Abstract: A rare-earth-doped optical fiber comprises a silica core region doped with a rare earth element and a cladding region adjacent the core region, characterized in that the core region is also doped with aluminum (Al) and fluorine (F). The presence of small amounts of F are effective to lower the refractive index, and hence the NA, of the core region even in the presence of significant amounts of Al (e.g.,>8 mol %). Thus, the fiber has both a relatively flat gain spectrum and a low NA (e.g.,<0.20). Also described are optical amplifiers that incorporate such fibers. Preferably the rare earth composition of the core includes at least erbium, and the core is also doped with germanium.