Compact Wafer-Bonded Fabry–Perot Cavity-Based Photothermal Spectroscopy for Gas Sensing

Abstract Currently available electrochemical and direct absorption gas sensor approaches suffer from weak molecular selectivity, challenges in miniaturization, and constraints in cost-effective large-scale manufacturing. In contrast, the integration of an interferometric cavity-assisted technique with photothermal spectroscopy (PTS) provides an alternative solution promising improved sensitivity and increased dynamic range. Here, we report a compact wafer-bonded Fabry–Perot (WB-FP) cavity fabricated using a Distributed Bragg Reflector (DBR) and demonstrate its use as a photothermal transducer for gas sensing. Unlike conventional manually assembled Fabry–Perot (FP) cavities, the proposed design defines cavity permenant alignment during fabrication, providing a pathway toward scalable and reproducible photonic gas sensors. In this architecture, a near-infrared (NIR) beam is collimated and propagated through a 500 μm spaced FP cavity to probe the photothermal effects induced by interactions between the target gas molecules and the optical pump beam inside the cavity volume. To demonstrate the applicability of the developed sensor system, the metrological figures of merit were evaluated using methane (CH4) as the targeted analyte across a range of concentrations. Targeting the absorption band centered at 6047 cm–1, the system achieves a minimum detection limit (MDL) of 42 ppm with a 1 s integration time. Allan deviation analysis further indicates an improved MDL of 5.1 ppm at an optimal integration time of 100 s, corresponding to a normalized noise-equivalent absorption (NNEA) coefficient of 3.27 × 10–7 cm–1 W Hz–1/2. These results highlight the potential of the wafer-bonded FP cavity as a scalable, compact transducer for photothermal gas sensing, bridging the route toward miniaturized, stable, cost-effective, and high-sensitivity gas technologies.