The disclosure relates to an infrared spectrometer comprising first and second opposing reflectors spaced apart by a spacing length, and a plurality of discrete concave reflecting facets, the reflecting facets being facets of at least one of the opposing reflectors. An infrared laser source is arranged to form a laser beam. The opposing reflectors are arranged such that the laser beam is reflected alternately from each of the opposing reflectors, including being reflected at least once by each of the reflecting facets. A detector is arranged to detect spectral properties of the laser beam after reflection from each of the plurality of reflecting facets, and an analyser then determines properties of a sample disposed between the first and second opposing reflectors from the detected spectral properties.