Effects of polarization in a distributed raman fibre amplifier

The need to exploit the large fibre bandwidth and increase the reach has seen the application of the Raman fibre amplifier (RFA) become indispensable in modern light wave systems. The success and resilience of RFAs in optical communication is deeply rooted in their unique optical properties and new technologies which have allowed the amplifier to come of age. However, the full potential of RFAs in optical communication and other applications are yet to be realized. More so are its polarization properties which still remain largely unexploited and have not been fully understood. In this work, fundamental issues regarding distributed RFA have been investigated with the aim of acquiring a better understanding of the amplifier polarization characteristics which have potential applications. In particular the effects of polarization mode dispersion (PMD) and polarization dependent loss (PDL) have been demonstrated both by simulation and experiment. The possibility of Raman polarization pulling in single mode fibres (SMFs) has also been addressed. Polarization sensitivity of RFA has been known for a long time but the clear manifestation of it has become evident in the advent of modern low PMD fibre. Unlike EDFAs which make use of special doped fibre, RFAs require no special fibre for operation. Besides, RFA uses a very long length of fibre and as such the fibre polarization characteristics come into play during amplification. In the demonstrations presented in this thesis a fibre of PMD coefficient < 0.05 pskm-1/2 was regarded as low PMD fibre while one having coefficient ≥ 0.05 pskm-1/2 was categorized to have high PMD unless otherwise stated. Several experiments were performed to evaluate the RFA gain characteristics with respect to fibre PMD and the system performance in the presence of noise emanating from amplified spontaneous emission (ASE). Analysis of Raman gain statistics was done for fibres of low and high PMD coefficients. The statistics of PDG and on-off gain were eventually used to demonstrate the extraction of PMD coefficients of fibres between 0.01- 0.1 pskm-1/2 using a forward pumping configuration. It was found that, at increasing pump power a linear relationship exists between forward and backward signal gain on a dB scale. The interaction of PDL and Raman PDG in the presence of PMD were observed at very fundamental level. It was found the presence of PDL serves to reduce the available on-off gain. It was also established that the presence of PMD mediates the interaction between PDG/PDL. When PMD is high it reduces PDG but the presence of PDL introduces a wavelength dependent gain tilting for WDM channels. Further analysis revealed that signal polarization is influenced by the pump SOP due to the pulling effect which is present even at moderate pump power

Recent Progress in Optical Fiber Research

This book presents a comprehensive account of the recent progress in optical fiber research. It consists of four sections with 20 chapters covering the topics of nonlinear and polarisation effects in optical fibers, photonic crystal fibers and new applications for optical fibers. Section 1 reviews nonlinear effects in optical fibers in terms of theoretical analysis, experiments and applications. Section 2 presents polarization mode dispersion, chromatic dispersion and polarization dependent losses in optical fibers, fiber birefringence effects and spun fibers. Section 3 and 4 cover the topics of photonic crystal fibers and a new trend of optical fiber applications. Edited by three scientists with wide knowledge and experience in the field of fiber optics and photonics, the book brings together leading academics and practitioners in a comprehensive and incisive treatment of the subject. This is an essential point of reference for researchers working and teaching in optical fiber technologies, and for industrial users who need to be aware of current developments in optical fiber research areas

Efeitos da polarização em sistemas de comunicação por fibras óticas

Doutoramento em FisicaIn this thesis we perform a detailed analysis of the state of polarization (SOP) of light scattering process using a concatenation of ber-coil based polarization controllers (PCs). We propose a polarization-mode dispersion (PMD) emulator, built through the concatenation of bercoil based PCs and polarization-maintaining bers (PMFs), capable of generate accurate rst- and second-order PMD statistics. We analyze the co-propagation of two optical waves inside a highbirefringence ber. The evolution along the ber of the relative SOP between the two signals is modeled by the de nition of the degree of co-polarization parameter. We validate the model for the degree of co-polarization experimentally, exploring the polarization dependence of the four-wave mixing e ect into a ber with high birefringence. We also study the interaction between signal and noise mediated by Kerr e ect in optical bers. A model accurately describing ampli ed spontaneous emission noise in systems with distributed Raman gain is derived. We show that the noise statistics depends on the propagation distance and on the signal power, and that for distances longer than 120 km and signal powers higher than 6 mW it deviates signi catively from the Gaussian distribution. We explore the all-optical polarization control process based on the stimulated Raman scattering e ect. Mapping parameters like the degree of polarization (DOP), we show that the preferred ampli cation of one particular polarization component of the signal allows a polarization pulling over a wavelength range of 60 nm. The e ciency of the process is higher close to the maximum Raman gain wavelength, where the DOP is roughly constant for a wavelength range of 15 nm. Finally, we study the polarization control in quantum key distribution (QKD) systems with polarization encoding. A model for the quantum bit error rate estimation in QKD systems with time-division multiplexing and wavelength-division multiplexing based polarization control schemes is derived.Nesta tese realizamos uma análise detalhada do processo de espalhamento do estado de polarização (SOP) da luz, obtido através da concatenação de vários controladores de polarização (PCs) baseados no enrolamento de fibra ótica. E proposto um emulador de dispersão dos modos de polarização (PMD), construído através da concatenação de PCs e fibras que mantêm a polarização, capaz de gerar corretamente a estatística da PMD de primeira e segunda ordens. Analisamos ainda a copropagação de dois feixes de luz em fibras de elevada birrefringencia. A evolução ao longo da fibra do SOP relativo entre os dois feixes de luz é modelada através da definição do parâmetro grau de copolarização. O modelo e validado experimentalmente, explorando a dependência na polarização do efeito de mistura de quatro ondas em fibras de elevada birrefringência. Estudamos também a interação sinal ruído mediada pelo efeito de Kerr em fibras óticas. É derivado um modelo que descreve o ruído gerado pela emissão espontânea amplificada em sistemas com ganho de Raman distribuído. Mostramos que a estatística do ruído varia com a distância de propagação e com a potencia do sinal, e que para distâncias superiores a 120 km e potências do sinal maiores que 6 mW esta deixa de ser descrita por uma distribuição Gaussiana. Analisamos o processo de controlo totalmente ótico da polarização baseado no efeito de espalhamento de Raman estimulado. Através do mapeamento do grau de polarização (DOP), mostramos que a amplificação preferencial de uma componente do sinal permite uma atração do SOP num intervalo de comprimentos de onda igual a 60 nm. A eficiência do processo é mais elevada em torno do comprimento de onda de ganho de Raman máximo, onde existe um intervalo de 15 nm para o qual o DOP tem valores praticamente constantes. Finalmente, fazemos um estudo do controlo do SOP em sistemas de distribuição de chaves quânticas (QKD) com codificação na polarização. E derivado um modelo que permite estimar a taxa de erro quântica em sistemas de QKD com esquemas de controlo do SOP baseados na multiplexagem no comprimento de onda e na multiplexagem no domínio temporal