Dual‐Mode Quartz‐Enhanced Spectroscopy Enabled by Mixed Heterodyne Demodulation for Frequency‐Mismatch‐Free Gas Sensing

ABSTRACT In this paper, a mixed‐frequency heterodyne demodulation (MHD) method is reported for the first time to improve the performance of quartz‐enhanced laser spectroscopy sensors. The MHD method employs frequency conversion modulation technology to achieve spectrum shifting. Signals at specific frequencies realize the improvement of signal‐to‐noise ratio (SNR) through a high‐Q QTF filter, and quadrature demodulation is performed on the filtered heterodyne signals, thereby enhancing the performance of the sensor. First, light‐induced thermoelastic spectroscopy (LITES) is used to verify the effectiveness of the MHD method, and then it is applied in the synergistic sensor of quartz‐enhanced photoacoustic spectroscopy (QEPAS) and LITES. Compared to the standalone QEPAS sensor and LITES sensor, the QEPAS‐LITES sensor demonstrates a signal‐to‐noise ratio improvement of 10.04 times and 7.29 times, respectively. Furthermore, this method solves the problem of frequency mismatch and provides the capability of synergistic operation within a wide frequency range of ∼24 kHz.