An optical sensor. The optical sensor comprises a substrate, a Fabry-Perot interferometer, and first and second photodetectors. The Fabry-Perot interferometer comprises a first mirror and a second mirror, and is mounted on the substrate such that light is transmitted through the interferometer to the substrate. The first and second photodetectors are configured to detect light transmitted through the etalon and the substrate. The first photodetector is sensitive to a first wavelength range, and the second photodetector is sensitive to a second wavelength range, and wherein the first and second wavelength ranges each correspond to a different mode of the interferometer.