A single-fiber optical readout of cantilever deflection caused by Lorentz force was investigated. A good agreement of the optical near field model with the measured nonlinear characteristic was found. The ac-excitation mode of the cantilever is preferable, due to the uncontrollable static light reflections in a single-fiber setup. The presented sensor design seems suitable for a measurement range of the magnetic flux density of at least 10 mT to 50T with moderate excitation current amplitudes. Two-fiber setups may reduce the background light and could enable dc excitation but require smaller offset between cantilever and fiber axis.