Closed-Loop Elliptically Polarized Atomic Magnetometer with Normalized Signal Feedback

In recent years, spin-exchange relaxation-free (SERF) atomic magnetometers (AMs) have emerged as promising tools for biomagnetism sensing due to their room-temperature operation, high sensitivity, and potential for miniaturization. Elliptically polarized SERF AMs, which utilize the circular component of the pump beam for polarization and the linear component for detection, are particularly well-suited for compact and high-performance biomagnetism applications. However, most existing elliptically polarized SERF AMs operate in open-loop mode, making the calibration factor sensitive to environmental magnetic fields, thereby compromising measurement accuracy. Although closed-loop control methods can be employed to stabilize calibration factors, they remain susceptible to internal parameter fluctuations, such as laser power instability and atomic density variations, which limits the robustness and accuracy of the measurements. To address these challenges, we propose a novel closed-loop control scheme that employs a normalized feedback signal derived from the ratio of the first-and second-harmonic components of the zero-order resonance. This method significantly mitigates the impact of laser ellipticity, power instability, frequency drift, and atomic density variations. Simulation and experimental results demonstrated that the proposed method enhanced the resistance to internal noise interference while also expanding the dynamic range of the closed-loop system. The proposed scheme enhances both accuracy and reliability while maintaining the intrinsic advantages of elliptically polarized SERF magnetometers, making it suitable for precision biomagnetism and ultra-low field sensing applications.