Thesis

High common-mode-noise-rejecting magnetic gradiometer for biomagnetic measurements

Creator
Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2026
Thesis identifier
  • T18096
Person Identifier (Local)
  • 202588448
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • This project sits within the field of atomic magnetometry and biomagnetic sensing, with a specific focus on developing and characterising a high common-mode-noise-rejecting magnetic gradiometer based on optically pumped magnetometers (OPMs). The work is motivated by the need to measure extremely weak magnetic signals, such as those produced by the heart and brain, which are often masked by environmental interference. Improving commonmode noise rejection is therefore essential for enabling reliable biomagnetic measurements in realistic laboratory, clinical, and potentially unshielded environments. Experimental data were collected using a two-channel Mx-type OPM gradiometer system. The setup included magnetically shielded vapour cells, Radio Frequency (RF) excitation coils, and VCSEL (Vertical-Cavity Surface-Emitting Laser)-based optical pumping. The system parameters were controlled through LabVIEW, with signals acquired via FPGA and exported for further analysis. MATLAB was used to process the data and generate plots showing amplitude, phase, relaxation rate, resonance frequency, and signal gradient as a function of VCSEL voltage. The results plot clear resonance behaviour linked to optical detuning across the caesium D1 transition. Peaks in amplitude, relaxation, and resonance frequency occur within a narrow voltage range, identifying the optimal operating region. Gradient analysis highlights the balance between signal strength and linewidth, revealing a voltage range where sensitivity is maximised. The experimental work primarily characterised the sensitivity of the system through the VCSEL voltage sweep, measuring amplitude, phase, relaxation, resonance frequency, and gradient as proxies for system performance. Direct quantitative measurement of the common-mode rejection ratio (CMRR) was not performed in this work. These findings provide insight into how optical pumping conditions influence magnetometer performance and stability. Understanding these relationships supports the optimisation of gradiometer operation and contributes toward the broader goal of improving noise rejection, sensitivity, and practical deployment of OPM-based systems for biomagnetic measurements.
Advisor / supervisor
  • Ingleby, Stuart
Embargo Note
  • This thesis is restricted to Strathclyde users only.

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