Measurement bandwidth enhancement of SERF atomic magnetometers using frequency response regulation

The spin-exchange relaxation-free (SERF) atomic magnetometer, grounded in quantum precision measurement, offers exceptional magnetic field sensitivity. Nonetheless, its measurement bandwidth is typically below 200 Hz, a constraint that hinders the capture of diagnostically significant high-frequency components in magnetocardiography (MCG). This paper introduces a measurement bandwidth enhancement strategy that exploits the intrinsic characteristics of the SERF magnetometer by integrating cascaded notch filtering with closed-loop control based on the zero crossing of the dispersive response. Theoretical and experimental analyses first reveal the time- and frequency-domain characteristics of high-frequency modulation noise, alongside an analysis of how conventional low-pass filter parameters distort the amplitude–frequency response. To mitigate the phase lag associated with traditional output filtering, a configuration of cascaded notch filters is specifically designed to suppress both the modulation noise and its associated harmonic components. Subsequently, a proportional–integral (PI) control system is constructed, leveraging the zero-crossing point of the dispersive response inherent to the modulated SERF magnetometer. A closed-loop feedback methodology is then deployed to further regulate the system's frequency response. Experimental results demonstrate an enhancement of the measurement bandwidth from 213.3 Hz to 632.3 Hz, while maintaining a closed-loop resonant peak below 1.6 dB. The open-loop and closed-loop sensitivities are recorded at 18.81fT/Hz^1/2 and 19.61fT/Hz^1/2, respectively. Furthermore, the correlation coefficient for high-frequency MCG waveforms exhibits an improvement, rising from 0.5165 to 0.8740.