Bandwidth Enhancement for Magnetic-Field-Modulation-Free SERF Magnetometers

Magnetoencephalography (MEG) using optically pumped magnetometers (OPMs) enables precise measurement of brain neural activity with enhanced signal strength and spatial detail. Most OPM-MEG systems use spin-exchange-relaxation-free (SERF) OPMs, which have a narrow bandwidth (<100 Hz), limiting their ability to measure broadband neural activity. Existing bandwidth enhancement methods for SERF-OPMs face challenges, including sensor interference, difficulties in miniaturizing light modulation devices, and insufficient theoretical explanations. To address these issues, this study introduces a magnetic-field-modulation-free (MFMF) SERF OPM that significantly enhances the response bandwidth of compact stand-alone SERF OPMs without magnetic field interference, enabling high-density detection. Specifically, we conducted a detailed theoretical analysis to develop a frequency response model that accounts for the large polarization gradient. We also applied heuristic optimization methods to accurately calculate the frequency response of OPMs with optically thick cells using only premeasured amplitude response. This approach minimizes amplitude attenuation and phase shift, achieving a flat frequency response (frequency response linearity <3% up to 1000 Hz) while maintaining a sensitivity of 20 fT/Hz $^{\text {1/2}}$ up to 500 Hz, typically. Our method achieves an approximately eightfold bandwidth increase for individual OPMs and enhances the common-mode rejection ratio (CMRR) of sensor array synthetic gradiometers by over fivefold. Thus, the enhanced compact MFMF SERF OPM, characterized by its broad response bandwidth, high sensitivity, and the absence of crosstalk effect, holds the potential for developing high-density detection arrays capable of more accurate and reliable biomagnetic imaging with exceptional spatial resolution across a broad frequency range.