Embedded Single-Fiber Gas Sensor with Dual Enhancement of Photoacoustic Excitation and Detection

Single-fiber photoacoustic (PA) gas sensing probes have been proven to have the characteristics of miniaturization, telemetry, and high sensitivity <i>in situ</i> detection. However, cantilever beams and diaphragms create an air damping effect and have mechanical miniaturization limitations, which hinder further improvement of the acoustic sensitivity. The double enhancement of PA excitation and detection based on microcavity multiple light reflection and ultrasensitive acoustic detection is applied to miniature single-fiber PA gas sensors to improve gas detection sensitivity. To leverage the high sensitivity of silicon cantilever beams, the embedded single-fiber novel structure design with an enlarged inner cavity reduces air damping at the resonance. The novel simple structure is characterized by inserting an optical fiber into a gold-plated PA microcavity to form a miniature optical cage, which amplifies local PA waves. The effects of inner cavity volume on the acoustic sensitivity of silicon cantilever beams and the Fabry-Perot (F-P) cavity length limitation based on the white-light interferometric demodulation method are analyzed by theory and simulation. The experimental results show a sensitivity of 3115 nm/Pa, achieving an 81 ppb methane detection limit with a 60 s integration time. The gas detection system exhibits 20 s response time and achieves a normalized noise equivalent absorption (NNEA) coefficient of 5.7 × 10^-9 cm^-1 W/Hz^1/2, establishing superior performance metrics for ultra-compact PA gas sensors.