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Journal of the Optical Society of America B

Wide-range and self-locking atomic magnetometer based on free spin precession

Pei-Xian Miao, Wenqiang Zheng, Shiyu Yang, Bin Wu, Bing Cheng, Jianhui Tu, Hong-Liang Ke, Wei Yang, Ji Wang, Cui Jing-zhong, Qiang Lin

Published Mar 4, 2019• 11 citations
Publisher

Abstract

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.

DOI
10.1364/josab.36.000819

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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.
We report a rubidium atomic magnetometer based on pump-probe nonlinear magneto-optical rotation. The rubidium vapor cell is placed in a five-layer magnetic shield with inner coils that can generate uniform magnetic fields along the direction of pump beam, and the cell is also placed in the center of a Helmholtz coil that can generate an oscillating magnetic field perpendicular to the direction of pump beam. The atoms are optically pumped by circularly polarized pump beam along a constant magnetic field in a period of time, then the pump beam is turned off and a /2 pulse of oscillating magnetic field for 87Rb atoms is applied. After the above process, the individual atomic magnetic moments become phase coherent, resulting in a transverse magnetization vector precessing at the Larmor frequency in the magnetic field. The linearly polarized probing beam is perpendicular to the direction of magnetic field, and can be seen as a superposition of the left and right circularly polarized light. Because of the different absorptions and dispersions of the left and right circularly polarized light by rubidium atoms, the polarization direction of probing beam rotates when probing beam passes through rubidium vapor cell. The rotation of the polarization is subsequently converted into an electric signal through a polarizing beam splitter. Finally, the decay signal related to the transverse magnetization vector is measured. The Larmor frequency proportional to magnetic field is obtained by the Fourier transform of the decay signal. The value of magnetic field is calculated from the formula:B=(2/) f, where and f are the gyromagnetic ratio and Larmor frequency, respectively. In order to measure the magnetic field in a wide range, the tracking lock mode is proposed and tested. The atomic magnetometer can track the magnetic field jump of 1000 nT or 10000 nT, indicating that the atomic magnetometer has strong locking ability and can be easily locked after start-up. The main performances in different magnetic fields are tested. The results show that the measurement range of the atomic magnetometer is from 100 nT to 100000 nT, the extreme sensitivity is 0.2 pT/Hz1/2, and the magnetic field resolution is 0.1 pT. The transverse relaxation times of the transverse magnetization vector in different magnetic fields are obtained, and the relaxation time decreases with the increase of the magnetic field. When the measurement range is from 5000 nT to 100000 nT, the magnetic field sampling rate of the atomic magnetometer can be adjusted in a range from 1 Hz to 1000 Hz. The atomic magnetometer in high sampling rate can measure weak alternating magnetic field at low frequency. This paper provides an important reference for developing the atomic magnetometer with large measurement range, high sensitivity and high sampling rate.
We present an implementation of an atomic magnetometer based on free spin precession of optically pumped Rb 87 atoms. The design features a microfabricated vapor cell with an effective volume of 7.1 mm 3 containing isotopically enriched Rb 87 and nitrogen buffer gas. The magnetometer is designed to operate in the Earth-field range, exhibiting a magnetic noise floor of about 147 fT / Hz at field strengths comparable to Earth's magnetic field, i.e., around 50 μ T . This is in close agreement with the Cramér-Rao lower bound of 118 fT / Hz . The good sensitivity as well as the high dynamic range enables a wide range of applications, including magnetocardiography and geomagnetic measurements.
Atomic magnetometer operating in spin-exchange-relaxation-free (SERF) regime has a wide range of applications due to its ultra-high sensitivity. However, the requirement of operating in near-zero magnetic field restricts some of the applications which need the atomic magnetometer to work in the high field environment such as monitoring the magnetic anomalies. A novel scheme of the chip-scale SERF atomic magnetometer operating in geomagnetic field without magnetic shield is demonstrated here. A 18 bits high-precise digital-to-analog converter (DAC) is applied to ensure the stability of the compensated magnetic field. The fluctuation of the compensated magnetic field is less than 8.3nT and a measurement sensitivity of 12.4pT/Hz1/2 is achieved in the laboratory environment without any magnetic shields.
The dead zones of atomic magnetometers bring great challenges for the mobile applications in high-sensitivity magnetic field measurement. To solve this fundamental problem, we present a dead-zone-free orientation-based atomic magnetometer employing free-induction-decay (FID) scheme. A modulation scheme combining synchronous optical pumping and radio frequency pulse manipulation is utilized to achieve the free precession of spin polarization in all magnetic field orientations, and the FID signals are detected in two directions with an orthogonal pump-probe light configuration. We perform a comprehensive theoretical analysis of spin polarization evolution for arbitrary magnetic field orientations, and both simulation and experimental results demonstrate that the fusion of information from the pump and probe beams enables the dead-zone-free measurement of the magnetometer, with the sensor exhibiting a sensitivity in the range of 0.6 – 1.4 pT / Hz 1 / 2 across all directions. The proposed scheme is of great significance for the advancement of pulsed atomic magnetometers and their various applications in mobile platforms.
Spin-exchange relaxation-free (SERF) atomic magnetometers are the promising non-cryogenic sensors to potentially replace the superconducting quantum interference device (SQUID) in the ultra-weak magnetic-field measurements. Although the SERF magnetometers have achieved a sensitivity of sub-femtotesla, the bandwidth is limited at a narrow range. In this work, we developed a closed-loop SERF magnetometer under proportional-integral (PI) control, the bandwidth could be adaptively tuned and extended above 1.2 kHz. A nearly 40-fold enhancement of the typical bandwidth was achieved in comparison with the open-loop system and the sensitivity was maintained at 15 fT/Hz1/2from 30 Hz to 750 Hz. Moreover, the magnetic field dynamic range could also be greatly improved. This proposed atomic magnetometer with higher bandwidth, sensitivity, and dynamic range has a good prospect in developing a compact, portable, and non-cryogenic magnetic field sensor and may find more applications in biomedical, nuclear magnetic resonance, and fundamental physics research.
Free-induction-decay (FID) magnetometers have evolved as simple magnetic sensors for sensitive detection of unknown magnetic fields. However, these magnetometers suffer from a fundamental problem known as a “dead zone,” making them insensitive to certain magnetic field directions. Here, we demonstrate a simple experimental scheme for the dead-zone-free operation of a FID atomic magnetometer. Using a single laser beam containing equal strength of linear- and circular-polarization components and amplitude modulation at a low-duty cycle, we have synchronously pumped the rubidium (87Rb) atoms with both first- and second-order frequency harmonics. Such a pumping scheme has enabled us to observe the free Larmor precession of atomic spins at a frequency of ΩL (orientation) and/or 2ΩL (alignment) in a single FID signal, depending on the direction of the external magnetic field. We observed that the amplitude of the FID signal does not go to zero for any magnetic field direction, proving the absence of dead zones in the magnetometer. The magnetometer has a sensitivity in the range of 3.2–8.4 pT/Hz in all directions. Our experimental scheme can be crucial in developing miniaturized atomic magnetometers for various practical applications, including geomagnetic applications.