[pt] Este trabalho visa estabelecer na prática a estatística do ganho dependente da polarização (PDG-Polarization Dependent Gain) em sistemas ópticos com amplificação Raman. A amplificação Raman depende fortemente da polarização relativa entre os fótons de bombeio e de sinal, que tem que ser paralelas para máximo ganho [1]. Portanto, a birrefringência é um importante limitador de desempenho de sistemas de longa distância amplificados por esta técnica visto que modifica os estados de polarização de forma diferente para cada comprimento de onda. A birrefringência varia aleatoriamente de acordo com as flutuações do ambiente onde está a fibra óptica, dando origem à dispersão dos modos de polarização, efeito este conhecido pela sigla PMD. Alguns experimentos recentes mostraram que o amplificador Raman não só depende do estado de polarização do sinal de entrada, mas também que o valor da dependência do ganho com a polarização (PDG- Polarization Dependent Gain) flutua devido a natureza aleatória da PMD [4,5]. É importante conhecer a estatística da PDG, sua relação com a PMD e como a PDG pode ser reduzida a níveis aceitáveis. Nesse trabalho será medida experimentalmente a distribuição estatística da PDG em fibras de dispersão deslocada e os resultados comparados com as previsões teóricas dadas por [2]. / [en] Raman amplifiers are very attractive because they provide a large and relatively flat gain over a wide bandwidth while maintaining a small noise figure, and they can be made using regular silica fiber. However, the Raman Gain coefficient is polarization sensitive and can be up to ten times higher when the signal and pump polarization states are parallel rather than perpendicular [1]. Usually fibers present some degree of residual asymmetry - because the fiber core is slightly out-of-round, or because of mechanical stress on the deployed fiber - and this causes polarization mode dispersion. The light traveling along one polarization axis moves slower or faster than the light polarized along the other axis. This effect distorts the signal and causes polarization fluctuations along the fiber. As the Raman gain is higher when the signal and pump polarization states are parallel these fluctuations of the relative polarization between signal and pump vary the instantaneous value of the Raman gain. So the Polarization Dependence Gain (PMG) is directly related to the PMD. Several experimental studies have shown not only that the gain of raman amplifiers depends on the state of polarization of the input signal but also that this polarization-dependent gain (PDG) fluctuates over a wide range because of the random nature of polarization mode dispersion (PMD) [4,5]. It is important to know the statistics of PDG, its relationship to the PMD, and how the PDG can be reduced to acceptable low levels. In this letter we will demonstrate experimentally the statistical distribution of the PDG given by [2]. This In this work the polarization dependent gain (PDG) fluctuations due to PMD in Raman amplified optical transmissions is experimentally demonstrated.

The incidence of gastrointestinal (GI) cancers has been steadily increasing in the UK since the mid 1970’s. These include cancers of the colon and oesophagus. Colon cancers have a high incidence rate, being the fourth most common cancer in the UK for both men and women. Oesophageal cancers in comparison are much rarer, however they have a poor survival rate primarily due to a late diagnosis. The key to improving survival for these cancers and many others is to detect and remove the disease at the early stages, to prevent the cancer from advancing. At present the ‘gold standard’ for diagnosis is a biopsy followed by histopathology. This technique is invasive, time consuming and highly subjective. It is therefore important to look towards non-invasive methods for early and rapid diagnosis. Optical techniques have begun to show such promise. By probing the interactions of tissues with light, diagnostic information is able to be obtained non-invasively. Techniques such as Raman spectroscopy utilise inherent molecular vibrations to extract biochemical information from tissues. Raman spectroscopy, however, is currently fundamentally limited by long acquisition times, due to the inherently weak signals produced. Using coherent Raman techniques such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), the molecular vibrations are coherently driven to provide an enhancement in signal. This thesis explored spectral signatures from snap frozen oesophageal sections in the fingerprint (450 cm-1 to 1850 cm-1) and high wavenumber (2800 cm-1 to 3050 cm-1) regions using spontaneous Raman and compared with spectra from hyperspectral SRS. The diagnostic potential for each technique was assessed for four major pathology groups, normal, Barrett’s oesophagus, dysplasia and adenocarcinoma. Samples were classified using a principal component fed linear discriminant analysis (PCA-LDA) approach with a leave-one-out cross validation. Comparisons were made to haematoxylin and eosin (H&E) stained sections. Raman in the fingerprint region was found to be the most promising for diagnosis. There were minimal changes in the high wavenumber region between pathology groups which was also reflected in the SRS spectra and proved to be insufficient for classification. Further comparisons were made between spontaneous and coherent Raman techniques using frozen colon sections. The morphological and structural information available was explored using a k-means cluster analysis. Both spontaneous and coherent Raman were able to distinguish important structural features in the colon, such as the epithelial cells that form the colonic glands and surrounding connective tissue. Both are important visual markers for cancer diagnosis in the current approach. SRS demonstrated higher spatial resolution and faster acquisition times in comparison to spontaneous Raman. This work has discussed the many advantages of using coherent Raman techniques for tissue applications, but has also highlighted some of the limitations for spectral measurements, arising from the complexity of the system.

La peau est un organe qui enveloppe le corps, elle est une barrière naturelle importante et efficace contre différents envahisseurs. Pour le traitement des maladies dermatologiques ainsi que dans l'industrie cosmétique, les applications topiques sur la peau sont largement utilisées. Ainsi beaucoup d'efforts ont été investis dans la recherche sur la peau visant à comprendre l'absorption moléculaire et les mécanismes rendant efficace la pénétration. Cependant, il reste difficile d'obtenir une visualisation 3D de haute résolution combinée à une information chimique- ment spécifique et quantitative dans la recherche sur la peau. La spectroscopie et la microscopie non-linéaire, incluant la fluorescence excitée à 2-photon (TPEF), la diffusion Raman spontanée, la diffusion Raman cohérente anti-Stokes (CARS) et la diffusion Raman stimulée (SRS), sont introduits dans ce travail pour l'identification sans ambiguïté de la morphologique de la peau et la détection de molécules appliquées de façon topique. Plusieurs méthodes quantitatives basées sur la spectroscopie et la microscopie non-linéaire sont proposées pour l'analyse chimique en3D sur la peau artificielle, ex vivo et in vivo sur la peau humaine. De plus, afin de s'adapter aux applications cliniques à venir, un design endoscopique est étudié pour permettre l'imagerie non-linéaires dans les endoscopes flexibles. / Skin is an organ that envelops the entire body, acts as a pivotal, efficient natural barrier to- wards various invaders. For the treatment of major dermatological diseases and in the cosmetic industry, topical applications on skin are widely used, thus many efforts in skin research have been aimed at understanding detailed molecular absorption and efficient penetration mechanisms. However, it remains difficult to obtain high-resolution visualization in 3D together with chemical selectivity and quantification in skin research. Nonlinear spectroscopy and microscopy, including two-photon excited fluorescence (TPEF), spontaneous Raman scattering, coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), are introduced in this work for unambiguous skin morphological identification and topical applied molecules detection. Sev- eral quantitative methods based on nonlinear spectroscopy and microscopy are designed for 3D chemical analysis in reconstructed skin, ex vivo and in vivo on human skin. Furthermore, to adapt to forthcoming clinical applications, an endoscopic design is investigated to bring nonlin- ear imaging in flexible endoscopes.

[pt] Neste trabalho é apresentado um estudo sobre a determinação matemática dos valores da dispersão cromática total, do zero de dispersão distribuído e efeitos não lineares em fibras ópticas de dispersão deslocada (DS). Uma metodologia para caracterização das dispersões cromáticas e para os zeros de dispersão local das fibras estudadas é elaborada com a utilização do modelo de fibra degrau equivalente e de aproximações entre as fibras ópticas dos enlaces trabalhados. A relação entre o diâmetro do campo modal e o zero de dispersão cromática distribuído segundo A. Rossaro [6] é discutida e comprovada através da realização de uma série de medidas práticas para a determinação dos efeitos não lineares das fibras de dispersão deslocada dos enlaces da Petrobrás. O objetivo deste trabalho é analisar estes efeitos prejudiciais a transmissão de sinais ópticos de alta velocidade apresentados nos cabos submarinos da Petrobrás entre o Porto de Imbetiba e as plataformas de Pargo I e Pampo I na Bacia de Campos, analisando os resultados obtidos e caracterizando as possibilidades de upgrade destes enlaces. / [en] This work presents a study on the mathematical determination of the total chromatic dispersion, zero dispersion wavelength and nonlinear optical effects in dispersion shifted (DS) fiber optic system. A methodology to determine the chromatic dispersion and zero dispersion wavelength is proposed using the equivalent step index model and the similarity of the total chromatic dispersion measured in fibers of the links under study. The relationship between the mode field diameter and the zero dispersion wavelength according to A. Rossaro [6] is discussed and proven with a series of practical measurements for the determination of the nonlinear effects of the dispersion shifted optical fibers of Petrobras links Imbetiba - Pargo and Imbetiba - Pampo of the Campos Basin. / [es] En este trabajo se presenta un estudio sobre la determinación matemática de los valores de dispersión cromática total, del cero de dispersión distribuido y los efectos no lineales en fibras ópticas de dispersión desplazada (DS). Se elabora una metodología para la caracterización de las dispersiones cromáticas y para los ceros de dispersión local de las fibras estudiadas; utililizando el modelo de fibra escalón equivalente y de aproximaciones entre las fibras ópticas de los enlace. Se discute la relación entre el diámetro del campo modal y el cero de dispersión cromática distribuido según A. Rosaro [6] se comprueba a través de la realización de una serie de medidas prácticas para la determinación de los efectos no lineales de las fibras de dispersión desplazada de los enlaces de la Petrobrás. EL objetivo de este trabajo es analizar los efectos perjudiciales a la transmisión de señales ópticos de alta velocidad que se presentan en los cables submarinos de la Petrobrás entre el Puerto de Imbetiba y las plataformas de Pargo I y Pampo I en la Bacia de Campos, analizando los resultados obtenidos y caracterizando las posibilidades de upgrade de estos enlaces.