a bragg reflector grating comprises a plurality of chirped grating sections , in which at least a first chirped grating section and a second chirped grating section have differing ranges of grating pitches . the combined range of grating pitches provided by the first and second chirped grating sections includes at least one discontinuity , such that the first and second chirped grating sections have one or more grating pitches in common and / or there are one or more ranges of grating pitches within the combined range of grating pitches that are absent .