All-fiber offset-core sandwich-structured gas sensor based on photothermal spectroscopy detection

In this paper, we present the first-ever application of an all-fiber offset-core sandwich-structured Fabry-Pérot (OSFP) sensor for gas detection. This optical fiber sensor, featuring an ultra-compact footprint with a cavity length of merely 150 μm and a volume of less than 1.4 nL, constitutes a low-finesse Fabry-Pérot (FP) cavity. Specifically, the sensor is configured as a miniature FP interferometer, which exhibits low detection limits, facile fabrication, and high stability. By leveraging photothermal wavelength modulation combined with interferometric phase demodulation, this sensor enables the detection of acetylene (C₂H₂) gas at parts-per-billion (ppb) levels. OSFP realizes highly stable gas detection without the need for a sophisticated feedback control system, achieving a minimum detection limit of 390 ppb for C₂H₂ gas and holding promise for future applications in medical testing and gas detection under harsh environments.