Highly sensitive CO₂-LITES sensor based on a self-designed low-frequency quartz tuning fork and fiber-coupled MPC

A highly sensitive carbon dioxide (CO₂) sensor based on light-induced thermoelastic spectroscopy (LITES) utilizing a self-designed low-frequency quartz tuning fork (QTF) and a fiber-coupled multipass cell (MPC) is reported in this paper. The QTF with a low resonant frequency of 8675 Hz and a high Q factor of 11,675.64 was used to improve its energy accumulation time and the sensor’s signal level. The MPC with the fiber-coupled structure and optical length of 40 m was adopted to significantly increase the gas absorbance and reduce the optical alignment difficulty as well as improve the robustness of the sensor system. A distributed feedback (DFB), near-infrared diode laser with an emission wavelength of 1.57 μm was used as an excitation source. The experimental results showed that this CO₂-LITES sensor had an excellent linear response to CO₂ concentrations. The minimum detection limitation (MDL) of this CO₂-LITES sensor was obtained to be 445.91 ppm, and it could be improved to 47.70 ppm (parts per million) when the integration time of the system reached 500 s. Further improvement methods for the detection performance of such sensors were also discussed.