Angiographic optical coherence tomography (OCTA) has rapidly found utility within many facets of medical research. Here OCTA algorithms are enabled on a commercial OCT system and verified through correlation with intra-vital light microscopy (IVM). While the vast majority of vessels were accurately measured, smaller vessels (< 30μm) have a tendency to appear dilated in comparison to IVM. The technique was also expanded upon to facilitate the imaging of subcutaneous murine fibrosarcoma tumours, negating the requirement for an intra-vital window. It was found that vessel measurement sensitivity was sufficiently high such that the morphologies of vessels within tumours expressing unique vascular endothelial growth factor (VEGF) isoforms could be differentiated, potentially providing a new angle of approach in the study of anti-angiogenic treatments. OCTA was then applied to human studies of atopic dermatitis, where it was found that metrics corresponding to vessel depth and morphology could be correlated with the sub-clinical severity of the condition. Knowledge of this could be utilised to observe the therapeutic response to treatment, past the point of clinical remission. A range of image-processing techniques were also developed, including automatic segmentation of the epidermal layer within skin being utilised to quantify the degree of epidermal thinning in response to applied skin strain, calculation of the skin capillary loop density and the response of skin vessels to temperature and pressure.

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This thesis investigates the stator mounted permanent magnet machines from the point of view of fault tolerant capability. The topologies studied are switched flux (and its derivatives C-Core, E-Core and modular), doubly salient and flux reversal permanent magnet machines. The study focuses on fault mode operation of these machines looking at severe conditions like short-circuit and irreversible demagnetization. The temperature dependence of the permanent magnet properties is taken into account. A complex multi-physics model is developed in order to assess the thermal state evolution of the switched flux machine during both healthy and faulty operation modes. This model couples the electro-mechanical domain with the thermal one, thus being able to consider a large range of operating conditions. It also solves issues such as large computational time and resources while still maintaining the accuracy. Experimental results are also provided for each chapter. A hierarchy in terms of fault tolerant capability is established. A good compromise can be reached between performance and fault tolerant capability. The mechanism of the magnet irreversible demagnetization process is explained based on magnetic circuit configuration. It is also found that the studied topology are extremely resilient against the demagnetizing influence of the short-circuit current and the magnet demagnetization is almost only affected by temperature.

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GaAsBi has recently attracted much attention due to its large band gap reduction, a less temperature dependence of the band gap, and the giant spin orbiting properties. The large band gap reduction of GaAsBi is explained by valence band anti-crossing (VBAC) model. It has been proposed that a resonant energy state is introduced in the valence band, and the interaction between that state and the original valence states leads to a splitting of the valence band into two sub-bands and therefore a reduction of the band gap. Molecular beam epitaxy (MBE) has been implemented to grow GaAsBi layers. Two growth conditions should be satisfied to get bismuth incorporated into GaAs: the growth temperature should be low enough (usually lower than 400 °C), and the As:Ga atomic flux ratio should be near stoichiometry. Four parameters can affect the final bismuth incorporation: growth rate, As:Ga atomic flux ratio, Bi:Ga atomic flux ratio, and growth temperature. The relationships have been investigated in the previous research, and also explored in this thesis. Absorption properties are the key properties of an optoelectronic device. Through measuring the photoresponse of GaAsBi based p-i-n heterojunction devices, the absorption coefficient as a function of the incident light energy is obtained. The results reveal that the absorption coefficient follows the square law of the Tauc relation, which indicates that the material is a direct band gap material. The diffusion length is a combination of the lifetime and the mobility of carriers, and it is the diffusion length that directly reflects the performance of carrier transportation. Especially in a device which requires high absorption of photons such as a solar cell, a long diffusion length becomes even more important. In this thesis, a model has been established to calculate the diffusion length of GaAsBi based on the photocurrent measurements, and results show that the diffusion length is around 1 μm. Photoluminescence (PL) measurements are used to decide the bismuth content based on the relation between the bismuth content and the band gap obtained from VBAC. Temperature dependence of the band gap of GaAsBi is also investigated using PL measurements. The s-shape of the PL peak position against the temperature reveals the existence of localized states. The source of localized states is claimed to be from bismuth clusters in some papers. Low temperature behaviours of GaAsBi have further been investigated using low temperature current-voltage measurements. It seems that the results also reflected the existence of localized states. For low noise avalanche photodiodes (APDs), it is important that the electron-initiated and hole-initiated impact ionization coefficients α and β are very different in magnitude. Photo-multiplication measurements were taken in this thesis to investigate the impact ionization property of GaAsBi p-i-n and n-i-p diodes. A relatively large ionization coefficient ratio α/β for GaAsBi heterojunctions was obtained, which shows a promising use of GaAsBi as low-noise electron-initiated APDs.

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Due to the increasing demand of high data rate of modern wireless communications, there is a significant interest in error control coding. It now plays a significant role in digital communication systems in order to overcome the weaknesses in communication channels. This thesis presents a comprehensive investigation of a class of error control codes known as Multiple Parallel Concatenated Gallager Codes (MPCGCs) obtained by the parallel concatenation of well-designed LDPC codes. MPCGCs are constructed by breaking a long and high complexity of conventional single LDPC code into three or four smaller and lower complexity LDPC codes. This design of MPCGCs is simplified as the option of selecting the component codes completely at random based on a single parameter of Mean Column Weight (MCW). MPCGCs offer flexibility and scope for improving coding performance in theoretical and practical implementation. The performance of MPCGCs is explored by evaluating these codes for both AWGN and flat Rayleigh fading channels and investigating the puncturing of these codes by a proposed novel and efficient puncturing methods for improving the coding performance. Another investigating in the deployment of MPCGCs by enhancing the performance of WiMAX system. The bit error performances are compared and the results confirm that the proposed MPCGCs-WiMAX based IEEE 802.16 standard physical layer system provides better gain compared to the single conventional LDPC-WiMAX system. The incorporation of Quasi-Cyclic QC-LDPC codes in the MPCGC structure (called QC-MPCGC) is shown to improve the overall BER performance of MPCGCs with reduced overall decoding complexity and improved flexibility by using Layered belief propagation decoding instead of the sum-product algorithm (SPA). A proposed MIMO-MPCGC structure with both a 2X2 MIMO and 2X4 MIMO configurations is developed in this thesis and shown to improve the BER performance over fading channels over the conventional LDPC structure.

In this thesis, different types of the magnetic lead screw systems are considered, and the effects of the key design parameters, such as magnet thickness, air-gap length, pole-pitch, number of pole-pairs, dimension etc. on the force/torque transmission are investigated. Moreover, research into the realisation of helical magnetisation distribution employing a novel impulse magnetisation process is undertaken, in order to reduce the complexity and cost of manufacture. This avoids the complex and/or time-consuming methods, which may require the assembly of a large number of small magnets to approximate helical magnetisation distribution. A prototype reluctance type magnetic screw system is realised, it consists of a double start mechanical screw and a permanent magnet nut equipped with impulse magnetised cylindrical permanent magnets, using purpose designed double-sided impulse magnetising fixture. Furthermore, a test-rig to measure the transmitted force is developed and used to compare the predicted and measured results.

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