A multimode optical fiber comprising: a multilayered silicate glass core region (12.1) having a refractive index profile shape parameter α for broadband operation at wavelengths in the range of 0.78 to 1.55 μm, and a cladding region (12.3) adjacent said core region, said core and cladding regions (12.1, 12.3) being configured to have an NA of about 0.2 and to support the simultaneous propagation of optical radiation in said core region (12.1) at at least two wavelengths within said range, said core region (12.1) being co-doped with aluminum oxide, phosphorous oxide, germanium oxide and fluorine, the concentrations and distribution of said co-dopants being radially varied so that the refractive index of said core region (12.1) is radially graded from a maximum at the center of said core region (12.1) to a minimum near the outer edge of said core region (12.1), said grading following an essentially parabolic profile, where the refractive index n c (r) of said core region (12.1) at any radius, r, less than a, the radius of said core region (12.1), is given by n c r = n c 1 1 − 2 Δ r / a α 1 / 2 where Δ = n c 1 2 − n c 2 2 / 2 n c 1 2, where the quantities n c1 and n c2 are the refractive indices of said core region (12.1) at r = 0 and r = a, respectively, characterised in that said core region (12.1) is co-doped with no more than: 6 mole % Al 2 O 3, 9 mole % P 2 O 5, 6 mole % GeO 2, 6 mole % fluorine, and including 90-100 mole % SiO 2, the concentrations and distribution of said co-dopants being radially varied so that the optimum profile shape parameter α opt of said fiber is essentially independent of wavelength in that said optimum profile shape parameter varies by no more than 0.01 within said range of 0.78 to 1.55 μm. a multilayered silicate glass core region (12.1) having a refractive index profile shape parameter α for broadband operation at wavelengths in the range of 0.78 to 1.55 μm, and a cladding region (12.3) adjacent said core region, said core and cladding regions (12.1, 12.3) being configured to have an NA of about 0.2 and to support the simultaneous propagation of optical radiation in said core region (12.1) at at least two wavelengths within said range, said core region (12.1) being co-doped with aluminum oxide, phosphorous oxide, germanium oxide and fluorine, the concentrations and distribution of said co-dopants being radially varied so that the refractive index of said core region (12.1) is radially graded from a maximum at the center of said core region (12.1) to a minimum near the outer edge of said core region (12.1), said grading following an essentially parabolic profile, where the refractive index n c (r) of said core region (12.1) at any radius, r, less than a, the radius of said core region (12.1), is given by n c r = n c 1 1 − 2 Δ r / a α 1 / 2 where Δ = n c 1 2 − n c 2 2 / 2 n c 1 2, where the quantities n c1 and n c2 are the refractive indices of said core region (12.1) at r = 0 and r = a, respectively, characterised in that said core region (12.1) is co-doped with no more than: 6 mole % Al 2 O 3, 9 mole % P 2 O 5, 6 mole % GeO 2, 6 mole % fluorine, and including 90-100 mole % SiO 2, the concentrations and distribution of said co-dopants being radially varied so that the optimum profile shape parameter α opt of said fiber is essentially independent of wavelength in that said optimum profile shape parameter varies by no more than 0.01 within said range of 0.78 to 1.55 μm. 6 mole % Al 2 O 3, 9 mole % P 2 O 5, 6 mole % GeO 2, 6 mole % fluorine, and including 90-100 mole % SiO 2, the concentrations and distribution of said co-dopants being radially varied so that the optimum profile shape parameter α opt of said fiber is essentially independent of wavelength in that said optimum profile shape parameter varies by no more than 0.01 within said range of 0.78 to 1.55 μm.