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Elson, Lucy Jade (2025) Advances in Alignment Based Optically Pumped Magnetometers: Techniques, Characterisation and Applications. PhD thesis, University of Nottingham.

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Abstract

Eddy current induction in objects is a technique that can be used for detection, characterisation and imaging of a sample. By using a primary radio frequency magnetic field eddy currents can be induced in an object which in turn produces a secondary magnetic field. This secondary magnetic field can then be measured using a magnetic field sensor such as a fluxgate magnetometer or an optically pumped magnetometer (OPM). The characteristics of the magnetic field sensor chosen need to be considered in order to ensure the noise floor, operational frequency and band width are sufficient. In this thesis, experiments and numerical simulations for the evaluation of conductive objects are carried out. A commercially available fluxgate magnetometer is used to detect and characterise a non-magnetic (aluminium) and magnetic (steel) sample. The frequency dependence (up to 1 kHz) and positional dependence of the secondary magnetic field is investigated.

Optically pumped magnetometers are capable of measuring oscillating magnetic fields with sensitivities in the fT/√Hz range in magnetically shielded and unshielded environments. In this thesis, there is a focus on alignment based magnetometers. Typically, alignment based magnetometers use paraffin coated vapour cells to extend the spin relaxation lifetimes of the alkali vapour. Although this works well, paraffin coated cells have the drawback of being hand-blown making the supply of the cells somewhat unreliable. Buffer gas cells have the ability to be micro-fabricated and hence have the potential to be mass produced. Here, the first implementation of a buffer gas cell in an alignment based OPM is presented. A single laser beam is used to pump and probe the atomic ensemble. Initially, a table-top set up is used to characterise the buffer gas cell’s capabilities in an alignment based magnetometer. A sensitivity of 310 fT/√Hz with a bandwidth of 800 Hz is found at a Larmor frequency of ωL ≈ 2π(10 kHz). This data is compared to a paraffin coated cell placed in the same set up.

Characterisation of OPMs and the dynamical properties of their noise is important for applications in real time sensing tasks. The spin noise spectroscopy of an alignment based magnetometer, using a paraffin coated cell, is presented. A stochastic model that predicts the noise power spectra when, as well as the the static magnetic field responsible for the Larmor precession, a white noise field is applied in the beam propagation direction. By experimentally varying the strength of the white noise applied as well as the linear-polarisation angle of incoming light, the theoretical model is verified. This work paves the way for alignment based magnetometers to become operational in real time sensing tasks.

The table-top set up is also used to utilise a spin aligned atomic ensemble for magnetometry at zero-field. An approach is introduced which involves evaluating how the linear polarisation of light rotates as it passes through the atomic vapour to null the magnetic field. Analytical expressions are derived for the resulting spin alignment and photodetection signals. The experimental results show good agreement with the theoretical predictions. The sensitivity and bandwidth of the magnetometer are characterised and the practical utility for medical applications is demonstrated by successfully detecting a synthetic cardiac signal.

The buffer gas vapour cell is then implemented into a compact and portable magnetometer sensor head that was developed. For the development of the portable OPM, a low noise and high bandwidth balanced photodetector (BPD) was designed. The final design had a shot noise limit of ∼ 4 μW below 1 MHz, which exceeds that of a commercial BPD that was used for comparison. The use of the home-made detector resulted in a lower overall noise floor in the magnetometer with sensitivities of 230 fT/√Hz at a Larmor frequency of ωL ≈ 2π(6 kHz) in magnetically shielded conditions and 865 fT/√Hz in unshielded conditions. Eddy current measurements were then carried out with the sensor in unshielded conditions where aluminium samples with a diameter as small as 1.5 cm were detected at a distance of 26.4 cm from the excitation coil and 23.9 cm from the sensing point of the magnetometer.

The portable OPM was developed with a future goal of imaging the conductivity of the heart with the use of magnetic induction tomography. High frequencies are required to measure induced fields in an object with a low conductivity. The portable OPM has a sensitivity of 825 fT/√Hz at ωL ≈ 2π(1.5 MHz) in shielded conditions. The use of a buffer gas vapour cell in the prototype OPM is a promising step towards this goal.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Fernholz, Thomas Jensen, Kasper
Keywords:	optically pumped magnetometers, eddy currents, magnetic fields
Subjects:	Q Science > QC Physics > QC501 Electricity and magnetism
Faculties/Schools:	UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID:	81253
Depositing User:	Elson, Lucy
Date Deposited:	31 Jul 2025 04:40
Last Modified:	31 Jul 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/81253

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