According to at least one embodiment a graded index multimode fiber comprises: (i) a silica based core co-doped with GeO2 and 1 to 12 mole % P2O5; the core having a dual alpha, α1 and α2, where 1.8≦α1≦2.4 and 1.9≦α2≦2.4 at the wavelength (λ) range between 840 and 1100 nm; and (ii) a silica based cladding region surrounding the core, wherein the fiber has a numerical aperture NA and 0.185≦NA≦0.25 (more preferably 0.185≦NA≦0.23). Preferably, the silica based cladding region surrounding the core has refractive index lower than that of pure silica.