Hourglass-Shaped Resonator-Based Fiber-Optic Photoacoustic Multipass Gas Sensor

An hourglass-shaped resonator based high-sensitivity fiber-optic photoacoustic sensor (FOPAS) for trace gas detection is proposed. To minimize the gas chamber volume of the sensor, the resonance tube is designed with an hourglass-shaped structure. In order to match the paths of multiple light reflections with the hourglass-shaped structure, a 4<i>f</i> focusing optical system is implemented to establish a multipass cell (MPC). This configuration significantly reduces the resonator volume while maintaining the multiple reflection conditions, thereby enhancing the photoacoustic signal. The light undergoes 52 reflections within the MPC and converges to a single point at the center of the resonance tube, achieving an optical path length of 5.2 m. The acoustic pressure signal generated in the sensor is detected using a highly sensitive fiber-optic Fabry-Perot (FP) cantilever microphone. The amplitude of the photoacoustic signal of the hourglass-shaped resonance tube is 2.7 times that of the cylindrical resonance tube. The amplitude of the photoacoustic signal after multiple reflections is 18 times that of a single reflection. The designed FOPAS achieves a minimum detection limit (MDL) of 1.4 ppb (1σ @100 s), and a normalized noise equivalent absorption (NNEA) coefficient of 3.6 × 10^-10 W cm^-1 Hz^-1/2 for C₂H₂ detection.