An angled Bragg grating optical waveguide section adjusts the spectral response of a transmission filter in an optical waveguide or in an optical fiber. To smooth the response of the filter, which is subject to modulation due to coupling to the various cladding modes, the pitch of the Bragg grating is caused to vary in a linear fashion along the length of the filter. To adapt the attenuation band to a required band, the cladding of the optical fiber is doped with a photosensitive material so that the grating is also formed in the cladding. The photosensitivity of the cladding is advantageously greater than that of the core, in a ratio in the order of 5:1.