Magnetic-field sensing using a magnetostrictively coupled multimode whispering-gallery-mode microlaser

We report an active magnetic sensor based on a magnetostrictively actuated multimode whispering-gallery-mode (WGM) microlaser with heterodyne beat-frequency readout. By mechanically coupling an erbium-doped stretchable WGM microcavity to a Terfenol-D, magnetic-induced magnetostrictive deformation is efficiently transduced into cavity strain, resulting in mode-dependent frequency shifts. Owing to the intrinsic dependence of the strain sensitivity of the WGM mode number, different lasing modes exhibit unequal frequency responses under the same applied magnetic field. We exploit the differential frequency shifts between two simultaneously lasing modes to generate a radio-frequency beat signal. This transduction converts small optical frequency shifts into a resolvable beat frequency while suppressing common-mode perturbations arising from thermo-optic effects. As a result, a magnetic sensor with a sensitivity of 6.02 MHz/mT and a microtesla-level detection limit is achieved. This detection limit represents a two-orders-of-magnitude improvement compared with direct optical spectral interrogation of lasing modes. Beyond magnetic sensing, the proposed heterodyne detection scheme provides a general framework for ultrasensitive sensing based on multimode WGM microlaser platforms.