Impact of micro-nano fiber diameter on optical transmission characteristics in insulating oil

Accurate monitoring of dissolved gases in insulating oil is of great significance for ensuring the stable operation of oil-immersed power equipment. Micro-nano fibers have attracted extensive attention due to their low-loss transmission, strong optical field confinement, and strong evanescent field effect, showing great potential in the application field of insulating oil gas detection. Since the refractive index of insulating oil is close to that of the optical fiber and a large proportion of evanescent field exists in the insulating oil, even a small change in the diameter of the micro-nano fiber can alter the optical field confinement and mode distribution. To address this, a realistic micro-nano fiber model is established in this work to investigate the influence of fiber diameter on the mode distribution and optical field transmission characteristics of micro-nano fibers immersed in insulating oil. Numerical simulation and experimental results show that, under oil-immersed conditions, the diameter range in which both the HE₁₁ and HE₁₂ modes coexist in the micro-nano fiber is 9.68~17.48 μm. As the waist diameter of the micro-nano fiber decreases, the energy fraction of the HE₁₁ and HE₁₂ modes in the evanescent field increases. Within the effective transmission range of the HE₁₂ mode, a smaller micro-nano fiber diameter leads to a larger interference signal induced by the photothermal phase change, thereby yielding higher sensitivity for acetylene gas detection. This study provides an important basis and data support for the application of micro-nano fiber sensors in condition monitoring of transformer oil.