Rare earth doped single polarization double clad optical fiber and a method for making such fiber

An optical fiber, comprising:

• (i) a rare earth doped silica based elongated core with a first refractive index (n1), a minimum dimension a, and a center air hole situated inside the core;
• (ii) a silica based moat abutting and at least substantially surrounding the core, the moat having a dimension d and a refractive index n2 wherein n2<n1 and the ratio of d/a is between 2.0 to 7.0;
• (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;
• and
• the optical fiber is a single polarization fiber with single polarization at the operating wavelength band and has SPB of at least 20 nm.