PET-Cantilever-Enhanced Fiber-Optic Photoacoustic Spectroscopy for Rapid Subppm Methane Detection

To address the challenges of low sensitivity and poor stability in conventional single-fiber photoacoustic spectroscopy, here, we present for the first time a polyethylene terephthalate (PET)-cantilever-enhanced fiber-optic photoacoustic spectroscopy (PET-CEFOPAS) system featuring an integrated PET cantilever and a dual-cavity structure that enables highly sensitive and rapid detection of trace CH₄. COMSOL Multiphysics finite element simulations reveal that the PET cantilever, benefiting from its low Young's modulus and flexible mechanical properties, exhibits an acoustic pressure sensitivity of 7334 nm/Pa. The theoretical resonance frequency (2638 Hz) closely matches the experimental result (2465 Hz), validating the reliability of the simulations. Experimental results further confirm that the PET cantilever combines high optical transmittance (>90% at 1550 and 1653.7 nm) with low reflectivity (<0.1). By optimizing the F-P cavity length (0.5 mm) and the photoacoustic cavity length (3.5 mm), the sensor achieves a synergistic enhancement of optical path length and sensitivity within an ultracompact volume of 10 μL. Allan deviation analysis shows that the PET-CEFOPAS system has excellent stability. The dual-cavity design reduces the CH₄ detection limit to 440 ppb (integration time of 132 s) and shortens the gas response time to 4.5 s, resolving the long-standing trade-off between sensitivity and response speed in miniaturized PAS systems. This work offers a new solution in fiber-optic PAS for high-sensitivity, rapid gas sensing.