Patent ID: 7844155

Claim:
An optical fiber comprising: a silica-based core, said core comprising an alkali metal oxide selected from the group consisting of K 2 O, Na 2 O, Li 2 O, Rb 2 O, Cs 2 O and mixtures thereof in an average concentration in said core between about 10 and 10000 ppm by weight wherein said core further comprises fluorine in an average amount greater than about 500 ppm by weight, and a silica-based cladding surrounding the core, said silica based cladding including at least one annular region having a lower refractive index delta than an outer cladding region of said cladding, said annular region comprises randomly distributed voids, fluorine, or mixtures thereof, and said annular region is spaced from said core, and comprises a relative refractive index Δ 3min <−0.1% compared to the outer cladding region.