Microscale Intrinsic Fiber Mode Vernier Photothermal Spectroscopy Gas Sensing

ABSTRACT The optical Vernier effect significantly improves sensitivity and resolution in fiber‐optic sensors, especially in Fabry–Pérot interferometers (FPIs). This study presents a novel mode Vernier photothermal spectroscopy (MV‐PTS) based on a dual‐mode anti‐resonant Bragg hollow‐core fiber (BHCF), which is spliced with a single‐mode fiber and a suspended core fiber (SCF) to construct an FPI. The dual‐mode configuration facilitates the mode Vernier effect (MVE) within a single FPI, enhancing the sensitivity of photothermal phase demodulation. Eccentric splicing of the BHCF and SCF enables regulation of the dual‐mode energy ratio to enhance the MVE, while constructing a natural gas channel to support direct trace gas detection. Monitoring the phase difference of the modes minimizes common‐mode noise, leading to a high signal‐to‐noise ratio of the photothermal signal. The excitation and demodulation processes occur within a compact 0.8 nL hollow core, improving energy conversion efficiency. The 1 mm‐long BHCF‐based F‐P cavity achieves a minimum detection limit of 12 parts per billion (ppb) for acetylene gas, demonstrating rapid responsiveness and long‐term stability. This compact sensor design overcomes challenges in achieving ultrasensitive gas detection with minimal light‐matter interaction length in intrinsic fiber structures, offering promising applications in gas sensing.