Ultrasensitive Single-Fiber Fabry-Pérot Photoacoustic Acetylene Sensor Enabled by a T-Type Acoustic Resonator.

Single-fiber photoacoustic spectroscopy is attractive for compact, remote, and intrinsically safe trace-gas sensing, but its sensitivity is limited by the short absorption length and weak photoacoustic excitation in a miniaturized sensing head. Here, we propose and demonstrate an ultrasensitive single-fiber Fabry-Pérot (F-P) photoacoustic acetylene sensor based on a T-type acoustic resonator. The resonator not only confines and resonantly enhances the locally generated photoacoustic wave but also provides a large-volume acoustic cavity for the ultrathin silicon cantilever, reducing the air-spring effect and viscous damping. The resulting F-P acoustic sensor achieves a sensitivity of 7460 nm/Pa and a noise-equivalent pressure of 0.295 μPa·Hz-1/2 at 2.7 kHz. Without dual-resonance matching or external optical amplification, the sensor achieves a detection limit of approximately 621 ppt and a normalized noise-equivalent absorption (NNEA) coefficient of about 1.35 × 10-9 cm-1·W·Hz-1/2, representing, to the best of our knowledge, the lowest reported NNEA value among single-fiber photoacoustic gas sensors to date. This work provides an effective acoustic-resonance-enhancement approach for addressing the sensitivity limitation of single-fiber photoacoustic sensing, paving the way for compact and ultrasensitive single-fiber gas sensors.