Low-dispersion single-mode optical fiber

A low-dispersion single-mode fiber, comprising:

• a core layer and a cladding,
• wherein the core layer has a relative refractive index difference Δ1 in a range from 0.30% to 0.65% and a radius R1 in a range from 2.5 μm to 4.5 μm, and
• wherein the cladding is divided into three cladding layers and one outer cladding from inside to outside, wherein a first cladding layer covers the core layer, and has a relative refractive index difference Δ2 in a range from −0.70% to −0.30% and a radius R2 in a range from 4.5 μm to 7.5 μm;
• a second cladding layer covers the first cladding layer, and has a relative refractive index difference Δ3 in a range from −0.20% to 0.25% and a radius R3 in a range from 7.0 μm to 12.0 μm;
• a third cladding layer covers the second cladding layer, and has a relative refractive index difference Δ4 in a range from −0.60% to 0.00% and a radius R4 in a range from 10.0 μm to 20.0 μm;
• and the outer cladding covers the third cladding layer, and is a layer made of pure silicon dioxide glass wherein the core layer is provided with an inner core layer, wherein the inner core layer is depressed at a central portion thereof and has a relative refractive index difference AO in a range from 0.10% to 0.55% and a radius Δ0 in arrange from 1.0 μm to 2.5 μm.