Magnetic sensor based on a Whispering Gallery Mode double-tailed silica microsphere

A vector magnetometer based on a stretchable Whispering Gallery Mode (WGM) silica Microsphere with a Q-factor of 10⁸ is reported. The used microsphere has two fiber tails enabling it to serve as a strain sensor. The Double-Tailed Microsphere (DTM) is mechanically coupled to a Terfenol-D Rod (TDR), which undergoes longitudinal strain in response to a change in the ambient magnetic field. The strain of the magnetostrictive rod induces mechanical stress on the DTM resulting in a spectral shift of the WGMs of the microsphere. The strong dependence of the magnetometer sensitivity on the sensor geometry is studied, and geometry considerations for optimizing sensor performance are discussed. By introducing and employing geometrical adjustments, applicable due to the unique structure of the magnetometer, the sensitivity improved by a factor of 33. A sensitivity of 7.73 pm/mT is reported with a detection limit of ∼1 µT. The conditions required to achieve a considerably higher sensitivity of ∼200 pm/mT are presented, suggesting an expected detection limit under 40 nT. The simplicity, low cost and lack of electrical wiring make it attractive for remote magnetic sensing. In addition, the possibility to concatenate multiple sensors to a single optical fiber makes the suggested magnetometer highly suitable for quasi-distributed vector magnetic sensing.