Browsing by Author "Forbes, Andrew."

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Applications of digital holograms for the selection and detection of transverse laser modes.
(2014) Ngcobo, Sandile.; Forbes, Andrew.
The transverse mode of generally available commercial lasers in most instances is not suitable for desired applications. Shaping the laser beam either extra-cavity, that is outside the laser resonator, or intra-cavity, which is inside the laser resonator, is required to force the laser beam or cavity to oscillate on a selected desirable single laser mode. The shaped laser beam’s spatial intensity profile and propagation properties would then be suitable for the desired application. The crux of the work presented in this thesis involves intra-cavity beam shaping where we generate desirable transverse modes from inside the laser resonator and detecting such mode using digital holograms. In Chapter 1 we discuss a novel technique of modal decomposition of an arbitrary optical light field into underlying superposition of modes. We show that it can be used to extract physical properties associated with the initial light field such as the intensity, the phase and M2, etc. We show that this novel modal decomposition approach that requires no a priori knowledge of the spatial scale of the modes which lead to an optimised modal expansion. We tested the new technique by decomposing arbitrary modes of a diode-pumped solid-state laser to demonstrate its versatility. In Chapter 2 we experimentally demonstrate selective generation of Laguerre-Gaussian (LG) modes of variable radial order from 0 to 5, with zero azimuthal order. To generate these customised LG modes from within the laser resonator we show that a specialised optical element in a form of an amplitude mask is required to be inserted inside the laser resonator. The amplitude mask is designed and fabricated to contain absorbing rings which are immutably connected to the desired LG mode. The geometry of the absorbing ring radii are selected to match and coincide with the location of the selected LG mode zero intensity parts inside the cavity. We show for the first time that the generated LG modes using this method are of high mode purity and a gain mode volume similar to the desired LG mode. The results provide a possible alternative route to high brightness diode pumped solid state laser sources. In Chapter 3 we show that we can overcome the disadvantage of the specialised optical element being immutably connected to the selection of a particular mode by experimentally demonstrating a novel digital laser capable of generating arbitrary laser modes inside the laser resonator. The digital laser is realised by intra-cavity replacing an end-mirror of the resonator with a rewritable holographic mirror which is an electrically addressed reflective phase-only spatial light modulator (SLM). We show that by calculating a new computer-generated holographic gray-scale image on the SLM representing the desired customized laser mode digitally, the digital laser resonator is capable of generating the desired laser modes on demand. The results provide a new laser that can generate customized laser modes. In Chapter 4 we show that the digital laser can be used as a test bed for conceptualizing, testing, and proving ideas. We experimentally demonstrate this by using a simple laser cavity that contains an opaque ring which is digitally programmed on the SLM and an adjustable circular aperture on the output coupler mirror. We show that by manually varying the diameter of the aperture without realignment of the laser, the generated laser modes can be tuned from a Gaussian mode to a Flat-top mode. This opens up new digital methods that can be used to test laser beam shaping techniques. In Chapter 5 we outline a simple laser cavity comprising an opaque ring and a circular aperture that is capable of producing spatially tuneable laser modes, from a Gaussian beam to a Flat-top beam. The tuneability is achieved by varying the diameter of the aperture and thus requires no realignment of the cavity. We demonstrate this principle using a digital laser with an intracavity spatial light modulator, and confirm the predicted properties of the resonator experimentally. In Chapter 6 we discuss the techniques used to intra-cavity generate and detect LG beams with a non-zero azimuthal index since they are known to carry orbital angular momentum (OAM), and have been routinely created external to laser cavities. We show that the few reports of obtaining such beams from laser cavities suffer from inconclusive evidence of the real electromagnetic field. In this Chapter we revisit this question and show that an observed doughnut beam from a laser cavity may not be a pure Laguerre–Gaussian azimuthal mode but can be an incoherent sum of petal modes, which do not carry OAM. We point out the requirements for future analysis of such fields from laser resonators. In Chapter 7 we conclude and discuss future work.
Digital control of light.
(2019) Majola, Nombuso.; Chetty, Naven.; Dudley, Angela.; Forbes, Andrew.
The objective of this research was to describe innovative ways in which digital holography can be applied in controlling laser light. The ability to control and manipulate a laser beam has become an extremely desirable feature since it enables improvement in the efficiency and quality of a number of applications. Methods of controlling light make use of optical components to change the properties of a light beam according to the function of that optical element; therefore, a particular arrange- ment of optical elements in a system controls light in a certain way. Technological advancements in the field of optics have developed a versatile device called a spatial light modulator (SLM), which is a novel instrument that employs computer gener- ated holographic patterns (or phase masks) to modulate the amplitude and /or phase of a laser beam and it can therefore perform the function of a number of optical elements. This research presents novel optical set-ups based on the phase-only liquid crystal spatial light modulator (LC-SLM) for generating, controlling and exploring different laser beam pat- terns. The thesis has three main sections, the first one is Holographic beam shaping, where a Gaussian beam was reshaped using an SLM to produce Vortex, Bessel or Laguerre-Gaussian beams. These beams were found to agree with theoretically generated beams. Secondly, we produce o -axis laser beams by constructing coherent superpositions of Gaussian and vortex modes and then use two measurement techniques, peak intensity ratio and modal decomposition technique, to obtain the constituent components of these fields. Finally, we investigate the propagation dynamics of Vortex and Laguerre-Gaussian beams by using a SLM to digitally propagate these beams in free space, and then perform mea- surements on the far field intensity pattern. The results show that the Laguerre-Gaussian beam suffers less spreading and beam distortion compared to the vortex beam in free space propagation.
End-pumped solid-state lasers.
(2010) Koen, Wayne Sean.; Forbes, Andrew.; Bollig, Christoph.
This dissertation consists of four sections, with the focus on near- and mid-infrared lasers using Yttrium Lithium Fluoride (YLF) crystals doped with various rare-earth ions as a gain medium. As introduction a general overview of the concepts pertaining to end-pumped solid- state lasers are presented. The basic principles, components and operation of lasers are discussed. Stimulated emission, laser gain media, pump sources and pump geometries are elaborated upon. Three-, four-, quasi-three- and quasi-four-level laser schemes are described. Finally, the advantages and disadvantages of end-pumping as opposed to side-pumping schemes for solid-state lasers are discussed. Thereafter, the design and results of a high-powered diode-end-pumped Nd:YLF laser is presented. In conjunction with previously demonstrated methods, the thermal fracture issues of Nd:YLF were addressed by utilizing the natural doping gradient along the boule of the crystal. This, in addition to a novel crystal mounting technique, resulted in the highest reported output power from a diode-end-pumped Nd:YLF laser as well as record pumping powers. In the third section, a compact Ho:YLF oscillator-ampli er system is reported. The novel setup utilised the unpolarised pump power from a bre-laser e ciently by using the pump light transmitted by the oscillator crystal to pump the ampli er crystal, which produced 21.3mJ at 1 kHz, with an M2 better than 1.1. Lastly, the conclusion is drawn that YLF as a host material can be used in a highly successful manner for high-power applications. Additionally, the novel pumping scheme implemented in the Ho:YLF oscillator-amplifier has been shown to be scalable by a subsequent system which delivered record performance.
Fabrication of high efficacy selective solar absobers.
(2012) Tile, Ngcali.; Forbes, Andrew.; Roro, Kittessa.
High efficiency tandem selective solar absorber materials of carbon in nickel oxide (C-NiO) composite were fabricated on an aluminium substrate using a simple and cost effective sol-gel process. The process involved preparation of carbon and nickel oxide precursor sols which were homogeneously mixed to form a final C-NiO precursor sol. The carbon precursor sol was prepared by dissolving sucrose (SUC) in 8 ml of distilled water. The NiO precursor sol was prepared by dissolving 7.5 g nickel acetate in 50 ml ethanol, then adding 6.3 g diethanol amine (DEA) to stabilise the solution followed by addition of a structure directing template of polyethylene glycol (PEG). The final C-NiO precursor sol was spin coated on pre-cleaned aluminium substrate to form thin films which were then heat treated in nitrogen ambient inside a tube furnace. The final heat treatment temperature of the sols was determined by thermal studies using thermo gravimetric analytic (TGA) and differential scanning calorimetric (DSC) techniques. TGA and DSC studies of the final precursor sol showed that the weight loss of the precursors stabilised at around 450 °C. The impact of the sol-gel process parameters namely heat treatment temperature, PEG content, SUC content as well as spin coating speed on the optical properties i.e. solar absorptance (αsol) and thermal emittance (εtherm) was investigated. It was found that the optical properties as well as photo-thermal conversion efficiency, η = αsol - εtherm, improved with an increase in heat treatment temperature in the range studied (300-550 °C). This is in good agreement with the results obtained from thermo-gravimetric analysis which showed the weight loss of the precursor to stabilise around a temperature of 450 °C. Results obtained from the Raman studies showed a progressive increase in the graphitic domain in C-NiO samples with an increase in temperature. Heat treatment temperatures above 450 °C gave the best optical properties. Scanning electron microscopy (SEM) results showed that samples that did not have PEG in the precursor sol were compact and an addition of PEG in the precursor sol caused an increase in the size and density of pores in the films produced which affected the optical properties. As a result, the optical properties increased with an increase in PEG content from 0 g to 2 g then decreased with further increase in PEG content. It was found that addition of SUC of up to 8 g in the sol did not change the optical properties of the fabricated materials because SUC contributed little carbon to the final composite material. Further increase in SUC content resulted in materials with poor photo-thermal conversion efficiency. An increase in spin coating speed did not change the absorptance of the materials but it improved their thermal emittance. The best spin coating speed was found to be 7000 RPM. A solar absorptance of 0.81 and thermal emittance of 0.06 have been achieved for an optimum sample in this study yielding a photo-thermal conversion efficiency of 0.75. The optimum sample fabricated in this study showed superior optical properties compared to the widely used commercial solar absorber paint. This suggests that the C-NiO composite material has the potential for possible use as a selective solar absorber in a solar collector.
Generation and detection of bessel beams.
(2015) Mhlanga, Thandeka.; Forbes, Andrew.; Konrad, Thomas.
In this dissertation we study the properties of Bessel-Gauss beams. Bessel-Gauss beams are created by the interference of plane waves lying on a cone, and have unique properties: they propagate without spreading and recover their phase and amplitude upon encountering an obstruction. These modes have found application in the manipulation of micro-particles, atomic dipole traps, and atomic guiding. As high-order Bessel-Gauss beams carry orbital angular momentum, they have been used as a basis for information encoding in both the classical and quantum regimes. We show how to generate these modes using axicons, and spiral ring-slits, which we implement digitally on a spatial light modulator. Using an all digital experimental setup we extract the information encoded in these modes in two dimensions, where we simultaneously detect the radial and azimuthal components of these beams. This detection tool is shown to be useful in studying Bessel-Gauss modes that have propagated through optical turbulence and that have been obstructed. Vector Bessel-Gauss beams are then generated and detected using a q-plate and polarized grating, respectively. We then apply the reconstruction property of the Bessel-Gauss modes in a quantum experimental setup, where we show that we can recover quantum entanglement after encountering an obstruction. We show that the digital spiral ring-slit can be used at the single photon level as a single pixel detector, to recover the phase and amplitude on an object in a ghost imaging setup.
Implementing Grover's search algorithm using the one-way quantum computing model and photonic orbital angular momentum.
(2011) Bassa, Humairah.; Konrad, Thomas.; Forbes, Andrew.
Standard quantum computation proceeds via the unitary evolution of physical qubits (two-level systems) that carry the information. A remarkably different model is one-way quantum computing where a quantum algorithm is implemented by a set of irreversible measurements on a large array of entangled qubits,, known as the cluster state. The order and sequence of these measurements allow for different algorithms to be implemented. With a large enough cluster state and a method in which to perform single-qubit measurements the desired computation can be realised. We propose a potential implementation of one-way quantum computing using qubits encoded in the orbital angular momentum degree of freedom of single photons. Photons are good carriers of quantum information because of their weak interaction with the environment and the orbital angular momentum of single photons offers access to an infinite-dimensional Hilbert space for encoding information. Spontaneous parametric down-conversion is combined with a series of optical elements to generate a four-photon orbital angular momentum entangled cluster state and single-qubit measurements are carried out by means of digital holography. The proposed set-up, which is based on an experiment that utilised polarised photons, can be used to realise Grover’s search algorithm which performs a search through an unstructured database of four elements. Our application is restricted to a two-dimensional subspace of a multi-dimensional system, but this research facilitates the use of orbital angular momentum qubits for quantum information processing and points towards the usage of photonic qudits (multi-level systems). We also review the application of Dirac notation to paraxial light beams on a classical and quantum level. This formalism is generally employed in quantum mechanics but the analogy with paraxial optics allows us to represent the classical states of light by means of Dirac kets. An analysis of the analogy between the classical and quantum states of light using this formalism, is presented.
Modelling diode-pumped solid-state lasers.
(2008) Bernhardi, Edward H.; Forbes, Andrew.; Bollig, Christoph.
This thesis consists of three main parts. An introduction to diode-pumped solid-state lasers, thermal modelling of solid-state lasers and rate-equation modelling of solid-state lasers. The first part explains the basic components and operation principles of a typical diode-end-pumped solid-state laser. The stimulated emission process, solid-state laser gain media, various pump geometries and a basic end-pumped laser resonator configuration are among the topics that are explained. Since thermal effects are one of the main limiting factors in the power-scaling of diode-pumped solid-state lasers, the second part of this thesis describes numerical and analytical thermal models that determine the thermal lens and thermally induced stresses in a laser crystal. As a first step, a time-independent numerical thermal model which calculates the three-dimensional temperature distribution in the laser crystal is implemented. In order to calculate the time dependent thermally induced stresses in a laser crystal, a coupled thermal-stress finite element analysis model was implemented. Even though some steady-state analytical solutions for simple crystal geometries do exist, the finite element analysis approach was taken so that the time dependent thermally induced stresses could be calculated for birefringent crystals of various geometries. In order to validate the numerical results, they are compared to experimental data and analytical solutions where possible. In the last part, the population dynamics inside the laser gain medium are described and modelled with a quasi-three-level rate-equation model. A comprehensive spatially resolved rate-equation model is developed and discussed. In order to simplify the implementation of the rate-equation model as a computer simulation, the spatial dependence of the laser parameters is ignored so that the model reduces to a singleelement plane-wave model. The simplified rate-equation model is implemented and solved numerically. The model is applied to a four-level CW and Q-switched Nd:YLF laser as well as a quasi-three-level QCW Tm:GdV04 laser. The models' predictions are thoroughly verified with experimental results and also with analytical solutions where possible.
Novel laser beams for optical trapping and tweezing.
(2011) Ismail, Yaseera.; Forbes, Andrew.
Optical trapping and tweezing has been around for the last 30 years and since found its place in the fields of physics and biology. Over the years this technique has advanced exceedingly and is a unique tool to carry out research in the micrometre and nanometre scale regime. The aim of this dissertation was to illustrate that an optical trapping and tweezing system is an effective tool for the manipulation of micron sized particles and that using such a system allows one the ability to accurately and precisely measure optical forces in the piconewton scale. A custom built single gradient optical trapping system was built to illustrate the manipulation of micron sized particles. Here we will highlight some of the key components of such a system and give an explanation of how these components affect the optical trap. To enhance this system, we exploit the ability to shape light and in particular laser light to generate novel laser beams. This was achieved using a diffractive optical element known as a spatial light modulator (SLM). A spatial light modulator is an electronically addressed optical element which when incorporated into an optical system effectively manipulates the phase of light in order to generate various novel laser beams. In particular these novel laser beams include Laguerre-Gaussian, Bessel and recently proposed Bessel-like beams. Each of these beams contains interesting properties which can be beneficially exploited. Laguerre-Gaussian beams are particularly known as ‘donut’ shaped beams since they have a central dark hole. Increasing the order of these Laguerre-Gaussian beams leads to an increase in the central dark region. These beams are of particular interest since they carry orbital angular momentum. This is not easily observed; however, when incorporated into the optical trapping system, leads to the rotation of trapped particles due to the transfer of photons carrying orbital angular momentum. Bessel and Bessel-like beams on the other hand are classes of beam that possess interesting non-diffracting and self-reconstructive properties upon encountering an obstacle. Here the generation and properties of these novel laser beams will be discussed in detail. Furthermore it is well known that these novel laser beams prove highly useful when incorporated into an optical trapping system hence we will illustrate the effects on a trapped particle when incorporating a Laguerre-Gaussian beam carrying a topological charge of one. It is expected that the trapped particle should rotate due to the transfer of orbital angular momentum. The knowledge gained from beam shaping and the means to trap micron sized particles optically allows one the ability to incorporate this technique in a number of fields, including the promising field of microfluidics. This is an emerging field that deals with investigating fluid properties at the nano and microlitre regime. Optical tweezers integrated into a microfluidic device are beneficial since they are an adequate tool for measuring fluid flow using Stokes’ Law.
Optimizing the synthesis of vanadium oxide nano-structures by plasma plume dynamics.
(2016) Masina, Bathusile Nelisiwe.; Forbes, Andrew.; Mwakikunga, B. W.
Abstract available in PDF file.
Photothermal refraction and focusing.
(1997) Forbes, Andrew.; Michaelis, Max M.
This thesis begins with an introduction to the interaction and refraction of light in continuous media. It is shown how these properties can be exploited to achieve focusing of parallel light rays in such a medium. Past work on Gas Lenses is reviewed, highlighting the progress in design of gas lenses, leading to a justification for the research described in the rest of the chapter. Original work by the author on the subject of continuous gas lenses at low and high pressure is then presented. Experiments show that gas lenses at low pressure have stable foci, but long focal lengths, while at high pressure two foci are produced, both of unstable character. These results are explained by a simple theory, and future applications of such lensing properties are presented. Chapter two introduces the concept of the Colliding Shock Lens (CSL), and presents shallow water wave simulations, conducted by the author, as a useful analogy to the interaction of shocks in the CSL. All the properties of the CSL lensing action are reproduced in the water simulations, yielding useful insight, by means of a simple experiment, into the physics of interacting shock waves. Chapter three presents original work by the author on the subject of multiple pulse thermal lensing. A theory is developed which predicts the behaviour of thermal lenses seen in an industrial laser chain. Experiments on thermallensing, as well as some solutions, are presented and discussed. Chapter four revises the theory of Zernike Polynomials and their application to the study of aberrations. Thermal aberrations are studied, including the aberrations introduced by thermal lensing and thermal blooming. The relationship between aberrations and subsequent beam quality and beam propagation is explored. Chapter five looks at the use of adaptive mirrors for mode matching. Although the theory of adaptive systems is well known, no-one has as yet tackled the problem of correcting for mode matching changes. A new way of thinking about mode matching is proposed, and the merits of this system, called characterisation space, are explained. Chapter six comprises the theory and design of a novel vacuum chamber which has applications in gas lens designs. All the gas lenses used in pressure experiments were housed in compressional vacuum chambers. The idea of a Tensional Vacuum Vessel (TVV) is introduced, and experiments show that such chambers are very successful low vacuum chambers. The advantages and applications of TVVs are discussed, specifically those relating to gas lens applications. At the end of this thesis it was apparent that more questions had been generated than answers. This is probably true of any study. Chapter seven therefore outlines some as yet unanswered questions, and gives some suggestions for starting points. Some of this work is presently being undertaken by the author.
Superpositions of light fields carrying orbital angular momentum.
(2012) Dudley, Angela.; Forbes, Andrew.; Konrad, Thomas.
The work presented in this thesis is centred on the generation of superimposed optical fields which each carry orbital angular momentum (OAM) and the development of OAM measurement techniques. Optical fields which carry OAM have found applications ranging from optical tweezing to quantum cryptography. Due to the fact that they offer a potentially infinite-dimensional state space, much interest has been generated in the measurement of OAM in optical fields, in order for higher-dimensional quantum information processing to be realised. In this study we generate superpositions of higher-order Bessel beams and show that even though we can create a field which carries no overall OAM, we can still witness an angular rotation in the intensity profile of the beam. We also develop two new OAM measurement techniques: (1) a robust odd-even-OAM interferometer and (2) a method to measure the OAM density of an optical field by means of a single spatial light modulator (SLM). In the first chapter we give an overview of the literature regarding optical OAM, followed by the derivation of the Helmholtz wave equation from Maxwell’s equations. We illustrate that helically-phased beams, having a phase factor of exp(ilθ), possess a well-defined OAM. Definitions for the fundamental Gaussian mode, as well as two OAM-carrying modes: Laguerre-Gaussian (LG) and Bessel-Gaussian (BG) modes are also given. Since a majority of this thesis involves generating superimposed OAM fields as well as the measurement of OAM, chapter 2 contains detailed discussions on the optical components used to generate and measure OAM. In section 2.9 we present one of our contributions to the field of OAM-measurement, which involves a stable Dove-prism embedded Mach-Zehnder interferometer, capable of sorting 41 OAM states into odd and even ports with a contrast ranging from 92% to 61%. We implement the Dove prism embedded Mach-Zehnder interferometer to mimic an amplitude damping channel for OAM states in chapter 3. Our device is useful in modelling a ‘lossy’ environment for OAM states. In chapter 4 we develop a new technique for the generation of superimposed Bessel beams through the use of a single digital hologram and theoretically and experimentally show that even though the superimposed Bessel beams can be constructed to produce no overall OAM, a rotation in the beam’s intensity profile is still present, as the field propagates. This rotation is due to the differing longitudinal wave-vectors present in the field and we make quantitative, experimental measurements of the angular rotation rates, which are in very good agreement with our theoretical predictions. We also show that the far-field of these superimposed Bessel beams, exhibit no rotation in their intensity profile and we offer a theoretical explanation for this occurrence. In chapter 5, we adapt our technique for generating superimposed Bessel beams to create non-diffracting speckle fields, which are known to possess optical vortices, and show that by controlling the standard deviation of the phase distribution within the digital hologram, we are able to control the evolution of the non-diffracting speckle field into a non-diffracting zero-order Bessel beam. Our final chapter contains a novel technique for the measurement of the OAM density of optical fields, by implementing two optical components: an SLM and a lens.