Abstract: Objective　Hydrogen energy is of paramount importance for constructing a clean and efficient modern energy system and achieving the strategic goals of carbon peaking and carbon neutrality. With the rapid scaling-up of hydrogen (H2) applications, the resulting surge in H2 consumption has made real-time leakage warning monitoring and purity analysis increasingly critical for safety and process control. This imposes stringent demands on hydrogen sensors, which must provide fast response over a wide concentration range while maintaining high robustness in complex industrial environments. To address this issue, this work aims to develop a high-precision, high-sensitivity online H2 detection system by integrating wavelength modulation spectroscopy (WMS) with a Herriott multipass gas cell. The system is designed to enable continuous, accurate, and rapid monitoring of H₂ over a broad concentration dynamic range, thereby providing a reliable technological solution for H2 leakage early warning and purity assessment in both laboratory and industrial settings. Methods　The sensing system is constructed based on a distributed feedback (DFB) diode laser with a center wavelength of 2121.83 nm, which precisely targets the strong near-infrared absorption line of H2 at 4712.90 cm− ¹. To enhance detection sensitivity, a Herriott multipass cell providing an effective optical path length of 2640 cm is employed, significantly increasing the light-gas interaction length. The core measurement strategy is to combine wavelength modulation spectroscopy with second-harmonic detection. A self-developed lock-in amplifier is used to demodulate the first-harmonic (1f) and second-harmonic (2f) signals from the detector output. Then, the 2f signal is normalized by the corresponding 1f signal (2f/1f), which is a well-established approach that can effectively compensate for laser power fluctuations and strongly suppresses interference caused by environmental stray light, window contamination, and mechanical vibration. The system calibration is performed at ambient temperature and pressure using a series of standard H2/N2 gas mixtures with concentrations ranging from 2% to 100%, establishing a linear relationship between the peak value of the 2f/1f signal and the known hydrogen concentration. The sensor performance is systematically evaluated through repeatability tests, long-term continuous observation, and Allan deviation analysis to determine linearity, stability, and ultimate detection limits. Furthermore, by conducting online detection of the concentrations of industrial by-product H2, the H2 purity of the raw byproduct gas mixture after treatment by absorption towers at different positions is obtained, providing valuable data support for H2 purity analysis. Results and Discussion The performance of the developed WMS-based H2 sensor was comprehensively characterized. Calibration experiments with standard gases demonstrated an excellent linear response with the 2f/1f signal peak value as a function of H2 concentration, yielding a determination coefficient R² of 0.999 with the calibration range. Long-term stability tests showed a maximum fluctuation of less than 0.51%, confirming the robustness of the 2f/1f normalization scheme. Allan deviation analysis was conducted to evaluate the detection sensitivity at different integration times. At a short integration time of 1 s, the sensor achieved a lower detection limit of 0.9%, which is sufficient for rapid H2 leakage warning. By extending the integration time to 169 s, the minimum detection limit was further improved to 0.0358%, demonstrating the system's capability for trace-level H₂ detection. These results indicate that the sensor can reliably distinguish the variation of H₂ concentration from sub-percent levels up to pure H₂, meeting the requirements for wide-concentration-range monitoring. The sensor was deployed for the first time to perform real-time H2 concentration measurements in controlled laboratory environments and complex gas mixtures arising from the wet-treatment process of aluminum dross. In the industrial setting, the system operated continuously and the measurement results were in remarkable agreement with those obtained from a gas chromatograph, with the relative deviation controlled within 5.5%. Most importantly, the response time of the WMS sensor reached the order of seconds, which represents a reduction of more than 99% compared with the typical response time of approximately 100 s of a gas chromatograph. This ultrafast response enables true online monitoring and allows for the capture of rapid concentration changes that are completely missed by slower analytical methods. The combination of broad dynamic range, fast response, high sensitivity, and insensitivity to optical disturbances makes this WMS-based system highly suitable for industrial H₂ safety surveillance and process control. Conclusions　A high-performance H₂ sensing system based on wavelength modulation spectroscopy has been successfully developed, which employs a 2121.83 nm DFB laser and a Herriott multipass cell to achieve precise and fast concentration measurements across the entire range relevant to hydrogen production, storage, transportation, and utilization. The sensor exhibits excellent measurement linearity (R² = 0.999), a detection limit of 0.9% at 1 s integration time sufficient for leakage warning, and a minimum detection limit of 0.0358% at 169 s integration time. The system was applied for the first time to real-time monitoring of H ₂ in the complex gas environment of aluminum dross wet processing, where it demonstrated outstanding agreement with gas chromatography (relative deviation within 5.5%) while shortening the response time by over 99%. These results validate that the developed system possesses the high sensitivity, long-term stability, and rapid response required for online H2 monitoring. The developed system provides a powerful technical means to support the safe and efficient use of H2 as a zero-carbon fuel, contributing to the advancement of hydrogen energy infrastructure and the realization of carbon neutrality goals.

Key words: absorption spectroscopy, rapid hydrogen detection, wavelength modulation spectroscopy, industrial byproduct hydrogen