A Single-Laser TDLAS Sensor with Dual-Gas Modulation and BiLSTM-Based Spectral Decoupling for CH ₄ /C ₂ H ₆ Detection

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is widely used in trace gas detection because of its high sensitivity and selectivity. However, two persistent challenges restrict its performance in multigas applications: spectral cross-interference and harmonic imbalance. These problems are particularly evident for methane (CH₄) and ethane (C₂H₆), which commonly coexist in natural gas and atmospheric environments and exhibit overlapping absorption features near 1680 nm. In this study, a single-laser TDLAS detection system is developed by integrating a dual-gas modulation strategy with a Bidirectional Long Short-Term Memory (BiLSTM)-based spectral decoupling approach. The proposed modulation scheme dynamically alternates harmonic responses to match the full width at half-maximum (FWHM) of CH₄ and C₂H₆, effectively eliminating the harmonic mismatch that limits simultaneous detection in conventional single-amplitude systems. The BiLSTM model further compensates for the cross-interference caused by the spectral overlap of CH₄ and C₂H₆, thereby enhancing both the accuracy and stability of concentration estimation. During a 200 s stability test, the proposed method maintained consistent performance, yielding standard deviations of 3.66 ppm for 1000 ppm of CH₄ and 0.56 ppm for 100 ppm of C₂H₆. The predicted CH₄ and C₂H₆ concentrations align closely with the reference values, achieving R² values of 0.99097 and 0.99819, respectively. These results confirm that the combined TDLAS-BiLSTM system significantly improves measurement precision and reliability in dual-gas sensing. Overall, this approach provides a practical and robust solution for accurate concentration retrieval under cross-sensitivity conditions and offers an effective strategy for multicomponent gas analysis.