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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Towards mid-infrared fibre lasers : rare earth ion doped chalcogenide glasses and fibres

Sakr, Hesham January 2016 (has links)
This Project is aimed at developing rare earth ion doped chalcogenide glasses targeting mid-infrared (MIR) fibre lasers, emitting in the wavelength region 4 - 5 μm. The work reported in this thesis has two objectives: (i) a study of the Ge-As-In-Se glass system when doped with a single species of rare earth (RE) ions, i.e. praseodymium (Pr3+) or cerium (Ce3+), or when co-doped with two rare earth ion species: Pr3+ and Ce3+, and (ii) a study of the effect of replacing a gallium (Ga) additive with an indium (In) additive on the physical and optical properties of the undoped and Pr3+ doped Ge-As-(Ga/In)-Se glasses and fibres. The MIR, i.e. 3 - 25 μm wavelength, offers to advance many photonics areas including bio-medical imaging spectroscopy for human tissue sensing in vivo for early cancer diagnosis. Low loss RE-ion doped MIR fibre lasers are potential pumps for MIR supercontinuum generation (SCG) sources for a compact MIR broadband device. Also, MIR fibre narrowband lasers offer potential new wavelengths for laser medical surgery. To date, there are no MIR rare earth ion doped glass fibre lasers emitting at wavelengths ≥ 4 μm. Selenide (Se)-based glasses, a member of the chalcogenide glass family, are known for their wide transparency up to 12 μm and good rare earth ion solubility. In the Project, an additive to the Ge-As-Se glass system of indium or gallium is considered to help decluster the rare earth ions and increase their solubility in the as-prepared Pr3+ doped Ge-As-(Ga/In)-Se glasses. However, an indium additive is concluded here to achieve a lower rare earth ion solubility limit than that obtained using the equivalent gallium additive in the Ge-As-(Ga/In)-Se glass systems. On the other hand, the photoluminescent intensity is concluded here to be approximately doubled when using an indium additive in Pr3+ doped Ge-As-In- Se, compared to the analogous gallium glasses. Furthermore, the decay lifetime, at the same emission wavelength of 4.7 μm, is found to be longer in the Pr3+ doped Ge-As-In-Se glasses when compared to the Pr3+ doped Ge-As- Ga-Se glasses. Overall, for a singly-doped Ge-As-In-Se glass system, Pr3+ offer wide photoluminescence spectral emission in the range 3 - 6 μm, which promotes this type of glass fibre as an active source for MIR laser emission in the target range of 4 - 5 μm. However, the photoluminescent decay lifetime, at 4.7 μm, of Pr3+ doped Ge-As-In-Se is concluded to decrease substantially with the number of thermal processes invoked to fabricate the glass-based fibres; a lifetime of 7 - 9 ms measured on the as-prepared fibres is compared to the decay lifetime of 9 - 10.1 ms that were found in the bulk glasses. Alternatively, the addition of Ce3+ in the Ge-As-In-Se glass system is concluded to offer a larger absorption cross-section than that of the Pr3+ in the wavelength range 3.5 - 5 μm. Co-doping the Pr3+ / Ce3+ in Ge-As-In-Se in order to enhance the MIR photoluminescence emission in the range 3 - 6 μm is also investigated. It is concluded that rare earth ions, in particular Ce3+ and / or Pr3+, doped chalcogenide glass fibres based on the Ge-As-In-Se glass system, developed through this Project, are strong candidates towards achieving MIR fibre lasers.
2

Analysis of the impact of external optical feedback on the performance of high-power and high-brightness laser diodes

Helal, Mohamad Anas January 2017 (has links)
The main motivation behind this work was to answer the following question: what is the impact of unintentional external optical feedback on laser performance. During the course of this PhD, however, two more questions were raised: how does the spectral external feedback (i.e. intentional feedback from a grating) enhances the spectral performance of the laser? And why does the beam quality of Distributed Bragg Reflector (DBR) tapered lasers degrade at >2.5 times the threshold current? To answer these two questions, new simulations were performed and more results were obtained. The impact of external feedback can be positive or negative. High-brightness lasers are mostly characterised under stand-alone conditions. Nevertheless, high-brightness lasers are almost always operated in external cavity configurations. The external cavity will produce reflections, whether they are intentional reflections off mirrors or gratings, or unintentional reflections off the optics elements. In order to have an accurate prediction of how the high-brightness laser will perform in an external optical system, a modelling tool, which is capable of self-consistently modelling the laser cavity and the fields propagating in the external optical system, is needed. In this work, an external cavity laser simulation tool was developed. This tool consists of an in-house 2.5D laser modelling tool, Speclase; a commercial coherent ray-tracing software, OpticStudio; and an interface software to bidirectionally couple the two different light modelling algorithms used by these tools: finite difference beam propagation (FD-BPM) method and coherent ray-tracing. The author was responsible for: developing the interface software – using Matlab - between Speclase and OpticStudio. Also, the author cooperated with Dr. Kaunga-Nyirenda to modify Speclase so that the coupling between Speclase and OpticStudio modelled the physics correctly. This way the software was able to produce external cavity designs to study the impact of external feedback, intentional and unintentional. The external cavity tool was used to investigate and analyse the impact of parasitic reflections on the performance of high-brightness diode lasers. It was also used to analyse how the Littrow cavity works, which is an example of how intentional feedback is used to improve laser performance. Finally, Speclase was used to investigate why the beam quality of high-brightness DBR tapered lasers degrade at higher power, i.e. at currents > 2.5 times the threshold current. This gave an insight on how the external optical feedback affects the degradation of beam quality. It also helped differentiate between the impact of external feedback coupled to the fundamental mode and to the higher-order vertical modes. This work contributes to the knowledge of how external feedback alters the performance of high-brightness laser diodes. It shows that unintentional feedback can lead to the degradation of output power, power conversion efficiency and beam quality. It also shows the significance of modal discrimination and external cavity coupling coefficients and how they can be used to mitigate the unwanted impacts of external optical feedback. The developed model also contributes to the optimisation of external cavity systems with intentional feedback to get optimum performance. It shows how spectral feedback coupled to the laser cavity can force the laser to favour a specific wavelength that is, by allowing the beam of that wavelength to couple into the RW section, while other wavelengths are spatially filtered out. This work also answers a significant question of why the beam quality of DBR tapered lasers degrade at currents 2.5 times the threshold current, although the back- facet reflectivity is patterned, allowing fields propagating outside the RW to escape the structure. The results show clearly how the patterned back facet reflectivity creates a diffraction of the back-propagating beam which allows the diffracted beam to exit the RW and couple into the taper section, exciting higher-order lateral modes.
3

Facial thermography for assessment of workload in safety critical environments

Marinescu, Adrian Cornelius January 2018 (has links)
The fast changing modern world is placing humans in positions we have not had time to evolve and adapt to by natural means, we are thus faced with the task of understanding our abilities and limitations, at both a physical and mental levels and design the world around us with these in mind. This is in line with the aim of the discipline of ergonomics (or human factors), to "optimize human well-being and overall system performance by contributing to the design and evaluation of task, jobs, products, environments and systems" (International Ergonomics Association 2014). This is a large task, spanning multiple other disciplines. The research presented in this thesis is in the area of workload, a concept used to describe the interaction between a task and an operator in terms of demand, perception of task and performance. Many tools and methods have been developed aiming at measuring workload, ranging from subjective measures, primary and secondary task measures, task analysis and physiological measures. The main focus of this research is on physiological methods of assessing workload in safety critical environments. Within the domain of physiological methods for workload assessment, many techniques have been explored over the years and will be presented in the thesis with their advantages and disadvantages. Despite all the efforts made to develop a reliable physiological measurement assessment method for workload, further research is needed; the research presented here focuses on facial thermography as a non-invasive, real-time assessment method for workload, coupled with other physiological measures such as heart rate, breathing rate and pupil diameter. The human physiological response to changes in workload has been examined in three studies which also explore the use of multiple physiological measures as a means of estimating the level of workload. While two of the studies were performed in laboratory conditions having students as participants, a third study was performed in an ecologically valid helicopter simulator in order to test the physiological reactions of highly trained individuals to changes in workload. The results indicate that facial thermography, especially nose area temperatures, as well as pupil diameter respond well to changes in workload and could be used as a noninvasive, real-time method of estimating workload. The flight simulator study revealed that even highly trained individuals have similar responses to changes in demand as the general public. This thesis contributes to the measurement and assessment of workload by using physiological measures, especially facial thermography and presenting the relative contribution of each of the measures in both laboratory and real-life scenarios.
4

Visual tracking : detecting and mapping occlusion and camouflage using process-behaviour charts

Chandesa, Tissa January 2013 (has links)
Visual tracking aims to identify a target object in each frame of an image sequence. It presents an important scientific problem since the human visual system is capable of tracking moving objects in a wide variety of situations. Artificial visual tracking systems also find practical application in areas such as visual surveillance, robotics, biomedical image analysis, medicine and the media. However, automatic visual tracking algorithms suffer from two common problems: occlusion and camouflage. Occlusion arises when another object, usually with different features, comes between the camera and the target. Camouflage occurs when an object with similar features lies behind the target and makes the target invisible from the camera’s point of view. Either of these disruptive events can cause a tracker to lose its target and fail. This thesis focuses on the detection of occlusion and camouflage in a particle-filter based tracking algorithm. Particle filters are commonly used in tracking. Each particle represents a single hypothesis as to the target’s state, with some probability of being correct. The collection of particles tracking a target in each frame of an image sequence is called a particle set. The configuration of that particle set provides vital information about the state of the tracker. The work detailed in this thesis presents three innovative approaches to detecting occlusion and/or camouflage during tracking by evaluating the fluctuating behaviours of the particle set and detecting anomalies using a graphical statistical tool called a process-behaviour chart. The information produced by the process-behaviour chart is then used to map out the boundary of the interfering object, providing valuable information about the viewed environment. A method based on the medial axis of a novel representation of particle distribution termed the Particle History Image was found to perform best over a set of real and artificial test sequences, detecting 90% of occlusion and 100% of camouflage events. Key advantages of the method over previous work in the area are: (1) it is less sensitive to false data and less likely to fire prematurely; (2) it provides a better representation of particle set behaviour by aggregating particles over a longer time period and (3) the use of a training set to parameterise the process-behaviour charts means that comparisons are being made between measurements that are both made over extended time periods, improving reliability.
5

On-chip ultra-fast data acquisition system for optical scanning acoustic microscopy using 0.35um CMOS technology

Dong, Peiliang January 2009 (has links)
Optical Scanning Acoustic Microscopy (OSAM) is a non-contacting method of investigating the properties and hidden faults of solid materials. This thesis presents an ultra-fast data acquisition system (DAQ) which samples and digitises the output signal of OSAM. The author's work includes the design of the clock source and the sampler, and integration of the whole system. The clock source is a unique pulse generator based on a 2.624GHz PLL with a Quadrature VCO (QVCO), which is able to generate 4 clock signals in accurate quadrature phase difference. The pulse generator used the 4-phase clocks to provide control pulses to the sampler. The pulses were carefully aligned to the clock edges by digital logic, so that jitters were reduced as much as possible. The required short time delay for the sampler was also provided by the pulse generator, and this was implemented by a smartly-controlled switch box which re-shuffles the 4-phase clocks. The presented sampler is a novel 10.496GSample/s Sub-Sampling Sample-and-Hold Amplifier (SHA). The SHA sampled the input, and transformed its spectrum down to a low-frequency range so that it can be digitised. Charge-domain sampling strategy and double differential switches were both developed in this circuit to significantly improve the sampling speed. The periodicity of the system input was exploited in repetitive sampling to reduce the noise. These designed modules were integrated into a DAQ for a 2x8 sensor array. A pseudo-parallel scanning strategy was presented to minimise the power consumption, and a current-based buffer was applied to deliver the control pulses into the array. The DAQ was implemented on-chip in a low-cost 0.35um standard CMOS process. The measurement results showed that the DAQ successfully achieved a sampling rate more than 10GS/s, with a maximum output resolution of approximately 6 bits.
6

Visually tracked flashlights as interaction devices

Green, Jonathan January 2008 (has links)
This thesis examines the feasibility, development and deployment of visually tracked flashlights as interaction devices. Flashlights are cheap, robust and fun. Most people from adults to children of an early age are familiar with flashlights and can use them to search for, select and illuminate objects and features of interest. Flashlights are available in many shapes, sizes, weights and mountings. Flashlights are particularly appropriate to situations where visitors explore dark places such as the caves, tunnels, cellars and dungeons that can be found in museums, theme parks and other visitor attractions. Techniques are developed by which the location and identity of flashlight projections are recovered from the image sequence supplied by a fixed camera monitoring a target surface. The information recovered is used to trigger audiovisual events in response to users' actions. Early trials with three prototype systems, each built using existing techniques in computer vision, show flashlight interfaces to be feasible both technically and from a usability point of view. Novel methods are developed which allow extraction of descriptions of flashlight projections that are independent of the reflectance of the underlying physical surface. Those descriptions are used to locate and recognise individual flashlights and support a multi-user interface technology. The methods developed form the basis of Enlighten, a software product marketed by the University of Nottingham spinoff company Visible Interactions Ltd. Enlighten is currently is daily use at four sites across the UK. Two patents have been filed (UK Patent Publication Number GB2411957 and US Patent Application Number 10/540,498). The UK patent has been granted, and the US application is under review.
7

The design and optimisation of nanophotonic devices using the Finite Element Method

Arca, Ahmet January 2010 (has links)
The aim of this thesis is to develop a technique which can be used in the reliable modelling, design and optimisation of practical suboptical wavelength sized photonic/plasmonic devices, which may involve arbitrary geometries on various scales. The technique involves the application of numerical electromagnetic simulation led by theoretical knowledge and physical insight to determine, design and optimise the operating mechanism of such devices. The work in this thesis contains a variety of problems/devices which involve arbitrary structures of different scales. This poses difficulties in both the fabrication and the modelling aspects of the design. The problems range in difficulty from those which can be simply and perfectly described via an analytical solution, to those which would be impractical to design using any other technique. The nature of the problems considered, i.e. the complicated geometry and the range of scales, necessitates the use of a flexible modelling technique. Finite Element Method (FEM) was found to be a valuable tool in the design and optimisation of the devices throughout this thesis, owing its success mainly to its versatility and flexible meshing abilities which allowed its operation in different length scales in an efficient manner. Three nanophotonic/plasmonic devices are considered in an effort to demonstrate the implementation and the application of the developed technique. The devices considered in this thesis demonstrate different challenges in the modelling and design while being of considerable interest in their own right as nanostructures for sensing and measurement. These devices are: A self-calibrated plasmon sensor, a plasmon resonator and an ultrahigh frequency optical acoustic surface wave detector. Whilst the first two devices are important as an application of plasmonics, the third device links the mechanical and optical processes together.
8

Application of PCA and Hough Transform to classify features in optical images

Inrawong, Prajuab January 2012 (has links)
Viewing fine features of object with optical instruments have become increasingly difficult as the dimensions of many features of interest have become smaller than the traditional optical resolution limit. Examples of these features can be found in semiconductor components and biological tissues. This has caused a move to non-optical methods such as scanning electron and atomic force microscopy techniques, or optical methods combined with signal processing techniques to provide clearer images of samples. This thesis presents a method to increase the resolution of an optical system. This is achieved by using principal component analysis (PCA). Once the PCA measured the object image parameters, the new clearer image can be reconstructed based on these parameters. This process works extremely well. Various aspects of samples measured by the PCA have been investigated, such as the shift of sample, the sample with different sizes, the orientation of sample and the impact of noise. These studies show that the technique is extremely robust, and has huge potential for general usage. The thesis also contains the detail of the Hough Transform technique which was used to provide the initial parameters to the PCA. From the analysis of the technique, it is concluded that the accurate measurement of the technique can be achieved by providing adequate templates of the object image for the system.
9

Visual tracking : from an individual to groups of animals

French, Andrew Peter January 2005 (has links)
This thesis is concerned with the development and application of visual tracking techniques to the domain of animal monitoring. The development and evaluation of a system which uses image analysis to control the robotic placement of a sensor on the back of a feeding pig is presented first. This single-target monitoring application is then followed by the evaluation of suitable techniques for tracking groups of animals, of which the most suitable existing technique is found to be a Markov chain Monte Carlo particle filtering algorithm with a Markov random field motion prior (MCMC MRF, Khan et al. 2004). Finally, a new tracking technique is developed which uses social motion information present in groups of social targets to guide the tracking. This is used in the new Motion Parameter Sharing (MPS) algorithm. MPS is designed to improve the tracking of groups of targets with coordinated motion by incorporating motion information from targets that have been moving in a similar way. Situations where coordinated motion information should improve tracking include animal flocking, people moving as a group or any situation where some targets are moving in a correlated fashion. This new method is tested on a variety of real and artificial data sequences, and its performance compared to that of the MCMC MRF algorithm. The new MPS algorithm is found to outperform the MCMC MRF algorithm during a number of different types of sequences (including during occlusion events and noisy sequences) where correlated motion is present between targets. This improvement is apparent both in the accuracy of target location and robustness of tracking, the latter of which is greatly improved.
10

Tellurite and fluorotellurite glasses for active and passive fibreoptic waveguides

O'Donnell, Matthew David January 2004 (has links)
Glasses systems based on TeO2-ZnO-Na2O (TZN), TeO2-WO3, and TeO2-Na2O-ZnF2 are reported here, with a number of other components added (PbO, GeO2, Nb2O5, Bi2O3, Er2O3, Yb2O3, PbF2, and ErF3). Glass formation was shown for the first time, to this author's knowledge, in the ternary system (90-x)TeO2-10Na2O-xZnF2 for x = 5 to 30 mol. %. Glass stability (Tx-Tg) was found to increase with ZnF2 addition, reaching a plateau of around 161oC at x = 25 mol. %. This could be due to competition of various phases to crystallise (NaZnF3 and Zn2Te3O8) as the eutectic is approached, with fluoride addition. These glasses are the most stable ZnF2 containing tellurite compositions reported to date, to the author's knowledge. As-received ZnF2 batch material was shown to contain a significant proportion of Zn(OH)F, identified by XRD. The as-received ZnF2 was fluorinated with (NH4)HF2, which produced a substantially more phase pure powder, with oxygen levels reduced from around 13.2 to 3.1 at. % from XPS spectra. By calculation from the O1s XPS peaks, the proportion of Zn(OH)F was reduced in the powders from 39.7 to 9.4 mol. %. A number of absorption bands in the infrared were identified by FTIR for the TeO2 -ZnO-Na2O glasses due to intrinsic lattice vibrations (visible in a 0.2 mm sample), and extrinsic impurity absorption, including: free OH (around 3.0 microns, 3300 cm-1), weakly hydrogen-bonded OH (around 3.3 microns, 3060 cm-1), and strongly hydrogen-bonded OH (around 4.8 microns, 2090 cm-1). For the series (90-x)TeO2-10Na2O-xZnF2, mol. %, x = 5 to 30 mol. % melted for 2 hours as bulk glasses, OH bands at 2900 cm-1 were reduced in intensity with ZnF2 addition due to self drying of the melt, from around 0.12 cm-1 (120 dB.m-1) for x = 5 mol. %, to around 0.02 cm-1 (20 dB.m-1) for x > 15 mol. %. Melting time also had a significant effect on drying of this series, with the loss at 2900 cm-1 reduced by two orders of magnitude when increased from 1 hour (around 705 cm-1, 705 dB.m-1) to 2 hours (0.01 cm-1, 10 dB.m-1). Refractive index of the series (90-x)TeO2-10Na2O-xZnF2, mol. %, x = 5 to 30 mol. % decreased linearly with ZnF2 addition, from 2.02 (x = 5 mol. %) to 1.85 (x = 30 mol. %), as the fluoride and zinc are less polarisable than oxygen and tellurium. Cohen-Grest viscosity modelling was used to predict the fibre drawing temperature (around 330oC, corresponding to a viscosity of 10^4.5 Pa.s), and fragility of the fluorotellurite core / clad pair (20 / 25 mol. % ZnF2), occurring at least 60oC < Tx. Fragility of these glasses was predicted to lie between oxide tellurite glasses (stronger) and fluorozirconate glasses (more fragile). Increasing melting time and fluorination resulted in a significant decrease in OH bands in the 700 nm to 2.5 micron region for unstructured 70TeO2-10Na2O-20ZnF2 mol. % fibre, from around 40 to 4 dB.m-1 at 2.5 microns as melting time was increased from 3 to 10 hours. Fibre optical loss for the 10 hour melt was relatively flat (around 5 dB.m-1) over the entire 700 nm to 2.5 micron region.

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