Dual-gas light-induced thermoelastic spectroscopy sensor based on mixed-frequency heterodyne demodulation

A novel dual-gas light-induced thermoelastic spectroscopy (DG-LITES) sensor based on mixed-frequency heterodyne demodulation (MHD) is reported. The DG-LITES sensor exploits the fundamental and first overtone vibration modes of a single self-designed low-frequency quartz tuning fork to achieve the high-performance simultaneous detection of methane (CH₄) and acetylene (C₂H₂). Using a frequency-division multiplexing mechanism, this technique creates dual detection channels based on a single sensing element. Furthermore, it uses the MHD method to convert photothermal signals at different frequencies into a unified intermediate frequency, thereby enabling synchronous demodulation with only one correlation demodulation unit. The DG-LITES sensor not only maintains system compactness but also effectively suppresses inter-channel crosstalk below 0.057%. Experimental results demonstrated that the DG-LITES sensor exhibited excellent linear responses to both CH₄ and C₂H₂ (R² > 0.999), with maximum nonlinearity errors as low as 1.39% and 1.48% full-scale span, respectively. The mean relative systematic errors were 0.95% and 0.93%, respectively, whereas the maximum relative errors were 1.8% and 2.5%, respectively. Allan deviation analysis validated the excellent long-term operational stability of both the CH₄ and C₂H₂ channels. The minimum detection limits for both channels were 0.13 and 2.93 ppm, with normalised noise-equivalent absorption coefficients of 2.73 × 10⁻⁹ and 9.61 × 10⁻⁸ cm^{−1·W·Hz−1/2}, respectively. This paper presents a universal sensing architecture that offers a novel solution to the long-standing trade-off between performance and system complexity in multi-gas detection.