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).