Circular near-field mapping of microwave magnetic fields using a projection-based diamond fiber probe

Accurate characterization of microwave (MW) near fields is essential for antenna diagnostics and electromagnetic engineering, yet conventional probes primarily access linear components and provide limited polarization information. Here, we demonstrate a fiber-integrated diamond probe based on nitrogen-vacancy (NV⁻) centers for projection-based mapping of MW magnetic fields with circular selectivity. The measurement exploits transition-selective coupling between Zeeman-resolved spin transitions and circular components of the MW magnetic field defined with respect to the NV quantization axis, such that the observed optically detected magnetic resonance (ODMR) contrast reflects a projection onto a spin-defined circular basis rather than the field magnitude. This framework enables a physically consistent description of polarization-dependent response in near-field environments without a unique propagation direction. Operating in a continuous-wave regime, the probe provides robust measurements without absolute calibration and yields spatial distributions directly comparable with simulations. Measurements of a double-ridge horn antenna show agreement with linear field distributions, while mapping of a helical antenna reveals circularly selective response consistent with local measurement geometry. These results are performed using practical antenna structures relevant to wireless communication systems, establishing a compact, quantum-enabled platform for polarization-sensitive microwave diagnostics in realistic electromagnetic environments.