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IEEE Transactions on Instrumentation and Measurement

Efficient Measurement of Free Precession Frequency in Bell–Bloom Atomic Magnetometers

Dongxu Bai, Yuanrui Zhou, Yongze Sun, Hongfei Yang, Yanzhang Wang

Published Jan 1, 2024• 2 citations
Publisher

Abstract

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$\mathrm {10~pT/}(\mathrm {Hz})^{1/2}$in the frequency range corresponding to the magnetic field of$4060~\mu T$, 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.

DOI
10.1109/tim.2024.3417539

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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 reach10 pT/(Hz)1/2\mathrm {10~pT/}(\mathrm {Hz})^{1/2}10 pT/(Hz)1/2in the frequency range corresponding to the magnetic field of4060 μT4060~\mu T4060 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.
In order to availably solve the crosstalk of the traditionally optically pumped magnetometer,in which an oscillating magnetic field drives transitions between Zeeman magnetic sublevels,an all-optical cesium(Cs)atomic magnetometer based on Bell-Bloom structure has been investigated and demonstrated experimentally in this paper.When there is an external magnetic field,by analyzing the Larmor precession spectrum of the polarized ground state atoms induced by a modulated pump laser,the atomic magnetometer with high sensitivity is achieved.First,we present the operating principle of the magnetometer with amplitude modulation and set up the experimental system by means of a three-layer magnetic shield.Then in the case of 100 nT field by optimizing the experimental parameters,i.e.,the frequency and intensity of the pump laser,we have obtained the atomic precession spectrum with line width(full width at half maximum)of 22 Hz and signal noise ratio(SNR)of 3.2×104 per unit hertz band width,and then have evaluated the magnetometer with an intrinsic sensitivity of 100fT/Hz1/2 and spatial resolution smaller than 1.2cm,while the experimentally closed loop test shows that the cesium atomic magnetometer would have the sensitivity of 120fT/Hz1/2 and response time of 300 ms.
The Bell-Bloom-type optically pumped atomic magnetometers are well suited for weak geomagnetic field detection. However, conventional single-beam pumping introduces an atomic spin polarization gradient, which limits the measurement accuracy and sensitivity. To address this issue, this paper proposes and experimentally demonstrates a Bell-Bloom-type rubidium FID magnetometer scheme integrating orthogonally polarized counter-propagating pumping and multi-pass probe detection. This design homogenizes the atomic spin polarization distribution and suppresses light shifts and power broadening effects induced by the pump beam. Meanwhile, the five-pass probe configuration significantly enhances the signal amplitude. Experimental results reveal that, compared with the traditional single-beam pumping and single-pass detection scheme, the proposed magnetometer achieves a remarkable improvement in magnetic field measurement accuracy, and the magnetic field sensitivity is improved from 18.9 pT/\sqrt{Hz} to 3.1 pT/\sqrt{Hz}. This work provides an effective technical approach and reference for optimizing the performance of atomic magnetometers and extending their applications in integrated arrays.
Vector field measurement is demonstrated with an all-optical scalar atomic magnetometer using intrinsic parameters related to its scalar operation. The Bell-Bloom type atomic magnetometer measures the Larmor precession of cesium atoms through on-resonant absorption of a probe beam. While the AC component of the probe signal is used for the field magnitude measurement, the probe DC signal contains information about the polar angle, defined as the angle between the magnetic field and the probe beam. Additional polar angle information is obtained from the light-shift-induced magnetic field caused by the frequency modulation of the probe beam. With a measurement time of 100 milliseconds, better than 0.02 degree sensitivity has been achieved using a commercial miniaturized sensor at the optimal sensor orientation. The angle measurement accuracy is checked against an optical encoder over the entire polar angle range of 0 to 180 degrees. Better than 1 degree error is recorded over most set polar angles. Azimuthal angle measurement is also exhibited with an additional orthogonally oriented sensor.
Vector field measurement is demonstrated with an all-optical scalar atomic magnetometer using intrinsic parameters related to its scalar operation. The Bell-Bloom type atomic magnetometer measures the Larmor precession of cesium atoms through on-resonant absorption of a probe beam. While the AC component of the probe signal is used for the field magnitude measurement, the probe DC signal contains information about the polar angle, defined as the angle between the magnetic field and the probe beam. Additional polar angle information is obtained from the light-shift-induced magnetic field caused by the frequency modulation of the probe beam. With a measurement time of 100 ms, better than 0.02° sensitivity has been achieved using a commercial miniaturized sensor at the optimal sensor orientation. The angle measurement accuracy is checked against an optical encoder over the entire polar angle range of 0°–180°. Better than 1° error is recorded over most set polar angles. Azimuthal angle measurement is also exhibited with two orthogonally oriented sensors.
We propose a scheme for a Bell–Bloom atomic magnetometer based on a single beam. In our approach, the light is periodically modulated synchronously with Larmor precession. As a result, the light that interacts with atoms is alternately polychromatic and monochromatic. When the light is polychromatic, the ±1st sidebands of the light are optically resonant with the atoms, and the atoms are polarized by pumping. When the light is monochromatic, a differential detection technique extracts the Faraday-rotation signal of the light, and the polarization of the light is probed. We demonstrate an experimental implementation of the Bell–Bloom magnetometer that achieves a sensitivity for magnetic-field detection of 0.25 pT/Hz1/2 at 1 Hz. Our scheme is suitable for developing compact, miniaturized magnetometers.
The magnetic field is one of the most fundamental and ubiquitous physical observables, often conveying rich information about its generation source. Atomic magnetometers have been proven to be high-potential techniques for weak magnetic field measurement in various applications. However, due to the lack of excellent frequency-locking ability against abrupt magnetic field changes, atomic magnetometers are limited to use in application scenarios, which require both wide-range and self-locking characteristics. In this paper, theoretical and experimental research for a wide-range and self-locking rubidium atomic magnetometer based on free spin precession is presented. The rubidium atomic magnetometer can track a magnetic field jump of 10,000 nT in the measurement range from 500 nT to 100,500 nT. We also investigate the bandwidth and dead zone of the atomic magnetometer. An atomic magnetometer with excellent self-locking ability in a wide range can be developed into new highly sensitive magnetic measuring devices, enabling geomagnetic measurement in real environments and monitoring the magnetic phase transitions or coercive fields of soft magnetic materials in very magnetic-shielding environments.