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Proceedings of IEEE Sensors/Proceedings of IEEE Sensors ...

Magnetic Field Compensation for An Optically Pumped Magnetometer Without Iteration

Yaqiong Niu, Chaofeng Ye

Published Oct 29, 2023• 1 citation
Publisher

Abstract

The most efficient way to improve the sensitivity of an optically pumped magnetometer (OPM) is to maximize the spin-polarization lifetime of the atoms. The spin exchange relaxation-free (SERF) OPM that operates in zero field environment is one of the most sensitive magnetic sensors. This paper proposes a new three-axis magnetic field compensation method for the single-beam OPM, which zeroes the magnetic field around the sensor to make it work in SERF regime. The method simultaneously employs two modulation magnetic fields and two low-frequency sweeping magnetic fields along two directions perpendicular to the laser beam during the compensation process. The output signal of the photodiode is demodulated at different frequencies, resulting in two outputs that reflect the three components of the magnetic field. This method does not require iterate to tune the three-axis field, which increases the speed of the compensation process significantly. The magnetic field resolutions of the method for the x-axis, y-axis and z-axis are 0.6 pT, 0.6 pT and 5 pt, respectively. A compact OPM with noise density about 20 fT / Hz1/2@10 Hz is developed and tested, demonstrating the feasibility of the method.

DOI
10.1109/sensors56945.2023.10325002

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The most efficient way to improve the sensitivity of an optically pumped magnetometer (OPM) is to maximize the spin-polarization lifetime of the atoms. The spin exchange relaxation-free (SERF) OPM that operates in zero field environment is one of the most sensitive magnetic sensors. This paper proposes a new three-axis magnetic field compensation method for the single-beam OPM, which zeroes the magnetic field around the sensor to make it work in SERF regime. The method simultaneously employs two modulation magnetic fields and two low-frequency sweeping magnetic fields along two directions perpendicular to the laser beam during the compensation process. The output signal of the photodiode is demodulated at different frequencies, resulting in two outputs that reflect the three components of the magnetic field. This method does not require iterate to tune the three-axis field, which increases the speed of the compensation process significantly. The magnetic field resolutions of the method for the x-axis, y-axis and z-axis are 0.6 pT, 0.6 pT and 5 pt, respectively. A compact OPM with noise density about 20 fT / Hz1/2@10 Hz is developed and tested, demonstrating the feasibility of the method.
In this study, we demonstrate a non-modulated triaxial magnetic field compensation method for spin-exchange relaxation-free (SERF) atomic magnetometers. We realize simultaneous compensation of the magnetic field along the pump and probe directions by maximizing the first order differential value of the zero-field resonance signal. The magnetic field perpendicular to the pump-probe plane is compensated by zeroing the DC response of the magnetometer. The zero-field resonance is obtained by applying a linearly changed magnetic field along the direction perpendicular to that of the pump-probe plane. The first order differential of the zero-field resonance is acquired using the second order central difference method in real time. Compared with the conventional compensation methods, this method does not involve modulation and demodulation. Therefore, it requires no lock-in amplifier, thus making it more applicable in miniature atomic devices. Moreover, this method compensates the magnetic field along the pump and probe directions using only one criterion. It avoids the cross-talk effect between the pump and probe directions in the presence of non-orthogonal coils. The experiment results show that the compensation resolution is 9 pT, 7 pT and 0.05 pT for the probe, pump, and direction perpendicular to the pump-probe plane, respectively.
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 magnetometers exhibit ultrahigh sensitivity for the magnetic field measurement, and their performance is further enhanced through zero-field parameter modulation. However, this modulation introduces additional parameter coupling that limits the performance of magnetometers in multi-axis measurement. In this paper, we develop a modified dual-axis output equation based on the nonlinear interaction between the optical pumping rate and modulation field amplitude, mediated by the spin-exchange relaxation and polarization. We employ a multi-objective optimization algorithm incorporating the implicit constraints among these parameters to enable efficient, simultaneous optimization of both the pump rate and modulation amplitude. Experimental results show that under optimal parameters obtained by the algorithm, the dual-axis signal responses improve by approximately 20% and 30% over conventional optimization methods, achieving 2.5 fT/Hz1/2magnetic measurement sensitivities for the y-axis and 4.0 fT/Hz1/2for the x-axis, respectively.
We propose a three-axis closed-loop optically pumped magnetometer with high sensitivity. The closed-loop magnetometer has a three-axis sensitivity of approximately 30 fT/Hz 1/2 using two orthogonal laser beams for pumping and probing the alkali metal atoms. In the closed-loop mode, the dynamic range is improved from ±5 nT to ±150 nT. The bandwidth is increased from about 100 Hz to over 2 kHz with 10 kHz modulation fields in x - and y -axes and another 6 kHz modulation field along the z -axis. Compared with single-axis or dual-axis magnetometers, the proposed magnetometer not only provides the direction and magnitude of the magnetic field but also has high robustness in a challenging environment. The magnetometer has applications in biomagnetic measurements, magnetic resonance imaging, and fundamental physics.
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).
A miniature wide-range three-axis vector atomic magnetometer under spin-exchange relaxation-free regime has been developed. With a cylindrical vapor cell of size 10 mm in diameter and length, we have constructed a single-beam configuration through a polarization-maintaining fiber carrying the laser beam to the sensor and a multimode fiber guiding the transmitted light field to the photodiode. A rotating field on the plane perpendicular to the direction of propagation of the laser and another modulation field in the pump direction, which are generated in the field programmable gate array using direct digital synthesis to measure the three components of the magnetic field simultaneously. The spin-exchange relaxation is suppressed in zero field controlled by a feedback system, thus a wide range and excellent accuracy of three-axis vector atomic magnetometer response is obtained. Consequently, the transverse magnetic field sensitivity of 350 fT/Hz1/2and longitudinal field sensitivity of 3pT/Hz1/2are achieved with a dynamic range of 60000 nT. This sensor with compact size, wide range and high sensitivity is particularly suitable for geomagnetic observatory and space exploration.