Abstract: An optical fiber, comprising: (i) a rare earth doped silica based elongated core with a first refractive index (n1) with an aspect ratio of 1:5 to 1; (ii) a silica based moat abutting and at least substantially surrounding the core, the moat having a refractive index n2, wherein n2<n1; (iii) a silica based inner cladding surrounding the moat, the inner cladding having a third refractive index (n3), wherein n1>n3; and n3>n2; (iv) a silica based outer cladding surrounding said inner cladding, the outer cladding having a fourth refractive index (n4), such that n4<n3; the optical fiber exhibits single polarization at the operating wavelength band.

Abstract: 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 ?m and delta % difference between the peak core delta and the cladding of less than 0.3%.