A gas sensor system for detecting and measuring the concentration of a gaseous molecular species from an environment having an ambient temperature and an ambient pressure, the system comprising: a laser light source operable to provide a mid-infrared coherent light, the laser light source or a portion thereof being disposed within a laser module; a single gas cell operable so as to receive the mid-infrared coherent light into an interior of the gas cell; a sample gas inlet port operable to direct a sample gas from the environment into the gas cell interior such that the mid-infrared coherent light is transmitted through the sample gas; a vacuum system or flow controller fluidically coupled to the gas cell operable to maintain the sample gas within the gas cell at a predetermined pressure less than the ambient pressure, wherein a wavelength of the mid-infrared coherent light coincides with a rotationally resolved absorption line of the gaseous molecular species and a line width of the mid-infrared coherent light is less than a line width of the rotationally resolved absorption line at the predetermined pressure; a photodetector within an optical module operable to receive the mid-infrared coherent light transmitted through the sample gas in the gas cell, the optical module comprising an enclosure and being optically coupled to the gas cell; one or more optical fibers (95, 98a, 98b) coupled to the laser module and to the optical module, wherein the one or more optical fibers are operable to direct the mid-infrared laser light from the laser module into the optical module; the optical module being configured to receive light from the one or more optical fibers (95, 98a, 98b), perform any required conditioning or filtering of said light and direct said light to the gas cell; a thermally insulated enclosure having the gas cell, the laser light source, the laser module and the optical module including the photodetector disposed therein; a heat source or heat exchanger operable to influence a temperature of the interior of the thermally insulated enclosure; characterized in that the system further comprises: a pressure sensor operable to sense a pressure of the sample gas within the gas cell; and a control system electronically coupled to the vacuum system and to the pressure sensor operable to maintain the sample gas within the gas cell at the predetermined pressure to within 133.3 Pa; wherein the gas cell is fluidically coupled to the optical module such that, in operation, the sample gas flows from the gas cell into the enclosure of the optical module. a laser light source operable to provide a mid-infrared coherent light, the laser light source or a portion thereof being disposed within a laser module; a single gas cell operable so as to receive the mid-infrared coherent light into an interior of the gas cell; a sample gas inlet port operable to direct a sample gas from the environment into the gas cell interior such that the mid-infrared coherent light is transmitted through the sample gas; a vacuum system or flow controller fluidically coupled to the gas cell operable to maintain the sample gas within the gas cell at a predetermined pressure less than the ambient pressure, wherein a wavelength of the mid-infrared coherent light coincides with a rotationally resolved absorption line of the gaseous molecular species and a line width of the mid-infrared coherent light is less than a line width of the rotationally resolved absorption line at the predetermined pressure; a photodetector within an optical module operable to receive the mid-infrared coherent light transmitted through the sample gas in the gas cell, the optical module comprising an enclosure and being optically coupled to the gas cell; one or more optical fibers (95, 98a, 98b) coupled to the laser module and to the optical module, wherein the one or more optical fibers are operable to direct the mid-infrared laser light from the laser module into the optical module; the optical module being configured to receive light from the one or more optical fibers (95, 98a, 98b), perform any required conditioning or filtering of said light and direct said light to the gas cell; a thermally insulated enclosure having the gas cell, the laser light source, the laser module and the optical module including the photodetector disposed therein; a heat source or heat exchanger operable to influence a temperature of the interior of the thermally insulated enclosure; characterized in that the system further comprises: a pressure sensor operable to sense a pressure of the sample gas within the gas cell; and a control system electronically coupled to the vacuum system and to the pressure sensor operable to maintain the sample gas within the gas cell at the predetermined pressure to within 133.3 Pa; wherein the gas cell is fluidically coupled to the optical module such that, in operation, the sample gas flows from the gas cell into the enclosure of the optical module. a pressure sensor operable to sense a pressure of the sample gas within the gas cell; and a control system electronically coupled to the vacuum system and to the pressure sensor operable to maintain the sample gas within the gas cell at the predetermined pressure to within 133.3 Pa; wherein the gas cell is fluidically coupled to the optical module such that, in operation, the sample gas flows from the gas cell into the enclosure of the optical module.