Enhanced Performance of Elliptically Polarized SERF Atomic Magnetometers through Optical Parameter Optimization and Light Frequency Modulation

Elliptically polarized spin-exchange relaxation-free (SERF) atomic magnetometers (AMs) are promising sensors for biomagnetism measurements due to their compact structure and high sensitivity. However, their practical advantages over single-beam circularly polarized SERF AMs remain unfully demonstrated, attributed to the use of a single beam of far-resonance elliptically polarized light. This leads to complex coupling, as ellipticity, intensity, and frequency simultaneously influence the pumping and detection processes, and an unavoidable fictitious magnetic field induced by AC-Stark shift and its gradient exist. To solve them, we improve elliptically polarized SERF AMs by decoupling and optimizing optical parameters, while concurrently suppressing the fictitious magnetic field. We first refine the input-output model by considering not only the first-harmonic of longitudinal polarization but also the DC and second-harmonic components. Based on the modified model, we simultaneously decouple and optimize the major axis orientation, ellipticity, intensity, and frequency of light by assessing their dependencies on light intensity. This approach led to an improved sensitivity of 6 fT/Hz^1/2, demonstrated in a ⁸⁷Rb vapor cell measuring 3×3×3 mm³. Furthermore, we implemented symmetric frequency modulation on the pump light, transforming the fictitious magnetic field and its gradient into a modulated field to alleviate their effects. This modulation also contributed to a reduction in low-frequency sensitivity by isolating slow fluctuations in light. The improved elliptically polarized SERF AMs hold promise for practical applications, especially in array-based biomagnetism measurements.