Design and Analysis of a High-Sensitivity Atomic Magnetic Gradiometer With Adjustable Baseline

Optically pumped magnetometers (OPMs) can replace superconducting quantum interference devices (SQUIDs) as the core sensor for biomagnetic measurements. Among various types of atomic magnetometers, the single-beam spin-exchange relaxation-free (SERF) magnetometer exhibits the most promise for miniaturization. Moreover, combined with the gradient configuration that only adds another measuring channel, the common-mode noise can be effectively suppressed. However, traditional studies use the fixed baseline distance, and there is insufficient comprehensive research on the effects of the baseline distance, measurement distance, and source size on the signal-to-noise ratio (SNR) of the gradiometer. Under this condition, a four-channel single-beam SERF gradiometer with an adjustable baseline distance of 5–60 mm and an adjustable measurement distance of 40–95 mm was constructed. The developed gradiometer achieved an impressive sensitivity of 4 fT/cm/ $\sqrt {\text {Hz}}$ using magnetic coils of various radii to simulate diverse sources. The experimental results indicate that the SNR can be significantly improved when the gradiometer is placed much closer to the source regardless of the source size. Meanwhile, the optimal baseline distance varies with the source size, and a smaller source requires a relatively longer baseline for better performance. We anticipate that these findings will provide general references to design the gradiometer in biomagnetic measurements, especially for applications that have different coexisting source sizes. Moreover, this study explores key parameters that affect the performance of optically pumped gradiometers and finds the parameters that may improve the biomagnetic measurements and other applications that must detect weak magnetic signals in noisy environments.