Patent ID: 6961128

Claim:
An apparatus for detecting cross-talk drift, the apparatus comprising: a wavelength selective device arranged to receive electromagnetic radiation of a first spectral distribution and provide electromagnetic radiation within a first spectral band at a first output and electromagnetic radiation within a second spectral band at a second output, the first spectral band spectrally overlapping with the second spectral band, wherein the wavelength selective device is athermal; a photodetector for receiving the electromagnetic radiation provided at the first and second outputs, wherein the electromagnetic radiation in the first spectral band is co-incident upon the photodetector with the electromagnetic radiation in the second spectral band; and a meter circuit for measuring a magnitude of a homodyne beat signal resulting from the coincidence of the electromagnetic radiation within the first and second spectral bands, the magnitude of the homodyne beat signal being indicative of a position of the first spectral distribution relative to the first and second spectral bands.