Improving measurement accuracy of CH4 and CO2 sensing system using passive locking achromatic multipass absorption cell

Highly accurate CH4 and CO2 sensing systems are required to monitor geochemical gases in the atmosphere to provide early warnings about earthquakes. Based on an achromatic model, an achromatic structure was devel-oped in this study using ridge-shaped windows to eliminate optical system dispersion, and a passive locking structure was designed based on a multilayer rotationally symmetrical model to lock the cavity. A passive locking achromatic multipass absorption cell (PLAMAC) based on achromatic and passive locking structures was developed to improve the optical transmission performance and environmental adaptability. A temperature control subsystem was employed to lock the PLAMAC state and control the gas temperature. A gas-sensing system was developed using PLAMAC to realize highly accurate measurements of CH4 and CO2. The PLAMAC enables chromatic aberration-free transmission in the wavelength range of 1.55- 2.35 mu m and achieves perfect locking at environmental temperatures from -10-40 degrees C. The systematic errors are 0.93%o@ 1653.7 and 0.91%o@ 2329.05 nm for 500 parts per million by volume (ppmv) CH4 and 0.92%o@ 1572.3 and 0.91%o@ 2004.5 nm for 1000 ppmv CO2 within the confidence interval of 98%. The PLAMAC system has a higher accuracy over a wide spectral range and greater environmental adaptability than the sensing system based on MAC-41. The experi-ments verified that the PLAMAC system has significant advantages in multi-gas exploration with wide temper-ature variations.