Sensitivity optimization for lock-in detection in nitrogen-vacancy center-based magnetic sensing using resonance frequency tracking method

The nitrogen-vacancy (NV) centers in diamonds, known for their exceptional sensitivity, spatial resolution, and temperature adaptability, exhibit excellent magnetic field sensing capabilities. To address the time-consuming feature of the frequency-sweeping method, the resonance frequency tracking (RFT) method, based on lock-in detection, has been proposed and widely employed for real-time magnetic field measurements. To further improve the accuracy of this method, this study reports a sensitivity optimization strategy designed to minimize the fluorescence shot-noise-limited sensitivity and the magnetic noise sensitivity. Specifically, based on an all-fiber optical lock-in detection system, we investigate the coupling effects between laser power and microwave power on the fluorescence shot-noise sensitivity and the impact of modulation parameters of lock-in detection on the magnetic noise sensitivity. Furthermore, both simulation and experimental approaches are employed to investigate the mechanisms for optimizing magnetic noise sensitivity under the influence of hyperfine coupling induced by the ¹⁴N isotope. These results provide concrete sensitivity optimization measures for lock-in detection systems using the RFT method, which can be extended to other lock-in detection applications.