Patent ID: 7203407

Claim:
An optical fiber, comprising: a rare earth doped silica based elongated core with a first refractive index (n 1 ) with an aspect ratio of 1.5 to 10 and a minimum dimension a; a silica based moat abutting and at least substantially surrounding said core, said moat having a dimension d and a refractive index n 2, wherein n 2< n 1 and the ratio of d/a is between 2.0 to 7.0; a silica based inner cladding surrounding the moat, the inner cladding having a third refractive index (n 3 ), wherein n 1> n 3; and n 3> n 2 a silica based outer cladding surrounding said inner cladding, the outer cladding having a fourth refractive index (n 4 ), such that n 4< n 3 ; the optical is a single polarisation fiber with a single polarization at the operating wavelength band.