Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by browse.metadata.advisor "Forbes, Andrew"

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    Intra–cavity laser beam shaping
    (Stellenbosch : University of Stellenbosch, 2010-03) Litvin, Igor A.; Forbes, Andrew; Rohwer, Erich G.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: There are many applications where a Gaussian laser beam is not ideal, for example, in areas such as medicine, data storage, science, manufacturing and so on, and yet in the vast majority of laser systems this is the fundamental output mode. Clearly this is a limitation, and is often overcome by adapting the application in mind to the available beam. A more desirable approach would be to create a laser beam as the output that is tailored for the application in mind – so called intra-cavity laser beam shaping. The main goal of intra-cavity beam shaping is the designing of laser cavities so that one can produce beams directly as the output of the cavity with the required phase and intensity distribution. Shaping the beam inside the cavity is more desirable than reshaping outside the cavity due to the introduction of additional external losses and adjustment problems. More elements are required outside the cavity which leads to additional costs and larger physical systems. In this thesis we present new methods for phase and amplitude intra– cavity beam shaping. To illustrate the methods we give both an analytical and numerical analysis of different resonator systems which are able to produce customised phase and intensity distributions. In the introduction of this thesis, a detailed overview of the key concepts of optical resonators is presented. In Chapter 2 we consider the well–known integral iteration algorithm for intra–cavity field simulation, namely the Fox–Li algorithm and a new method (matrix method), which is based on the Fox–Li algorithm and can decrease the computation time of both the Fox–Li algorithm and any integral iteration algorithms. The method can be used for any class of integral iteration algorithms which has the same calculation integrals, with changing integrants. The given method appreciably decreases the computation time of these algorithms and approaches that of a single iteration. In Chapter 3 a new approach to modeling the spatial intensity profile from Porro prism resonators is proposed based on rotating loss screens to mimic the apex losses of the prisms. A numerical model based on this approach is presented which correctly predicts the output transverse field distribution found experimentally from such resonators. In Chapter 4 we present a combination of both amplitude and phase shaping inside a cavity, namely the deployment of a suitable amplitude filter at the Fourier plane of a conventional resonator configuration with only spherical curvature optical elements, for the generation of Bessel–Gauss beams as the output. In Chapter 5 we present the analytical and numerical analyses of two new resonator systems for generating flat–top–like beams. Both approaches lead to closed form expressions for the required cavity optics, but differ substantially in the design technique, with the first based on reverse propagation of a flattened Gaussian beam, and the second a metamorphosis of a Gaussian into a flat–top beam. We show that both have good convergence properties, and result in the desired stable mode. In Chapter 6 we outline a resonator design that allows for the selection of a Gaussian mode by diffractive optical elements. This is made possible by the metamorphosis of a Gaussian beam into a flat–top beam during propagation from one end of the resonator to the other. By placing the gain medium at the flat–top beam end, it is possible to extract high energy in a low–loss cavity.
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    Novel implementation of a phase-only spatial light modulator for laser beam shaping
    (Stellenbosch : Stellenbosch University, 2016-03) Burger, Liesl; Forbes, Andrew; Litvin, Igor; Rohwer, Erich G.; Stellenbosch University. Faculty of Science. Dept. of Physics
    ENGLISH ABSTRACT : The phase-only spatial light modulator (SLM) has revolutionized the field of laser beam shaping. In this thesis we describe in detail the considerations necessary to build a “digital laser” which incorporates an SLM into a laser cavity as an intracavity element to dynamically generate a wide variety of custom beams. We then present a theoretical analysis of the healing of petal-like (or Laguerre-Gaussian) beams using rotational considerations, and using digital laser technology to demonstrate this healing experimentally. We extend our investigation into self-healing beams with the theoretical derivation of a new type of Bessellike beams, which retains a concentric ring structure on propagation, and which self-heal axially. These beams are generated using an SLM, and self-healing is demonstrated experimentally.
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    Tailoring quantum entanglement of orbital angular momentum
    (Stellenbosch : Stellenbosch University, 2014-12) McLaren, Melanie; Forbes, Andrew; Rohwer, Erich G.; Roux, Filippus Stefanus; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: High-dimensional quantum entanglement offers an increase in information capacity per photon; a highly desirable property for quantum information processes such as quantum communication, computation and teleportation. As the orbital angular momentum (OAM) modes of light span an infinite-dimensional Hilbert space, they have become frontrunners in achieving entanglement in higher dimensions. In light of this, we investigate the potential of OAM entanglement of photons by controlling the parameters in both the generation and measurement systems. We show the experimental procedures and apparatus involved in generating and measuring entangled photons in two-dimensions. We verify important quantum tests such as the Einstein, Podolsky and Rosen (EPR) paradox using OAM and angle correlations, as well as a violation of a Bell-type inequality. By performing a full state tomography, we characterise our quantum state and show we have a pure, highly entangled quantum state. We demonstrate that this method can be extended to higher dimensions. The experimental techniques used to generate and measure OAM entanglement place an upper bound on the number of accessible OAM modes. As such, we investigate new methods in which to increase the spiral bandwidth of our generated quantum state. We alter the shape of the pump beam in spontaneous parametric down-conversion and demonstrate an effect on both OAM and angle correlations. We also made changes to the measurement scheme by projecting the photon pairs into the Bessel-Gaussian (BG) basis and demonstrate entanglement in this basis. We show that this method allows the measured spiral bandwidth to be optimised by simply varying the continuous radial parameter of the BG modes. We demonstrate that BG modes can be entangled in higher dimensions compared with the commonly used helical modes by calculating and comparing the linear entropy and fidelity for both modes. We also show that quantum entanglement can be accurately simulated using classical light using back-projection, which allows the study of projective measurements and predicts the strength of the coincidence correlations in an entanglement experiment. Finally, we make use of each of the techniques to demonstrate the effect of a perturbation on OAM entanglement measured in the BG basis. We investigate the self-healing property of BG beams and show that the classical property is translated to the quantum regime. By calculating the concurrence, we see that measured entanglement recovers after encountering an obstruction.