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A single-mode optical fiber has a cut-off wavelength of 1260 nm or less, a zero-dispersion wavelength in the range of 1300 nm to 1324 nm, a zero-dispersion slope of 0.093 ps/nm2/km or less, a mode field diameter at a wavelength of 1310 nm in the range of 5.5 μm to 7.9 μm, and a bending los

A single-mode optical fiber has a cut-off wavelength of 1260 nm or less, a zero-dispersion wavelength in the range of 1300 nm to 1324 nm, a zero-dispersion slope of 0.093 ps/nm2/km or less, a mode field diameter at a wavelength of 1310 nm in the range of 5.5 μm to 7.9 μm, and a bending loss of 0.5 dB or less at a wavelength of 1550 nm, the bending loss being produced when the fiber is wound around a 10-mm radius for 10 turns.

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The invention claimed is: 1. A single-mode optical fiber comprising: a central core which has a radius r1 and a refractive index n1; an inner cladding which is provided around an outer circumference of the central core and has a radius r2 and a refractive index n2; a trench portion which is provide

The invention claimed is: 1. A single-mode optical fiber comprising: a central core which has a radius r1 and a refractive index n1; an inner cladding which is provided around an outer circumference of the central core and has a radius r2 and a refractive index n2; a trench portion which is provided around an outer circumference of the inner cladding and has a radius r3 and a refractive index n3; and an outer cladding which is provided around an outer circumference of the trench portion and has a radius r4 and a refractive index n4, wherein: description="In-line Formulae" end="lead"n1>n4≧n2>n 3,description="In-line Formulae" end="tail" a cut-off wavelength is not greater than 1260 nm, a zero-dispersion wavelength is between 1300 nm and 1324 nm, a zero-dispersion slope is not greater than 0.093 ps/nm2/km, a mode field diameter at a wavelength of 1310 nm is between 5.5 μm and 7.9 μm, a bending loss produced if the fiber is wound around a 10-mm radius for 10 turns is not greater than 0.5 dB at a wavelength of 1550 nm, description="In-line Formulae" end="lead"1.52/r1r3-r2)/r1 1 is a relative refractive index difference of the central core, Δ2 is a relative refractive index difference of the inner cladding, and Δ3 is a relative refractive index difference of the trench portion, and the following conditions are satisfied: description="In-line Formulae" end="lead"0.40%≦Δ1≦0.85% description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"-0.20%≦Δ2≦0.00% description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"-1.0%32. description="In-line Formulae" end="tail" 3. The single-mode optical fiber according to claim 1, wherein the radius r3 of the trench portion is between 6 μm and 20 μm. 4. The single-mode optical fiber according to claim 1, wherein the radius r4 of the outer cladding is between 28 μm and 64 μm. 5. A single-mode optical fiber, comprising: a central core which has a radius r1 and a refractive index n1; an inner cladding which is provided around an outer circumference of the central core and has a radius r2 and a refractive index n2; and an outer cladding which is provided around an outer circumference of the inner cladding and has a radius r4 and a refractive index n4, wherein: description="In-line Formulae" end="lead"n1>n4>n2,description="In-line Formulae" end="tail" a cut-off wavelength is not greater than 1260 nm, a zero-dispersion wavelength is between 1300 nm and 1324 nm, a zero-dispersion slope is not greater than 0.093 ps/nm2/ km, a mode field diameter at a wavelength of 1310 nm is between 5.5 μm and 7.9 μm, and a bending loss produced if the fiber is wound around a 10-mm radius for 10 turns is not greater than 0.5 dB at a wavelength of 1550 nm. 6. The single-mode optical fiber according to claim 5, wherein Δ1 is a relative refractive index difference of the central core, Δ2 is a relative refractive index difference of the inner cladding with reference to the outer cladding, and the following conditions are satisfied: description="In-line Formulae" end="lead"0.42%≦Δ1≦r2/r1 ≦5.0description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"-1.0%≦Δ2≦-0.05%. description="In-line Formulae" end="tail" 7. The single-mode optical fiber according to claim 5, wherein y=(r2/r1)��|Δ2|, and the following conditions are satisfied: description="In-line Formulae" end="lead"1.4��Δ1-0.8≦y≦ 1.4��Δ1-0.05description="In-line Formulae" end="tail" description="In-line Formulae" end="lead"y≧0.075%.description="In-line Formulae" end="tail" 8. The single-mode optical fiber according to claim 5, wherein the radius r2 of the inner cladding is between 4.5 μm and 16 μm. 9. The single-mode optical fiber according to claim 5, wherein the radius r4 of the outer cladding is between 28 μm and 64 μm.