Bend insensitive optical fibers with low refractive index glass rings

An optical fiber comprising a silica-based glass portion having an outer diameter of less than about 120 μm, the glass portion comprising a core, an inner cladding, and a low index ring, wherein:

• the core comprises an index of refraction n1, a relative refractive index Δ1% relative to the pure silica glass and a radius r1;
• the inner cladding surrounds the core and extends from r1 to r2 such that the inner cladding has a radial thickness of r2−r1 <40 μm and the inner cladding comprises silica based glass, an index of refraction n2, a relative refractive index percent Δ2% relative to pure silica glass, wherein n1>n2 and the difference between Δ1% and Δ2% (Δ1% - Δ2%) is greater than about 0.1%;
• and
• the low index ring is an outer most glass portion of the optical fiber surrounding and directly contacting the inner cladding and extending from r2 to a radius r3 such that the low index ring has a radial thickness of r3−r2 ≦about 20 μm, wherein the low index ring comprises silica-based glass co-doped with boron and fluorine, a third index of refraction n3 and a third relative refractive index percent Δ3% relative to pure silica glass, wherein n2>n3,Δ2% >Δ3% and Δ3% is less than about -0.5%;
• and
• a coating surrounding the low index ring, wherein the coating comprises a relative refractive index percent Δc% relative to pure silica glass and Δc% >Δ2%.