An optical fiber including: (i) a silica based, rare earth doped core (12) having a first index of refraction n1; (ii) a silica based inner cladding (14) surrounding the core and having a second index of refraction n2, such that n1>n2, the inner cladding having a plurality of air holes (24, 26) extending longitudinally through the length of the optical fiber; (iii) a silica based outer cladding (16) surrounding the inner cladding and having a third index of refraction n3, such that n2>n3; wherein the optical fiber supports a single polarization mode within the operating wavelength range.