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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.

Computation and measurement of the degree of spatial coherence

Nichols, P. D. January 1983 (has links)
No description available.

Whole field, real time photorefractive holography for imaging through turbid media using sources of diverse spatial and temporal coherence

Ansari, Zunaira January 2002 (has links)
No description available.

Structural evolution of the Val d'Agri fault system, Southern Apennines, Italy

Gambini, Stefano January 2003 (has links)
No description available.

Visual texture integration processes and the role of selective attention

O'Donnell, Helen Louise January 2000 (has links)
No description available.

Can sense of coherence predict therapeutic outcome of a brief guided self-help intervention?

Williams, Mhairi Elizabeth January 2010 (has links)
Background: The construct sense of coherence (SOC) is proposed to explain the variation in the way people cope and it has been linked with positive mental health. Evidence suggests that level of SOC may be able to predict therapeutic outcome. There is a lack of evidence regarding individual predictors for treatment response of guided self-help services. Therefore, SOC is an important construct to consider. Objectives: The purpose of this study was to investigate whether a client’s sense of coherence at the start of a guided self-help intervention could predict their therapeutic outcome. The possibility that coping style mediated this relationship was also examined. Method: The study employed a longitudinal survey design. Participants were patients aged 30-64 years attending a guided self-help service for mild-moderate psychological difficulties. Participant data was collected pre and post intervention (3 weeks to 3 months after initial appointment). Results: A significant negative association was found between SOC and pre intervention anxiety and depression scores. No significant relationship was found between SOC and post intervention anxiety and depression scores (therapeutic outcome). Multiple regression analysis found that sense of coherence and coping style were not significant predictors of therapeutic outcome. Conclusions: It is important to determine the causality of SOC’s relationship with mental health because if SOC can be influenced via psychological intervention this may promote positive mental health and effective coping. Therefore, further research is required to determine if SOC has clinical application.

A data-driven methodology for motivating a set of coherence relations

Knott, Alistair January 1996 (has links)
The notion that a text is coherent in virtue of the `relations' that hold between its component spans currently forms the basis for an active research programme in discourse linguistics. Coherence relations feature prominently in many theories of discourse structure, and have recently been used with considerable success in text generation systems. However, while the concept of coherence relations is now common currency for discourse theorists there remains much confusion about them, and no standard set of relations has yet emerged. The aim of this thesis is to contribute towards the development of a standard set of relations. We begin from an explicitly empirical conception of relations: they are taken to model a collection of psychological mechanisms operative during the tasks of reading and writing. This conception is fleshed out with reference to psychological theories of skilled task performance, and to Rosch's notion of the basic level of categorisation. A methodology for investigating these mechanisms is then presented, which takes as its starting point a study of cue phrases- the sentence/clause connectives by which they are signaled. Although it is conventional to investigate psychological mechanisms by studying human behaviour, it is argued here that evidence for the constructs modelled by relations can be sought in ananalysis of the linguistic resources available for marking them explicitly intext. The methodology is based on two simple linguistic tests: the test for cue phrases and the test for substitutability. Both tests are functional in inspiration: the former test identifies a heterogenous class of phrases used for linking one portion of text to another; and the later test is used to discover when a writer is willing to substitute one of these phrases for another. The tests are designed to capture the judgements of ordinary readers and writers, rather than the theoretical intuitions of specialised discourse analysts. The test for cue phrases is used to analyse around 20 pages of naturally occuring text, from which a corpus of over 20 cue phrases is assembled. The substitutability test is then used to organise this corpus into a hierarchical taxonomy, representing the substitutability relationship between every pair of phrases. The taxonomy of cue phrases lends itself neatly to a model of relations as feature-based constructs. Many cue phrases can be interpreted as signalling just some features of relations, rather than whole relations. Small extracts from the taxonomy can be used systematically to determine the alternative values of single features; complex relation definitions can then be formed by combining the values of many features. The thesis delivers results on two levels. Firstly,it sets out a methodology for motivating a set of relation definitions, which rests on a systematic analysis of oncrete linguistic data, and demands a minimum of theoretical assumptions. Also provided are the relation definitions which result from applying the methodology. The new definitions give an interesting picture of the variation that exists amongst cuephrases, and offers a number of innovative insights into text coherence.

Energy considerations, propagation in random medium and imaging in scalar coherence theory

Skinner, Thomas Junior January 1964 (has links)
Thesis (Ph.D.)--Boston University / After a review of scalar coherence theory the infinite-time averages of coherence theory are interpreted in terms of the conserved quantities of any scalar wave field and these averages are related to the finite-time averages of physical measurements. In particular it is shown that the infinite-time average of the square of the wave field (intensity) is proportional to the average energy flux under many, but not all, circumstances. By application of the energy conservation law for the scalar wave field it is shown that the average energy flux through a plane surface depends not only upon the intensity but also upon the correlation of the field. An explicit example of this dependence is given. Further the assumptions which underlie the association of infinite-time averages with actual physical measurements are given explicitly. After a review of ensemble coherence theory, an ergodic theorem which relates the time and ensemble average coherence theories is presented. In particular it is shown that if the boundary conditions are ergodic (i.e., the ensemble averages on the boundary are equal to the time averages), then the solutions are also ergodic. In Chapter VI the investigation is turned from the formal properties of coherence theory to an application of the theory to a particular problem in wave motion - the propagation of waves in a random medium. Random medium here means any region of space for which the speed of the waves varies randomly in both space and time. By invoking an ergodic hypothesis the time and space varying medium is replaced by an ensemble of media which vary only in space. For such media it is shown that the ensemble average behavior depends only upon the ensemble averaged, two-point cross correlation of the fluctuations in the index of refraction of the media. Further, an approximate solution to the problem of the propagation of a wave beam of finite cross -section and divergence (spread) through a slab of Gaussianly correlated random medium is given. The principle conclusions are that the slab produces mainly forward scattering and that the scattered wave is highly coherent. In Chapter VII a wave theory of imaging is developed from the equations of motion for the coherence. From this theory explicit representations for imaging in the cases of coherent and incoherent illumination are derived. These representations show that imaging with coherent illumination is basically nonlinear in terms of what is observable in the object plane and observable in the image plane. In Chapter VIII various consequences of the nonlinear equation governing imaging with coherent illumination are exhibited. Lacking a general theory of nonlinear equations, the properties of coherent imaging cannot be summarized by giving a Green's function or transfer function. Instead the nature of the images of simple geometrical objects are given in some detail. From examples of coherent imaging it is concluded that the two most striking features are that the images of edges "ring" and appear to be shifted in relation to the same images under incoherent illumination. By "ring" it is meant that the image of an edge is not a monotonic change in intensity, but rather, that interference fringes occur in the neighborhood of the location of the edge. Further, coherent imaging is linearized by restricting the object to be of low contrast. Having linearized coherent imaging, it is directly compared with incoherent imaging. Finally, a particular problem in optics is analyzed - that of the sparkle of laser light. It is shown that all the observed properties of the sparkle can be explained by considering the nature of the image of a coherently illuminated, rough surface. By a rough surface it is meant any surface which deviates randomly from a plane (or other simple geometrical surface that could be constructed to be coincident with a wavefront of the illumination). It is shown that the image of a coherently illuminated rough surface is as if incoherent illumination were used - edge ringing and shifting are not present - but that the image is modified by random fluctuations in intensity. These random fluctuations give the image a speckled appearance. It is shown that this speckling is Rayleigh distributed and the dependence of the statistical parameters upon the nature of the imaging system (lens) is exhibited.

Experimental study of holography with partially coherent light.

January 1976 (has links)
Title also in Chinese. / Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaves 92-93.

Photon sources for linear optical quantum computing

Lee, James January 2019 (has links)
Quantum photonic technologies have many exciting applications including secure commu- nication, quantum enhanced measurement and quantum computing. Linear optical quantum computing (LOQC) is a technology of particular interest - especially given that recent progress in on-chip waveguide technology removes the requirement for complex and costly bulk optics setups and state-of-the-art detectors have detection efficiencies of over 90%. Arguably the largest remaining technological hurdle for LOQC is the development of a suitable photon source. A suitable source would produce single, indistinguishable photons deterministically. Additionally, it would be beneficial if the generated photons were entangled, as this can significantly reduce the degree of multiplexing needed to implement LOQC. Quantum dots are a suitable candidate system for these photon sources as they exhibit bright single photon emission and can act as the interface between light and a trapped spin qubit. These properties have resulted in proposals to generate multi-photon entangled states suitable for use in LOQC. This thesis presents some of the progress we have made towards the creation of a suitable photon source. After introducing the background material, we demonstrate pulsed resonant excitation using a single-electron-charged quantum dot. Deterministic excitation is demonstrated by performing Rabi oscillations and Ramsey interference in the excitonic population. We also investigate Ramsey interference in a Faraday geometry magnetic field and observe a variety of beats and oscillations in the interferograms. We develop a model to explain our results and conclude that controlling the phase between the two Ramsey interference pulses allows a degree of control over the state of the trapped spin. We then also demonstrate the coherent optical manipulation of a trapped spin in a Voigt geometry magnetic field. Once we have presented the manipulation of the excitonic state and the state of the trapped spin, we proceed to investigate the properties of the light produced by the resonant excitation of a quantum dot. Hong Ou Mandel interference experiments allow us to probe the indistinguishability of the photons resulting from the resonant excitation of the negative trion transition. Repeating the measurement using light generated from a similar system (this time with a trapped hole rather than a trapped electron) that is embedded in a micropillar cavity, we find that the cavity enhancement of the transition results is higher indistinguishabilities. We make use of this bright source of indistinguishable photons to perform an on-chip quantum enhanced measurement and observe the phase superresolution associated with N00N states. In the final experimental chapter, we propose and implement a scheme to generate multi- qubit single photon states. We show that by repeatedly driving a micropillar-cavity-enhanced Raman transition of a single-hole-charged quantum dot in a Voigt geometry magnetic field it is possible to coherently superpose a photon across multiple time bins. The scheme is conceptually similar to proposed schemes for producing multi-photon entangled states. Lastly, we propose a scheme that makes use of the capabilities shown in the three experimental chapters to overcome several of the experimental difficulties associated with generating multi-photon entangled states.

How to run a semiconductor diode laser in a stable way

Arnesson, Fredrik January 2012 (has links)
Interferometry and holography are two well-known methods for measuring distances, positions, vibrations, index of refraction etc. In these methods a coherent light source is used to create interference between different parts of the light. Since the wavelength of the light is used as reference it is possible to achieve very good accuracy in the measurements. The need of small and cheap light sources for these applications is large and an interesting alternative would be to use ordinary semiconductor diode lasers. These are unfortunately not designed to give sufficiently good coherence. In this Master Thesis work investigations of how the coherence of semiconductor diode lasers is affected by changes in temperature, injection current and between different individuals are performed. A Michelson interferometer is used to create an interference pattern where the contrast then can be analyzed. The contrast is related to the coherence of the laser, i.e., good coherence will give high contrast. The results show that in order to drive the laser in a stable way it is better to hold the temperature constant and varying the injection current until the wanted output power is achieved instead of doing the opposite. The results also indicate that the best coherence is achieved for low temperatures (around 10 OC) and high injection currents (around 80 mA). During these conditions a contrast of 70 % -80 % is achieved. The result of this Master Thesis work gives a hint on how to run a semiconductor diode laser in a stable way.

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