Patent ID: 7680381

Claim:
A optical fiber which is single-mode at 1550 nm comprising a core, an inner cladding and a low index ring, wherein: the core comprises silica-based glass, an index of refraction n 1 , a relative refractive index percent Δ 1 % relative to pure silica glass, a radius; the inner cladding surrounds the core and extends from r 1 to a radius r 2 such that the inner cladding has a radial thickness of r 2 -r 1 , wherein the inner cladding comprises silica-based glass, an index of refraction n 2 , a relative refractive index percent Δ 2 % relative to pure silica glass, wherein n 1 >n 2 , Δ 1 %>Δ 2 % and the difference between Δ 1 % and Δ 2 % (Δ 1 %−Δ 2 %) is greater than about 0.1%; and the low index ring surrounds and directly contacts the inner cladding and extends from r 2 to a radius r 3 such that the low index ring has a radial thickness of r 3 -r 2 <20 μm, wherein the low index ring comprises silica-based glass co-doped with boron and fluorine, a third index of refraction n 3 and a third relative refractive index percent Δ 3 % relative to pure silica glass, wherein n 2 >n 3 , Δ 2 %>Δ 3 % and Δ 3 % is less than about −0.7%; and the single-mode optical fiber having a cable cutoff wavelength of less than 1500 nm.