NV nanodiamond doped fiber for magnetic field mapping

. The advances in fluorescent diamond-based magnetic field sensors have led this technology into the field of fiber optics. Recently, devices employing diamond nanobeams or diamond chips embedded on an optical fiber tip enabled achieving fT-level sensitivities. Nevertheless, these demonstrations were still confined to operation over localized magnetic field sources. A new approach of volumetric incorporation of nanodiamonds into the optical fiber core enables optical fibers sensitive to magnetic field at any point along the fiber length. We show that information on the perturbed spin state of a diamond nitrogen-vacancy color center can be transmitted over a macroscopic length in an optical fiber, in presence of noise from large concentration of the color centers along the fiber. This is exploited in optical readout at the fiber output not only of the magnetic field value, but also spatially variable information on the field, which enables the localization of its source.


Introduction
The significant advances in fluorescent diamond-based magnetic field sensors have enabled the progress of this technology into the fiber optics, with demonstrators of fiber-based devices with sub-nT sensitivities achievable over highly localized targets.More recently, elaborate devices employing diamond nanobeams or diamond chips embedded on a specially micro-machined optical fiber tip enabled achieving even fT-level sensitivities [1,2].Nevertheless, these impressive demonstrations are still confined to operation over localized magnetic field sources.
A new approach of integration of the NV-rich diamond particles with optical fibers, involving volumetric incorporation of nanodiamonds into the fiber core has been proposed in 2020 by the group of prof.Heike Ebendorff-Heidepriem [3], which essentially enables optical fibers sensitive to magnetic field at any point along the fiber length.Despite this unique functionality, the current magnetometric applications of such structures have been limited to reading magnetic fields over localized targets [3].So the question is, whether the information on the local magnetic field contained in the NV fluorescence propagating as a guided fiber mode could be preserved over a fiber length and enable the localization of the magnetic field source.Notably, spatially-variable magnetic field information readout using spin-manipulation in color centers has been demonstrated only once [4].This involved use of multiple diamond sensor and detector chips along a detection line.
In this work, we are demonstrating distributed magnetic field sensing over a 13-cm long section of a magnetically sensitive fiber with nanodiamond particles placed into the fiber core.Recording of optically detected magnetic resonance (ODMR) spectra is carried over the length of the fiber, which is scanned with a single microwave (MW) antenna mounted on a translation stage.The sensor's capability to localize the magnetic field source is verified with two ODMR recordings with a magnet placed at different positions along the fiber, which was operated with the color centers excitation and fluorescence collection at the opposite ends of the fiber.

Experimental setup
The optical fiber was developed by using combination of the dip-coating deposition of nanodiamond particles on the surface of a glass rod and a modified stack-and-draw fiber development method [5].It begins with dip coating of a glass rod (F2 glass from Schott) in a suspension of NV-rich diamonds with mean particle size of 750 nm.The diamond-coated rod is then drawn down from diameter of 30 mm into canes with a diameter of 0.5 mm.A stack of 790 canes is then drawn into a fiber core preform.This, after insertion into a glass tube with lower refractive index, is drawn into the final fiber.The fiber with an optical core diameter of 50 μm is a multimode fiber with nanodiamonds distributed along entire length of its core.
The fiber used was 35 cm long, with attenuation measured at nearly 50 dB/m at a wavelength of 780 nm.It was pumped by a laser set at a wavelength of 532 nm.Fluorescence at the fiber output was collected through a high-pass filter which enabled the collection of light over wavelengths from 600 nm to about 850 nm.
For the MW part of the experiment, the generator signal was fed to a MW resonant antenna with a central opening for the fibre (3 mm wide), which was moved along the fiber.The magnetic field source was a cylindrical neodymium magnet.The applied laser pumping excited all NV ─ centers in diamonds along the whole fiber, but only a small fraction of diamonds were subjected to MW field from the antenna at a time, resulting in a low ODMR contrast (below 0.1%).These differs from other diamond-based devices for magnetic field measurement, where the entire area or population of diamond particles is subjected to the magnetic field and the MW field at once [1,2].
The magnetic field sensitivity of the described sensor was 26.6 /√ .Such sensitivity is not directly comparable to the most recent results obtained, where nTto fT-level sensitivities have been reported [1,2], but those results have been obtained using localized magnetic field sources.Sensitivity in the µT range has been routinely reported for many fiber based magnetic field sensors with nanodiamond particles deposited on the tip or tapered fiber sections [6].

Results
Distributed magnetic field measurements were performed with the neodymium magnet positioned either close to the input end of the fiber sensor or around the middle of the fiber length.In each of these cases the distance between the fiber and the magnet was about 4 cm.The fiber was scanned along 13 cm of its length, with the step size set at 5 mm.
Figure 1 shows the contour maps obtained by stacking individual ODMR spectra recorded at every 5-mm step of the antenna travel on the translation stage, i.e. the horizontal axis represents the MW frequency and the position along the fiber is recorded along the vertical axis.The colorbar scale is a normalized NV ─ fluorescence intensity at the fiber sensor output.In both measurements the fiber was pumped at one end, and the fluorescence was collected at the other end.Particular ODMR spectra were assigned step-by-step to the specific positions along the fiber, as the antenna scanned the fiber during its travel along the fiber length.At each step, the ODMR spectrum assigned to that particular position was recorded at the fiber output.This was done in order to prove that the wave guiding functionality of a fiber can be used even when the entire fiber is magnetically sensitive.The data shown in Fig. 1 reveal that near the position of the magnet (fixed during the measurement) ODMR spectra becomes broadened the closer it is to the magnet.

Conclusion
The ability to measure magnetic field distributions with good sensitivity and spatial resolution is very important, particularly in disciplines exploiting spin properties and quantum effects [7].The main highlight of our experiment is the use of a lossy and noisy waveguide of a macroscopic length for a remote optical read-out of spin information of the localized NV ─ defects in diamonds positioned fiber core.This was achieved with a good spatial resolution, with read out taking place at one physical location and with assignment of variable spatial information.This is accomplished by a physical displacement of the spin perturbance caused by localized MWs along the length of the fibre.By using this fact it is easy to imagine improving the setup by integration of the antenna with the fiber, along with an electronic addressing of the chosen antenna regions.This would significantly shorten the measurement time down to a duration determined in principle by the antenna switching electronics.Such sensors would be of great convenience in spintronics or in computing applications using magnetic switching [8].

Fig. 1 .
Fig. 1.Contour maps of ODMR signals from the magnetically sensitive optical fiber, obtained at the fiber output during longitudinal scanning of the fiber with the movable MV field antenna (blue ring around yellow fiber): a) when the magnet ( grey rectangle) is placed close to the fiber input, b) when the magnet is placed away from either end of the fiber.