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IEEE Sensors Journal

Suppressing Spatial Inhomogeneity in Spin Polarization and Light Shift in Atomic Magnetometer Array Using Frequency-Paired Counter-Propagating Pumps

Jin Peng, An‐Ning Xu, Yuanjiang Tang, Runbing Li, Mingsheng Zhan, Yong‐Chun Liu, Bei Liu, L. Deng

Published Aug 20, 2024• 5 citations
Publisher

Abstract

In spin-exchange-relaxation-free (SERF) atomic magnetometers (AMs), the attenuation of circularly polarized pump beam introduces spatial-dependent artificial magnetic fields in atomic spin polarization and light shift. In general, such a spatial-dependent effective magnetic field, especially prevalent in single-cell magnetometer arrays, cannot be uniformly compensated over the entire excitation region, resulting in a complex distributed compensation mask effect. In addition, any external effective-field-compensation methods inevitably complicate the final analysis of the unknown magnetic field measured. Here, we report a new excitation scheme that can substantially suppress spatial nonuniformity due to pump beam attenuation. Using counter-propagating pump beams with symmetrically red- and blue-detuning-paired frequencies, we demonstrate a significant reduction of spatial inhomogeneity in atomic spin polarization and light shift in comparison with the usual single-frequency unidirection and counter-propagating pump schemes. Employing this red-/blue-detuning-paired pump scheme, we construct a single-cell, four-channel SERF magnetometer array with high spatial homogeneity. We demonstrate ~10-fT/Hz$^{{1}/{2}}$sensitivity in the 5–45-Hz band for all four channels, with a gradient sensitivity of 2 fT/Hz1/2$^{{1}/{2}}$/cm in the same band. This new scheme enables the development of single-vapor-cell, high-uniformity multichannel magnetic field mapping devices with potential applications in magnetoencephalography (MEG) and magnetocardiography (MCG), or in searching for exotic spin-dependent interactions.

DOI
10.1109/jsen.2024.3443284

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In spin-exchange-relaxation-free (SERF) atomic magnetometers (AMs), the attenuation of circularly polarized pump beam introduces spatial-dependent artificial magnetic fields in atomic spin polarization and light shift. In general, such a spatial-dependent effective magnetic field, especially prevalent in single-cell magnetometer arrays, cannot be uniformly compensated over the entire excitation region, resulting in a complex distributed compensation mask effect. In addition, any external effective-field-compensation methods inevitably complicate the final analysis of the unknown magnetic field measured. Here, we report a new excitation scheme that can substantially suppress spatial nonuniformity due to pump beam attenuation. Using counter-propagating pump beams with symmetrically red- and blue-detuning-paired frequencies, we demonstrate a significant reduction of spatial inhomogeneity in atomic spin polarization and light shift in comparison with the usual single-frequency unidirection and counter-propagating pump schemes. Employing this red-/blue-detuning-paired pump scheme, we construct a single-cell, four-channel SERF magnetometer array with high spatial homogeneity. We demonstrate ~10-fT/Hz1/2^{{1}/{2}}1/2sensitivity in the 5–45-Hz band for all four channels, with a gradient sensitivity of 2 fT/Hz1/21/2^{{1}/{2}}1/2/cm in the same band. This new scheme enables the development of single-vapor-cell, high-uniformity multichannel magnetic field mapping devices with potential applications in magnetoencephalography (MEG) and magnetocardiography (MCG), or in searching for exotic spin-dependent interactions.
The ultrasensitive magnetometer has a vital importance in fundamental research and applications. Currently, the spin-exchange relaxation-free (SERF) atomic magnetometer has been reported with a sensitivity around the level of fT/Hz 1/2 . To enhance the sensitivity, a gradiometer configuration has usually been introduced to cancel the common-mode noise between two separate channels. However, the signal and response from different channels are not the same due to the attenuation of the pump beam. Here, we proposed a counter-propagating optical sideband pumping method to polarize the atoms, using the electro-optic modulator to modulate the single-pump beam, generating two symmetrically red- and blue-detuned sidebands of frequency. This scheme leads to a significant reduction of undesirable effects coming along with the optical pumping, such as light shifts and spatial inhomogeneity in atomic spin polarization. With the help of this pumping scheme, the two channels have the same magnetic response, and we have built a gradiometer atomic magnetometer with a sensitivity of 0.5 fT/Hz 1/2 ranging from 5 to 40 Hz. Our results propose the possibility of creating larger arrays of atomic magnetometers (AMs) with high sensitivity and spatial resolution based on single-vapor cells for magnetocardiography and magnetoencephalography imaging or searching for exotic spin-dependent interactions.
The light shift in a spin-exchange relaxation-free (SERF) atomic magnetometer (AM) generated by detuning of circularly polarized pump light, results in deviations of orthogonality measurement and attenuation of magnetometer response. In this paper, we propose and systematically investigate a dual-axis method to suppress light shift based on a self-developed single-beam compact SERF AM capable of measuring magnetic field components along two orthogonal sensitive axes. By sequentially observing the non-sensitive axis response and applying a magnetic field along the pump light direction, the virtual magnetic field induced by light shift is compensated and is rigorously demonstrated as a dominant component in this direction. Experimental results show a significant reduction in crosstalk between the dual sensitive axes, improving accuracy in resonant frequency determination of87Rb atoms and orthogonality evaluation of triaxial coils. Furthermore, the performance of dual-axis measurement in the SERF AM is tested, showing a more notable enhancement than that of single-axis measurement.
In recent years, arrayed atomic magnetometers have been continuously applied in biomagnetic scenarios. However, the crosstalk between adjacent sensors introduced by the modulated magnetic fields significantly compromises the imaging accuracy of arrayed atomic magnetometers. To solve this problem, a detuned square-wave optical modulation zero-field atomic magnetometer configuration is developed and we provide an analytical solution of this method. In this scheme, the circularly polarized pump light is used to polarize alkali metal atoms longitudinally, and a beam of detuned square-wave modulated circularly polarized light is applied transversely to generate a light-shift modulation instead of the coil-generated modulated magnetic field. Then the transverse magnetic field is measured by the optical absorption method of longitudinally circularly polarized pump light. This scheme maintains the inherent advantage of high sensitivity in spinexchange relaxation-free (SERF) atomic magnetometers. Additionally, the all-optical configuration eliminates the need for additional modulated magnetic fields or radio frequency (RF) fields, thereby mitigating crosstalk issues associated with modulated magnetic fields in array applications. Consequently, this scheme exhibits great potential for arraying and is expected to be employed in magnetocardiography (MCG) and magnetoencephalography (MEG).
Abstract The chip-scale hybrid optical pumping spin-exchange relaxation-free (SERF) atomic magnetometer with a single-beam arrangement has prominent applications in biomagnetic measurements because of its outstanding features, including ultrahigh sensitivity, an enhanced signal-to-noise ratio, homogeneous spin polarization and a much simpler optical configuration than other devices. In this work, a miniaturized single-beam hybrid optical pumping SERF atomic magnetometer based on a microfabricated atomic vapor cell is demonstrated. Although the optically thin Cs atoms are spin-polarized, the dense Rb atoms determine the experimental results. The enhanced signal strength and narrowed resonance linewidth are experimentally proven, which shows the superiority of the proposed magnetometer scheme. By using a differential detection scheme, we effectively suppress optical noise with an approximate five-fold improvement. Moreover, the cell temperature markedly affects the performance of the magnetometer. We systematically investigate the effects of temperature on the magnetometer parameters. The theoretical basis for these effects is explained in detail. The developed miniaturized magnetometer has an optimal magnetic sensitivity of 20 fT/Hz 1/2 . The presented work provides a foundation for the chip-scale integration of ultrahighly sensitive quantum magnetometers that can be used for forward-looking magnetocardiography (MCG) and magnetoencephalography (MEG) applications.
In recent years, atomic magnetometers (AMs) have demonstrated sub-femtotesla sensitivities and have emerged as potential replacements for superconducting quantum interference devices (SQUIDs). In an AM, a circularly polarized pump laser aligns the electron spins in a cloud of alkali metal vapor. Changes occur in the optical properties of the vapor when its collective moment interacts with an external magnetic field, resulting in a measurable polarization rotation of a linearly polarized probe beam. We present a unique pump/probe scheme in which a pump beam tuned to the87Rb D1 line (795 nm) and a probe beam tuned to the D2 line (780 nm) share a single optical axis. A dichroic waveplate is used to circularly polarize the pump beam while leaving the probe beam linearly polarized. Based upon this scheme, we have developed a small profile, fiber-coupled AM. By independently optimizing pump/probe power and detuning, we are able to achieve intrinsic single-channel sensitivity of <; 5 fT/√Hz, which is suitable for demanding applications such as magnetoencephalography (MEG). Furthermore, through straightforward adaptations of the current design, it should be possible to create dense, reconfigurable arrays of AMs for whole-head MEG.
Ultra-sensitive multi-channel optically pumped atomic magnetometers based on the spin-exchange relaxation-free (SERF) effect are powerful tools for applications in the field of magnetic imaging. To simultaneously achieve ultra-high spatial resolution and ultra-high magnetic field sensitivity, we proposed a high-resolution multi-channel SERF atomic magnetometer for two-dimensional magnetic field measurements based on a digital micro-mirror device (DMD) as the spatial light modulator for a single vapor cell. Under the optimal experimental conditions obtained via spatial and temporal modulation of the probe light, we first demonstrated that the average sensitivity of the proposed 25-channel magnetometer was approximately 25fT/Hz 1/2 with a spatial resolution of 216µm. Then, we measured the magnetic field distribution generated by a gradient coil and compared the experimentally obtained distributions with those calculated via finite element simulation. The obtained g value of 99.2% indicated good agreement between our experimental results and the theoretical calculations, thereby confirming that our proposed multi-channel SERF magnetometer was effective at measuring magnetic field distributions with an ultra-high spatial resolution.