Academic literature on the topic 'High-power sources'

High-Power Microwaves (HPM) can be used to intentionally disturb or destroy electronic equipment at a distance by inducing high voltages and currents. This thesis presents results from experiments with a narrow band HPM source, the vircator. The high voltages needed to generate HPM puts the vircator under great stress, especially the electrode materials. Several electrode materials have been tested for endurance and their influence on the characteristics of the microwave pulse. With the proper materials the shot-to-shot variations are small and the geometry can be optimized in terms of e.g. output power or frequency content. Experiments with a resonant cavity added to the vircator geometry showed that with proper tuning of the cavity, the frequency content of the microwave radiation is very narrow banded and the highest radiated fields are registred. Since HPM pulses are very short and have high field strengths, special field probes are needed. An HPM pulse may shift in frequency during the pulse so it is very important to be able to compensate for the frequency dependence of the entire measurement system. The development and use of a far-field measurement system is described.<br>QC 20110616

At starting, electrical motors require large power and current. This may not be a problem in a large electrical system but it may be very severe for a limited power source like a solar array. If a direct approach is taken the array rating must be 5-6 times the motor rating in order to start the motor and its high inertia high friction load. Batteries have been used to store energy and supply that energy for starting. Batteries need maintenance and their low efficiency is a problem too. In this study a new type of controller has been suggested and developed for the use with D.C. motors. Computer simulation showed promising results. The controller uses the array power, which is equal to the rated power of the motor, for starting and for running condition. Experimental results showed that the theoretical results are applicable.

This thesis reports on theoretical and experimental studies of wavelength-selective waveguide structures for high-power Nd3+- and Yb3+-doped fibre lasers. Cladding-pumped high-power fibre lasers based on these novel waveguide designs and operating at desired unconventional wavelengths were investigated through numerical simulations and fibre laser experiments. Rare earth doped fibres have typically multiple emission bands of different effective strengths. Stimulate emission from strong bands dominates over, and via a reduced population inversion normally even suppresses, emission from weaker bands in conventional step-index waveguides. For efficient emission and laser operation on the weaker emission bands, it is necessary to suppress unwanted stimulated emission on the strong transitions by preventing power from building up at the unwanted wavelengths. Discrete "bulk" (non-waveguide) devices at a single or a few points are ineffective, if the gain at unwanted wavelength is sufficiently high to generate high-power amplified spontaneous emission even between filters. In such cases, waveguide structures which reduce the gain at unwanted wavelengths and prevent build-up of unwanted emission can be considered. The fibre itself acts as a distributed wavelength-selective filter, and a compact all-fibre laser can be made. For short-wavelength operation when the gain at longer wavelengths needs to be suppressed, a helical core fibre is proposed. This induces a large bending loss at unwanted longer wavelengths while the bending loss at desired shorter wavelength remains relatively low. The required bending loss properties, for efficient operation at the desired shorter wavelength, can be achieved by designing the helix pitch and offset along with fibre core diameter and NA (numerical aperture). A Nd3+-doped helical fibre laser operating at 0.92 μm was investigated through computer simulations. Alternatively, there are fibres in which the fundamental mode can be cut off at a certain wavelength. I have studied fibres with a W-type refractive index profile and fibres with a hollow (air-filled) central region surrounded by a core and then a region with depressed refractive index, known as depressed-clad hollow fibre. With these fibre designs, the doped core guides the desired shorter wavelength but not the unwanted longer wavelengths. Nd3+-doped W-type fibre lasers operating at 0.92 μm were simulated and experimentally demonstrated. Also Yb3+-doped depressed-clad hollow fibre lasers operating at 0.98 μm were simulated and experimentally demonstrated. For long wavelength operation, with a suppressed gain at shorter wavelengths, modified W-type designs are proposed. By designing the refractive index profile and using ring-shaped gain regions, the net gain on an intrinsically weak long-wavelength transition may become larger than that on an intrinsically stronger short-wavelength transition. Adopting this technique, Nd3+-doped fibre amplifiers and lasers operating at 1.38 μm were simulated. While fibre lasers that generate a nearly diffraction-limited single-mode beam are normally targeted, a multimode output is often obtained, e.g., in development stages with nonideal fibres. Then it is important to characterise the modal properties of the beam. For this, two different modal power decomposition methods based on intensity measurements are proposed. The first method is based on a tomography technique that uses a Wigner function followed by an inverse Radon transform. The second method adopts a wavelength-sweeping optical source which induces beat patterns after propagation through a certain length of fibre. The feasibilities of the two proposed ideas were verified through numerical simulations.

This thesis describes an investigation of the use of low pressure plasmas for the generation of high power microwaves. Previous research has shown that the efficiency of a high power microwave ("HPM") source such as a BWO is enhanced by the introduction of a low pressure plasma into the oscillator cavity. The principle aim of this thesis is to extend the use of low pressure plasmas to the whole HPM system. Electron beams with current densities of the order of 20 A cm-2 can be generated in a cold cathode glow discharge at low gas pressures. Results are presented which show the effects of magnetic fields and electrode spacing on the I-V characteristics of a DC glow discharge electron gun. A glow discharge electron gun with an operating voltage of 350 kV has been designed and tested. A new kind of RP plasma cathode is proposed in which electrons are drawn from an RF discharge in a low pressure gas. An analysis of the production of an annular RF plasma cathode using a microwave-excited helical slow-wave structure is presented. Experimental results show that the RF plasma cathode yields electron current densities an order of magnitude higher than does a solid cathode. Examples of the implementation of the RF plasma cathode in a number of components of an HPM system are given. The propagation of electromagnetic waves in plasma-loaded waveguides of circular cross-section has been modelled. Numerical solutions are presented for the case of slow-waves in a longitudinally-magnetised plasma waveguide. Propagation below the cut-off frequency of the waveguide is generally possible and, according to the configuration, the propagating waves may be used for plasma generation or for RF power transmission. A new kind of high power microwave waveguide switch, based on the properties of plasma waveguides, is proposed. The design of new kind of magnetron, the "Glow Discharge Inverted Magnetron" ("GDIM"), is presented. The GDIM is an inverted magnetron with the resonant structure located on the cathode. The resonant cavities are used as a source of glow discharge electron beams, which gives high power operation without requiring relativistic voltages.

High-Power Microwaves (HPM) can be used to intentionally disturb or destroy electronic equipment at a distance by inducing high voltages and currents.This thesis presents results from simulations and experiments with a narrow band HPM source, the vircator. The high voltages needed to generate HPM puts the vircator under great stress, especially the electrode materials. Several electrode materials have been tested for endurance and their influence on the characteristics of the microwave pulse. With the proper materials the shot-to-shot variations are small and the geometry can be optimized in terms of e.g. output power or frequency content. Experiments with a resonant cavity added to the vircator geometry showed that with proper tuning of the cavity, the frequency content of the microwave radiation is very narrow banded and in this case the highest fields are generated. The vircator can be built in different geometries. Four different vircator types are investigated and the coaxial vircator is found to have advantages as a high radiated power and the possibility to vary the polarization during operation.Since HPM pulses are very short and have high field strengths, special field probes are needed. An HPM pulse may shift in frequency during the pulse and therefore it is very important to be able to compensate for the frequency dependence of the entire measurement system. The development and use of a far-field measurement system is described.<br><p>QC 20121122</p>

High-power, stable, high-repetition-rate, picosecond ultraviolet (UV) sources are of crucial importance for a variety of applications, such as atmospheric sensing, spectroscopy or optical data storage. Further, precise material processing or laser patterning requires high energy sources with ultrashort pulses for increased accuracy. Nonlinear, single-pass, frequency conversion sources present a highly effective and simplified approach to cover the UV spectral regions inaccessible to lasers, offering potential solutions for many of the applications mentioned above. The development of high-average-power UV sources through third- and fourth-harmonic generation (THG and FHG, respectively) of 1064 nm fiber lasers in nonlinear crystals is of particular importance due to their compact footprint, high efficiency, long lifetime, excellent stability and cost-effective design. The features of these sources are strongly dependent on the choice of the nonlinear crystal. For UV generation, this choice is particularly challenging when low-intensity picosecond pulses at high repetition rates are involved. Borate-based birefringent crystals are the most viable candidates for UV generation in the absence of suitable periodically-poled nonlinear materials, and are readily available. This thesis presents the development of High-power, stable, high-repetition-rate, picosecond ultraviolet (UV) sources are of crucial importance for a variety of applications, such as atmospheric sensing, spectroscopy or optical data storage. Further, precise material processing or laser patterning requires high energy sources with ultrashort pulses for increased accuracy. Nonlinear, single-pass, frequency conversion sources present a highly effective and simplified approach to cover the UV spectral regions inaccessible to lasers, offering potential solutions for many of the applications mentioned above. The development of high-average-power UV sources through third- and fourth-harmonic generation (THG and FHG, respectively) of 1064 nm fiber lasers in nonlinear crystals is of particular importance due to their compact footprint, high efficiency, long lifetime, excellent stability and cost-effective design. The features of these sources are strongly dependent on the choice of the nonlinear crystal. For UV generation, this choice is particularly challenging when low-intensity picosecond pulses at high repetition rates are involved. Borate-based birefringent crystals are the most viable candidates for UV generation in the absence of suitable periodically-poled nonlinear materials, and are readily available. This thesis presents the degenerating the second- and third-harmonic of a high-power, picosecond Yb-fiber laser at 1064 nm, delivering excellent stability and high quality beam profile. Moreover, efforts to refine the THG efficiency led to a successful improvement of the aforementioned fiber-based source at 355 nm. This was achieved by deploying a single-pass second-harmonic generation (SHG) under noncritical phase-matching in LiB3O5 (LBO) crystal, which considerably enhanced the output power and improved the overall performance with regard to stability and beam quality in the green, that are technologically important for a diverse range of technological applications. The obtained results at 355 nm confirm the viability of BIBO as a highly attractive material for efficient generation of low-intensity, high-average-power picosecond pulses in the UV. Further, we demonstrated a fiber-based high-repetition-rate UV source at 266 nm based on single-pass FHG in BBO crystal in a simple and practical design. Using direct single-pass SHG of 1064 nm in LBO as a pump source for the BBO crystal, 1.7 W of output power at 266 nm was generated in a high beam quality with excellent stability and spectral features. This compact and robust design represents the highest single-pass efficiency and average power of a MHz-repetition-rate picoseconds UV source at 266 nm ever demonstrated.<br>Las fuentes estables de luz pulsada en el ultravioleta (UV) en el régimen de picosegundos (ps) con altas frecuencias de repetición y de alta potencia juegan un papel crucial en gran cantidad de aplicaciones. Alguno ejemplos son la detección atmosférica de gases, técnicas de espectroscopia o el almacenamiento óptico de datos. Además, las nuevas técnicas de procesado de materiales y de grabado láser requieren estas fuentes de pulsos ultracortos de alta potencia para conseguir los más altos niveles de precisión. En este sentido, las fuentes no lineales de radiación basadas en la conversión de frecuencias de paso único presentan las mejores características para cubrir dicha región espectral a día de hoy inaccesible a los láseres convencionales, ofreciendo un amplio abanico de soluciones para todas las aplicaciones anteriormente mencionadas. El desarrollo de estas fuentes de luz de alta potencia basadas en la generación del tercer y cuarto armónico (THG y FHG del inglés) de láseres de fibra de 1064 nm son de gran interés gracias a su compacto diseño, alta eficiencia, larga vida, excelente estabilidad y buena relación calidad-precio. Las características de estas fuentes están determinadas por la elección del cristal no lineal que se utilice. La generación de radiación UV presenta particulares dificultades cuando se trata de pulsos de ps a baja intensidad a altas frecuencias de repetición. En estos casos, los cristales birrefringentes de la familia de los boratos son los mejores candidatos para la generación de esta radiación dada la ausencia de materiales no lineales periódicamente polarizados adecuados para este fin. En esta tesis se presenta el desarrollo de varias fuentes de UV de alta potencia basadas en la conversión de frecuencias, empleando diferentes configuraciones experimentales así como distintos cristales no lineales, construyendo diseños compactos, fiables y de bajo coste. En concreto, se escogieron los cristales relativamente nuevos, ß-BaB2O4 (BBO) y BiB3O6, (BIBO), para nuestras fuentes de UV. Estos presentan mejoras sustanciales en las propiedades ópticas, térmicas y de ajuste de fases para THG y FHG. Por otro lado, en esta tesis se utilizó un láser de fibra de iterbio a 1064 nm como fuente de bombeo. Primeramente, se demostró una nueva fuente de UV de 355 nm comprendida por dos etapas en un innovador esquema multicristal. Este incluye dos cristales BIBO que amplifican eficientemente los efectos inducidos por su propia birrefringencia. Esta fuente generó simultáneamente el segundo y tercer armónico de un láser de fibra de iterbio a 1064 nm de alta potencia, presentando una excelente estabilidad con un perfil en el haz de alta calidad. En segundo lugar, gracias a los esfuerzos para incrementar la eficiencia del THG se obtuvieron mejoras sustanciales respecto a la anterior fuente de UV de 355 nm. La generación del segundo armónico (SHG del inglés) se realizó implementando un cristal LiB3O5 (LBO), con ajuste de fases no crítico de paso único y por ello se incrementó la potencia y se mejoraró la estabilidad y la calidad del haz de 532 nm. Posteriormente se procedió a sumar las frecuencias ¿1064 nm y 532 nm¿, obteniendo unos resultados a 355 nm que confirman la viabilidad del BIBO como un excelente material para generar eficientemente pulsos de ps de baja intensidad en el UV. Por último, también demostramos una fuente de radiación de 266 nm con alta frecuencia de repetición basado en FHG de paso único usando un cristal BBO mediante un diseño simple y práctico. Utilizando el SHG de 1064 nm de paso único generado en un cristal LBO como fuente de bombeo para el cristal BBO, pudimos generar hasta 1.7 W de potencia de salida a 266 nm, con un haz de gran calidad y excelentes características espectrales y de estabilidad. Este diseño compacto y robusto presenta la mayor eficiencia de paso único y potencia media en una fuente de 266 nm de ps con frecuencia de repetición de MHz jamás demostrada<br>Les fonts estables de llum polsada en l'ultraviolat (UV) en el règim de picosegons amb altes freqüències de repetició i d'alta potència, juguen un paper crucial en gran quantitat d'aplicacions. Algun d'aquests exemples són la detecció atmosfèrica de gasos, tècniques d'espectroscòpia o l'emmagatzemat òptic de dades. A més a més, les noves tècniques de processat de materials i de gravat làser requereixen aquestes fonts de polsos ultracurts d'alta potència per aconseguir els més alts nivells de precisió. En aquest sentit, les fonts de radiació no lineal basades en la conversió de freqüències de pas únic presenten les millors característiques per cobrir aquesta regió espectral a dia d'avui inaccessible als làsers convencionals, oferint un ampli ventall de solucions per totes les aplicacions anteriorment esmentades. El desenvolupament d'aquestes fonts de llum d'alta potència basades en la generació del tercer i quart harmònic (THG i FHG per les seves sigles en anglès) del làser de fibra de 1064 nm són de gran interès gràcies al seu compacte disseny, alta eficiència, llarga vida, excel·lent estabilitat i bona relació qualitat-preu. Les característiques d'aquestes fonts estan fortament determinades per l'elecció del cristall no lineal que s'utilitzi. La generació de radiació UV presenta particulars dificultats quan es tracta de polsos de picosegons a baixa intensitat amb altes freqüències de repetició. En aquests casos, els cristalls birefringents de la família dels borats són els candidats més atractius per la generació d'aquesta radiació donada l'absència de materials no lineals periòdicament polaritzats adequats per aquesta finalitat. En aquesta tesi es presenta el desenvolupament de diverses fonts d'UV d'alta potència basades en la conversió de freqüències, emprant diferents configuracions experimentals així com diferents cristalls no lineals, construint dissenys compactes, fiables i de baix cost. En concret, es van escollir els cristalls relativament nous, β-BaB2O4 (BBO) i BiB3O6, (BIBO), per les nostres fonts d'UV. Aquests presenten millores substancials pel que fa a les propietats òptiques, tèrmiques i d'ajust de fases per THG i FHG. D'altra banda, en els treballs presentats en aquesta tesi es va utilitzar un làser de fibra d'iterbi a 1064 nm com a font de bombeig. En primer lloc, es va demostrar una nova font d'UV de 355 nm que consta de dues etapes en un simple i innovador esquema multi-cristall. Aquest inclou dos cristalls BIBO que amplifiquen eficientment els efectes induïts per la seva pròpia birefringència. Aquesta font va generar simultàniament el segon i tercer harmònic d'un làser de fibra d'iterbi a 1064 nm d'alta potència, presentant una excel·lent estabilitat amb un perfil al feix d'alta qualitat. A més a més, es van fer servir al màxim les tècniques per un òptim enfocament i el consegüent augment de l'eficiència. En segon lloc, els esforços per incrementar l'eficiència del THG van resultar en millores substancials respecte l'anterior font d'UV de 355 nm. La generació del segon harmònic (SHG per les seves sigles en anglès) es va realitzar mitjançant la implementació d'un cristall LiB3O5 (LBO), que presenta un ajust de fases no crític de pas únic. Gràcies a aquesta acció, es va realçar la potència i es van millorar les característiques de sortida de la font com l'estabilitat i la qualitat del feix de 532 nm, les quals són importants per diverses aplicacions tecnològiques. Posteriorment es va procedir, com en el cas anterior, a sumar les freqüències –1064 nm i 532 nm–, obtenint uns resultats a 355 nm que confirmen la viabilitat del BIBO com un excel·lent material per generar eficientment polsos de picosegons de baixa intensitat en el UV. Per últim, també vam demostrar una font de radiació de 266 nm amb un alta freqüència de repetició basat en FHG de pas únic utilitzant un cristall BBO mitjançant un disseny simple i pràctic. Utilitzant el SHG de 1064 nm de pas únic generat en un cristall LBO com a font de bombeig per el cristall BBO, va poder generar fins a 1.7 W de potència de sortida a 266 nm, presentant un feix de gran qualitat amb unes excel·lents característiques espectrals i d'estabilitat. Aquest disseny és compacte i robust, presenta la major eficàcia de pas únic i potència mitja en una font de 266 nm de picosegons amb freqüència de repetició de MHz mai demostrada fins ara.

Raman fibres and rare-earth doped silica fibres have been investigated for many years as gain media for the amplification of optical signals in telecommunications thanks to their broad gain bandwidth. They are now widely used for that application. It is only recently that power scaling of fibre laser sources has taken place with the development of double-clad fibres and highpower laser diode pump sources. Cladding-pumped fibre lasers are now a rapidly expanding and emerging technology with a wide range of applications, where high-power and high-brightness laser sources are required. Nevertheless, so far, most high-power lasers have been based on highly efficient ytterbium-doped fibre, while progress to power-scale other rare-earth doped fibres and fibre Raman laser has been much more modest. This thesis can be divided into two main themes. The first concerns the power-scaling and study of erbium-ytterbium doped fibre laser sources for optical amplification or as laser sources. The second theme concerns the development and study of high-power Raman fibre lasers and amplifiers based on the novel concept of a cladding-pumped Raman fibre. The themes are jointed in that the cladding-pumped Er:Yb doped lasers, developed in the first theme, are used as pump sources in this second part for the Raman devices. They are also jointed in that they both concern power-scaling of “eye-safe” sources at around 1.6 mm. Firstly, in collaboration with co-workers, high-power, large core, erbium-ytterbium doped fibre laser sources are developed. Output powers in excess of 70 W are obtained. Good beam quality output is achieved thanks to a tapered fibre section. The taper is compatible with standard single-mode fibre which enabled the realization of tuneable fibre lasers free from bulk external grating. The laser’s tuning characteristics are investigated in the C- and L-band range. Subsequently, a master-oscillator power-amplifier (MOPA) based on large core Er:Yb doped fibres is developed for the generation of high-energy pulses. The details of the MOPA are studied and presented. With careful design considerations, pulses free from non-linear effects, with energy up to 1 mJ and peak powers up to 6.6 kW, with narrow spectral linewidth, are obtained at 1535 nm. Secondly, using a double-clad fibre, consisting of raised index, germanium doped, core and inner cladding, with a pure silica outer cladding, a high-power CW single mode Raman fibre laser, pumped by a multi-mode erbium-ytterbium doped fibre laser, is demonstrated for the first time. The laser slope efficiency is 67% and the output power is in excess of 10 W. An experimental and theoretical study of the laser is performed. Then, the pulse amplification in a cladding-pumped Raman fibre is studied in a single pass amplifier configuration. The effects on the laser performance of the pump and Stokes seed powers, fibre length and four-wave mixing are presented. The Stokes’s small-signal gain can be as high as 50 dB. Using this configuration, 700 ns long pulses are amplified up to 10 mJ which shows that, potentially, optical pulses could be amplified to much higher energy. Finally, these results together, let predict that, soon, cladding-pumped Raman fibre could be used as direct brightness converter.

Les systèmes laser en fibre optique de verre fluoré se sont placés en tête de file pour la génération de lumière cohérente dans l’infrarouge moyen, de 2 à 5 μm. En particulier, les lasers à fibre opérant à 3 μm ont attiré une attention considérable puisqu’ils permettent le développement d’applications en spectroscopie, en contre-mesure militaire et en médecine. De ce fait, ces lasers ont connu des progrès considérables en termes de puissance, de qualité de faisceau, de fiabilité et de compacité dans la dernière décennie. Cette thèse s’inscrit dans cette démarche d’accroissement des performances des lasers à fibre infrarouges opérant à 3 μm. Ainsi, elle présente différents systèmes laser en fibre de verre fluoré et détaille les composants tout-fibre qui ont permis d’atteindre des records d’efficacité énergétique, de puissance et d’énergie par impulsion.Trois types de sources laser, chacune ayant établi un record de performance, ont été investiguées. Tout d’abord, une efficacité laser record de 50% a été obtenue à partir d’un laser à fibre opérant à 2.8 μm en utilisant le principe de cascade laser à 2.8 et 1.6 μm. Ensuite, un amplificateur à fibre, basé sur des fibres dopées à l’ion erbium et aux ions holmium et praséodyme, a permis d’amplifier des impulsions picosecondes à 3 μm d’une source laser à l’état solide pour obtenir une énergie par impulsion (122 μJ) et une puissance moyenne (2.45 W) records. Finalement, une cavité laser tout-fibre dopée à l’ion erbium opérant autour de 3 μm, dépourvue d’épissures fusionnées, a mené à la démonstration d’une puissance laser recordde 41.6 W. D’autre part, cette thèse a ciblé différents obstacles limitant l’accroissement de la puissance des lasers à fibre opérant à 3 μm, et a permis d’identifier des pistes de solutions pour pallier ces limitations. En l’occurrence, la photodégradation de l’extrémité de sortie des lasers à fibre à 3 μm, causée par la diffusion de la vapeur d’eau ambiante, limite la durée de vie et la puissance maximale de ce type de laser. Ainsi, le dernier volet de cette thèse a été consacré à l’étude expérimentale de la photodégradation d’embouts de protection à base de verre fluoré ou d’oxyde. Cette étude a mené au développement d’une nouvelle méthode permettant d’inhiber la diffusion de la vapeur d’eau dans les embouts de protection. Cette thèse représente une avancée majeure dans le contexte de l’accroissement de la puissance des sources laser en fibre optique opérant à 3 μm et démontre leur potentiel indéniable pour remplacer d’autres types de lasers dans une multitude d’applications.<br>Fluoride fiber laser technology is one of the noteworthy tools for generating coherent mid-infrared signal between 2 to 5 μm that has made outstanding progress over the last decade interms of compactness, reliability, high beam quality, and output power. In the mid-infrared spectral region, laser emission near 3 μm is crucial for many applications such as spectroscopy, counter measures and medicine. In addition, there has always been an increasing demand for higher laser output parameters to open new doors for potential applications.This dissertation presents a series of experimental studies of fluoride fiber laser systems, either in continuous wave or pulsed regime, and of their critical in-fiber components to achieve a laser emission with high slope efficiency, output power, and pulse energy near 3 μm. During this PhD project, three main 3 μm-class fluoride fiber laser sources, each representing at least one record output parameter in their own category, have been investigated. First, the highest optical-to-optical efficiency (50%) at 2.8 μm was achieved from a diode-pumped fiber laser cavity by cascaded transitions of 2.8 μm and 1.6 μm in a low-doped erbium fluoride fiber. Then, active media based on erbium and holmium/praseodymium zirconium fluoride fibers seeded by a sub-ns solid-state laser enabled to achieve highest pulse energy (122μJ)and average power (2.45 W) from a picosecond fiber laser amplifier operating near 3 μm. Lastly, the highest average power 3 μm-class laser (41.6 W) has been demonstrated by using asplice-less heavily erbium-doped fluoride fiber medium. The major problems during the high-power laser operation have been investigated and potential solutions were proposed. The most common problem of all the high-power 3 μm fiber laser demonstrations is the degradation of the fiber tips due to OH migration, which limits the output power and can lead to catastrophic failures. Therefore, in the last part of the PhD project, the performance of fluoride- and oxide-based endcap components under high-power 3 μm laser emission has been experimentally investigated and a novel endcapping method was proposed for suppressing the OH migration. Experimental studies in this PhD project represents a significant advance for further power scaling of 3 μm fluoride fiber laser sources and shows their potential to replace other laser technologies.

Thesis (PhD)--Stellenbosch University, 2002.<br>ENGLISH ABSTRACT: One of the most difficult problems in microwave dielectric heating is the generation and control of field and heating patterns. A technique allowing the synthesis of different, pre-determinable heating patterns by interference modulation is proposed. The proposed concept may be described by the term 'interference modulation'. Interference modulation is a technique which enables particular patterns, called features, to be obtained by signals from several sources interfering with each other. The relative phases of the signals are modulated, by which process known features may be selected. Weights are assigned to these features, which may be combined over time to form a new heating pattern. Phase changes may then be used to switch to specific, known features, with weights which will determine the contribution of each feature to the desired overall pattern. In the practical implementation described, magnetron tubes are the sources. Each of these narrow-band high-power sources was injection locked to a low power control signal. The control signals are derived from a reference source and their phase is set to select a corresponding feature. Calculation and measurement showed that reliable locking occurs with a control signal power of at least 3% of the magnetron's emitted power. Measurements of patterns were carried out with materials formed into sheets and blocks. Some were chemically prepared to reveal the overall heating pattern. The observed patters, simulations and field measurements concur, thus validating the concept and operation of the proposed topology.<br>AFRIKAANSE OPSOMMING: Een van die moeilikste probleme in mikrogolf diëlektriese verhitting is die opwekking en beheer van veld- en verhittingspatrone. 'n Tegniek wat voorsiening maak vir die sintese van verskillende, voorafbepaalde verhittingspatrone deur interferensie word hier voorgestel. Die voorgestelde beginsel kan beskryf word deur die term "interferensie modulasie". Interferensie modulasie is 'n tegniek wat spesifieke patrone, genoem kenmerke, moontlik maak deur seine van verskillende bronne met mekaar te laat interfereer. Bekende kenmerke kan geselekteer word deur die relatiewe fases van die seine te moduleer. Gewigte word aan hierdie kenmerke toegeken wat oor tyd gekombineer kan word om nuwe verhittingspatrone te vorm. Faseveranderings kan dan gebruik word om na 'n spesifieke, bekende kenmerk te skakel met gewigte wat die bydrae van elke kenmerk van die verlangde algehele patroon bepaal. Magnetrons word gebruik as bronne in die praktiese implimentering wat beskryf word. Elkeen van hierdie nouband, hoë drywing bronne is injeksie-gesluit met 'n lae drywing beheersein. Die beheerseine is afgekoppel van 'n verwysingsbron en hul fases is gestelom 'n ooreenstemmende kenmerk te verkry. Berekening en meting toon dat betroubare sluiting voorkom wanneer die beheersein ten minste 3% van die magnetron se uittree-drywing is. Metings van patrone is gemaak met materiale wat in lae en blokke gevorm is. Sommiges is met chemikalië voorberei sodat die algehele verhittingspatroon gesien kan word. Die waargeneemde patrone, simulasies en veldmetings stem goed ooreen en bevestig die beginsel en werking van die voorgestelde topologie.