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IEEE Transactions on Instrumentation and Measurement

Dual-Axis Closed Loop of a Single-Beam Atomic Magnetometer: Toward High Bandwidth and High Sensitivity

Junjian Tang, Yueyang Zhai, Binquan Zhou, Bangcheng Han, Gang Liu

Published Jan 1, 2021• 94 citations
Publisher

Abstract

Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed cross-talk effect, achieved by applying a rotating modulation field and using of the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 × 4 × 4 mm 87Rb cell, we achieve magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis to the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the cross-talk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for the noninvasive biomagnetic imaging.

DOI
10.1109/tim.2021.3112797

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Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed cross-talk effect, achieved by applying a rotating modulation field and using of the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 × 4 × 4 mm 87Rb cell, we achieve magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis to the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the cross-talk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for the noninvasive biomagnetic imaging.
Atomic magnetometer (AM) operated in a spin-exchange relaxation-free (SERF) regime features superior sensitivity and non-cryogenic operation, and thus is competitive with the best superconducting quantum interference devices. Previously, SERF AM with fT/Hz 1/2 level sensitivity commonly acted as a single-axis sensor. Here we demonstrate a dual-axis SERF AM capable of simultaneously and independently detecting x - and y -field components with a sensitivity of 20 fT/Hz 1/2 . As there is no necessity to worry about the cross-talk effects arising from field modulations, the dual-axis scheme proposed here is of particular interest to AM array and hence the biomagnetic applications.
Atomic magnetometers operated in the spin-exchange relaxation-free (SERF) regime are the promising sensor to replace superconducting quantum interference devices (SQUIDs) in the biomagnetism field. The SERF magnetometer with compact size and good performance is crucial to the new generation of wearable magnetoencephalography (MEG) system. In this paper, we developed a compact and closed-loop SERF magnetometer with the dimensions of 15.0×22.0×30.0 mm3 based on a single-beam configuration. The bandwidth of the magnetometer was extended to 675 Hz while the sensitivity was maintained at 22 fT/Hz1/2. A nearly 3-fold enhancement of the bandwidth was obtained in comparison with the open-loop control. The implementation of the closed-loop control also greatly improved the dynamic range, enabling the magnetometer to be robust against the disturbance of the ambient field. Moreover, the magnetometer was successfully applied for the detection of human α-rhythm and auditory evoked fields (AEFs), which demonstrated the potential to be extended to multi-channel MEG measurements for future neuroscience studies.
Spin-exchange relaxation-free (SERF) atomic magnetometers are the promising non-cryogenic sensors to potentially replace the superconducting quantum interference device (SQUID) in the ultra-weak magnetic-field measurements. Although the SERF magnetometers have achieved a sensitivity of sub-femtotesla, the bandwidth is limited at a narrow range. In this work, we developed a closed-loop SERF magnetometer under proportional-integral (PI) control, the bandwidth could be adaptively tuned and extended above 1.2 kHz. A nearly 40-fold enhancement of the typical bandwidth was achieved in comparison with the open-loop system and the sensitivity was maintained at 15 fT/Hz1/2from 30 Hz to 750 Hz. Moreover, the magnetic field dynamic range could also be greatly improved. This proposed atomic magnetometer with higher bandwidth, sensitivity, and dynamic range has a good prospect in developing a compact, portable, and non-cryogenic magnetic field sensor and may find more applications in biomedical, nuclear magnetic resonance, and fundamental physics research.
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.
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.
Spin-exchange relaxation-free (SERF) atomic magnetometers usually adopt Faraday modulation to achieve better performance but suffer from laser intensity noise, thermal noise generated by the Faraday modulator, and nonmagnetic technical noise. These noises limit the sensitivity of the SERF magnetometer. We demonstrate a SERF magnetic gradiometer with dual-beam and closed-loop Faraday modulation. Operating in the SERF regime, the gradiometer utilizes an additional Faraday modulator rotation feedback to suppress probe laser intensity noise and thermal noise associated with the Faraday modulator, and it simultaneously uses dual-beam difference to cancel common-mode nonmagnetic technical noises. A gradient sensitivity of 14 fT/Hz1/2 per 1 cm gradiometer base length was achieved using Cs atoms.