Mode-Phase-Difference Photothermal Spectroscopy Assisted by a Bent Biconically Tapered Microfiber for Gas Sensing

Photothermal spectroscopy based on an optical pump-probe configuration has been widely used for the detection of characteristic gases in microfibers. However, the performance of existing photothermal gas sensors remains insufficient to satisfy the requirements of many high-precision applications. In this study, we propose a gas sensing method based on mode-phase-difference photothermal spectroscopy assisted by a bent biconically tapered microfiber. The sensing element consists of a tapered optical fiber with a locally bent taper region. By precisely tuning the local bending angle using a piezoelectric transducer, the interfering modes and their power distribution within the fiber can be effectively tailored, thereby significantly enhancing the photothermal detection efficiency. Using a microfiber with a diameter of 2 μm and a length of 2 cm, highly accurate detection of C₂H₂ is achieved, with a noise-equivalent concentration as low as 27 ppb and a dynamic range approaching 6 orders of magnitude. During continuous operation over 4 h, the signal fluctuation remains within 1.1%, and the response time is 15 s. Owing to its high sensitivity, wide dynamic range, excellent long-term stability, and fast response, this method provides a compact, cost-effective, and high-performance fiber-optic gas sensing strategy for characteristic gas detection, showing considerable potential for practical applications.