According to one example of the invention an optical fiber comprises: (i) silica based, rare earth doped core having a first index of refraction n.sub.1; (ii) at least one silica based cladding surrounding the core and having a second index of refraction n.sub.2, such that n.sub.1>n.sub.2; wherein at least one of the core or cladding is doped with Al.sub.2O.sub.3, such that the ratio of max wt % to min wt % of Al.sub.2O.sub.3 concentration is less than 2:1.