Highly Sensitive Noninvasive Hemispherical Fiber Optic Photoacoustic Sensor Utilizing Dual Radial and Angular Resonant Modes for Simultaneous Multigas Detection

This paper presents a hemispherical fiber optic photoacoustic sensor (H-FOPAS) for simultaneous multigas detection, aiming to address the limitations of traditional spherical photoacoustic sensors that struggle to efficiently utilize both angular and radial resonances for multigas sensing. The sensor integrates a hemispherical acoustic cavity with two silicon cantilever microphones (SCMs) featuring distinct Fabry-Perot (F-P) cavity lengths, enabling high-sensitivity dual-channel photoacoustic signal detection, and utilizes white-light interferometry (WLI) based frequency division multiplexing (FDM) for demodulating the composite interference spectrum originating from the two different F-P cavity lengths. The sensor's design utilizes hemispherical resonator principles to simultaneously excite and detect dual resonant modes corresponding to radial and angular resonances for high-sensitivity dual gas sensing. The performance of the sensor is validated through finite element simulations and experimental measurements, demonstrating excellent linearity and high sensitivity for detecting gases such as acetylene (C₂H₂) and methane (CH₄). Experimental results show a clear linear relationship between signal amplitudes and gas concentrations, with correlation coefficients exceeding 0.999. The sensor exhibits a detection limit as low as 3.6 ppb for C₂H₂ and 100 ppb for CH₄, demonstrating its capability for high-precision trace dual-gas monitoring. This hemispherical design provides the merits of high acoustic confinement while allowing noninvasive access to the resonance modes, ensuring efficient multigas detection in a compact configuration.