An optical fiber, comprising: (i) a rare earth doped silica based elongated core with a first refractive index (n.sub.1) 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 n.sub.2, wherein n.sub.2n.sub.3; and n.sub.3>n.sub.2; (iv) a silica based outer cladding surrounding said inner cladding, the outer cladding having a fourth refractive index (n.sub.4), such that n.sub.4