Four-Channel Optically Pumped Magnetometer for MEG Sensor Arrays

Background

Optically Pumped Magnetometers (OPMs) operating in the Spin-Exchange Relaxation-Free (SERF) regime are highly sensitive magnetic field sensors, with sensitivities as low as 0.16 ft/√Hz and 0.54 ft/√Hz. OPMs are based on the interaction between spin-polarized atoms and magnetic fields, where the angular momentum of a pump light beam is transferred to atoms (typically alkali metal vapors) to achieve spin polarization. This spin polarization interacts with the magnetic field through Larmor precession, and by optically detecting the projection of the spin polarization along the direction of the probe beam, the external magnetic field can be determined. In the high atomic density and near-zero magnetic field SERF regime, spin-exchange collisions that cause polarization relaxation are strongly suppressed, significantly improving the OPM’s sensitivity.

In recent years, the application of OPMs in biomagnetism, especially in the measurement of human brain magnetic fields (magnetoencephalography, MEG), has gained increasing attention. Traditional MEG instruments primarily use Superconducting QUantum Interference Devices (SQUIDs) for signal detection, but these require liquid helium for cooling, which presents various drawbacks. In contrast, OPMs can be miniaturized and lightweight, enabling magnetic field detection on the scalp, improving spatial resolution, and facilitating the construction of wearable sensor arrays that allow scanning of subjects in motion.

Paper Origin

This paper was co-authored by Joonas Iivanainen and six other authors from Sandia National Laboratories and the Center for Quantum Information and Control at the University of New Mexico. It was published in the journal Optics Express on May 6, 2024.

Research Content

This paper introduces a new four-channel SERF-OPM sensor developed by the authors and describes in detail its design improvements, experimental methods, and performance evaluation.

2. Sensor Overview

The new four-channel OPM sensor is based on previous research (reference [25]) and incorporates a combined dichroic pump/probe scheme with an integrated optical axis design. Several improvements have been made to the new sensor, including lowering the vapor cell operating temperature, enhancing the probe optics components, reducing optical power requirements, and designing new electromagnetic coils for the sensor head to provide three-axis magnetic field control.

2.1 Design Improvements

The new sensor uses a 795 nm circularly polarized light to optically pump 87Rb atoms and operates in the SERF regime, measuring the spin polarization components via photodetection to detect the external magnetic field. Compared to previous designs, the new sensor has been optimized in several ways:

1. Vapor Cell Operating Temperature: Lowered from the previous 150°C to 135°C.
2. Probe Optics Components: The probe module has been improved, with each channel using independent probe optics to reduce optical crosstalk and increase accuracy.
3. Electromagnetic Coil Design: The new electromagnetic coil design enhances field uniformity and three-axis control capabilities.

3. Sensor Head Coil Design and Experimental Characteristics

3.1 Coil Design and Simulation

A pair of transverse Bx and By coils and a longitudinal Bz coil were designed on the surface of the sensor housing. These coils were optimized using numerical stream functions and target field methods to ensure a uniform magnetic field at the target location while minimizing magnetic field leakage in adjacent areas.

3.2 Experimental Measurements

After installing the coils, the authors measured the efficiency and uniformity of the coils using various experimental methods, including Free Induction Decay (FID) and Mz mode measurements to assess the gradient expansion effects of the magnetic field. Experimental results indicated that the optimized coils achieved the expected goals in terms of field uniformity and efficiency.

4. Sensor Performance Evaluation

The performance of the new OPM sensor was evaluated in various ways, including magnetic sensitivity, bandwidth, and noise source analysis. Experimental results showed that the new sensor performed excellently in terms of noise, magnetic sensitivity, and common-mode rejection ratio, with sensitivity reaching an average of 12.3 ft/√Hz in the 10-44 Hz range.

5. Conclusion

The new four-channel SERF-OPM sensor introduced in this paper features several improvements in both sensing performance and structural design. Notably, significant advantages were demonstrated in lowering operating temperature, reducing optical power requirements, and enhancing the performance of probe optical components. The research not only enhances the sensitivity and stability of MEG systems but also provides a technical foundation for future multi-sensor integration. The authors plan to further optimize the sensor in larger scale shielded rooms to achieve higher quality MEG signal recordings in the future.

Significance of the Study

The new four-channel OPM sensor developed in this study performs excellently in multiple technical parameters and is expected to provide higher resolution and accuracy in magnetic field detection for brain science research and clinical applications. Unlike traditional SQUID systems, OPMs do not require low-temperature cooling, are lightweight and flexible, and are especially suitable for research and detection in mobile states, demonstrating significant potential for the future.