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IEEE Transactions on Instrumentation and Measurement

Triaxial Vector Operation in Near-Zero Field of Atomic Magnetometer With Femtotesla Sensitivity

Fei Lu, Jixi Lu, Bo Li, Yeguang Yan, Shaowen Zhang, Kaifeng Yin, Mao Ye, Bangcheng Han

Published Jan 1, 2022• 23 citations
Publisher

Abstract

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 of$B_{z-\mathrm {offset}}= 8$nT and$B_{x-\mathrm {offset}} = 2$nT. The bandwidth alongx- axis was extended to 232 Hz under$B_{z-\mathrm {offset}} = 32$nT, which can further be improved by increasing$B_{z-\mathrm {offset}}$. 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 increasing$B_{z-\mathrm {offset}}$and$B_{x-\mathrm {offset}}$, 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.

DOI
10.1109/tim.2022.3162282

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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 display a single-beam triaxial spin-exchange relaxation-free (SERF) atomic magnetometer by utilizing two orthogonal transverse modulation fields perpendicular to the light. The magnetic field’s three-axis vector components can be extracted simultaneously and independently from the transmitted light by lock-in amplifiers with reference to the different demodulated frequencies. We find that the residual magnetic field is a significant factor that could affect triaxial coupling, and specially the influence of the z -axis remanence on xy -axis coupling is studied in detail. Under the optimal parameters, the three-axis sensitivities are 20 fT Hz − 1 / 2 in the x - and y - axis, and 230 fT Hz − 1 / 2 in the z -axis. Meanwhile, the three-axis bandwidths are 110 Hz, 138 Hz and 211 Hz in the x -, y - and z -axis, respectively. The proposed single-beam triaxial SERF atomic magnetometer could be used to construct a compact and low-cost atomic sensor that is particularly suitable for the vector measurement of the biomagnetic such as magnetoencephalography and magnetocardiography.
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.
Abstract High-sensitivity triaxial magnetic field measurements are crucial in various scientific and industrial applications, with spin-exchange relaxation-free (SERF) atomic magnetometers (AMs) standing out for their exceptional sensitivity. However, conventional methods for achieving triaxial measurements in SERF magnetometers typically involve multiple modulated magnetic fields, which introduce crosstalk challenges in array-based configurations. We propose a static bias magnetic field assisted optical modulation triaxial AM to address this limitation. In this configuration, the detuned circularly polarized light is utilized to achieve synchronous modulation of the fictitious magnetic field and pumping rate while incorporating a static bias magnetic field to generate a transverse projection component. We develop a comprehensive triaxial response model that fully considers the effects of the longitudinal spin polarization deflection. Building upon this model, we derive the relationship between the transverse spin component and the triaxial magnetic field and introduce parameter optimization methods to obtain optimal parameters, thus enabling triaxial measurements of 12 fT Hz −1/2 , 10 fT Hz −1/2 , and 15 fT Hz −1/2 along the x-, y -, and z -axes. Our approach offers significant advantages over existing methods, particularly in minimizing crosstalk and mitigating spin-exchange relaxation introduced by multiple modulated magnetic fields. Moreover, the effective detection of simulated magnetocardiography signals highlights the practical applicability and reliability of the proposed method in real-world scenarios.
A single-beam atomic magnetometer being operated near zero-field and measuring three-axis fields simultaneously is demonstrated. We produce a rotating field on the x-0-y plane with the frequency of 90 Hz and a modulation field in the z axis at 130 Hz. The rotating field enables a nonzero z axis output when the transverse fields are zeroed using feedback systems. Based on the phase difference of π/2, x and y axes fields can be measured using one lock-in amplifier. Magnetic field sensitivities of 300 fT/Hz1∕2 in x and y axes and 3 pT/Hz1∕2 in the z axis are achieved.
The high-sensitivity three-axis detection of magnetic fields is widely used in various applications.Our study demonstrates an atomic magnetometer detecting vector magnetic field, of which the core components are one glass cell and two elliptically polarized laser beams.The light-intensity noise is suppressed by differential detection technology, so a higher sensitivity is obtained compared with our previous work.The three-axis average sensitivities of the atomic magnetometer simultaneously reached 14 fT/Hz 1/2 along the x axis, 11 fT/Hz 1/2 along the y axis, and 25 fT/Hz 1/2 along the z axis.Furthermore, the atomic magnetometer's dynamic range was further improved to 150 nT, and its bandwidth was increased to over 200 Hz using a closed-loop control system.The proposed three-axis atomic magnetometer with a large dynamic range and a wide bandwidth holds great promise for biomagnetic measurement in a more challenging environment.
Spin-exchange relaxation-free (SERF) atomic magnetometers operated under a near-zero magnetic field are used for vector magnetic field measurements with high sensitivity. Previously, the cross-axis coupling error evoked by a nonzero background magnetic field has been verified to be adverse in modulated single-beam magnetometers. Here, in a dual-beam unmodulated SERF magnetometer, we propose a somewhat different solution model for the cross-axis coupling effect where the field of interest couples with the interference field. Considering two cases where the transverse or longitudinal background field exists, the cross-axis coupling effect dependence on multiple factors is investigated here based on the dynamic response under a background magnetic field within ±5 nT. The theoretical and experimental investigation suggests that it has an adverse impact on the output response amplitude and phase and tilts the sensitive axis by several degrees, causing a measurement error on the dual-beam magnetometer. To suppress this effect, the background magnetic field is compensated through the PI closed-loop control. The coupling effect is effectively suppressed by 1.5 times at the 10–40 Hz low-frequency band and the sensitivity reaches 2.4 fT/Hz1/2.