A modular optically pumped magnetometer system

TL;DR

The modular optically pumped magnetometer platform addresses the need for dense, high-bandwidth biomagnetic sensor arrays by enabling rapid reconfiguration of light-source, sensor, and beam-distribution modules around a shared light source. It leverages SERF-based Hanle resonance in vapor cells and coil-based offset/modulation to achieve high sensitivity and bandwidth in compact, shielded modules. Key results include a gradient sensitivity of $10 fT/cm/sqrt(Hz)$, module bandwidths near $213(5)$ Hz and $219(4)$ Hz, MEG alpha-band measurements, and a spinal cord phantom test with improved high-frequency fidelity and lower delays compared to a commercial sensor. The platform supports rapid prototyping and benchmarking of dense OPM arrays for MEG, magnetomyography, and magnetospinography in clinical and industrial settings.

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 $\mathrm{fT/cm/\sqrt{Hz}}$. 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.

Figures (6)

• Figure 1: An example of a multi-axis magnetic gradiometer connecting 4 sensor modules in a $2\times 2$ array configuration with light source and beam distribution modules. The laser beam is conditioned with a lens, a linear polariser (LP) and a quarter-waveplate ($\lambda/4$). After passing through vapour cells, the beam power is monitored with photodiodes (PD). A non-polarising 50:50 beam splitter (50:50 BS) and a mirror ensure that the probe light from a single beam is delivered to all sensors.
• Figure 2: A schematic (top) and a photograph (bottom) of the single-axis gradiometer modular OPM consisting of a light source module and two sensor modules. Modules can be snapped together to produce different array configurations.
• Figure 3: The 1200 frequency-compensated linear spectral density of the demodulated signals from sensor module 1 (blue), sensor module 2 (red) and from the balanced photodiodes gradiometer configuration (yellow). The gradiometer configuration removes common-mode noise between the two sensors, which can be seen by the noise reduction at 50Hz, and in the broader peak in the 5575 band. Sensor module 1 has a noise-floor of 65fT/√Hz, sensor module 2, 83fT/√Hz, and the gradiometer configuration 47fT/√Hz in the 545 band.
• Figure 4: The magnetometer response amplitudes for sensor modules 1 (blue) and 2 (red) as a function of applied signal frequency. The data points are fitted to the response of a single order low pass filter. Dashed lines indicate a -3dB bandwidth of 213(5)Hz for module 1, and 219(4)Hz for module 2.
• Figure 5: Spectrogram of the 812 alpha-band response to a participant opening/closing their eyes. The overlaid white trace denotes the audio cue trigger signal from the stimulus PC. High values of the trigger signal indicate the time when the participant is instructed to close their eyes, the low value is when the participant has their eyes open. The spectrogram colour scheme details the high periods of activity in yellow, and the low periods in green. A peak in activity of 1pT/√Hz is observed in the the 812 region during the third eyes-closed time period.