Patent ID: 7541573

Claim:
An optical sensor comprising: a sensing element comprising: a first wavelength selective filter having a first wavelength filtering response that varies with an effective modal index experienced by light propagating in the filter; and a sample window arranged to receive a sample of fluid such that the presence of a sample of fluid affects the effective modal index experienced by light propagating in the filter and hence modifies the first wavelength filtering response; the sensing element arranged to receive light from a light source and to output the light after filtering; and an analysing element comprising a second wavelength selective filter having a second wavelength filtering response, wherein the second wavelength filtering response overlaps with the first wavelength filtering response, and wherein the analysing element is arranged to receive light output by the sensing element and to output the light after filtering in dependence on the extent of the overlap of the first and second wavelength filtering responses for detection by an optical power detector.