The primary objective of this work was to develop a laser source for fundamental investigations in the field of laser – materials interactions. In particular it is supposed to facilitate the study of the influence of the temporal energy distribution such as the interaction between adjacent pulses on ablation processes. Therefore, the aim was to design a laser with a highly flexible and easily controllable temporal energy distribution. The laser to meet these demands is an SBS-laser with optional active mode-locking. The nonlinear reflectivity of the SBS-mirror leads to a passive Q-switching and issues ns-pulse bursts with µs spacing. The pulse train parameters such as pulse duration, pulse spacing, pulse energy and number of pulses within a burst can be individually adjusted by tuning the pump parameters and the starting conditions for the laser. Another feature of the SBS-reflection is phase conjugation, which leads to an excellent beam quality thanks to the compensation of phase distortions. Transverse fundamental mode operation and a beam quality better than 1.4 times diffraction limited can be maintained for average output powers of up to 10 W. In addition to the dynamics on a ns-timescale described above, a defined splitting up of each ns-pulse into a train of ps-pulses can be achieved by additional active mode-locking. This twofold temporal focussing of the intensity leads to single pulse energies of up to 2 mJ at pulse durations of approximately 400 ps which corresponds to a pulse peak power of 5 MW. While the pulse duration is of the same order of magnitude as those of other passively Q-switched lasers with simultaneous mode-locking, the pulse energy and pulse peak power exceeds the values of these systems found in the literature by an order of magnitude. To the best of my knowledge the laser presented here is the first implementation of a self-starting mode-locked SBS-laser oscillator. In order to gain a better understanding and control of the transient output of the laser two complementary numerical models were developed. The first is based on laser rate equations which are solved for each laser mode individually while the mode-locking dynamics are calculated from the resultant transient spectrum. The rate equations consider the mean photon densities in the resonator, therefore the propagation of the light inside the resonator is not properly displayed. The second model, in contrast, introduces a spatial resolution of the resonator and hence the propagation inside the resonator can more accurately be considered. Consequently, a mismatch between the loss modulation frequency and the resonator round trip time can be conceived. The model calculates all dynamics in the time domain and therefore the spectral influences such as the Stokes-shift have to be neglected. Both models achieve an excellent reproduction of the ns-dynamics that are generated by the SBS-Q-switch. Separately, each model fails to reproduce all aspects of the ps-dynamics of the SBS-laser in detail. This can be attributed to the complexity of the numerous physical processes involved in this system. But thanks to their complementary nature they provide a very useful tool for investigating the various influences on the dynamics of the mode-locked SBS-laser individually. These aspects can eventually be recomposed to give a complete picture of the mechanisms which govern the output dynamics. Among the aspects under scrutiny were in particular the start resonator quality which determines the starting condition for the SBS-Q-switch, the modulation depth of the AOM and the phonon lifetime as well as the Brillouin-frequency of the SBS-medium. The numerical simulations and the experiments have opened several doors inviting further investigations and promising a potential for further improvement of the experimental results: The results of the simulations in combination with the experimental results which determined the starting conditions for the simulations leave no doubt that the bandwidth generation can primarily be attributed to the SBS-Stokes-shift during the buildup of the Q-switch pulse. For each resonator round trip, bandwidth is generated by shifting a part of the revolving light in frequency. The magnitude of the frequency shift corresponds to the Brillouin-frequency which is a constant of the SBS material and amounts in the case of SF6 to 240 MHz. The modulation of the AOM merely provides an exchange of population between spectrally adjacent modes and therefore diminishes a modulation in the spectrum. By use of a material with a Brillouin-frequency in the GHz range the bandwidth generation can be considerably accelerated thereby shortening the pulse duration. Also, it was demonstrated that yet another nonlinear effect of the SBS can be exploited: If the phonon lifetime is short compared to the resonator round trip time we obtain a modulation in the SBS-reflectivity that supports the modulation of the AOM. The application of an external optical feedback by a conventional mirror turns out to be an alternative to the AOM in synchronizing the longitudinal resonator modes. The interesting feature about this system is that it is ― although highly complex in the physical processes and the temporal output dynamics ― very simple and inexpensive from a technical point of view. No expensive modulators and no control electronics are necessary. Finally, the numerical models constitute a powerful tool for the investigation of emission dynamics of complex laser systems on arbitrary timescales and can also display the spectral evolution of the laser output. In particular it could be demonstrated that differences in the results of the complementary models vanish for systems of lesser complexity. / Ziel der Arbeit ist die Entwicklung einer Laserstrahlquelle, die zur Untersuchung von Laser-Material-Wechselwirkungen eingesetzt werden soll. Im Mittelpunkt des Interesses steht hierbei der Einfluss der zeitlichen Pulsstruktur des Lasers auf Materialabtragsprozesse. Daraus ergibt sich die Anforderung einer möglichst hohen Flexibilität in der Pulsstruktur des Lasers sowie einer möglichst guten Strahlqualität. Eine gute Strahlqualität zeichnet sich durch eine homogene räumliche Intensitätsverteilung aus und ist Voraussetzung für eine gezielte Energiedeponierung auf dem Material. Diese Anforderungen wurden mit einem SBS-Laser erfüllt. Dabei handelt es sich um einen Laser, der einen SBS-Spiegel verwendet, dessen Reflektivität auf der Streuung des Lichts an Schallwellen beruht, die von dem einfallenden Licht selbst erzeugt werden. Als Resultat hat dieser Spiegel eine Reflektivität, die abhängig von der Energie des eingestrahlten Lichts ist. In einem Laser verwendet führt diese energieabhängige Reflektivität zu einer sogenannten Güteschaltung, die sich in der Ausbildung von kurzen Lichtpulsen mit Dauern von etwa 100 Nanosekunden äußert. Die Abstände zwischen den Pulsen, die Pulsdauern und die Pulsenergien können sehr leicht durch die Randbedingungen, wie etwa die Reflektivität der konventionellen Spiegel des Lasers, gesteuert werden. Durch eine zusätzliche, aktiv herbeigeführte Verlustmodulation im Laserresonator wird eine Aufspaltung der Nanosekundenpulse in eine Reihe von Pulsen mit Dauern von nur noch einigen 100 Pikosekunden erreicht. Diese Technik ist unter dem Begriff Modenkopplung bekannt. Es liegt jetzt also eine doppelte Pulsstruktur vor: Nanosekundenpulse, die sich jeweils aus vielen Pikosekundenpulsen zusammensetzen. Durch diese doppelte zeitliche Bündelung der Ausgangsleistung werden während der Pulse Spitzenleistungen von bis zu 5 MW erreicht. Die Pulsenergien der ps-Pulse erreichen bis zu 2 mJ. Diese Werte liegen um den Faktor 10 über denen vergleichbarer Systeme. Meines Wissens ist dies die erste Umsetzung eines selbsttätigen SBS-Lasers mit zusätzlicher Modenkopplung. Um die verschiedenen Einflüsse auf diese Emissionsdynamik besser verstehen und kontrollieren zu können, wurden zwei Modelle mit komplementären Ansätzen entwickelt, auf deren Basis diese Dynamik durch numerische Simulationen wiedergegeben werden kann. Insbesondere können auf diese Weise auch die Einflüsse von einzelnen Materialparametern isoliert betrachtet werden, was im Experiment im allgemeinen nicht möglich ist. Der SBS-Laser wurde bereits erfolgreich in Laser-Materialbearbeitungsexperimenten eingesetzt. So konnte beispielsweise gezeigt werden, dass sich die Bearbeitungsdauer beim Wendelbohren durch die Verwendung von Pulszügen, also einer Reihe kurz aufeinander folgender Pulse, gegenüber dem Einsatz von Pulsen mit gleichmäßigen Abständen erheblich verbessern lässt.

An ongoing experiment is described to demonstrate the principle of multi-pulse laser wakefield acceleration, in which a plasma wakefield is resonantly excited by a train of laser pulses, spaced by the plasma wavelength. Particle-in-cell simulations of the initial single-pulse experimental setup are presented, in order to calculate the expected signal. Preliminary results are presented and future plans, based on work done so far, are discussed. Part of this work involves the implementation of a single-shot wakefield diagnostic - frequency-domain holography, which records the phase shift caused by passage of a probe pulse through the plasma. This implementation is described in detail, along with the associated analysis procedure. Practical difficulties encountered while implementing the diagnostic are discussed, along with possible ways of mitigating them in the future. A method is presented by which the noise level in the resulting phase measurements can be predicted, much more accurately than any previously published method for this technique. Methods of generating pulse trains for use in future multi-pulse laser wakefield acceleration experiments are presented. These include techniques proposed for use in this demonstration experiment, as well as one intended for use in a dedicated high-efficiency, high repetition-rate, multi-pulse driver laser. This last method, based on programmable pulse shaping using a spatial light modulator, requires a suitable mask to be computed based on the parameters of the required pulse train; an algorithm is described to perform this computation.

Orientador: Luis Eduardo Evangelista de Araujo / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-14T22:09:31Z (GMT). No. of bitstreams: 1 Soares_AntonioAugusto_D.pdf: 11187832 bytes, checksum: 3ffb3d078fef8dd053b760fc0342ec6f (MD5) Previous issue date: 2009 / Resumo: Nesta tese apresentamos um estudo teórico da interação coerente entre trens de pulsos ultracurtos e sistemas atômicos simples de dois e três níveis, este último na configuração L. Primeiramente avaliamos a situação em que um trem de pulsos ultracurtos não só sonda um sistema de três níveis na configuração L, mas também leva o átomo à configuração do átomo vestido. Nessas condições estudamos o efeito do acúmulo coerente de excitação na transparência eletromagneticamente induzida (EIT) experimentada pelo trem de pulsos ultra-curtos. Mostramos que os parâmetros do trem de pulsos ( área do pulso, diferença de fase entre pulsos consecutivos e período de repetiçãco dos pulsos) determinam a dinâmica da formação da EIT. Em seguida estudamos os efeitos da propagação do trem de pulsos ultracurtos através de um meio atômico estendido, constituído por sistemas de dois e três níveis na configuraçãco L. Mostramos que no caso em que o meio é constituído por sistemas de dois níveis a frequência central dos pulsos é rapidamente removida devido à propagação, prejudicando o efeito de acúmulo coerente. Já para o meio constituído por sistemas de três níveis o efeito acumulativo é observado e coerência é transferida ao meio, levando a uma sobreposição linear entre os níveis fundamentais, sobreposição esta que está associada à formação da EIT. Após um número suficientemente grande de pulsos o meio se torna transparente aos pulsos subsequentes que se propagam sem sofrer alterações em seu perfil temporal e espectral. Outra situação que estudamos nesta tese é aquela onde o sistema atômico é excitado por um campo contínuo monocromático levando o átomo à configuração do "átomo vestido". Este sistema é, então, investigado por um trem de pulsos ultracurtos. Neste caso mostramos que a utilização do trem de pulsos ultracurtos permite a realização de espectroscopia de alta resolução dos estados atômicos vestidos induzidos pelo campo contínuo monocromático / Abstract: In this thesis we present a theoretical study about the coherent interaction between a train of ultrashort pulses with two- and three-level atomic systems, this last in the L configu-ration. Firstly, we investigated the situation at which a train of ultrashort pulses not only probes the three-level system in the L configuration, but also drives the atom to its dressed configura-tion. Under these conditions, we studied the effects of the coherent accumulation of excitation on the electromagnetically induced transparency (EIT) of the train of ultrashort pulses. We showed that the pulse train parameters (area, repetition period, and phase between successive pulses) play a significant role in establishing EIT. Then, we study the effects of propagation through an extended sample of two- and three-level atoms. We showed that in the two-level case, absorption of the pulse¿s resonant frequency by the atoms quickly compromises the accumulation of excitation. In the three-level case, the accumulative effect occurs, and the pulse train transfers coherence between the two lower states of the atoms, driving population into a dark superposition state. Such a superposition is related to the EIT formation and after a large enough number of pulses, the medium becomes transparent to the driving pulses. Later pulses in the train propagate through the atomic medium with both their amplitude and temporal profile preserved. Another configuration that we investigated in this thesis is that at which the atomic system is excited by a monochromatic cw laser that drives the atom to its dressed configuration. Such a system is then probed by an ultrashort pulse train. In this case, we showed that an ultrashort pulse train can be used to perform high-resolution spectroscopy of dressed atomic states / Doutorado / Física Atômica e Molecular / Doutor em Ciências

Nesta tese estudamos a dinâmica das não linearidades ópticas em compostos orgânicos, mais especificamente em amostras de bis-ftalocianina de itérbio, derivados do álcool furfurílico e compostos azo-aromáticos. Para que a dinâmica dos processos ópticos não lineares pudesse ser investigada nesses materiais, desenvolvemos uma extensão experimental à técnica de varredura-Z convencional, denominada de varredura-Z com trem de pulsos. Nesta técnica, o trem de pulsos de um laser Q-switched/mode-locked é convenientemente empregado para o estudo da dinâmica de processos não lineares no intervalo de 10 a 1000 ns, permitindo a discriminação entre não linearidades rápidas e processos lentos ou acumulativos. Os resultados obtidos através desta técnica nos diversos materiais estudados, complementados com medidas ópticas lineares, tornaram possível a determinação da origem dos processos não lineares refrativos e absorcivos, permitindo também a determinação dos parâmetros espectroscópicos do estado excitado. / This work reports the dynamic nonlinear optical properties of organic compounds, namely, ytterbium bis-phthalocyanine, furfuryl alcohol derivatives and azo-aromatic compounds. In order to carry out this investigation we developed an experimental extension to the standard Z-scan technique, named Z-scan with pulse trains. In this method, pulse trains contained in Q-switched/mode-locked lasers can be advantageously used to study dynamic optical nonlinearities in the 10-1000 ns time interval, allowing the discrimination between fast and slow (cumulative) nonlinear effects. The results obtained in the compounds studied with this technique, complemented with linear optical measurements, permitted the nonlinear refraction and absorption origin to be determined, allowing the characterization of excited state spectroscopic parameters.

Nesta tese estudamos a dinâmica das não linearidades ópticas em compostos orgânicos, mais especificamente em amostras de bis-ftalocianina de itérbio, derivados do álcool furfurílico e compostos azo-aromáticos. Para que a dinâmica dos processos ópticos não lineares pudesse ser investigada nesses materiais, desenvolvemos uma extensão experimental à técnica de varredura-Z convencional, denominada de varredura-Z com trem de pulsos. Nesta técnica, o trem de pulsos de um laser Q-switched/mode-locked é convenientemente empregado para o estudo da dinâmica de processos não lineares no intervalo de 10 a 1000 ns, permitindo a discriminação entre não linearidades rápidas e processos lentos ou acumulativos. Os resultados obtidos através desta técnica nos diversos materiais estudados, complementados com medidas ópticas lineares, tornaram possível a determinação da origem dos processos não lineares refrativos e absorcivos, permitindo também a determinação dos parâmetros espectroscópicos do estado excitado. / This work reports the dynamic nonlinear optical properties of organic compounds, namely, ytterbium bis-phthalocyanine, furfuryl alcohol derivatives and azo-aromatic compounds. In order to carry out this investigation we developed an experimental extension to the standard Z-scan technique, named Z-scan with pulse trains. In this method, pulse trains contained in Q-switched/mode-locked lasers can be advantageously used to study dynamic optical nonlinearities in the 10-1000 ns time interval, allowing the discrimination between fast and slow (cumulative) nonlinear effects. The results obtained in the compounds studied with this technique, complemented with linear optical measurements, permitted the nonlinear refraction and absorption origin to be determined, allowing the characterization of excited state spectroscopic parameters.

Negative cloud-to-ground (CG) lightning flashes transport negative charge from cloud to ground. Negative ground flashes typically involve various processes identified as preliminary breakdown, stepped leader, return stroke, dart leader, dart-stepped leader, subsequent return stroke, and cloud activity between strokes, such as regular pulse trains and chaotic pulse trains. These processes can be identified through their electromagnetic field signatures. The main focus of this thesis is to document the features and understand the origin of electromagnetic fields, especially the chaotic pulse trains, generated by lightning flashes. Electric field measurements have been used to study lightning flashes in Sweden. The equipment was a parallel flat plate antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. Four simultaneous measurements were made: wideband measurement of the E-field (the vertical component) and its time derivative dE/dt, and two narrowband measurements of the E-field, centred around 3 MHz and 30 MHz. Fourier and wavelet transforms were used in the analysis of the measured data. The results show that preliminary breakdown pulses are stronger radiators at 3 MHz and 30 MHz than are the return strokes. A comparison of our results with those of previous studies obtained in different geographical regions clearly shows that the strength of preliminary breakdown pulses decreases with decreasing latitude. It is higher in the temperate regions (Sweden, for instance) and lower in the tropical regions. A comparison of the time derivatives of preliminary breakdown pulses and of the narrow bipolar pulses shows that the physical origin of these two types of pulse is different, even though they may have similar appearances in the broadband fields. This thesis introduces a new procedure to estimate the zero-crossing time of the lightning-generated radiation fields. The procedure is based on the fact that the time integral of the radiation fields generated by a discharge event whose duration is finite is equal to zero, and the zero-crossing time corresponds to the time when the peak of the integral is reached. In addition to tabulating the various statistical parameters and features of Chaotic Pulse Trains (CPTs), it is shown that these pulse trains are created by the simultaneous propagation of several dart-stepped leader type discharges in the cloud. Each dart-stepped leader type discharge generates a Regular Pulse Train (RPT), and these pulse trains combine randomly in time to generate CPTs. This conclusion is based on the results obtained by numerical simulations and by analysing the signatures of these pulse trains using Fourier and wavelet transformations. The results presented in this thesis show that electromagnetic fields, even those measured from a single station, can be used to extract information concerning the physical processes that gave rise to these fields.

Ce mémoire de thèse présente les travaux effectués pour la réalisation de sources optiques fibrées d’impulsions picosecondes cadencées à 40 GHz dans la bande C des télécommunications. Dans une première partie, nous présentons des études numériques et expérimentales mises en place pour la génération d’un train d’impulsions cadencé à 40 GHz par la compression non-linéaire d’un battement sinusoïdal via un processus de mélanges à quatre ondes multiples. Afin d’obtenir des impulsions stables, le battement sinusoïdal initial est obtenu par la modulation en intensité d’un signal continu grâce à un modulateur Mach-Zehnder piloté au point nul de transmission. Nous démontrons également l’amélioration de la qualité des impulsions générées par la suppression de la diffusion Brillouin stimulée grâce à la mise en place d’isolateurs optiques dans la ligne fibrée de la source. Nous présentons ensuite la génération d’impulsions ultra-courtes grâce à un compresseur non-linéaire composé de quatre étages fibrés. Le train d’impulsions obtenu est alors codé puis multiplexé jusqu’à un débit optique de 160 Gbit/s. Dans la dernière partie, nous présentons les démarches mises en place en vue d’un transfert technologique, telles que la réalisation d’un prototype de la source, la recherche d’antériorité et l’étude de marché. / This thesis presents the work carried out on the realization of fibered 40-GHz picosecond optical pulse sources in the telecommunications C-band. In the first part, we present a numerical and experimental study of the generation of 40-GHz pulse trains thanks to the nonlinear compression of an initial beat-signal by multiple Four-Wave Mixing process. Enhanced temporal stability is achieved by generating the sinusoidal beating thanks to a Mach-Zehnder modulator driven at its zero-transmission working point. In order to improve the quality of the generated pulses, we also demonstrate the suppression of stimulated Brillouin back-scattering by inserting several optical isolators into the compression line. In the next part, we present the generation of low duty-cycle pulse trains by using a nonlinear compressor line based on 4 segments of fiber. The generated pulse trains have been encoded and then multiplexed to achieve a high bit rate signal (160 Gb/s). In the last part, we present the technology transfer steps of this optical source, such as creating a prototype of the source, prior art search and market research.

Cette thèse présente l’étude théorique et expérimentale de la génération et de la mise en forme d’impulsions térahertz (THz). Dans un premier temps, nous avons étudié la génération d’impulsions THz par redressement optique d’impulsions laser femtosecondes intenses dans des cristaux semiconducteurs de ZnTe. Nous avons mis en évidence une forte dépendance de l'efficacité de ce processus de génération avec l'intensité de l'impulsion laser génératrice. Ceci se traduit en particulier par un décalage progressif du spectre vers les basses fréquences et par une évolution anormale de l'énergie THz avec l'intensité laser. Ces comportements résultent d'une combinaison de trois phénomènes : la déplétion de l’impulsion laser au cours de sa propagation dans le cristal, l’absorption de l'onde THz par les porteurs libres créés par absorption à deux photons et une modification de la condition d’accord de phase. Dans un deuxième temps, nous avons développé une approche analytique permettant de générer des impulsions THz de formes particulièrement intéressantes pour les expériences de spectroscopie cohérente ou de contrôle cohérent. Nous avons notamment généré des paires d'impulsions verrouillées en phase, des trains d'impulsions, ainsi que des impulsions THz accordables de grande finesse spectrale. Cette technique repose sur le redressement optique d’impulsions laser femtosecondes mises en forme à l’aide d’un masque à cristaux liquides placé dans le plan de Fourier d’une ligne à dispersion nulle. Afin de démontrer la validité de notre approche, nous avons également développé un programme de simulation qui donne des résultats en très bon accord avec l'expérience. / This thesis deals with the theoretical and experimental study of the generation and shaping of terahertz (THz) pulses. At first, we have studied the generation of THz pulses by optical rectification of intense femtosecond laser pulses in semiconductor ZnTe crystals. We have demonstrated a strong dependence of the efficiency of the generation process with the intensity of the laser pulse. This is evidenced by a progressive shift of the spectrum towards lower frequencies and an abnormal evolution of the THz energy with laser intensity. These behaviors result from a combination of three phenomena: the depletion of the laser pulse during its propagation in the crystal, the absorption of THz wave by free carriers created by two-photon absorption and a change of the phase matching condition. Secondly, we have developed an analytical approach to generate THz shaped pulses particularly interesting for coherent spectroscopy or coherent control. In particular, we have generated phase-locked THz pulses pairs, pulse trains and tunable narrow-band THz pulses. This technique is based on optical rectification of shaped femtosecond laser pulses generated by a liquid crystal Fourier filter. To demonstrate the validity of our approach, we have also developed a program giving results in very good agreement with experiment.