Design of a NDIR gas sensor with two non-symmetric Fabry-Perot absorber-structures working as IR-emitter and IR-detector

Every gas (e.g. CO2) absorbs IR-radiation at individual gas specific IR-wavelengths. Non-dispersive infrared (NDIR) gas sensors exploit this property for gas monitoring. Such sensors are used in various applications, e.g. for control of air quality in office buildings or cars. This is a big market for low cost sensors. A NDIR sensor consists basically of three components: an IR-emitter, a chamber containing the sample gas, and an IR-detector with a filter for the observed wavelength. Commercially available systems use broadband IR-emitters (e.g.: micro-lamps) in combination with thermopile or pyroelectric detectors fabricated with a narrowband gas-specific IR-filter, e.g., an interference filter. We devised a concept for a simple and cost-effective NDIR-gas sensor based on two non-symmetric Fabry-Perot absorberstructures as IR-emitter and as IR-detector where no additional interference filter is needed. The presented sensor combines thin layer technology with optical sensing techniques. The system was first analyzed using ray tracing models based on a Monte Carlo method in order to model the response function of the system’s sample chamber. For our results, the sample gas is CO2 where the major absorption is centered around 4.26μm.