An optical apparatus comprises at least one primary diffraction grating and at least one reference diffraction grating each formed on or within a common grating substrate. The reference diffraction grating is arranged so as to diffract and disperse spatially according to wavelength a reference optical signal incident on the reference diffraction grating at an input incidence angle. The primary diffraction grating is arranged so as to diffract and disperse spatially according to wavelength an input optical signal incident on the primary diffraction grating at the input incidence angle. The reference and primary diffraction gratings exhibit at least one differing grating structural parameter. The reference and primary diffraction gratings are arranged so that a diffracted and spatially dispersed reference optical signal having at least one known wavelength component defines at least one spatial wavelength calibration reference for the diffracted and spatially dispersed input optical signal.