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arXiv:2106.05877 · Quantum Science and Technology

A modular optically pumped magnetometer system

Thomas Coussens, Aikaterini Gialopsou, C. Abel, Mark G. Bason, Tim M. James, W Evans, Michael T. M. Woodley, Daniel Nightingale, Denilson Nicolau, L Page, Fedja Oručević, P. Krüger

Published May 21, 2024• 11 citations
arXivHTMLPDF

Abstract

Abstract To address the demands in healthcare and industrial settings for spatially resolved magnetic imaging, we present a modular optically pumped magnetometer (OPM) system comprising a multi-sensor array of highly sensitive quantum magnetometers. This system is designed and built to facilitate fast prototyping and testing of new measurement schemes by enabling quick reconfiguration of the self-contained laser and sensor modules as well as allowing for the construction of various array layouts with a shared light source. The modularity of this system facilitates the development of methods for managing high-density arrays for magnetic imaging. The magnetometer sensitivity and bandwidth are first characterised in both individual channel and differential gradiometer configurations before testing in a real-world magnetoencephalography environment by measuring alpha rhythms from the brain of a human participant. We demonstrate the OPM system in a first-order axial gradiometer configuration with a magnetic field gradient sensitivity of 10 fT / cm / Hz at a baseline of 4.5 cm. Single-channel operation achieved a sensitivity of 65 fT / Hz . Bandwidths exceeding 200 Hz were achieved for two independent modules. The system’s increased temporal resolution allows for the measurement of spinal cord signals, which we demonstrate by using phantom signal trials and comparing with an existing commercial sensor.

DOI
10.1088/2058-9565/ad4e60

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Abstract To address the demands in healthcare and industrial settings for spatially resolved magnetic imaging, we present a modular optically pumped magnetometer (OPM) system comprising a multi-sensor array of highly sensitive quantum magnetometers. This system is designed and built to facilitate fast prototyping and testing of new measurement schemes by enabling quick reconfiguration of the self-contained laser and sensor modules as well as allowing for the construction of various array layouts with a shared light source. The modularity of this system facilitates the development of methods for managing high-density arrays for magnetic imaging. The magnetometer sensitivity and bandwidth are first characterised in both individual channel and differential gradiometer configurations before testing in a real-world magnetoencephalography environment by measuring alpha rhythms from the brain of a human participant. We demonstrate the OPM system in a first-order axial gradiometer configuration with a magnetic field gradient sensitivity of 10 fT / cm / Hz at a baseline of 4.5 cm. Single-channel operation achieved a sensitivity of 65 fT / Hz . Bandwidths exceeding 200 Hz were achieved for two independent modules. The system’s increased temporal resolution allows for the measurement of spinal cord signals, which we demonstrate by using phantom signal trials and comparing with an existing commercial sensor.
We demonstrate a compact single-beam optically pumped magnetometer (OPM) employing a pure 87Rb microfabricated vapor cell with 500 Torr buffer gas. The residual magnetic field is actively minimized to below 400 pT through a combination of mu-metal magnetic shielding and internal compensation coils. We systematically characterize the OPM response as a function of laser detuning and optical intensity, and derive the magnetic-field sensitivity from the signal slope and noise amplitude spectral density. A best sensitivity of 11 fT/Hz^1/2 is achieved near the resonance transition at an optimal optical intensity, using a small cell volume of only 0.027 cm^3. Furthermore, a miniaturized OPM prototype was developed using a folded flexible printed circuit board (FPCB) to integrate the vapor cell, a compact heater, and magnetic-field coils into a cubic sensing head. Two coil configurations patterned directly on the FPCB were investigated. Under optimized operating conditions, the OPM prototype achieved a sensitivity of 66 fT/Hz^1/2. These results demonstrate that high-buffer-pressure single-beam OPMs can achieve excellent sensitivity, establishing a pathway toward compact and scalable arrays.
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.
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.
The ability to measure the magnetic fields around celestial bodies enables various insights into processes and phenomena deep within such bodies.The use of magnetometry in space missions dates back to the 1950s, and a number of different technologies have been used to measure these fields, ranging from fluxgates to Anisotropic Magneto Resistance (AMR) sensors.Optically-pumped magnetometers (OPMs) represent the state-of-the-art in magnetic field sensitivity. 1 Such sensors rely upon the interaction of laser light to spin polarise a warm vapour of neutral atoms.Detecting the precession of these spins, again with laser light, is a direct way to measure a magnetic field and is relatable to fundamental constants.Improvements in quantum technology within the last decade have led to the miniaturisation of these sensors.We have now reached the point where compact, lightweight, and low power devices offer greater sensitivity to magnetic fields than any other technology.In this work, we identify the following four key space application areas in which OPMs may offer significant advantages over existing technologies: Environments in which high sensitivity is needed at low magnetic fields (10s of nT), full vector read-out with high directional sensitivity and reduced SWaP compared to helium magnetometers, miniaturised and lightweight scalar magnetometers to complement vector fluxgate measurements, vector or scalar sensors that need high (40 dB) gradiometer performance.This work identifies OPM configurations for each area and makes a first estimate of size, weight and power requirements for each.By enabling improvements in magnetic field sensitivity, this represents an exciting prospect for precision magnetic field measurements while also saving mass and reducing complexity.
Cardiomagnetometry is a growing field of noninvasive medical diagnostics that has triggered a need for affordable high-sensitivity magnetometers. Optical pumping magnetometers (OPMs) are promising candidates for satisfying that need since it has been demonstrated that they can be used to map the magnetic field of the beating human heart. We discuss the principle of a phase-detecting OPM and describe the procedures used to optimize its performance. The optimized OPM has an intrinsic magnetometric sensitivity of 63 fT/Hz^1/2 and a measurement bandwidth of 140 Hz with a spatial resolution of 28 mm, measured in a weakly shielded environment. We further discuss the fundamental limitations of frequency- and phase-detecting magnetometers on the basis of information theory.
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.