Miniaturized and highly-sensitive fiber-optic photoacoustic gas sensor based on an integrated tuning fork by mechanical processing with dual-prong differential measurement

A proof-of-concept gas sensor based on a miniaturized and integrated fiber-optic photoacoustic detection module was introduced and demonstrated for the purpose of developing a custom tuning-fork (TF)-enhanced photoacoustic gas sensor. Instead of piezoelectric quartz tuning fork (QTF) in conventional quartz-enhanced photoacoustic spectroscopy (QEPAS), a low-cost custom aluminum alloy TF fabricated by mechanical processing was employed as a photoacoustic transducer and the vibration of TF was measured by fiber-optic Fabry-Pérot (FP) interferometer (FPI). The mechanical processing-based TF design scheme greatly increases the flexibility of the TF design with respect to the complex and expensive manufacture process of custom QTFs, and thus it can be better exploited to detect gases with slow vibrational-translational (V-T) relaxation rates and combine with light sources with poor beam quality. The resonance frequency and the quality factor of the designed custom TF at atmospheric pressure were experimentally determined to be 7.3 kHz and 4733, respectively. Dual-prong differential measurement method was proposed to double the photoacoustic signal and suppress the external same-direction noise. After detailed optimizing and investigating for the operating parameters by measuring H₂O, the feasibility of the developed sensor for gas detection was demonstrated with a H₂O minimum detection limit (MDL) of 1.2 ppm, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of 3.8 × 10⁻⁸ cm⁻¹ W/Hz^1/2, which are better than the QTF-based photoacoustic sensors. The proposed gas sensing approach combined the advantages of QEPAS and fiber-optic sensing, which can greatly expand the application domains of PAS-based gas sensors.