Research on magnetic field sensing based on whispering gallery modes microbubble resonator

In this paper, we design a magnetic field sensing scheme based on whispering gallery mode (WGM) microbubble resonator infiltrated with magnetic fluid (MF) and experimentally verify this scheme. MF can be injected directly into the natural microfluidic channel of the microbubble to interact with the evanescent field effectively. We analyze the optical field distribution of WGMs before and after filling with MF and the influence of the wall thickness of the microbubble on the magnetic sensing sensitivity by COMSOL simulation. The microbubble cavities with thin wall thicknesses ranging from 2 to 3 mu m are fabricated using CO2 laser heating method. The magnetic field sensing performance of the microbubble resonator filled with MF is researched experimentally. With the growth of the applied magnetic field intensity, the refractive index of MF increases, which contributes to the redshift of the resonant wavelength. In the magnetic field range from 6 to 22 mT, the magnetic field intensity sensitivity is 2.15 pm/mT and the detection limit is 0.03 mT. When the magnetic field intensity exceeds 22 mT, the refractive index of MF reaches a saturation value, thus the resonance wavelength no longer shifts as the magnetic field intensity increases. Furthermore, the stability of this sensing system is verified experimentally by monitoring the variation of resonant wavelength in 290 s with the constant magnetic field intensity of 6 mT. This work proposes a novel magnetic sensing scheme based on WGM microbubble and demonstrated the feasibility of this scheme. Compared with existing optical fiber magnetic sensors, our proposed sensor reduces the preparation cost and simplifies the fabrication process by taking full advantage of the natural microfluidic channel of the microbubble. Besides, it has other advantages of small volume, high reliability and low detection limit, which shows great value of application in the field of weak magnetic sensing.