Dual-Layer Spherical Coil: A Novel Design Method for Self-Shielded Uniform Field Coil

Optically pumped magnetometers (OPMs) have emerged as a promising magnetic sensor for magnetoencephalography and magnetocardiography (MEG and MCG), owing to their low cost, high spatiotemporal resolution, and excellent magnetic-field sensitivity. Self-shielded coils—which generate a highly uniform internal field while rapidly decaying external fields—serve critical roles in OPM development: as in-magnetic shielding room (MSR) standard magnetic sources, they enable distortion-free uniform field for OPM calibration; as in-probe modulation magnetic sources, they provide stable, low-crosstalk modulation field. The external field decay and internal field uniformity of these coils are key performance metrics. To overcome the limitations inherent in conventional cylindrical self-shielded coil topologies, this article proposes a dual-layer spherical self-shielded coil structure and optimizes its geometry with respect to the field of the target region. Theoretical analysis shows that compared to common cylindrical designs, the proposed spherical structure reduces the minimum crosstalk-free distance by 50% when used as a modulation source, and expands the uniform field region by a factor of 1.9 when used as a standard source within MSR. Experimental validation corroborates these predictions, proving the efficacy of the spherical coil topology and optimization methodology in advancing OPM performance and suppressing crosstalk.