Fiber-Tip Fabry-Perot Interferometric Photothermal Flexible Gas Sensor with Capillary Microcavity

The fiber-optic gas sensor is an ideal choice for trace gas detection due to its high sensitivity, telemetry, and miniaturization. However, advanced technical methods based on a long optical path or functional coating to improve sensitivity limit the trace gas detection application of confined spaces and fast response. A fiber-tip Fabry-Perot (F-P) interferometric photothermal (PT) flexible gas sensor based on capillary microcavity is proposed for fast gas detection. An inner diameter of 130 μm capillary is used to align and fix the single-mode fiber and the short fiber coated with a gold reflective film, forming a high-contrast F-P cavity with a cavity length of about 800 μm. The volume of the gas chamber is only 10 nL. The capillary microcavity structure is optimized by simulation analysis and experiment. When the cavity length is 392-935 μm, the PT signal increases with the increase of the cavity length. The excitation light interference suppression and high-speed PT phase demodulation can be realized by a truncated spectral white-light interference demodulation method. The experimental results show that the sensor has a minimum detection limit of 0.8 ppm C₂H₂ and a response time of only 1 s.