Hybrid Photoacoustic Approach for Simultaneous Methane and Hydrogen Detection via Optical Absorption and Acoustic Frequency Tracking.

Trace detection of methane (CH4) and hydrogen (H2) is critical for energy infrastructure monitoring and safety management. However, conventional photoacoustic spectroscopy (PAS) approaches rely primarily on optical absorption and often require carrier gases or complex configurations for H2 detection. Here, we report a hybrid photoacoustic sensing strategy that combines optical absorption-based detection with acoustic property tracking, enabling simultaneous and carrier-gas-free detection of CH4 and H2. In this approach, CH4 is quantified using second harmonic (2f) wavelength modulation spectroscopy, while H2 is detected via first harmonic (1f) frequency tracking based on sound-speed-induced resonance shifts. A multi-pass resonant photoacoustic cell is employed to enhance sensitivity, and in situ temperature compensation is achieved using a fiber Bragg grating (FBG) array to mitigate temperature-dependent variation in the acoustic response. The system achieves minimum detection limits of 17.7 ppb for CH4 and 1.39 ppm for H2 at an integration time of 100 s, without the use of carrier gases. The normalized noise equivalent absorption coefficient for CH4 detection reaches 1.07 × 10-9 cm-1·W·Hz-1/2. This hybrid sensing framework provides a generalizable approach for multi-gas detection by integrating optical and thermodynamic sensing mechanisms, offering a new pathway for high-sensitivity and robust gas analysis in complex environments.