| Summary: | Radio-frequency optically pumped magnetometers (RF OPMs) are capable of measuring oscillating magnetic fields with high sensitivity in the fT/sqrt(Hz) range. Two types of RF OPMs are presented in this thesis. The first is a portable, orientation-based RF OPM with 600 fT/sqrt(Hz) sensitivity at 10 kHz in unshielded conditions, a new benchmark for a portable unshielded RF OPM, and 200 fT/sqrt(Hz) sensitivity at 10 kHz in shielded conditions, close to the spin projection noise limit. Eddy current measurements were performed with this OPM to remotely detect aluminium disks with diameters as small as 1.5 cm at distances of ~25 cm from both the excitation coil and the OPM, demonstrating the possibility of using OPMs for remote sensing. Off-axis measurements were performed with this OPM to illustrate how the OPM readout can be interpreted for remote sensing. All aspects of the theory, experimental setup and results relevant for this orientation-based RF OPM and eddy current measurements are presented in this thesis.
The second OPM presented is a table-top alignment-based RF OPM in shielded conditions using a buffer gas cell. The benefit of alignment-based magnetometers over orientation-based RF OPMs is that they require only one laser beam, making them compact and robust. Until now, the alignment-based magnetometer had only been used with hand-blown paraffin-coated cells, but not with buffer gas cells that can be produced on a mass scale using microfabrication techniques. We present here an alignment-based magnetometer using a buffer gas cell (Cs and N2). This one-beam RF OPM with a buffer gas cell obtained a sensitivity of 325 fT/sqrt(Hz) with an 800 Hz bandwidth to 10 kHz oscillating magnetic fields and is calculated to be close to the spin projection noise limit. The non-linear Zeeman splitting is observed with both the buffer gas cell and a paraffin-coated cell. These results open up the possibility for commercialisation and further miniaturisation of RF OPMs.
We derive a set of equations for the off-axis detection of electrically conductive spheres for the arbitrary positioning of the sphere and magnetometer. The equations are interpreted with a focus on predicting the expected signals for the imaging of the electrical conductivity of the human heart using RF OPMs to potentially help diagnose atrial fibrillation more effectively in the future. The optimal setups are discussed.
Details on how to design compact, low-noise balanced photodetectors are discussed. Several printed circuit board designs are presented and tested. The performance of the balanced photodetector exceeds that of a commercial balanced photodetector at low frequencies, being shot noise limited for powers as low as 3 uW at frequencies of 3 kHz.
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