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.