A Dual-Enhancement Fiber-Optic Photoacoustic Spectroscopy Sensor Based on a Spherical–Cylindrical Coupled Resonator with an Integrated Multipass Cell for Sub-ppb C2H2 Detection

Although conventional spherical resonators are effective for photoacoustic spectroscopy (PAS), their sensitivities are fundamentally limited because the required detection apertures disrupt the optimal acoustic resonance. To address this limitation, this work presents a novel dual-enhancement fiber-optic PAS sensor that integrates a multipass cell (MPC) within a novel spherical–cylindrical (SC) coupled resonator for ultrahigh-sensitivity gas detection. The coupled resonator design is engineered to resolve the aperture conflict by incorporating a cylindrical waveguide, allowing noninvasive photoacoustic signal detection, yielding a 1.72-fold acoustic enhancement. Simultaneously, the optical enhancement is realized by integrating a two-mirror MPC into the coupled cavity, which extends the effective optical path length and enhances the photoacoustic signal by a factor of over 15. Together, these dual enhancements enable a 26-fold overall sensitivity improvement compared with conventional spherical resonator-based PAS systems. Consequently, the system achieves 223-parts-per-trillion (ppt) detection for C₂H₂, with a normalized noise equivalent absorption coefficient as low as 9.8 × 10^–10 cm^–1 W/Hz^1/2. To the best of our knowledge, this performance represents the highest sensitivity and lowest detection limit reported to date for a spherical resonator-based PAS sensor. This work resolves the inherent design trade-off in spherical resonators between structural integrity and detection sensitivity. The proposed dual-enhancement fiber-optic PAS, featuring a compact, all-optical, and robust design, provides a powerful solution for ultrasensitive gas detection in complex or harsh environments, holding significant promise for broad engineering and practical applications.