Photothermal Detection of Trace Acetylene for Transformer DGA Monitoring Using an Adiabatically Tapered Optical Fiber

In the field of optical sensing of characteristic defect gas in transformers, hollow-core photonic crystal fiber (HC-PCF) gas cells suffer from high fabrication costs and complex manufacturing processes. Meanwhile, fabricating non-adiabatic tapered fiber gas cells requires precise control of geometric parameters, such as the length of the transition region and the waist diameter. Moreover, these structures inherently exhibit common-path interference, making them susceptible to environmental noise and limiting detection sensitivity. To address these issues, we proposed a low-cost and easily fabricated adiabatic tapered fiber gas cell, fabricated from standard single-mode fiber using a flame-brushing tapering technique. We fabricated the adiabatic tapered fibers with a waist diameter of 2.2 μm by optimizing the stretching speed and distance of the stepper motors, effectively avoiding the common-path interference caused by mode coupling in non-adiabatic tapered fibers. This process is simple and achieves a production cost that is only 1% of HC-PCF gas cells. Additionally, we developed a heterodyne Mach–Zehnder interferometric (MZI) sensing system for hydrocarbon detection. We moved the beat frequency carrier signal to 100 MHz by introducing an acousto-optic modulator (AOM) to shift the reference light frequency, significantly reducing low-frequency environmental noise disturbances on both the sensing and reference arms. Based on this system, we achieved a detection limit of 0.6 ppm for acetylene gas, with a linear correlation coefficient (R²) of 0.996 for the sensor response. This sensing approach provides a highly sensitive and low-cost technical solution for the real-time, online monitoring of acetylene gas in power transformers.