Patent ID: 7536076

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, LiO 2 , Rb 2 O, Cs 2 O and mixtures thereof in an average concentration in said core between about 50 and 500 ppm by weight, said core further comprising chlorine and fluorine, wherein the average concentration of fluorine in said core is greater than the average concentration of alkali metal oxide in said core and the average concentration of chlorine in said core is greater than the average concentration of alkali metal oxide in said core; and wherein the core of said fiber comprises a first region located along the centerline of the core which contains chlorine in a minimum amount which is less than 100 ppm, and a second core region surrounding said first region, said second region containing a peak chlorine concentration greater than 500 ppm, and a silica-based cladding surrounding and directly adjacent the core.