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IEEE Sensors Journal

Towards the Non-Zero Field Cesium Magnetic Sensor Array for Magnetoencephalography

Mikhail V. Petrenko, Sergei P. Dmitriev, Anatoly S. Pazgalev, Alexei E. Ossadtchi, Anton K. Vershovskii

Published Jun 15, 2021• 25 citations
Publisher

Abstract

Magnetic sensors developed for application in magnetoencephalography must meet a number of requirements; the main ones are compactness, sensitivity and response speed. We present a quantum optically pumped atomic sensor with cell volume of 0.5cm3 that meets these requirements and is operable in nonzero magnetic fields. The ultimate sensitivity of the sensor was estimated as (using the criteria of the ratio of the slope of the magnetic resonance signal to the shot noise spectral density) to be better than 5 fT/Hz1/2. The actual sensitivity, measured in a gradiometric scheme, reaches 13 fT/Hz1/2 per sensor. We also present a novel and fast algorithm for optimization of the geometric properties of non-zero field sensor array with respect to maximization of the information transfer rate for cortical sources.

DOI
10.1109/jsen.2021.3089455

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Magnetic sensors developed for application in magnetoencephalography must meet a number of requirements; the main ones are compactness, sensitivity and response speed. We present a quantum optically pumped atomic sensor with cell volume of 0.5cm3 that meets these requirements and is operable in nonzero magnetic fields. The ultimate sensitivity of the sensor was estimated as (using the criteria of the ratio of the slope of the magnetic resonance signal to the shot noise spectral density) to be better than 5 fT/Hz1/2. The actual sensitivity, measured in a gradiometric scheme, reaches 13 fT/Hz1/2 per sensor. We also present a novel and fast algorithm for optimization of the geometric properties of non-zero field sensor array with respect to maximization of the information transfer rate for cortical sources.
Abstract A variant of the scheme of a magnetometric sensor based on cesium atomic vapor is proposed and experimentally investigated. The sensor uses magnetic resonance excitation by modulated light of an hyperfine optical pumping that is transverse to the magnetic field. It is shown that, for a cell with a volume of 0.125 cm3, the variational sensitivity of such a scheme, estimated from the ratio of the steepness of the signal in the center of the magnetic resonance to the shot noise of the detecting radiation, reaches a level of <10 fT/Hz1/2 in the frequency band width of the order of 850 Hz. The sensor, which does not emit radio frequency fields, is designed for use in magnetoencephalographic complexes. Possible ways to improve the performance of the scheme for detecting relatively fast (~4.2 kHz in a field of 0.1 mT) signals of the precession of proton magnetic moments in promising ultra-weak field tomography schemes are considered.
Abstract We demonstrate the possibility of using vertical-cavity surface emitting lasers with intracavity contacts and a rhomboidal oxide current aperture for creating compact optically pumped 133Cs atomic magnetometer operating in non-zero magnetic fields, which is promising to use in magnetoencephalographic systems. The magnetic resonance parameters were studied in a two-beam MX optically pumped magnetic field sensor scheme based on the effect of the magnetic resonance line narrowing at high laser pumping and high concentrations of alkali-metal atoms. The ultimate sensitivity of OPM was estimated by the ratio of the magnetic resonance steepness to the probe light shot noise level. It is shown that a magnetic sensor based on the vertical-cavity surface emitting lasers and a compact (0.125 cm3 volume) Cs vapor cell can achieve a shot noise-limited sensitivity better than 15 fT in 1 Hz bandwidth. Developed intracavity contacted VCSELs suitable for use in compact atomic magnetometers for magnetoencephalographic systems.
Abstract The magnetic field is one of the most fundamental and ubiquitous physical observables, carrying information about all electromagnetic phenomena. For the past 30 years, superconducting quantum interference devices (SQUIDs) operating at 4 K have been unchallenged as ultrahigh-sensitivity magnetic field detectors1, with a sensitivity reaching down to 1 fT Hz-1/2 (1 fT = 10-15 T). They have enabled, for example, mapping of the magnetic fields produced by the brain, and localization of the underlying electrical activity (magnetoencephalography). Atomic magnetometers, based on detection of Larmor spin precession of optically pumped atoms, have approached similar levels of sensitivity using large measurement volumes2, 3, but have much lower sensitivity in the more compact designs required for magnetic imaging applications4. Higher sensitivity and spatial resolution combined with non-cryogenic operation of atomic magnetometers would enable new applications, including the possibility of mapping non-invasively the cortical modules in the brain. Here we describe a new spin-exchange relaxation-free (SERF) atomic magnetometer, and demonstrate magnetic field sensitivity of 0.54 fT Hz-1/2 with a measurement volume of only 0.3 cm3. Theoretical analysis shows that fundamental sensitivity limits of this device are below 0.01 fT Hz-1/2. We also demonstrate simple multichannel operation of the magnetometer, and localization of magnetic field sources with a resolution of 2 mm.
Abstract We present a design for a multi-channel optically pumped zero-field magnetometer utilizing a 200-μm-thick Rubidium vapor cell. The vapor cell and its housing are designed to reduce the minimal distance between a magnetic sample and the sensing volume to about 1 mm, to optimize the effective spatial resolution. The thin vapor cell, filled with 2 atm of nitrogen as a buffer gas reduces the volume across which the magnetic field is averaged. The vapor cell is fully illuminated by a single laser beam, and the transmitted light is imaged onto a 4 x 4 photodiode array, allowing for simultaneous measurement of a magnetic field distribution with up to 16 channels. The performance of the magnetometer is studied for all channels. It is shown that the sensor can operate in the spin-exchange relaxation-free regime with a projected photon-shot noise limited noise floor of about 1 pT/Hz1/2 for a sensitive voxel size of approximately 600 μm x 600 μm x 200 μm.
A cesium atomic magnetometer based on laser-pumped rf resonance has been investigated and demonstrated experimentally. Atoms are polarized and pumped to dark states by circularly polarized light. When there exists a magnetic field, the corresponding magnetization will be precessing around the magnetic field at Larmor frequency. By means of adding a resonant rf magnetic field, the atoms will be depolarized and absorb photons again. By detecting the spectrum of the transmitted light, one can obtain the information about the external magnetic field. We build an 894nm external cavity diode laser with a frequency stabilization device, and a low noise magnetic field measurement environment. After the optimization of the magnetometer parameter and closed-loop measurement, an ultimate sensitivity of 19 fT/Hz1/2 and an intrinsic sensitivity of 1.8 pT/Hz1/2 have been achieved with the spatial resolution smaller than 2 cm.
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.