Open-closed single-tube on-beam tuning-fork-enhanced fiber-optic photoacoustic spectroscopy

A proof-of-concept on-beam tuning-fork-enhanced photoacoustic sensor based on an open-closed single-tube acoustic-microresonator (AmR) was proposed and investigated for the first time, to the best of our knowledge. Due to the high acoustic amplification effect, the open-closed AmR improved the detection sensitivity by 54 times with respect to the bare tuning fork (TF). Compared to traditional dual-tube/single-tube on-beam spectrophone configuration, the developed approach significantly facilitates the laser beam alignment and reduces the sensor size and gas consumption. A 6.6 kHz low-frequency custom aluminum alloy TF was employed as the acoustic transducer to detect the photoacoustic signal. The vibration of TF was measured by a fiber-optic Fabry-Pérot (FP) interferometer (FPI). The modulation depth, tube length and laser power were experimentally optimized and evaluated in detail. An acetylene (C₂H₂) 1σ minimum detection limit (MDL) of 6.3 ppb was obtained with a high laser power of ∼ 500 mW, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 6.5 × 10^-9 cm^-1 W/Hz^1/2. The compact spectrophone size and all-fiber measurement method can make the PAS-based sensor have great application prospects in dissolved gases detection in transformer oil, remote gas detection, space-limited gas detection, and etc.