Highly sensitive magnetic field microsensor based on direct laser writing of fiber-tip optofluidic Fabry–Pérot cavity

In this paper, a polymer micro-Fabry–Pérot interferometer (FPI) fabricated via direct laser writing using two-photon polymerization techniques on a single mode fiber tip is proposed, designed, simulated, and experimentally demonstrated as a magnetic field probe. The sensor comprises a tapered waveguide with a length of 100 µm and a diameter of 1 µm along the axis connecting the two reflecting surfaces of the micro-FPI open cavity. The cavity is filled with a magnetic fluid (MF) changing the evanescent coupling and consequently leading to a change in the phase of the FPI. To improve the signal reflection, a thin layer of Au is coated on the device before fabricating the probe sensor, which is then sealed in a MF-filled glass tube. The experimental results show that the proposed probe sensor has a low temperature sensitivity and a sensitivity to magnetic field as high as 1.54 nm/mT, and the magnetic field measurement accuracy is ∼649.4 μT within a range from 1 mT to 8 mT. The microsensor, which is very stable and easy to fabricate, can be used as a probe to detect weak magnetic fields.