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High-Sensitivity rf Detection Using an Optically Pumped Comagnetometer Based on Natural-Abundance Rubidium with Active Ambient-Field Cancellation

Physical Review Applied

Bainbridge, Jonathan E.; Claussen, Neil C.; Iivanainen, Joonas; Schwindt, Peter S.

To detect a specific radio-frequency (rf) magnetic field, rf optically pumped magnetometers (OPMs) require a static magnetic field to set the Larmor frequency of the atoms equal to the frequency of interest. However, unshielded and variable magnetic field environments (e.g., an rf OPM on a moving platform) pose a problem for rf OPM operation. Here, we demonstrate the use of a natural-abundance rubidium vapor to make a comagnetometer to address this challenge. Our implementation builds upon the simultaneous application of several OPM techniques within the same vapor cell. First, we use a modified implementation of an OPM variometer based on 87Rb to detect and actively cancel unwanted external fields at frequencies 60Hz using active feedback to a set of field control coils. We exploit this stabilized field environment to implement a high-sensitivity rf magnetometer using 85Rb. Using this approach, we demonstrate the ability to measure rf fields with a sensitivity of approximately 9fTHz-1/2 inside a magnetic shield in the presence of an applied field of approximately 20μT along three mutually orthogonal directions. This demonstration opens up a path toward completely unshielded operation of a high-sensitivity rf OPM.

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TYPE Journal Article YEAR 2022
DOIOSTIScopus

Calibration and Localization of Optically Pumped Magnetometers Using Electromagnetic Coils

Sensors

Iivanainen, Joonas; Borna, Amir B.; Zetter, Rasmus; Carter, T.R.; Stephen, Julia M.; Mckay, Jim; Parkkonen, Lauri; Taulu, Samu; Schwindt, Peter S.

In this paper, we propose a method to estimate the position, orientation, and gain of a magnetic field sensor using a set of (large) electromagnetic coils. We apply the method for calibrating an array of optically pumped magnetometers (OPMs) for magnetoencephalography (MEG). We first measure the magnetic fields of the coils at multiple known positions using a well‐calibrated triaxial magnetometer, and model these discreetly sampled fields using vector spherical harmonics (VSH) functions. We then localize and calibrate an OPM by minimizing the sum of squared errors between the model signals and the OPM responses to the coil fields. We show that by using homogeneous and first‐order gradient fields, the OPM sensor parameters (gain, position, and orientation) can be obtained from a set of linear equations with pseudo‐inverses of two matrices. The currents that should be applied to the coils for approximating these low‐order field components can be determined based on the VSH models. Computationally simple initial estimates of the OPM sensor parameters follow. As a first test of the method, we placed a fluxgate magnetometer at multiple positions and estimated the RMS position, orientation, and gain errors of the method to be 1.0 mm, 0.2°, and 0.8%, respectively. Lastly, we calibrated a 48‐channel OPM array. The accuracy of the OPM calibration was tested by using the OPM array to localize magnetic dipoles in a phantom, which resulted in an average dipole position error of 3.3 mm. The results demonstrate the feasibility of using electromagnetic coils to calibrate and localize OPMs for MEG.

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TYPE Conference Poster YEAR 2022
DOIOSTIScopus
19 Results
19 Results