Disclosed is an optical fiber having a silica-based core comprising an alkali metal oxide selected from the group consisting of K.sub.2O, Na.sub.2O, LiO.sub.2, Rb.sub.2O, Cs.sub.2O 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 a silica-based cladding surrounding and directly adjacent the core. By appropriately selecting the concentration of alkali metal oxide dopant in the core and the cladding, a low loss optical fiber may be obtained.