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Review of Scientific Instruments

In situ triaxial magnetic field compensation for the spin-exchange-relaxation-free atomic magnetometer

Jiancheng Fang, Jie Qin

Published Oct 1, 2012• 69 citations
Publisher

Abstract

The spin-exchange-relaxation-free (SERF) atomic magnetometer is an ultra-high sensitivity magnetometer, but it must be operated in a magnetic field with strength less than about 10 nT. Magnetic field compensation is an effective way to shield the magnetic field, and this paper demonstrates an in situ triaxial magnetic field compensation system for operating the SERF atomic magnetometer. The proposed hardware is based on optical pumping, which uses some part of the SERF atomic magnetometer itself, and the compensation method is implemented by analyzing the dynamics of the atomic spin. The experimental setup for this compensation system is described, and with this configuration, a residual magnetic field of strength less than 2 nT (±0.38 nT in the x axis, ±0.43 nT in the y axis, and ±1.62 nT in the z axis) has been achieved after compensation. The SERF atomic magnetometer was then used to verify that the residual triaxial magnetic fields were coincident with what were achieved by the compensation system.

DOI
10.1063/1.4756046

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The spin-exchange-relaxation-free (SERF) atomic magnetometer is an ultra-high sensitivity magnetometer, but it must be operated in a magnetic field with strength less than about 10 nT. Magnetic field compensation is an effective way to shield the magnetic field, and this paper demonstrates an in situ triaxial magnetic field compensation system for operating the SERF atomic magnetometer. The proposed hardware is based on optical pumping, which uses some part of the SERF atomic magnetometer itself, and the compensation method is implemented by analyzing the dynamics of the atomic spin. The experimental setup for this compensation system is described, and with this configuration, a residual magnetic field of strength less than 2 nT (±0.38 nT in the x axis, ±0.43 nT in the y axis, and ±1.62 nT in the z axis) has been achieved after compensation. The SERF atomic magnetometer was then used to verify that the residual triaxial magnetic fields were coincident with what were achieved by the compensation system.
The spin-exchange-relaxation-free(SERF)atomic magnetometer with ultrahigh sensitive becomes the most effective measurement to analyze the component of the planet in space exploration.In order to improve the sensitivity of the SERF atomic magnetometer,we must avoid the influence on the SERF regime disturbed by environment magnetic field.In this paper we design an active compensation system based on in-situ magnetic measurement to compensate the magnetic disturbance.Firstly,we get the three-axis magnetic field information according to the measurement of pop-laser,then control the current of coil to generate a magnetic field which is opposite and equal to the disturbance,to keep the SERF regime.Finally,we validat the method with the atomic magnetometer experimental setup.The result shows that compared with the manual compensation method,the system can track the magnetic compensation point in real time,reduce the signal noise and compensate the magnetic disturbance successfully,validating the efficiency of the active compensation system and establishing the technique foundation of the production.
Maintaining a near-zero magnetic field condition is a requisite for atomic magnetometer under the spin-exchange relaxation-free (SERF) regime. Therefore, the precise determination of the residual magnetic fields is of great significance. Herein, we proposed an in-situ measurement method of residual magnetic fields in a dual-beam atomic magnetometer based on a transient response of the transverse polarization along the probe beam direction after switching off the pump beam. Proceeding from the Bloch equation, we established a complete evolution expression of the polarization vector under any static residual magnetic field with the pump beam switched off. On this basis, we analyzed the transient response of the transverse polarization by numerical simulation under different situations of residual magnetic fields. Besides, we also considered two possible conditions when the residual magnetic fields are relatively large or small, respectively. Accordingly, we presented an efficient and convenient estimation method of the residual magnetic fields based on certain time characteristics of the transient response. We verified the feasibility of our method by experiment and achieved a triaxial residual magnetic fields measurement within 0.5 s. This method can realize an in situ and real-time measurement of the residual magnetic fields in a fast and convenient way. Furthermore, this method can evaluate the effectiveness of different magnetic field compensation methods without introducing any modulation or extra devices.
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.
We propose a novel method, which leverages the atomic signal itself, to achieve rapid in situ magnetic field compensation in an unknown geomagnetic environment for an unshielded spin-exchange relaxation-free atomic magnetometer. This method employs a strong bias magnetic field aligned with the pump beam direction to maximize the strength of the magnetic field resonance signal. The compensation process is divided into three sequential stages, each focusing on one of the x, y, and z axes by adjusting the magnitude of the bias magnetic field. As a result, the zero magnetic field is identified within 11 seconds, with compensation resolution smaller than 3.0 nT, primarily constrained by the laser power stability. This approach not only enhances the applicability of atomic magnetometers in unshielded environment but also proves effective for in situ magnetic field compensation in shielded environment, such as those used in magnetoencephalography, magnetocardiography, and space science.
Compact atomic magnetometer has become an important approach for noise measurement of magnetic shielding systems as well as biomagnetic measurement. In this study, we propose a novel method that enables time-shard triaxial magnetic field measurements in near-zero magnetic field through a compact high-performance atomic magnetometer with femtotesla sensitivity. With pump–probe orthogonal configuration, the magnetometer can be operated without optical and magnetic modulation. They-axis magnetic field measurement was realized in zero field, whereas thex- andz- axes magnetic fields were measured by the cross-application of the bias magnetic fields operating in near-zero field. The maximum response amplitude and sensitivity with bandwidths of 46.8 Hz (x- axis), 10.5 Hz (y-axis), and 10.9 Hz (z-axis) are achieved under bias magnetic fields ofBz−offset=8B_{z-\mathrm {offset}}= 8Bz−offset​=8nT andBx−offset=2B_{x-\mathrm {offset}} = 2Bx−offset​=2nT. The bandwidth alongx- axis was extended to 232 Hz underBz−offset=32B_{z-\mathrm {offset}} = 32Bz−offset​=32nT, which can further be improved by increasingBz−offsetB_{z-\mathrm {offset}}Bz−offset​. The linear interval of measurable magnetic field are between −11.7 to +11.7 nT (x- axis), −0.8 to +0.8 nT (y-axis), and −28.2 to +28.2 nT (z-axis). Forx- andz-axes, the measurable interval can further be improved by increasingBz−offsetB_{z-\mathrm {offset}}Bz−offset​andBx−offsetB_{x-\mathrm {offset}}Bx−offset​, respectively. The triaxial sensitivities achieved through our method reached 3.1 ± 0.3 fT/Hz1/2(x- axis), 2.0 ± 0.1 fT/Hz1/2(y- axis), and 6.0 ± 0.5 fT/Hz1/2(z- axis). This system is measured in a low-noise ferrite magnetic shield to ensure extra-low magnetic noise condition. Furthermore, the non-modulation triaxial vector operation demonstrated in this study is advantageous for reducing interference and spin-exchange relaxation caused by magnetic field modulation.
Abstract We demonstrate a method for quickly and automatically detecting all three components of a remanent magnetic field around a shielded spin-exchange relaxation-free (SERF) atomic magnetometer (AM) using the trisection algorithm (TSA) for zero-field resonance (ZFR). To satisfy the measurement of AMs, a resonance light of the 87 Rb D1 line with a spectral width of less than 1MHz is converted to circular polarization by a linear polarizer and a quarter-wave plate. After the light beam has passed through the alkali metal vapor cell, the residual magnetic field can be measured by searching for triaxial ZFR optical peaks. The TSA stably reduces the measurement time to 2.41 s on average and improves the measurement accuracy, significantly outpacing existing methods. The weighted averages of all measurements with corresponding uncertainties are (−15.437 ± 0.022)nT, (6.062 ± 0.021)nT, and (−14.158 ± 0.052)nT on the x -, y -, and z -axes, respectively. These improvements could facilitate more extremely weak magnetic studies in real time, such as magnetoencephalography (MEG) and magnetocardiography (MCG) measurements.