Current sensors are widely used in modern power electronic systems due to their advantages including high sensitivity, great precision, excellent stability, etc. In the electric power industry, they are extremely important in power measurement, electrical protection and control systems. However, due to the low sensitivity of current transformers, Roche coils and Hall sensors, optical systems seem ideal for current sensing because of their resistance to electro-magnetic interference and fast response. In addition, the problems of miniaturization, process simplicity and high sensitivity of current sensors have not yet been solved. Non-contact current sensors based on whispering-gallery mode (WGM) optical microcavities have the advantages of simplified structure, high sensitivity, low detection limit and small size. The sensor designed has the potential in realizing the intelligent monitoring of the current status for practical applications in the fields of wind power generation, smart grids as well as electric vehicles.

A section of thin-walled quartz tube is intercepted to prepare a whispering-gallery mode microcapillary cavity by the method of arc discharge. The microcapillary cavity has a tiny curvature with a surface nanoscale axial photonic structure, which is able to bind more optical modes and improve the storage time of the optical modes, so that the microcapillary cavity with ultra-high quality (Q) factor is prepared. A tapered fiber with a waist diameter of 2 μm is prepared for excitation of whispering-gallery modes in the microcapillary cavity using the heat-and-pull technique. Subsequently, we propose a non-contact current sensor based on the ultra-high Q-factor microcapillary cavity. The copper wire with the diameter of 80 μm is put in the center of the microcapillary cavity, and 50% Fe₃O₄ and 100% Fe₃O₄ are filled into the microcapillary cavity, respectively, for comparative study. Subsequently, the optical instrument is adjusted to make the microcapillary cavity coupled with the tapered optical fiber and the measurement circuit is connected. The time interval is set as 90 s. The resonant wavelength shift of the microcapillary cavity with the change of the current can be observed and recorded by the oscilloscope. Finally, the sensitiveness of the microcapillary cavity and its detection limit for the three cases, i.e., the hollow microcapillary cavity, the microcapillary cavity with 50% Fe₃O₄, and the microcapillary cavity with 100% Fe₃O₄, are compared.

The resonance spectrum of the microcapillary cavity is measured using the experimental device, and the highest Q-factor of 3.45×10⁷ is obtained by Lorentz fitting (Fig.5). The hollow microcapillary cavity is tested by setting the current interval to 20 mA. Firstly, when the current is increased from 0 to 300 mA, the resonant wavelength is shifted by 0.1393 nm, and the sensitivity is calculated to be1.547 nm/A² with a current detection limit of 1.874×10^-8 A²/nm (Fig.6). Secondly, by adding 50% magnetic nanoparticles of Fe₃O₄ into the microcapillary cavity, the resonant wavelength is shifted by 0.1034 nm, with a sensitivity of 4.039 nm/A² and a current detection limit of 7.176×10^-9 A²/nm when the current is increased from 0 to 160 mA, showing an enhanced sensitivity and a higher precision of detection limit (Fig.7). Finally, by increasing the magnetic nanoparticles in the microcapillary cavity to 100%, when the current is increased from 0 to 30 mA, the resonant wavelength is shifted by 0.0973 nm, the sensitivity is 10.811 nm/A², and the current detection limit is 2.94×10^-9 A²/nm. The sensor sensitivity and current detection limit show nearly one order of magnitude improvement with respect to the hollow microcapillary cavity, and also have a significant enhancement in comparison with the case of 50% nanoparticles.

A non-contact current sensor based on ultra-high Q-factor whispering-gallery mode microcapillary cavity is investigated. The mode spectrum of microcapillary cavity is stably excited and regular, and the highest Q-factor value reaches 3.45×10⁷. By filling the microcapillary cavity with Fe₃O₄ nanoparticle magnetic fluids, the current sensing sensitivity can reach up to 10.811 nm/A², and the current detection limit reaches 2.936×10^-9 A²/nm, showing a very high current sensing detection performance. The proposed microcapillary cavity non-contact current sensor has the advantages of high sensitivity, high precision, good signal linearity and fast response. The sensor has a simple structure, small size, and low power consumption, and it is not subject to electromagnetic interference, which provides a new path for the application of microcavity in non-contact current detection. It can be applied to the condition assessment of equipment such as smart transformers, switchgear circuit breakers and insulating devices, etc. It can be used for online monitoring of partial discharges, harmonic currents, faults and leakage currents, etc. It provides a new path for current sensing in the power industry and consumer electronics.