A highly sensitive TDLAS sensor based on a multi-pass cell with dense four-concentric circle dot pattern: The potential of hydrogen sensing

This paper reports a high-sensitivity tunable diode laser absorption spectroscopy (TDLAS) sensor, utilizing a multi-pass cell (MPC) with a dense four-concentric-circle spot pattern to achieve an extended optical path length (OPL) of 37.8 m. To evaluate the sensor's performance, hydrogen (H₂) was chosen as the target gas for measurement. To realize long OPL within a compact structure, we developed a MPC model based on the vector reflection theory and validated its design through simulations. The design attains a ratio of optical path length to volume (RLV) of 13.85 cm⁻², effectively overcoming the challenge of weak H₂ absorption lines. A 2121.83 nm fiber-coupled continuous-wave distributed feedback (CW-DFB) laser serves as the light source, paired with a custom 3D-printed collimation module enabling automatic optical alignment to streamline experimentation. Further, a dual-hidden-layer shallow neural network (SNN) algorithm reduces spectral noise while preserving signal integrity, eliminating distortion. Before applying spectral noise reduction, the sensing system achieved a minimum detection limit (MDL) of 0.3 % in practical measurements with an integration time of 0.2 s. After algorithm-based noise reduction, the noise level was decreased significantly. These results demonstrate that the innovative MPC design holds strong potential for H₂ sensing applications.