A compact unshielded optically-pumped magnetic gradiometer

Optically-pumped magnetic gradiometers (OPGs) play a crucial role in applications such as magnetic anomaly detection and bio-magnetic measurements. This study classifies current OPGs into four types based on their differential modes: voltage, frequency, optical rotation, and magnetic field differential modes. We introduce the concept of inherent Common-Mode Rejection Ratio (CMRR) and analyze the differences between the inherent CMRR and the measured CMRR, as well as the upper limit of inherent CMRR. We point out that although magnetic field differential method has the potential to increase inherent CMRR by a factor of 1+AF, the difference between the feedback gains is often neglected, which may set the limit of inherent CMRR. We designed and fabricated a compact, unshielded OPG with a specially designed scheme to minimize the distance between the sensing heads and the magnetic source. Measurement results demonstrate a measured CMRR of 1200@1Hz and a sensitivity of approximately 5 pT/cm/\sqrt{Hz} from 1 Hz to 100 Hz.