An optical fiber (10), comprising: (i) a rare earth doped silica based elongated core (12) with a first refractive index (n1 with an aspect ratio of 1.5 to 10; (ii) a silica based moat (13) abutting and at least substantially surrounding the core, the moat having a refractive index n2, wherein n21; (iii) a silica based inner cladding (14) surrounding the moat, the inner cladding having a third refractive index (n3), wherein n1>n3; and n3>n2; (iv) a silica based outer cladding (16) surrounding said inner cladding, the outer cladding having a fourth refractive index (n4), such that n43; the optical fiber exhibits single polarization at the operating wavelength band.