An optical sensor comprises at least two planar Bragg gratings defined on a single substrate and arranged to receive light from a light source, each grating having a wavelength filtering response that varies with an effective modal index experienced by light propagating in the Bragg grating and a Bragg wavelength different to those of the other gratings, and at least one sample window overlying one or more of the gratings that can receive a sample of fluid that affects the effective modal index and response of the grating, the gratings filtering the light and outputting the filtered light for spectral analysis, from which the refractive index and related properties of the fluid can be determined. One or more of the gratings can be a reference grating used to compensate for temperature and other disturbances to the sensors. Gratings may have individual sample windows for testing separate fluid samples, or may share a common window so that a single fluid can be tested using several gratings.