Optically pumped magnetometers (OPMs) exploiting alkali metal vapours for accurate, precise magnetometry have benefited from improvements in components and techniques in recent years. Microfabrication of alkali cells and chip-scale lasers allow mass-production of compact sensors, and feedback an spin-preparation techniques, such as light-narrowing, allow enhanced performance, comparable with cryogenic SQUID magnetometers. I will introduce two OPM modalities developed at Strathclyde for geomagnetic operation- the digital alkali-spin maser and geophysical free-precession magnetometer. I will discuss the potential impacts of using these sensors for geophysical applications, including Global Navigation Satellite System (GNSS)-denied positioning, monitoring of space weather and magnetic anomaly detection. I will present developments in microfabrication and digital signal processing which will enable their widespread adoption.
Zero-field optically pumped magnetometers (OPMs) have emerged as an important technology in the realm of biomagnetic research, providing extremely small magnetic field detection capabilities, femtotesla-level, contained in a non-cryogenic compact form factor. Often, compact zero-field OPMs extract single or two-axis magnetic information, typically with a sensing bandwidth of < 100 Hz. The resolution of multiple axes of magnetic field is particularly important for accurate representation of radial components of biomagnetic fields. However, the presence of multi-axis static magnetic fields across the OPM causes measurement errors that degrade signal resolution. Here, we utilise our compact caesium single beam zero-field OPM to address these limitations. We magnetically modulated along both transverse axes of the sensor, at unique frequencies, to extract all axes static-field information. Active feedback can be realised through a lock-in detection scheme at fMod,x/y for the x- and y-axes, and at 2fMod,x for the beam axis, z. Operation in this scheme allows for the extraction of three-axis magnetic field information from only a single beam and highlights the importance of active feedback in high-sensitivity biomagnetic applications. The portable sensor also demonstrates a bandwidth with a -3 dB point at ≃ 1600 Hz. The combination of high bandwidth and the capability to extract three-axis magnetic fields opens up exciting prospects for resolving high-frequency biomagnetic signals.
Optically pumped magnetometers (OPMs) are becoming common in the realm of biomagnetic measurements. We discuss the development of a prototype zero-field cesium portable OPM and its miniaturized components. Zero-field sensors operate in a very low static magnetic field environment and exploit physical effects in this regime. OPMs of this type are extremely sensitive to small magnetic fields, but they bring specific challenges to component design, material choice, and current routing. The miniaturized cesium atomic vapor cell within this sensor has been produced through integrated microfabrication techniques. The cell must be heated to 120°C for effective sensing, while the sensor external faces must be skin safe ≤40 ° C making it suitable for use in biomagnetic measurements. We demonstrate a heating system that results in a stable outer package temperature of 36°C after 1.5 h of 120°C cell heating. This relatively cool package temperature enables safe operation on human subjects which is particularly important in the use of multi-sensor arrays. Biplanar printed circuit board coils are presented that produce a reliable homogeneous field along three axes, compensating residual fields and occupying only a small volume within the sensor. The performance of the prototype portable sensor is characterized through a measured sensitivity of 90 fT / Hz in the 5 to 20 Hz frequency band and demonstrated through the measurement of a cardiac magnetic signal.
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