Cavity-enhanced Absorption Spectroscopy in the Near- and Mid-infrared for Gas Sensing

Chemical gas-phase analysis is significant in physics, chemistry, space science, industrial process control as well as in atmospheric applications. Fundamental requirements for chemical analysis and gas sensing include sensitivity, selectivity, portability and affordability. The application of cavity-enhanced laser absorption spectroscopy (CEAS) techniques in the near- and mid-infrared for gas sensing was studied, including the mode-locked cavity-enhanced absorption spectroscopy (ML-CEAS), off-axis integrated cavity output absorption spectroscopy (OA-ICOS), and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). In terms of ML- CEAS technique, a near-infrared Pound-Drever-Hall (PDH)-based mode-locked cavity-enhanced gas sensor system was developed for water vapor (H₂O) detection; a mid-infrared cavity-enhanced H₂CO sensor system was demonstrated using a continuous-wave (CW) interband cascade laser (ICL) as light source. In terms of OA-ICOS technique, a near-infrared sensor system based on an ultra-compact cage-based absorption cell was proposed for highly sensitive and accurate acetylene (C₂H₂ ) detection; by combining frequency division multiplexing assisted wavelength modulation spectroscopy (FDM-WMS) and OA-ICOS, a near-infrared dual-gas sensor system was demonstrated for simultaneous chemical gas-phase detection of C₂H₂ and methane (CH₄ ). In terms of IBBCEAS technique, a broadband cavity-enhanced sensor system in combination with a Fourier-transform spectrometer (FTS) in the near-infrared region was demonstrated for CH₄ detection; a near-infrared broadband cavity-enhanced sensor system was demonstrated for the first time for C₂H₂ and CH₄ detection using an energy-efficient light emitting diode (LED) with a central emission wavelength at 1650 nm and a light power of ~ 16mW employing two detection schemes, i.e., NIRQuest In GaAs spectrometer and scanning monochromator combined with phase-sensitive detection. The three techniques demonstrated can be properly selected due to their pros and cons for practical gas sensing applications and can also be capable for remote gas sensing.