A large mode area fiber amplifier suitable for high power applications includes a core region specifically configured to allow for high power operation while also limiting the amount of SBS that is generated. The composition of the core region is selected to include a dopant (such as aluminum) in selected areas to reduce the acoustic refractive index of the core and limit the spatial overlap between the acoustic and optical fields. The acoustic refractive index is also structured so that the acoustic field is refracted away from the central core area. In one embodiment, the core may comprise a depressed index center portion and surrounding ring core area, with the center portion including the aluminum doping and the ring formed to have a diameter less that the phonon decay length for the operating wavelength(s).