Cort N. Johnson

Magnetoencephalography with a two-color pump-probe, fiber-coupled atomic magnetometer

The authors have detected magnetic fields from the human brain with a compact, fiber-coupled rubidium spin-exchange-relaxation-free magnetometer. Optical pumping is performed on the D1 transition and Faraday rotation is measured on the D2 transition. The beams share an optical axis, with dichroic optics preparing beam polarizations appropriately. A sensitivity of <5 fT/Hz is achieved. Evoked responses resulting from median nerve and auditory stimulation were recorded with the atomic magnetometer. Recordings were validated by comparison with those taken by a commercial magnetoencephalography system. The design is amenable to arraying sensors around the head, providing a framework for noncryogenic, whole-head magnetoencephalography.

Multi-sensor magnetoencephalography with atomic magnetometers

The authors have detected magnetic fields from the human brain with two independent, simultaneously operating rubidium spin-exchange-relaxation-free magnetometers. Evoked responses from auditory stimulation were recorded from multiple subjects with two multi-channel magnetometers located on opposite sides of the head. Signal processing techniques enabled by multi-channel measurements were used to improve signal quality. This is the first demonstration of multi-sensor atomic magnetometer magnetoencephalography and provides a framework for developing a non-cryogenic, whole-head magnetoencephalography array for source localization.

Four-channel optically pumped atomic magnetometer for magnetoencephalography.

We have developed a four-channel optically pumped atomic magnetometer for magnetoencephalography (MEG) that incorporates a passive diffractive optical element (DOE). The DOE allows us to achieve a long, 18-mm gradiometer baseline in a compact footprint on the head. Using gradiometry, the sensitivities of the channels are < 5 fT/Hz1/2, and the 3-dB bandwidths are approximately 90 Hz, which are both sufficient to perform MEG. Additionally, the channels are highly uniform, which offers the possibility of employing standard MEG post-processing techniques. This module will serve as a building block of an array for magnetic source localization.

A two-color pump probe atomic magnetometer for magnetoencephalography

In recent years, atomic magnetometers (AMs) have demonstrated sub-femtotesla sensitivities and have emerged as potential replacements for superconducting quantum interference devices (SQUIDs). In an AM, a circularly polarized pump laser aligns the electron spins in a cloud of alkali metal vapor. Changes occur in the optical properties of the vapor when its collective moment interacts with an external magnetic field, resulting in a measurable polarization rotation of a linearly polarized probe beam. We present a unique pump/probe scheme in which a pump beam tuned to the 87Rb D1 line (795 nm) and a probe beam tuned to the D2 line (780 nm) share a single optical axis. A dichroic waveplate is used to circularly polarize the pump beam while leaving the probe beam linearly polarized. Based upon this scheme, we have developed a small profile, fiber-coupled AM. By independently optimizing pump/probe power and detuning, we are able to achieve intrinsic single-channel sensitivity of < 5 fT/√Hz, which is suitable for demanding applications such as magnetoencephalography (MEG). Furthermore, through straightforward adaptations of the current design, it should be possible to create dense, reconfigurable arrays of AMs for whole-head MEG.

Atomic magnetometer for human magnetoencephalograpy.

We have developed a high sensitivity (<5 fTesla/{radical}Hz), fiber-optically coupled magnetometer to detect magnetic fields produced by the human brain. This is the first demonstration of a noncryogenic sensor that could replace cryogenic superconducting quantum interference device (SQUID) magnetometers in magnetoencephalography (MEG) and is an important advance in realizing cost-effective MEG. Within the sensor, a rubidium vapor is optically pumped with 795 laser light while field-induced optical rotations are measured with 780 nm laser light. Both beams share a single optical axis to maximize simplicity and compactness. In collaboration with neuroscientists at The Mind Research Network in Albuquerque, NM, the evoked responses resulting from median nerve and auditory stimulation were recorded with the atomic magnetometer and a commercial SQUID-based MEG system with signals comparing favorably. Multi-sensor operation has been demonstrated with two AMs placed on opposite sides of the head. Straightforward miniaturization would enable high-density sensor arrays for whole-head magnetoencephalography.