBL_G and optical microbend loss characteristic F.sub.μBL_O.

A low-dispersion single-mode optical fiber includes a core and a cladding covering the core. The core has a relative refractive index difference of 0.30-0.65% and a radius of 2.5-4.5 μm. The cladding layer including first, second, third cladding layers and an outer cladding arranged sequentially from inside to outside. The first cladding layer covers the core, and has a relative refractive index difference of −0.70% to −0.30% and a radius of 4.5-7.5 μm. The second cladding layer covers the first cladding layer, and has a relative refractive index difference of −0.20% to 0.25% and a radius of 7.0-12.0 μm. The third cladding layer covers the second cladding layer, and has a relative refractive index difference of −0.60% to 0.00% and a radius of 10.0-20.0 μm. The outer cladding covers the third cladding layer, and is a layer made of pure silicon dioxide glass.

An optical fiber includes a glass portion, a primary coating layer, and a secondary coating layer. In the optical fiber, a value of microbend loss characteristic factor F.sub.μBL_GO is 2.6 ([GPa.sup.−1.Math.μm.sup.−10.5.Math.dB/turn].Math.10.sup.−27) or less, when represented by
F.sub.μBL_GO=F.sub.μBL_G×F.sub.μBL_O
by using geometry microbend loss characteristic F.sub.μBL_G and optical microbend loss characteristic F.sub.μBL_O.

A low-dispersion single-mode optical fiber includes a core and claddings covering the core. The core has a relative refractive index difference of 0.30-0.65% and a radius of 2.5-4.5 μm. The claddings include first, second, third cladding layers and an outer cladding arranged sequentially from inside to outside. The first cladding layer covers the core, and has a relative refractive index difference of −0.70% to −0.30% and a radius of 4.5-7.5 μm. The second cladding layer covers the first cladding layer, and has a relative refractive index difference of −0.20% to −0.25% and a radius of 7.0-12.0 μm. The third cladding layer covers the second cladding layer, and has a relative refractive index difference of −0.60% to 0.00% and a radius of 10.0-20.0 μm. The outer cladding covers the third cladding layer, and is a layer made of pure silicon dioxide glass.

The optical fiber comprises a glass fiber including a core and a cladding. The cladding includes an inner cladding surrounding an outer periphery of the core, a trench surrounding an outer periphery of the inner cladding, and an outer cladding surrounding an outer periphery of the trench. The inner cladding has a refractive index lower than a refractive index of the core. The trench has a refractive index lower than the refractive index of the inner cladding. The outer cladding has a refractive index higher than the refractive index of the trench and lower than the refractive index of the core. The core is doped with germanium. The inner cladding has an average chlorine mass concentration of 500 ppm or more and 5000 ppm or less.

An optical fiber with large effective area, low bending loss and low attenuation. The optical fiber includes a core, an inner cladding region, and an outer cladding region. The core region includes a spatially uniform updopant to minimize low Rayleigh scattering and a relative refractive index and radius configured to provide large effective area. The inner cladding region features a large trench volume to minimize bending loss. The core may be doped with Cl and the inner cladding region may be doped with F.