High-sensitivity and low-interference gas analyzer with feature extraction from mid-infrared laser absorption-modulated signal

In this paper, we propose a gas analyzer that uses a quantum cascade laser (QCL) and achieves high sensitivity and gas selectivity with simple configuration and signal processing. Feature quantities are extracted from an absorption-modulated signal, which is obtained by the logarithmic conversion of a detector signal receiving a wavelength-modulated laser light. The extracted feature quantities are used for the determination of target and interfering gas concentration with simple simultaneous linear equations. As a result of the demonstration of CO gas measurement with a gas analyzer consisting of a 4.6 μm pulsed QCL, a small-volume Herriott cell with a path length of 5 m and a thermopile as a photodetector, it is shown that the limit of detection is 2.0 ppb at the integration time of 1 s and that interference by N2O can be eliminated. It is also shown that various disturbances such as spectral shift due to laser wavelength drift and spectral broadening due to partial pressure change of coexisting gases can be corrected by extracting appropriate additional feature quantities.