This thesis describes the design and implementation of several novel, nanosecond pulsed, intracavity optical parametric oscillators for the generation of terahertz radiation. The application of the intracavity approach in the context of terahertz optical parametric oscillators has been demonstrated for the first time, and the pump wave energy required was thereby reduced by an order of magnitude. The terahertz wave was tunable from under 1THz up to 3THz with a free running linewidth of ~50GHz and pulse energies up to ~20nJ (pulses were a few nanoseconds in duration). The terahertz beam profile was of Gaussian shape and could be focussed down to 2.3 times the diffraction limited spot size (M² values of 2.3 and 6.7 in the components of the beam parallel and perpendicular to the silicon prism array output coupler respectively). Developments of this intracavity source with regard to the linewidth are also reported. Implementation of etalons in the optical (laser and OPO) cavities was shown to be a promising technique that brings the terahertz linewidth down below 1GHz (close to the transform limit of nanosecond pulses) while retaining the tuning range and beam characteristics of the free running system. Close to Fourier transform limited pulses were obtained (<100MHz linewidth) via an injection seeding technique, although with significantly increased system complexity. A deleterious effect caused by the mode beating of a multimode host laser was also discovered, in that sidebands were induced on the seeded downconverted wave. This has wider implications in the field of intracavity OPOs. Finally, quasi-phasematching techniques implementing periodically poled lithium niobate were investigated as a way to lower the downconversion threshold energy requirement (by collinear propagation of the optical waves), and also to extract the terahertz wave rapidly from the (highly absorbing in the terahertz region) lithium niobate crystal. The existence of two phasematching solutions arising from the bidirectionality of the grating vector was identified as a serious design constraint in the context of an OPO where either solution can build up from noise photons, and so prefers the solution with the lowest walkoff of the downconverted waves - possibly resulting in unextractable terahertz radiation. Quasi-phasematching with an orthogonal grating vector (with identical but opposite phasematching solutions) was demonstrated and cascaded downconversion processes observed and characterised. These cascaded processes are permitted by the collinearality of the optical waves and may allow efficiency improvements through overcoming the quantum defect limit. This research has resulted in four peer reviewed papers in respected journals, and the intracavity terahertz OPO has been licensed to a company who have commercialised the technology (M Squared Lasers, Glasgow).

<p>In this report we describe simulation of the surface current density on a PEC cylinder and the diffracted field for a line source above a finite PEC ground plane as a means to verify the Nero2D program. The results are compared with the exact solution and give acceptable errors. A terrain model for a communication link is studied in the report and we simulate the wave propagation for terrain with irregular shapes and different materials. The Nero2D program is based on the fast multipole method (FMM) to reduce computation time and memory. Gaussian sources are also studied to make the terrain model more realistic</p>

We present Bessel function and Gaussian beam models for a study of microwave power deposition in bounded and inhomogeneous lossy media. The aim is to develop methods that can accurately simulate practical results commonly found in electromagnetic hyperthermic treatment, which is a noninvasive method. The Bessel function method has a closed form solution and can be used to compute accurate results of electromagnetic fields emanating from applicators with cosinusoidal aperture fields. On the other hand, the Gaussian beam method is approximate but has the capability to simplify boundary value problems and to compute fields in three-dimensions with extremely low CPU time (less than 30 sec). Although the Gaussian beam method is derived from geometrical optics theory, it performs very well in domains outside the realm of geometrical optics which stipulates that aperture dimension/λ ≥ 5 in the design of microwave systems. This condition has no relevance to the Gaussian beam method since the method shows that a limit of aperture dimension/ λ ≥ 0.9 is possible, which is a very important achievement in the design and application of microwave systems. Experimental verifications of the two theoretical models are integral parts of the presentation and show the viability of the methods.

In this report we describe simulation of the surface current density on a PEC cylinder and the diffracted field for a line source above a finite PEC ground plane as a means to verify the Nero2D program. The results are compared with the exact solution and give acceptable errors. A terrain model for a communication link is studied in the report and we simulate the wave propagation for terrain with irregular shapes and different materials. The Nero2D program is based on the fast multipole method (FMM) to reduce computation time and memory. Gaussian sources are also studied to make the terrain model more realistic

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

A short history of acousto-optics research is presented along with a general description of how light and sound interact. The Multiple Scattering model is derived and used with a Gaussian light beam to observe the distortion in light beam profile within the sound field. Numerical results are presented for comparison to previous studies using thick holograms and two orders of light. The results from using two light orders are compared to four light order results. A Hamming sound amplitude distribution is introduced as a possible way to reduce the amount of light beam profile distortion. / Master of Science

Real-time holographic recording using an optical heterodyne scanning technique was proposed by Poon in 1985. The first part of this dissertation provides a detailed theoretical treatment of the technique, based on a Gaussian beam analysis. Topics to be addressed include the derivations of the optical transfer function (OTF) and impulse response of the scanning holographic recording system, reconstructed image resolution and magnification, methods of carrier frequency hologram generation and experimental verification of the recording technique based on careful measurements of a hologram corresponding to a simple transmissive slit. Furthermore, computer simulations are presented pertaining to the incoherent nature of the scanning holographic process and it is shown that this new technique can be used to reduce the effects of bias buildup common in conventional incoherent holographic methods. The reconstruction of holograms generated by the heterodyne scanning technique is then considered in the second part of the dissertation. The primary concentration is on real-time reconstruction using an electron beam addressed spatial light modulator (EBSLM). For comparison, experimental coherent reconstruction methods are presented as well. Additional topics to be addressed are the spatial frequency limitations of the EBSLM and the derivation of the overall incoherent point spread function (PSF) for the holographic imaging (recording/reconstruction) system. Based upon the derived overall PSF, the reconstructed real image of a simple slit object is formulated, compared to, and shown to be consistent with experimental observations. / Ph. D.

In this thesis, an analysis of the scattering of a rough perfect electric conductor (PEC) surface under illumination by a Gaussian beam using the Kirchhoff approximation is presented. The analysis assumes a source distribution which yields a Gaussian beam solution as a radiated field. This field is used to excite a current density on the surface using the Kirchhoff approximation. A vector potential approach utilizes this current to calculate the fields scattered by the surface. The analysis is carried out for the backscatter case and near-normal incidence in order to reduce the final numerical evaluation to a two-dimensional integration. The normalized radar cross-section (NRCS) is calculated and compared with the result for plane wave illumination. The analysis explores the effects of varying the source aperture size, rough surface correlation length and rms height on the NRCS. An asymptotic evaluation of the mean squared field is presented, as well as the mathematical form of the fourth moment of the scattered field. As a further study, the NRCS of a rough surface under a Gaussian tapered plane wave illumination is presented. The interplay of the beam spot and correlation length for such illuminated surfaces is discussed. / Master of Science

Thesis (PhD)--Stellenbosch University, 2014. / 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. / AFRIKAANSE OPSOMMING: Hoë-dimensionele kwantumverstrengeldheid bied ’n toename in inligtingskapasiteit per foton. Hierdie is ’n hoogs wenslike eienskap vir kwantum inligting prosesse soos kwantum kommunikasie, berekening en teleportasie. Omdat die orbitale hoekmomentum (OAM) modusse van lig ’n oneindig dimensionele Hilbertruimte beslaan, het dit voorlopers geword in die verkryging van verstrengeling in hoër dimensies. In die lig hiervan, ondersoek ons die potensiaal van OAM verstrengeling van fotone deur die parameters in beide die generering en meting stelsels te beheer. Ons toon die eksperimentele prosedures en apparaat wat betrokke is by die generering en die meet van verstrengelde fotone in twee dimensies. Ons verifieer kwantumtoetse, soos die Einstein, Podolsky en Rosen (EPR) paradoks vir OAM en die hoekkorrelasies, sowel as ’n skending van ’n Bell-tipe ongelykheid. Deur middel van ’n volledige toestand tomografie, karakteriseer ons die kwantum toestand en wys ons dat dit ’n suiwer, hoogs verstrengel kwantum toestand is. Ons toon ook dat hierdie metode uitgebrei kan word na hoër dimensies. Die eksperimentele tegnieke wat tydens die generasie en meet van OAM verstrengeling gebruik is, plaas ’n bogrens op die aantal toeganklik OAM modusse. Dus ondersoek ons nuwe metodes om die spiraal bandwydte van ons gegenereerde kwantum toestand te verhoog. Ons verander die vorm van die pomp bundel in spontane parametriese af-omskakeling en demonstreer die uitwerking daarvan op beide OAM en die hoekkorrelasies. Ons het ook veranderinge aan die meting skema gemaak deur die foton pare op die Bessel-Gauss (BG) basis te projekteer. Ons wys dat hierdie metode die gemeetde spiraal bandwydte kan optimeer deur eenvoudig die kontinue radiale parameter van die BG modes te verander. Ons demonstreer dat BG modusse verstrengel kan word in hoër dimensies as die heliese modusse, wat algemeen gebruik word, deur berekeninge te maak en te vergelyk met lineêre entropie en vir beide modusse. Ons wys ook dat kwantumverstrengling akkuraat nageboots kan word, met behulp van die klassieke lig terug-projeksie, wat die studie van projeksie metings toelaat en voorspel die krag van die saamval korrelasies in ’n verstrengeling eksperiment. Ten slotte, gebruik ons elk van die tegnieke om die effek van ’n storing op OAM verstrengling wat in die BG basis gemeet is, te demonstreer. Ons ondersoek die self-genesingseienskap van BG bundels en wys dat die klassieke eienskap vertaal na die kwantum-gebied. Deur die berekening van die konkurrensie (concurrence), sien ons dat die gemeetde verstrengeling herstel word nadat ’n obstruksie ondervind is.

Thesis (MSc)--University of Stellenbosch, 2003. / ENGLISH ABSTRACT: The efficient manipulation of various nonlinear optical processes frequently requires the shaping of the laser beams used for these processes. Three beam shaping techniques were investigated in this thesis. The focussing of Gaussian laser beams was investigated analytically, in order to efficiently manipulate the focussed beam characteristics. The beam-shaping characteristics of a diffractive optical element (DOE) was investigated numerically, which illustrates the beamshaping capability of the DOE, and identifies the critical parameters in experimental situations. The use of a waveguide as beam shaping device was investigated analytically and experimentally, and characterized for use with the available tunable laser sources. A Raman laser, or Raman shifter, employs stimulated vibrational Raman scattering to generate laser radiation at shifted frequencies. The waveguide was successfully applied as a beam shaping device in the Raman laser system, for optimisation of the process. The Raman laser system was investigated experimentally and characterized for use with the available tunable laser sources. The successful generation of laser radiation at shifted frequencies illustrates the usefulness of the system for generating tunable red-shifted frequencies. The results of this work allow the simple and efficient application of the Raman laser to generate laser radiation at shifted frequencies, in particular tunable infrared laser radiation which is desirable for molecular spectroscopy. / AFRIKAANSE OPSOMMING: Nie-liniêre optiese prosesse kan meer effektief benut word deur die vervorming van die laserbundels wat gebruik word in die prosesse. In hierdie tesis word drie laserbundel-vervormings tegnieke ondersoek. Die fokussering van Gaussiese laserbundels word analities ondersoek, om die gefokusseerde bundel se eienskappe effektief te manipuleer. Die bundel-vervormings eienskappe van ’n diffraktiewe optiese element word numeries ondersoek, wat die effektiwiteit van die bundelvervorming en die sensitiewe parameters in die sisteem uitwys. Die gebruik van ’n golfgeleier as ’n bundel-vervormings tegniek word ook analities en eksperimenteel ondersoek, en gekarakteriseer vir gebruik met die gegewe golflengte-verstelbare laser sisteme. ’n Raman laser, wat gestimuleerde vibrasionele Raman verstrooiing gebruik om laser lig te genereer by Stokes-verskuifde frekwensies, word ondersoek. Die golfgeleier word effektief gebruik as ’n bundel-vervormings tegniek in die Raman laser, om die bogenoemde nie-liniêre proses te optimeer. Die Raman laser was eksperimenteel ondersoek en gekarakteriseer vir gebruik met die gegewe golflengte-verstelbare lasers. Laser lig by verskuifde golflengtes is suksesvol gegenereer, wat die bruikbaarheid van die sisteem illustreer. Van belang is spesifiek verstelbare infrarooi laser lig, wat gebruik kan word in die laser-spektroskopie van molekules. Die resultate van hierdie werk lei tot die eenvoudige en effektiewe gebruik van die Raman laser, om langer golflengtes in die infrarooi gebied te genereer met ’n gegewe laser in die sigbare gebied.