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

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.