An optical waveguide fiber having a high threshold for stimulated Brillouin scattering. According to some embodiments of the invention, the optical fiber comprises: (a) a rare earth doped core having a refractive index profile and a centerline, the core including at least two adjacent core regions including different amounts of updopants, such that the longitudinal acoustic field velocities within the two core regions differ by at least 0.2%; and (b) a cladding layer surrounding and directly adjacent the core. The said fiber has MFD of greater than 12 .mu.m and delta % difference between the peak core delta and the cladding of less than 0.3%.