An optical fiber, comprising: (i) a core having a core center and a radius or a width a, (ii) a cladding surrounding the core, and (iii) at least one stress member situated proximate to the fiber core within the cladding, said stress member comprising silica co-doped with F and at least one dopant selected from the list consisting of: GeO.sub.2, P.sub.2O.sub.5, Y.sub.2O.sub.3, TiO.sub.2 and Al.sub.2O.sub.3, wherein distance b between the stress member and the core center satisfies the following equation: 1.ltoreq.b/a<2.