Disclosed are optical gain fibers which include an erbium-containing core and a cladding surrounding the core and which have ripple of less than about 25 % over about a 40 nm wide window or ripple of less than about 15 % over about a 32 nm wide window, or both. In one embodiment, the optical gain fibers are pumpable at 980 nm and at 1480 nm. In another embodiment, the optical gain fibers are fusion sliceable. In yet another embodiment, the core includes oxides erbium; the cladding includes silicon dioxide; and the optical gain fiber has a passive loss of less than about 0.5 % of the peak absorption of the erbium absorption band in the vicinity of 1530 nm. The optical gain fibers of the present invention have a wider gain window, improved flatness across the gain window, and/or increased gain as compared to conventional optical gain fibers. Accordingly, they are useful in amplifying optical signals, particularly signals which need to be repeatedly amplified over the course of a long-haul transmission, without losses in the signal quality that are commonly encountered in conventional optical signal amplifying methods.