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A general discussion on propagation of an electromagnetic wave in a random medium is presented. Emphasis is placed on the bistatic scattering problem. The first phase of the investigation is focused on a random continuum. This is an extension of work done by de Wolf [19] recently. He derives a formal expression for the enhancement factor of the electromagnetic flux at large angles (excluding backscatter) from an extended weakly random medium. Enhancement describes the factor by which the singly-scattered flux is modified when the effects of cumulative forward scatterings are taken into account before and after one large-angle scattering. Explicit results are calculated here for a two-dimensional geometry describing cylindrical scattering from a slab of width L filled with a uniformly turbulent dielectric described by a power-law spectrum in the inertial subrange. The results show that the enhancement factor is close to unity beyond the mean free path of the small-angle scatterings and it increases when the medium width L exceeds the mean free path of a large-angle scattering. This result is extended for a generalized power-law structure function of the dielectric permittivity fluctuation, which shows a possibility of using the cumulative forward and single large-angle scattered flux to detect the statistical properties of a random continuum. The negligence of the Fresnel terms in the expression of the scattered flux is justified by including those in the phase term and investigating the resulting effects. This investigation reveals that the inclusion of the Fresnel terms makes the scattered flux complex, an error arising from the truncation of the higher-order phase terms, which is not observed when the Fresnel terms are neglected. The second phase of the investigation involves discrete random media. A mathematical model is developed for the purpose of deriving an integral equation of the coherent field and autocorrelation function in the very general case of an electromagnetic wave propagating in a medium of densely packed nontenuous particles. All orders of N-tuple particle correlation function are included. The resulting equations are a generalization of those derived recently by Tsolakis et al. [23]; the four lowest-order terms of each equation include those of Tsolakis et al.'s equations which incorporate only binary correlation between particles. The mathematical model is then used to derive an expression for the scattered flux of an electromagnetic wave under a first-order cumulative forward and single large-angle scattering approximation. The resulting expression is valid at high frequencies under Twersky's approximation [5]. It is shown that the discrete scatterer case may be treated by an approach similar to the continuum case by using the new formulation. / Ph. D.

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.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Lasers are commonly used in the industry for various applications such as laser cutting, laser drilling, lithography, medical applications, surface cleaning and a myriad of other applications. In any application of a laser the beam properties are significant. In the paint removal application discussed in this thesis, the beam properties of the laser beam can have a large impact on the efficiency of the paint removal process. The pulse energy or the average output power of the laser is normally an important parameter in laser materials processing applications. The spatial profile or intensity distribution of the beam also has an influence on the process. The propagation of the laser beam from the laser to the working point is also significant in applying the laser beam to the material. In the ideal scenario one would like to control all the parameters of the laser in terms of the output, in energy or output power, the propagation of the laser beam and the intensity distribution of the beam. The process of laser-based paint removal is no different to this. In this process a TEA CO2 laser is used for the ablation of paint from a substrate. In this application high pulse energy is required from the laser together with good beam propagation properties for delivery of the beam over a long distance. For this application the multimode beam of the TEA CO2 laser can be applied for the paint removal. The multimode beam has sufficiently high pulse energy for the paint removal process, but is not suitable for propagating over long distances through a beam path with a finite aperture. Furthermore the multimode beam does not have a uniform energy intensity distribution. It would therefore be ideal if the TEA CO2 laser could be designed with a custom beam that has a uniform intensity distribution, high pulse energy and good beam propagation. These requirements lead to the study of flattened irradiance profile laser beams. In this thesis flattened irradiance profile beams in the form of Flattened Gaussian beams are investigated. The theory of the Flattened Gaussian profile as well as the propagation of the beam is investigated. Furthermore the generation of such a beam internally to the laser resonator is studied. In succession to this a custom laser resonator was designed and implemented on the TEA CO2 laser. The resulting Flattened Gaussian Beam was characterised and applied to the application of laser paint removal. It was finally shown that the Flattened Gaussian Beam could be successfully generated and applied with equal success in the application of laser paint removal. / AFRIKAANSE OPSOMMING: Lasers word algemeen in die industrie gebruik vir toepassings soos laser snywerk, laser boorwerk, litografie, mediese toepassings, oppervlakreiniging en verskeie ander. In enige toepassing van 'n laser is die eienskappe van die laserbundel van groot belang vir die proses. In die verf verwydering toepassing wat bespreek word in hierdie tesis het die bundel eienskappe 'n groot invloed op die effektiwiteit van die verf stropings proses. Die pulsenergie of uitset drywing van die laser is gewoonlik 'n belangrike parameter in 'n materiaalverwerkings toepassing. Die ruimtelike profiel of energie intensiteitprofiel van die bundel het ook 'n invloed op die proses. Die voortplanting van die bundel vanaf die laser na die werkspunt het ook 'n beduidende invloed op die toepassing van die laserbundel op die materiaal. In die ideale geval sal mens graag al die parameters van die laserbundel soos pulsenergie of drywing, die bundel voortplanting en energie intensiteitprofiel wil beheer. Die toepassing van die laser vir verfverwydering vereis ook die beheer van hierdie unieke parameters wat reeds genoem is. In hierdie proses is 'n TEA CO2 laser gebruik vir die verwydering van verf van 'n substraat. Die toepassing vereis hoë pulsenergie saam met goeie bundel voortplantingseienskappe vir lewering van die bundel oor lang afstande. Die multimode bundel van die laser kan gebruik word vir hierdie toepassing. Die multimode bundel bevat genoegsame energie vir die verfstropings proses maar is nie geskik vir voortplanting oor lang afstande deur 'n bundelpad wat 'n beperking op die bundel grootte het nie. Verder het die multimode bundel ook nie 'n uniforme energie intensiteitprofiel nie. Dit sou ideal wees as die TEA CO2 laser toegerus kon word met 'n toepassingsgerigte bundel wat hoë puls energie, goeie bundel voortplanting en 'n uniforme intensiteitprofiel het. Hierdie vereiste het gelei tot die studie van laserbundels met 'n uniforme plat energie intensiteitprofiel. In hierdie tesis word plat intensiteit bundels in die vorm van plat Gaussiese bundels ondersoek. Die teorie van plat Gaussiese bundels sowel as die voortplanting van hierdie bundels word hier ondersoek. Verder word die opwekking van hierdie bundels intern tot die laserresonator ook ondersoek. Na die ondersoek is daar oorgegaan in die ontwerp en implementering van 'n doelgemaakte resonator op 'n TEA CO2 laser. Die resonator het 'n plat Gaussiese bundel as uitset gelewer. Die bundel was gevolglik gekarakteriseer en aangewend in 'n verfstropings toepassing. Ten einde is daar getoon dat 'n plat Gaussiese bundel suksesvol opgewek en toegepas kon word.

Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: This study involved the analysis of crystallization kinetics by means of a unique and newly developed Solution crystallization analysis by laser light scattering (Scalls) technique. In the main study we compared two commercial linear low-density polyethylene (LLDPE) polymers (PE-1- octene and PE-1-hexene) and studied the effect of short-chain branching on the solution crystallization of these complex polymer systems. Characterization of the polymers was done by nuclear magnetic resonance spectroscopy (NMR) and high-temperature gel permeation chromatography (HT-GPC). The second study involved the fractionation of a PE-1-hexene copolymer by temperature rising elution fractionation (Tref) and analyzing the solution crystallization of the different temperature fractions. This resulted in important details on the different molecular regions present in the polymer. A third additional study was done on the compatibility in polyolefin blends. Two different blends were prepared: isotactic polypropylene (iPP) – low density polyethylene (LDPE) blend and iPP – polypropylene impact copolymer (PPIC) blend. It was found that co-crystallization only occurred for the iPP - PPIC blends. Phase separation occurred for the iPP – LDPE blends, resulting in the formation of two phases for all blend compositions. Solution crystallization analysis is usually measured by the conventional Crystallization Analysis Fractionation (Crystaf) technique. In this study all crystallization data were compared with Crystaf results and a good correlation was found between the results obtained by Crystaf and Scalls. The major advantages of the Scalls technique are that, results similar to that of Crystaf can be acquired with much shorter analysis times and Scalls also allows for the measurement of solution melting of the crystallized polymer solutions. / AFRIKAANSE OPSOMMING: Hierdie studie het die analise van kristallisasie kinetika behels met behulp van die unieke en nuut ontwikkelde oplossing kristallisasie analise deur laser lig verstrooiing (Scalls) tegniek. In die hoof studie het ons twee kommersïele liniêre lae-digtheid polietileen (LLDPE) polimere (PE-1-okteen en PE-1-hekseen) vergelyk en die effek van kort-ketting vertakking op kristallisasie in oplossing van hierdie komplekse polimeer sisteme bestudeer. Karakterisering van die polimere was gedoen met kern magnetiese resonans spektroskopie (KMR) en hoë-temperatuur gel permeasie kromatografie (HT-GPC). Die tweede studie het die fraksionering van ‘n PE-1-hekseen ko-polieer met behulp van temperatuurstyging eluering fraksionering (Tref) behels asook die analisering van kristallisasie in oplossing van die verskillende temperatuur fraksies. Belangrike informasie oor die verskillende molekulêre areas teenwoordig in die polimeer was verkry. ‘n Derde addisionele studie was gedoen op die versoenbaarheid in poliolefin mengsels. Twee verskillende mengsels was voorberei: isotaktiese polipropileen (iPP) – lae digtheid polietileen (LDPE) mengsel en iPP – polipropileen impak ko-polimeer (PPIC) mengsel. Daar was gevind dat ko-kristallisasie slegs in die iPP – PPIC mengsel plaasgevind het. Fase skeiding het plaasgevind in die iPP – LDPE mengsels wat tot twee fases gelei het vir alle mengsel komposisies. Kristallisasie in oplossing word gewoonlik gemeet met die konvensionele kristallisasie analise fraksionering (Crystaf) tegniek. In hierdie studie was al die kristallisasie data met Crystaf resultate vergelyk en ‘n goeie korrelasie was gevind tussen die resultate van Crystaf en Scalls. Die grootste voordele van die Scalls tegniek is dat resultate soortgelyk aan diè van Crystaf kan verkry word met baie korter analises en Scalls laat ook toe vir die meting van smeltpunt van die gekristalliseerde polimeer oplossings.

Approved for public release; distribution is unlimited / Thermo mechanical modeling and simulation of a satellite and intercontinental ballistic missile assumes importance due to the increased interest in assessing the potential of such attacks. Effective and innovative methods are sought in assessing the structural integrity of such structural components. In this study, we present modeling and simulation aspects of two generic models loaded by high energy laser beam. We present an application of MSC software in modeling thermo-mechanical behavior, both steady state and transient behavior of satellite and missile structures. Thermal energies used for simulation correspond to high energy laser flux available at low earth orbits as reported in literature. A brief review of the concepts involved is outlined. The analysis is performed under several scenarios that include thermal failures due to steady state as well as transient thermal exposures. The thermal exposure times and locations are varied to assess typical failure modes of the structure. Analysis is done in order to define suitable material thicknesses that will make a satellite or a ballistic missile hardened enough to withstand these specific amounts of energy. Other parameters of interest pertaining to this study are the pulse width, and resulting transient phenomena affecting the behavior. Temperature gradients as well as resulting thermal stresses and thermal deformations are reported in this study. / Outstanding Thesis

Performance and reliability of solid state laser diodes in the IR region exceeds those in the visible and UV part of the light spectrum. Single frequency visible and UV laser diodes with higher than 500 mW power are not available commercially. However we successfully stabilized a multi-longitudinal mode IR laser to 860 mW single frequency. This means high efficiency harmonic generation using this laser can produce visible and UV laser light not available otherwise. In this study we examined three major leading nonlinear crystals: PPMgO:SLN, PPKTP and PPMgO:SLT to generate blue light by second harmonic generation. We achieved record high net conversion efficiencies 81.3% using PPMgO:SLT (~500 mW out), and 81.1% using PPKTP (~700 mW out). In both these cases an external resonance buildup cavity was used. We also studied a less complicated single pass waveguide configuration (guided waist size of ~ 5 um compared to ~60 um) to generate blue. With PPMgO:SLN we obtained net 40.4% and using PPKT net 6.8% (110mW and 10.1 mW respectively).

This study was conducted to examine the effect of flow turbulence on sound waves propagating across a velocity field. The resulting information can be used to determine the potential for increasing the accuracy of an ultrasonic flowmeter, and understand the data scatter typically seen when using an ultrasonic flowmeter. A modification of the Ray Trace Method was employed which enabled the use of multiple rays in a very fine grid through a flow field. This technique allowed for the computation of the statistical variation of the propagation times for sound pulses traversing a flow field. The statistical variation was studied using two flow fields: 1) a uniform flow field with a superimposed vortex street and 2) an experimentally measured channel flow. The uniform flow field with a superimposed vortex street allowed for the examination of the effects of a large-scale flow structure on sound wave propagation, and for the verification of the analysis technique. Next by using the measured turbulent channel flow, as an example, the statistical variation of sound pulse propagation time was computed for flow likely to be encountered in actual flow measurement situations. Analysis was also conducted to determine the maximum allowable repetition rate of measurements with regard to the optimal time of flight measurements. Both the propagation time of a sound pulse moving across a uniform flow field with superimposed vortex street, and the resultant computed flow were observed to vary at the same frequency of the vortex street. Further, the magnitude of the variations was proportional with the strength of the individual vortices in the vortex street. A sound pulse propagating back and forth across a measured turbulent channel flow, afforded individual time difference variation from the mean propagation time of up to 5%. It was shown that a minimum variation occurred when the sound pulses were transmitted at a 75 degree angle to the flow axis. It was also determined that the average speed of sound in a flow field affected the final flow measurements by decreasing the measured delta time difference between the upstream and downstream propagating sound waves, and therefore the measured flow. The width of the sound path also contributed to decreasing the variation of the individual measurements by integrating over a larger sound path. These findings suggest that turbulence in a flow field affects ultrasonic flowmeter measurements by creating differences in the propagation times of individual sound pulses. Thus, turbulence and large-scale flow structures can result in variations in volumetric flow rate determination made by an ultrasonic flowmeter system.

Protein-DNA crosslinking is an important method to study protein-DNA interactions. Crosslinking by short pulsed UV lasers is a potentially powerful tool that results in efficient crosslinking, apparently by a two photon process. However, the major problem in using UV laser crosslinking is that the conditions which lead to high crosslinking efficiency also result in high DNA damage. Previously, it has been shown that a combination of femtosecond laser pulses at two different wavelengths, in the UV (266 nm) and the visible range (400 nm), increases the effective crosslinking yield (i.e. higher crosslinking yields with reduced DNA damage). This new strategy has the advantage that the intensity of the UV pulse for the first excitation step can be kept low, leading to lower UV-induced DNA damage and the second pulse at a visible wavelength can provide enough energy for the UV excited bases to cross their ionization threshold without damaging the DNA. The objective of this thesis project was to develop a novel UV laser cross-linking technique that would permit higher effective crosslinking yields with the commonly used pulses in the nanosecond (ns) range. To serve this purpose we tried to extend the two-wavelength femto second laser irradiation approach to longer duration pulses. We chose MBP-PIF3 protein and its target G-box DNA motif as a model system. Before ultraviolet irradiation of the protein-DNA complexes in vitro, the specific binding interaction of purified MBP-PIF3 protein with the G-box DNA motif was studied by Electrophoretic Mobility Shift Assay (EMSA). We irradiated the PIF3/DNA complexes with different laser systems (i.e. Nd:YAG and Dye lasers) and their combinations. We were expecting to see that the combination of UV laser pulses (260nm) with longer wavelength dye laser pulses (480nm) will produce higher effective crosslink yields relative to the yield from the UV pulses alone.