The remarkable quantum spin properties of negatively charged nitrogen-vacancy centers present in nanodiamond can be precisely controlled by applying microwaves, magnetic fields, and light, enabling their potential as high-precision sensors. Such sensors exploiting nanodiamond-functionalized optical fibers have been demonstrated to enable reaching sub-nT magnetic field sensitivities over localized magnetic field sources, but their potential for distributed sensing remains largely unexplored. The volumetric incorporation of nanodiamond particles into the optical core of the fiber creates the possibility of developing fibers that are sensitive to the magnetic field over their entire length. That approach combines the advantages of robust axial fixation of nanodiamonds with a direct spatial overlap of their fluorescence with the guided mode of the fiber.
During the course of the QUNNA project we've developed three different optical fibres with integrated nanodiamonds. Two of those used dip coating method of nanodiamond deposition in order to fabicate a step-index fiber with the optical core volumetrically functionalized with nanodiamonds, and a suspended core fiber with 750 nm-diameter nanodiamonds located centrally in the 1.5 µm-core. For the third fibre we deposited nanodiamonds from a solvent suspension into a anti-resonant hollow core fiber. We've then tested their application for ODMR-based and microwave-free magnetic field gradiometry.