1. Introduction
Gas sensors based on absorption spectroscopy are attracting great interest in a variety of applications related to the detection and control of exhaust gases originating from combustion and other industrial devices [1–5]. The ability of these sensors to simultaneously monitor multiple species is of utmost importance. In many cases, the concentration ratio is more informative than the concentrations of individual components. This is due to the depend-ence of the latter on possible dilution by other gases at the point of measurement. For instance, the ratio of CO2to CO is a well-known indicator of the completeness of combustion, so measurement of both gases is often required in technical combustion equipmentor its exhaust gases. Accurate and simultaneous measurement of these gases, along with a precise temperature determination, is crucial to evaluating the environmental and performance as pectsof combustion and gas-reforming devices.