The present work describes a theoretical and experimental investigation of the stimulated Raman effect excited by the focused output of a slow, Q-switched ruby laser in the self-focusing liquids benzene, nitrobenzene, chlorobenzene, and carbon disulphide. Time relationships between the exciting pulse, the transmitted laser pulse, and the time resolved stimulated Raman spectra have been studied using a high speed streak camera in conjunction with fast photodiodes. The spontaneous Raman spectra of these liquids have been investigated using a photoelectric Raman spectrometer and a method is outlined for measuring the relative values of the stimulated Raman gain coefficients. It is experimentally shown that, on a nanosecond time scale, stimulated Raman lines are not generated simultaneously but in the sequence Stokes, Anti-Stokes, and second harmonic Stokes. Further, the time delay between the start of first and second harmonic Stokes is shown to be dependent on the rate of rise of the exciting pulse. It was found that during stimulated Raman generation the laser pulse transmitted through the liquid was heavily distorted and that each liquid produced its own characteristic pulse distortion. No distortion was found in the absence of stimulated Raman generation. Time correlations were found between the duration of features of the pulse distortion and the duration of first Stokes and second harmonic Stokes. These correlations show that forward stimulated Raman generation is controlled by the transmitted laser pulse not the exciting pulse. The threshold for the onset of pulse distortion is shown to be dependent on the rate of rise of the exciting pulse whilst, in all the liquids, apart from carbon disulphide, the 'cut-off' threshold is dependent on the peak power of the incident pulse. Investigation of the pulse distortion thresholds for the range of liquids produced relative values which did not agree with those predicted from either the optical Kerr coefficient or the calculated stimulated Raman gain coefficients. A theoretical model of the self-focusing of a focused beam in a medium for which both electrostriction and the optical Kerr effect are significant is presented and has been used to explain the anomalous threshold results. Within the experimental scatter of the results obtained, this model appears to explain the observed threshold effects. Since the forward stimulated Raman generation was weak in comparison to the transmitted laser pulse and followed this distorted pulse rather than the exciting pulse, it is concluded that stimulated Raman is not the dominant mechanism in the interaction. A brief review is presented of the theory and predictions of steady and non-steady state Brillouin scattering. Although the phonon lifetimes for the other liquids appear to be too short to be of significance, the results are similar in form to those of carbon disulphide. An explanation based on multiphoton absorption is suggested to explain the anomalous behaviour of these liquids. It is concluded that the observed effects in the forward stimulated Raman process depend upon the development in time of the non-linear field dependent self-focusing of the exciting beam and the strong backward scattering process.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents the analysis and implementation of a connection between two power converters, applied to street lighting with LEDs. This connection is called optimized cascade. The main idea of this connection is the reduction of the processed energy by the second converter, increasing this way the system efficiency. Besides that, the electrolytic capacitor is eliminated, aiming the life-span increase of the converter. To do that, the capacitance is reduced, causing a low frequency ripple, which is compensated by the second converter. This way, it is possible to unify high eficiency with long life-span on this converter. Such characteristics are considered fundamentals on LED driver. To prove the idea, two prototypes were built, one based on feedback control scheme and the other on a feedback plus feedforward control scheme. Both had shown good results, however the feedback plus feedforward controller proved to be more effiective on the active compensation of the low frequency ripple. In the end, a comparison between the two most used topologies on LED driver with the optimized cascade was performed. The results had shown a possible application range, advantages e limitations of this idea. / Este trabalho apresenta a análise e implementação de uma conexão entre dois conversores de potência, com aplicação para iluminação pública com LEDs. Essa conexão foi chamada de cascata otimizada. A principal ideia dessa conexãoo é a minimização da energia processada pelo segundo conversor, aumentando assim a eficiência do sistema. Além disso, o capacitor eletrolítico é eliminado desta topologia com o intuito de aumentar a vida útil do sistema. Para tal, a capacitância é reduzida, sendo a ondulação de baixa frequência, proveniente de tal redução, compensada ativamente pelo segundo conversor. Com isso, é possível unir alta eficiência e durabilidade, características consideradas fundamentais em um driver para LEDs. Para provar a ideia, dois protótipos foram desenvolvidos, um baseado em controle feedback e o outro baseado na união de um controle feedback com feedforward. Ambos mostraram bons resultados, o entanto o controle feedback mais o feedforward mostrou-se mais eficaz na compensação ativa da ondulação de barramento. Por fim, uma comparação entre as duas topologias mais utilizadas em drivers para LED e a cascata otimizada foi realizada. Os resultados obtidos mostraram sua possível faixa de aplicação, bem como vantagens e limitações.