The Bell–Bloom atomic magnetometer is a high-sensitivity magnetic field measuring instrument. It utilizes the free precession signal of polarized atoms to obtain the magnetic field measurement. However, the complexity of the free precession signal makes it difficult for the magnetometer to measure magnetic field information, which severely limits the application of the magnetometer. In this study, to solve this problem, a free precession frequency (FPF) measurement system is designed for the Bell–Bloom atomic magnetometer, which successfully achieves accurate, fast, and efficient frequency counting of the free precession signal. Based on the analysis results of signal characteristics, the FPF measurement system was designed and tested. The equivalent instrument sensitivity can reach
10 pT/(Hz)1/2in the frequency range corresponding to the magnetic field of
4060 μT, and the system can work continuously for a long time. The system operates stably with independence within the temperature range of 275 350 K and can be further enhanced through an external temperature control system. Regarding measurement stability, the median repeatability of frequency measurement results exceeds 0.6 Hz. The FPF measurement system effectively addresses the complexities associated with the free precession signal and meets the magnetic field measurement requirements of the Bell–Bloom atomic magnetometer.
An in situ simultaneous measurement of magnetic coil constants and nonorthogonal angles method is proposed using the atomic magnetometer. Based on the transient response of the atomic spin polarization under the different magnetic fields, we establish the corresponding measurement model of magnetic coil constants and nonorthogonal angles through the relationship between the Larmor precession frequency and the external magnetic field. By this model, we experimentally obtained the coil constants of x , y, and z axes are, respectively, 9.04 ± 0.10, 8.94 ± 0.07, and 4.34 ± 0.02 nT / V in our system. The nonorthogonal angles are 89.04 deg between x axis and y axis, 88.60 deg between y axis and z axis, and 91.15 deg between z axis and x axis. This method can be used to simultaneously measure magnetic coil constants and nonorthogonal angles without relying on external instruments, which provides a basic guarantee for the accurate calculation sensitivity of spin-exchange-relaxation-free atomic magnetometers, especially for miniaturized atomic magnetometers.
Spin-exchange-relaxation-free (SERF) atomic magnetometer is the most sensitive magnetic detector in the world at present due to the fact that the broadening of spin-exchange collision is eliminated by operating at high alkali-mental density and low magnetic field. On that condition, the precession frequency of high-density alkali vapor becomes a constant fractional value of Larmor frequency, and the width of magnetic-resonance line gets narrower. This paper analyzes the principle of SERF regime and proposes an innovation based on measuring the slowing-down Larmor frequency directly instead of the complicated conventional method based on the width of magnetic resonance line. We calculated and simulated the slowing-down precession frequency and linewidth of magnetic resonance on certain conditions. The experimental results have verified that the method is valid to identify the regime of SERF of atomic magnetometer via detecting the slowing-down precession frequency.