Compact and full-range carbon dioxide sensor using photoacoustic and resonance dependent modes

A compact and robust optical excitation photoacoustic sensor with a self-integrated laser module excitation and an optimized differential resonator was developed to achieve high sensitivity and full linear range detection of carbon dioxide (CO₂) based on dual modes of wavelength modulated photoacoustic spectroscopy (WMPAS) and resonant frequency tracking (RFT). The integrated laser module equipped with three lasers (a quantum cascade laser (QCL), a distributed feedback laser (DFB) and a He-Ne laser) working in a time-division multiplexing mode was used as an integrated set of spectroscopic sources for detection of the designated concentration levels of CO₂. With the absorption photoacoustic mode, the WMPAS detection with the QCL and DFB sources was capable of CO₂ detection at concentrations below 20 %, yielding a noise equivalent concentration (NEC) as low as 240 ppt and a normalized noise equivalent absorption coefficient (NNEA) of 4.755 × 10^-10 W cm^-1/√Hz, and dynamic range as great as 11 orders of magnitude. Higher concentration detection ranges (20 %-100 %) of CO₂ were investigated using the RFT mode with an amplitude-stabilized He-Ne laser and a mechanical chopper. With the dual modes of WMPAS and RFT, the optical excitation sensor achieved full-range CO₂ detection, with an R² ≥ 0.9993 and a response time of 5 seconds. The compact and full-range CO₂ sensor combines the advantages of WMPAS and RFT and offers a solution for high sensitivity, linearity and full-range CO₂ detection.