Optical path coupling optimization analysis of compact flat circular multi-pass cell for hydrogen leakage laser absorption spectroscopy detection.

Ensuring reliable hydrogen monitoring is vital to the safety of fuel cell systems, particularly when operation takes place in confined spaces. Conventional detection methods are often limited by optical loss, insufficient sensitivity, and delayed response. To overcome these limitations, we design a miniaturized flat circular multi-pass cell that maximizes optical path length and irradiance utilization while maintaining a small chamber volume. The configuration consists of a planar reflector array combined with a semi-closed housing, where a microporous cover promotes stable and uniform gas exchange. With optimized incident and reflection angles, the beam traces a spiral two-layer trajectory, yielding a total optical path of 1.54 m inside a cell of only 18.85 ml. A comparative study of polygonal reflection layouts identifies the 20-spike geometry as the most efficient compromise between structural complexity and optical efficiency, leading to merely 8.743% signal attenuation after multiple reflections. Unlike traditional spherical or parabolic mirror cells, this design not only suppresses loss effectively but also enhances average irradiance by nearly threefold. The tolerance analysis of the model is carried out to ensure that the beam propagates stably in the cell without deviation. Simulation further reveals that increasing beam divergence from 0.03° to 0.075° enlarges the spot size and aggravates optical loss. Owing to its compactness and reduced energy dissipation, the proposed structure enables direct in situ hydrogen detection within fuel cells, providing improved spatial efficiency and faster measurement response.