An optical fiber comprising: (i) a chlorine doped silica based core comprising a core alpha (α)>10, and maximum refractive index delta Δ1max % and Cl concentration >1 wt %; (ii) a cladding surrounding the core, the cladding comprising: (a) an inner cladding region adjacent to and in contact with the core and having a refractive index delta Δ2 and a minimum refractive index delta Δ2min such that Δ2min