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International Conference on Optical Communications and Networks (ICOCN)

Efficient Fluorescence Coupling Integrated Fiber Magnetometer Based on Nitrogen Vacancy Centers

Yuhao Li, Li Xia, Junchang Huang

Published Jul 26, 2024• 0 citations
Publisher

Abstract

A magnetic field sensor based on a fiber-optic integrated probe at the center of a nitrogen vacancy (NV) is proposed and demonstrated. Fiber optics are utilized to replace all conventional spatial optics while efficiently enabling laser excitation and fluorescence collection from diamond. Optical models were developed to study the fluorescence collection efficiency of diamond. The fiber-coupled probe was simulated and its efficiency was improved to 3.062% compared to the 0.607% fluorescence coupling efficiency of the fiber. The optical probe magnetic resonance (ODMR) phenomenon was successfully observed and significantly improved the magnetic field sensitivity to 5.3 nT/Hz1/2, which proved the feasibility and performance compared with the conventional confocal NV-centered magnetometer.

DOI
10.1109/icocn63276.2024.10648298

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A magnetic field sensor based on a fiber-optic integrated probe at the center of a nitrogen vacancy (NV) is proposed and demonstrated. Fiber optics are utilized to replace all conventional spatial optics while efficiently enabling laser excitation and fluorescence collection from diamond. Optical models were developed to study the fluorescence collection efficiency of diamond. The fiber-coupled probe was simulated and its efficiency was improved to 3.062% compared to the 0.607% fluorescence coupling efficiency of the fiber. The optical probe magnetic resonance (ODMR) phenomenon was successfully observed and significantly improved the magnetic field sensitivity to 5.3 nT/Hz1/2, which proved the feasibility and performance compared with the conventional confocal NV-centered magnetometer.
Efficient collection of photoluminescence arising from spin dynamics of nitrogen vacancy (NV) centers in diamond is important for practical applications involving precise magnetic field or temperature mapping. These goals may be realized by the integration of nanodiamond particles with optical fibers and volumetric doping of the particles alongside the fiber core. That approach combines the advantages of robust axial fixation of NV diamonds with a direct spatial overlap of their fluorescence with the guided mode of the fiber. We developed a suspended core silicate glass fiber with 750 nm-diameter nanodiamonds located centrally in the 1.5 µm-core cross-section along its axis. The developed fiber probe was tested for its magnetic sensing performance in optically detected magnetic resonance measurements using a 24 cm-long fiber sample, with the NV excitation and fluorescence collection from the far ends of the sample and yielding optical readout contrast of 7% resulting in 0.5 µT·Hz-1/2 magnetic field sensitivity, two orders of magnitude better than in earlier designs. Thanks to its improved fluorescence confinement, the developed probe could find application in magnetic sensing over extended fiber length, magnetic field mapping or gradiometry.
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The nitrogen-vacancy (NV) center in diamond is a prime candidate for quantum sensing technologies. Here, we present a fully integrated and mechanically robust fiber-based endoscopic sensor with a tip diameter of 1.25 mm . On its tip, a direct laser-writing process is used to secure a diamond containing NV centers above the fiber’s core inside a polymer structure. Additionally, a metallic direct laser-written antenna structure next to the fiber facet allows efficient microwave manipulation of NV-center spins. The sensor achieves a shot-noise-limited magnetic-field sensitivity of 5.9 nT / Hz using a 15 − μ m -sized microdiamond at a microwave power of 50 mW and an optical power of 2.15 mW . Using lock-in techniques, we measure a sensitivity of 51.8 nT / Hz . Furthermore, we introduce a dual-fiber concept that enables, in combination with a direct laser-written structure, independent guiding of excitation and fluorescence light and thus reduces background autofluorescence. Moreover, controlled guiding of excitation light to the diamond while avoiding sample illumination may enable operation in light-sensitive environments such as biological tissue. While the demonstrated sensitivity is achieved using a single-fiber configuration, the dual-fiber approach provides a path toward integrating smaller diamonds, where autofluorescence would otherwise limit performance. We demonstrate the capability of vector magnetic-field measurements in the type of magnetic field used in state-of-the-art ultracold quantum gas experiments, opening a potential arena in which high resolution and high sensitivity are required.
We experimentally demonstrate a simple and robust optical fiber based method to achieve simultaneously efficient excitation and fluorescence collection from Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We fabricate a suitable micro-concave mirror that focuses scattered excitation laser light into the diamond located at the focal point of the mirror. At the same instance, the mirror also couples the fluorescence light exiting out of the diamond crystal in the opposite direction of the optical fiber back into the optical fiber within its light acceptance cone. This part of fluorescence would have been otherwise lost from reaching the detector. Our proof-of-principle demonstration achieves a 25 times improvement in fluorescence collection compared to the case of not using any mirrors. The increase in light collection favors getting high signal-to-noise ratio optically detected magnetic resonance signals and hence offers a practical advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV sensor system by replacing some bulky optical elements in the optical path with a I x 2 fiber optical coupler in our optical system. This reduces the complexity of the system and provides portability and robustness needed for applications like magnetic endoscopy and remote-magnetic sensing. Published by AIP Publishing.
We experimentally demonstrate a simple and robust optical fiber based method to achieve simultaneously efficient excitation and fluorescence collection from Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We fabricate a suitable micro-concave mirror that focuses scattered excitation laser light into the diamond located at the focal point of the mirror. At the same instance, the mirror also couples the fluorescence light exiting out of the diamond crystal in the opposite direction of the optical fiber back into the optical fiber within its light acceptance cone. This part of fluorescence would have been otherwise lost from reaching the detector. Our proof-of-principle demonstration achieves a 25 times improvement in fluorescence collection compared to the case of not using any mirrors. The increase in light collection favors getting high signal-to-noise ratio optically detected magnetic resonance signals and hence offers a practical advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV sensor system by replacing some bulky optical elements in the optical path with a 1 × 2 fiber optical coupler in our optical system. This reduces the complexity of the system and provides portability and robustness needed for applications like magnetic endoscopy and remote-magnetic sensing.
Using an optical fiber to both excite the nitrogen-vacancy (NV) center in diamond and collect its fluorescence is essential to build NV-based endoscope-type sensor. Such endoscope-type sensor can reach inaccessible fields for traditional NV-based sensors built by bulky optical components and extend the application areas. Since single NV's fluorescence is weak and can easily be buried in fluorescence from optical fiber core's oxide defects excited by the green laser, fixing a micrometer size diamond containing high-density NVs rather than a nanodiamond containing single NV or several NVs on the apex of an optical fiber to build an endoscope-type sensor is more implementable. Unfortunately, due to small numerical aperture (NA), most of the optical fibers have a low fluorescence collection efficiency, which limits the sensitivity and spatial resolution of the NV-based endoscope-type sensor. Here, using a tapered optical fiber (TOF) tip, we significantly improve the efficiency of the laser excitation and fluorescence collection of the NV ensembles in diamond. This could potentially enhance the sensitivity and spatial resolution of the NV-based endoscope-type sensor. Numerical calculations show that the TOF tip delivers a high NA and has a high NV excitation and fluorescence collection efficiency. Experiments demonstrate that such TOF tip can obtain up to over 7-fold excitation efficiency and over 15-fold fluorescence collection efficiency of that from a flat-ended fiber (non-TOF) tip.