Spectroscopic Sensing System Based on Loop-Enhanced Flexible Rectangular Waveguide Cell with Multi-Pass Transmission

This study presents a novel gas sensing system based on a loop-enhanced hollow waveguide (LE-HWG) cell utilizing a fabricated flexible rectangular hollow waveguide (FR-HWG). By integrating multi-pass transmission, the system avoids the sensitivity constraints of conventional single-pass HWG cells. The design merges the optical path length (OPL) extension capability of the multi-pass cell (MPC) with the rapid response of HWG. The fabricated FR-HWG was coiled in a circular shape with a partially overlapping splice interface to enable conveniently controlled light recirculation. Theoretical analysis predicted a 4.5× sensitivity enhancement under optimized overlap ratio and low waveguide loss. Ray-tracing simulations confirmed the beam homogenization capability and effectiveness of the proposed structure. Experiments of CO₂ gas sensing demonstrated the sensitivity improvement, achieving a limit of detection (LOD) of 214.7 ppb using a low-cost broadband infrared source and tunable Fabry-Pérot (FP) detector. The circular configuration reduced device size, and the 7.22 g ultralight design enhanced portability. The FR-HWG exhibits beam homogenization functionalities, enabling spatially uniform optical coupling and potential applications in laser processing. This work establishes a portable gas sensor paradigm that combines high sensitivity and cost efficiency.