An optical waveguide suitable for a slanted Bragg grating (SBG) to be written thereinto has a refractive index profile defining a core and a cladding and a specific photosensitivity profile. This photosensitivity profile W(r) is such that, for a given SBG writing angle .theta., the coupling of the fundamental mode into itself as counterpropagation in the waveguide, K(.theta.), is substantially zero and such that its derivative K'(.theta.) with respect to the angle .theta. is also substantially equal to zero. The zero derivative, and also the zero coupling of the fundamental mode as a function of the angle, means that a widened reflection pocket about the grating write angle is guaranteed.