Abstract: An infrared laser spectrometer employs a laser and a thermoelectrically cooler detector. The spectrometer uses a monolithic ring mirror with a single aperture that serves to accept the input laser illumination and the output optical signal. The laser is tunable. The number of passes of the input laser illumination can be controlled, so as to define a laser path length. In some embodiments, the ring mirror is open to the atmosphere, and in other embodiments the ring mirror is closed from the ambient atmosphere to allow samples of known origin to be measured in the spectrometer.