Abstract: Objective Hydrogen (H2) is a critical raw material for numerous industrial processes and applications, including the petroleum and natural gas industry, chemical plants, and the steel industry. High concentrations of H2 can reduce the partial pressure of oxygen in the atmosphere, leading to respiratory distress, which in turn can cause asphyxiation and may produce anesthetic effects. More importantly, H2 is a flammable and explosive gas. When the volume concentration of H2 in a mixture ranges between 4% and 75.6%, it becomes highly prone to explosion, posing significant risks during production, storage, and transportation. The existing H2 sensors on the market are not suitable for detection in reactive, corrosive, or dust-laden gas stream enviroments, and often suffer from poor repeatability, stability, and sensitivity. Consequently, they fail to meet the fundamental requirements for industrial hydrogen sensors, including high sensitivity, online detection, rapid response, low cost, acid resistance, and non-contact operation. To overcome these limitations, we propose a highly sensitive, compact, tunable diode laser absorption spectroscopy (TDLAS) sensor incorporating a dense multi-pass cell. Compared to traditional Herriott-type multi-pass cells, the novel dense multi-pass cell achieves significant improvements in detection sensitivity while enabling miniaturization. Methods The laser beam emitted by a distributed feedback (DFB) laser is collimated by a fiber collimator and enters the multi-pass cell at a specific angle. The current and temperature of the laser are controlled by a custom-made laser driver board. A sawtooth wave and a sine wave are superimposed via an adder and input to the laser driver board to achieve wavelength tuning and modulation, thereby scanning the H2 line at 2121.83 nm, which is the strongest absorption line in this spectral region. The output from the photodetector is demodulated using an AD630 phase-locked amplifier chip. The demodulated 2f signal is acquired by an analog-to-digital converter (ADC) and transmitted to the microcontroller. The multipass cell comprises two spherical mirrors coated with dielectric films. By adjusting the incidence angle of the beam and the spacing between the two relecting mirrors, a seven-ring spot pattern can be obtained. With a mirror spacing of 11 cm, achieving an effective optical path length of 26 m is achieved. With the equivalent optical path length, the gas sample volume within the cavity of this design is only one-third of that in a traditional Herriott-type multi-pass cell. This design ensures high reflectivity counts while avoiding interference effects caused by light spot overlap, ultimately achieving a highly sensitive and compact gas sensor. Results and Discussion As the modulation depth increases, the amplitude of the 2f signal first rises, then stabilizes, and finally gradually decreases, reaching its peak at a modulation amplitude of 0.2 V. To enhance the detection sensitivity of H2, we set the optimal modulation amplitude to 0.2 V for subsequent data measurements. The fitting results reveals a linear response between the 2f signal and H2 concentration, with a linear correlation coefficient R² of 0.998, indicating excellent linear characteristics of the sensor in H2 detection. Furthermore, the linear function obtained can be applied for quantitative analysis of unknown H2 concentrations. Wavelet denoising is applied to the 2f signal of 9% H2. Analysis of 2f signals before and after denoising reveals that when the a signal-to-noise ratio (SNR) of 1 is taken as the criterionultimate for the detection limitsensitivity, the SNR and minimum detection limit increase from 26 and 0.35% to 49 and 0.18%, respectively. The detection limit at an integration time of 0.8 s is 0.21%, which is close to the detection limit of 0.18% calculated using the SNR method, thus confirming the reliability of the detection limit of the H2 sensor system. As the system integration time increases, sensitivity improves and the detection limit decreases, reaching the theoretical minimum detection limit of 0.025% at an integration time of 259 s. After fitting the concentration data histogram with a Gaussian linear function, the measurement accuracy is determined to be 0.33% based on the half-width at half-maximum (HWHM). The high precision and sensitivity demonstrate the performance of the designed sensor. Conclusions In order to achieve real-time monitoring of H2 and prevent accidents such as explosions caused by H2 leakage, we designed a TDLAS in-situ non-contact H2 sensor based on a compact multi-pass cell. A seven-ring light spot pattern is obtained using a spherical mirrors coated with a dielectric film, achieving an effective optical path length of 26 m with a gas sample volume as small as 240 cm³, thereby enhancing the detection sensitivity of the system. The sensor features compact and integrated structure, easy operation and portability, making it highly suitable for various industrial environments. As for the sensor, the experimental measurements for the relationship between H2 concentration and 2f signal show a linear correlation coefficient R² of 0.998, and when wavelet denoising is applied to the 2f signal, the system's signal-to-noise ratio and the minimum detection limit are improved from 26 to 49, and from 0.35% to 0.18%, respectively. System stability is analyzed using Allan variance analysis, and the results reveals that a minimum detection limit for H2 is 0.025% at an integration time of 259 s. The sensor not only provides a stable, highly sensitivity, and high precision tool for H2 detection, but also offers a potential approach for detecting other weakly absorbing trace gases.

Key words: tunable diode laser absorption spectroscopy, wavelength modulation, infrared spectroscopy, hydrogen; multi-pass cell