A compact unshielded optically pumped magnetic gradiometer

Optically pumped magnetic gradiometers (OPG) play a crucial role in applications such as magnetic anomaly detection and biomagnetic measurements. This study classifies current OPG 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 discrepancy between the inherent CMRR and the measured CMRR, as well as the upper limit of inherent CMRR. We point out that although the magnetic field differential method has the potential to increase the inherent CMRR by a factor of 1 + A(s)F(s), where A(s) and F(s) represent the forward and feedback channel complex gain, respectively, the often-overlooked difference between feedback gains between channels may impose a practical limit on the achievable inherent CMRR. We designed and fabricated a compact, unshielded OPG with a specially designed scheme to minimize the distance between the sensing head and the magnetic source. The measurement results demonstrate gradient sensitivity of 0.36 pT/cm/Hz with a CMRR of 8178@1 Hz in a shielded environment and 5 pT/cm/Hz with a CMRR of 1200@1 Hz in an unshielded environment.