Controllable gap in microsphere resonator integrated with a deformable ferrofluid droplet for magnetic field sensing

Abstract

Abstract A magnetic field sensor based on silica microsphere resonator and ferrofluid is proposed and experimentally demonstrated in this paper. A silica fiber taper with a waist diameter of 2 μm is employed to couple the evanescent light into the microsphere with a diameter of 31.21 μm. Whispering gallery modes (WGMs) are excited inside the microsphere resonator and circulate inside the equatorial plane. The essence of the sensing concept is to control the coupling gap between fiber taper and microsphere using the magnetically induced deformation of ferrofluid droplet. The shape-varying ferrofluid droplet is operated under different magnetic fields to observe the resonance shift and Q-factors of WGMs. As the magnetic field rises gradually, a longer resonant wavelength shifts with a decreasing Q-factor and a high maximum sensitivity of 120.9 pm/mT in the vertical magnetic field. On the contrary, resonance under the parallel magnetic field shifts to shorter wavelength with an increasing Q-factor and a low maximum sensitivity of 16.4 pm/mT. All experimental results are in good agreement with the theoretical model, which provides a disparate approach to integrate photonics with ferrofluid for signal processing and sensing applications.