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Photonic Sensors

Fast In-Situ Triaxial Remanent Magnetic Field Measurement for Single-Beam SERF Atomic Magnetometer Based on Trisection Algorithm

Tengyue Long, Bangcheng Han, Xinda Song, Yuchen Suo, Le Jia, Wen Ren

Published Jan 1, 2022• 26 citations
Publisher

Abstract

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.

DOI
10.1007/s13320-023-0684-y

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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.
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.
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.
We demonstrate a single-beam atomic magnetometer (AM) capable of measuring a three-axis magnetic field with high-sensitivity, achieved by applying a small DC offset field and a high frequency modulation field. To satisfy the miniaturization demand of AMs, an elliptically polarized light detuned by 50 GHz from the resonance transition center is employed. The circularly polarized component is used to polarize the alkali-metal atoms, while the linearly polarized light is used to detect the dynamics of the polarized spin under a magnetic field. Based on theoretical analysis, parameters that significantly affect the performance are optimized, and a sensitivity of 20 fT/Hz 1/2 in x -axis, 25 fT/Hz 1/2 in y -axis, 30 fT/Hz 1/2 in z -axis is achieved with a miniature 4 × 4 × 4 mm 87 Rb vapor cell. Moreover, we also verify that the operation principle of AMs can be used to null background magnetic fields in-situ with isotropic compensation resolution of 6.7 pT, which provides an effectively precise method for zeroing ambient magnetic field. The high-sensitivity operating of an elliptically-polarized-laser-based magnetometer provides prospective futures for constructing a compact, low-cost AM, which is particularly applicable for non-invasive bio-magnetic imaging such as array-based magnetoencephalography (MEG) and magnetocardiography (MCG).
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.
This study proposes an in situ method for measuring the equilibrium spin polarization and its longitudinal spatial distribution in a spin-exchange relaxation-free (SERF) atomic magnetometer. The total relaxation rate was determined by analyzing the steady-state response of the magnetometer to the triaxial magnetic field. The transfer coefficient between spin polarization along the probe beam axis and the steady-state response was derived from the optical depth of the probe light and the incident/transmitted power through the vapor cell. By applying a DC magnetic field along the sensitive axis and combining the total relaxation rate with the transfer coefficient, the equilibrium spin polarization value under zero-field conditions was obtained. The equilibrium spin polarization distribution along the pump beam propagation direction was measured, showing consistent trends with simulation results, with a maximum relative error of less than 5%. The expanded uncertainty of the proposed equilibrium spin polarization measurement method ranges from 0.8% to 9.7%. The advantages of this method include no requirement for additional optical field or magnetic field modulation, immunity to temperature and temperature gradients, and the capability to directly determine the equilibrium spin polarization value.
Atomic magnetometers (AMs) are used in various fields, including magnetocardiography and magnetoencephalography. In AMs, accurately distinguishing and measuring the light-shift equivalent magnetic field generated by the pump beam from the remanent field within the central area of a magnetic shield has been a significant challenge. This paper presents a novel measurement method that addresses this issue using87Rb AMs. By exploiting the distinctive responses of the magnetometer to oscillating magnetic fields under different pump photon spin polarization vectors s and employing a fitting technique at the resonance peak, we determined both the residual field and light shift. Furthermore, we conducted comprehensive numerical simulations to analyze the light shift of the pump beam under various conditions, e.g., photon spin vectors, intensities, and frequency detuning. Additionally, experimental measurements were performed to investigate the pressure broadening effect on the D1 line of the cell. The proposed method allows precise measurement of remanent fields while eliminating the influence of light shift; thus, it is useful for high-precision atomic device measurements in diverse experimental settings.