Abstract: Apparatus for filtering optical radiation, which apparatus comprises a waveguide (50), and which waveguide (50) comprises a grating (51) having a first reflection wavelength band (52) having a first average wavelength (53) and a first group delay (54), and a second reflection wavelength band (55) having a second average wavelength (56) and a second group delay (57), wherein the first group delay (54) and the second group delay (57) are different in at least a portion of the second reflection wavelength band (55).

Abstract: Apparatus for filtering optical radiation at an operating wavelength (19), which apparatus comprises a grating (1) written into a waveguide (2), and which grating (1) has a first end (3), a second end (91), a first bandwidth (24), a maximum re-flectivity (29), a first group delay variation (17) defined with respect to the first end (3), and a second group delay variation (18) defined with respect to the second end (91), wherein the first and second group delay variations (17), (18) are with respect to the first bandwidth (24), the maximum reflectivity (29) is greater than 50%, the first group delay variation (17) is between 0.1 ps and 100 ps, and the second group delay variation (18) is between 0.1 ps and 100 ps.