Robust frequency calibration of a large-area high-Q resonator in magnetic imaging with spin ensembles in diamond

High-Q resonators with a large uniform field are widely used in magnetic imaging, but the microwave (MW) frequency shifts differently because of the near-field coupling. In this study, the major factors influencing of the frequency shifts were analyzed in a confocal system with nitrogen vacancy centers. We demonstrated a frequency calibration method involving adjustments of the applied magnetic field. This technique maintained the uniformity of the MW field with robustness to system noise, without changing the MW power. The effective imaging region reached 1.3 ' 1.3 mm² with a uniformity of 92.7%. This method preserves the stable operation of spin ensembles in magnetic imaging systems.