An optical fiber including: (i) a silica based, rare earth doped core having a first index of refraction n.sub.1; (ii) a silica based inner cladding surrounding the core and having a second index of refraction n.sub.2, such that n.sub.1>n.sub.2, said inner cladding having a plurality of air holes extending longitudinally through the length of said optical fiber; (iii) a silica based outer cladding surrounding said inner cladding and having a third index of refraction n.sub.3, such that n.sub.2>n.sub.3, wherein said optical fiber supports a single polarization mode within the operating wavelength range.