Parameter optimisation of miniaturised SERF magnetometer below relaxation rate saturation region

The miniaturised spin-exchange relaxation-free (SERF) magnetometer has promising applications in biomagnetic measurements owing to its ultrasensitive magnetic field measurement capacity. In this paper, we propose a single-beam parameter-optimised SERF magnetometer that operates below the relaxation rate saturation region. Based on the relaxation rate model under zero-field resonance, the relationship between the magnetometer optimal working point and multiphysical parameters, including the optical power density, cell temperature, and nitrogen density, was studied. The experimental results verified the accuracy of the law deduced from the model. By adjusting the parameters according to our experimental results, a sensitivity of 17 fT/Hz^1/2 (@31.5Hz) and bandwidth of 110 Hz were obtained optimally in a 5.5 cm³ magnetometer. The superiority of the optimised parameters in the miniaturised SERF magnetometer is verified. We found that the relaxation rate saturation region results from light absorption saturation, which leads to a performance decline of the SERF magnetometer. This relaxation rate saturation phenomenon widely exists in miniaturised SERF magnetometers, and the study of this phenomenon is helpful for further applications in arrayed integration of biomagnetic measurement.