Portable single-beam cesium zero-field magnetometer for magnetocardiography

Abstract

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.