Sensing of Magnetic-Field Gradients with Nanodiamonds on Optical Glass-Fiber Facets

We demonstrate a photonic sensor of the magnetic field and its gradients with remote readout. The sensor is based on optically detected magnetic resonance (ODMR) in nanodiamonds with nitrogen-vacancy color centers that are covalently attached as a thin film on one facet of an optical fiber bundle. By measuring ODMR signals from a group of individual fibers in an ∼0.5-mm-wide imaging bundle, differences of local magnetic field strengths and magnetic field gradients are determined across the plane of the bundle facet. The measured gradients are created by direct electric currents flowing in a wire placed near the nanodiamond film. The measurement enabled the determination of the net magnetic field corresponding to various current directions and their corresponding magnetic field gradients. This demonstration opens up a perspective for compact fiber-based endoscopy, with additional avenues for remote and sensitive magnetic field detection with submicrometer spatial resolution under ambient conditions.