Orthogonal phase modulation and Lissajous mode decoupling in light-induced thermoelastic spectroscopy for real-time multi-component gas sensing

This paper proposed an optical detection mechanism that utilized orthogonal phase modulation (OPM) and Lissajous mode decoupling within a light-induced thermoelastic spectroscopy (LITES) sensor for the first time, enabling real-time multi-component gas sensing. By establishing a two-dimensional forced vibration model of a quartz tuning fork (QTF) under dual-path OPM, it revealed the vibration mode coupling mechanism induced by non-ideal phase and amplitude conditions, thereby providing a theoretical basis and optimization pathway for suppressing channel crosstalk. In the OPM-LITES sensor, two continuous-wave distributed feedback lasers were modulated by sinusoidal waves with the same frequency (f₀/2, where f₀ is the resonant frequency of the QTF) but a phase difference of 45°. The combined beam, after wavelength division multiplexing, co-excited the self-designed low-frequency QTF. The piezoelectric signal generated by the QTF underwent orthogonal demodulation via a lock-in amplifier (LIA). The concentration information for the two gases was independently retrieved from the decoupled X-component and Y-component of the LIA. Experimental results demonstrated that the OPM-LITES sensor exhibited excellent linear responses to both methane (CH₄) and acetylene (C₂H₂), with average relative systematic errors of 0.54% and 0.79%, and maximum relative errors of 1.72% and 1.38%, respectively. Allan deviation analysis indicated that the minimum detection limits (MDLs) for CH₄ and C₂H₂ reached 0.32 ppm and 0.29 ppm, with normalized noise-equivalent absorption coefficients of 5.09 × 10⁻⁹ cm⁻¹·W·Hz^−1/2 and 6.18 × 10⁻⁹ cm⁻¹·W·Hz^−1/2, respectively. This study not only provided a simple and efficient solution for real-time multi-component gas detection but also established a novel theoretical framework for mode decoupling and signal processing in spectroscopic sensing.