A planar photonic waveguide incorporating a Bragg grating having an asymmetric reflectance spectrum (200). In one embodiment, “red” side lobes (210) are suppressed relative to “blue” side lobes (220). In another embodiment, blue side lobes are suppressed relative to red side lobes. The waveguide has a core with dimensions which vary along the length of the grating so as to produce the asymmetric reflectance spectrum. Where red side lobes are suppressed, the width of the core is greatest in the middle of the grating and decreases symmetrically towards each end.