Single mode optical fiber

An optical transmission fiber, comprising:

• a central core and an outer optical cladding, the central core having a first refractive index difference (Δn1) as compared to the outer optical cladding;
• a first inner cladding immediately surrounding the core, the first inner cladding having a second refractive index difference (Δn2)as compared to the outer cladding;
• a second, buried, inner cladding disposed between the first inner cladding and the outer cladding, the second inner cladding having a third refractive index difference (Δn3) as compared to the outer cladding,
• wherein the second refractive index difference (Δn2) is between −1.2×10−3 and 2.0×10−3,
• wherein the third refractive index difference (Δn3) is less than −3×10−3, and
• wherein the second inner cladding contains Germanium in a weight concentration of between 0.5% and 7%.