In Situ Triaxial Magnetic Field Compensation for the Spin-Exchange-Relaxation-Free Optically Pumped Magnetometer

The most efficient way of improving the sensitivity of an optically pumped magnetometer (OPM) is to maximize the spin-polarization lifetime, which requires the miniaturization of the magnetic field around the sensor. This article proposed a novel method of compensation the magnetic field for a compact spin-exchange-relaxation-free (SERF) OPM. The method employs modulation and low-frequency sweeping magnetic fields along the two directions that are perpendicular to the laser beam simultaneously. The output signal of the photodiode is demodulated at different frequencies, resulting in an output signal correlated to the ${x}$ -axis modulation ( $\beta _{x}$ ) and an output signal correlated to the ${y}$ -axis modulation ( $\beta _{y}$ ). Then, the three components of the magnetic field are adjusted until $\beta _{x} $ and $\beta _{y}$ become sinusoids with zero bias. The proposed method can monitor the magnetic fields of the three axes simultaneously and does not require iteration during the process, which increases the speed of magnetic field compensation. The feasibility of the proposed method is demonstrated experimentally. The experimental results show that the magnetic field compensation resolutions of the ${x}$ -axis, ${y}$ -axis, and ${z}$ -axis are 0.6, 0.6, and 5 pT, respectively. The noise density of the compact OPM is about ${20} \text {fT}/\text {Hz}^{{1}/{2}}$ .