Compact and robust mid-infrared laser-based gas sensor for portable and real-time measurements

Abstract

We propose a gas sensor using quantum cascade lasers which achieves high sensitivity and gas selectivity using a compact configuration and simple signal processing. This sensor features a small-volume Herriott cell and a simple concentration quantification algorithm. Our invented Herriott cell offers 5 m optical path length with only 40 mL internal volume. To preform concentration determination, feature quantities are extracted from an absorption signal by correlating the measured absorption signal with predetermined reference signals. The extracted feature quantities are used for the determination of target and interfering gas concentrations with simple simultaneous linear equations. The method is also capable of correcting for various disturbances, such as spectral shift due to laser wavelength drift and spectral broadening due to partial pressure changes of coexisting gases, by extracting additional, appropriate, feature quantities. This concentration quantification algorithm dramatically reduces calculation time compared to conventional spectral curve fitting methods because hundreds of spectral data points are replaced with a small number of feature quantities used during calculation. This approach allows the construction of a compact, fast-responding and robust gas sensor. We demonstrate a multicomponent and real-time measurement with a prototype of the proposed gas sensor. The prototype showed sub-ppm detection limits for NO, NO₂ and N₂O with 0.1 s integration time, and the interferences from high concentrations of H₂O and CO₂ could be removed completely. Furthermore, it was also shown that the prototype demonstrates excellent robustness against ambient temperature changes and spectral broadening effects caused by coexisting gases.