Αντικείμενο αυτής της διπλωματικής εργασίας είναι η διερεύνηση των τεχνικών διαφορισμού στους ψηφιακούς δέκτες των σύγχρονων ασύρματων δικτύων. / Purpose of this paper is the examination of combining techniques at digital receivers of modern wireless networks.

The main objective of this thesis is to explore the use of volume holographic elements recorded in photo-thermo-refractive (PTR) glass for power scaling of narrow linewidth diffraction-limited fiber lasers to harness high average power and high brightness beams. Single fiber lasers enable kW level output powers limited by optical damage, thermal effects and non-linear effects. Output powers can be further scaled using large mode area fibers, however, at the cost of beam quality and instabilities due to the presence of higher order modes. The mechanisms limiting the performance of narrow-linewidth large mode area fiber lasers are investigated and solutions using intra-cavity volume Bragg gratings (VBG) proposed. Selfpulsations-free, completely continuous-wave operation of a VBG-stabilized unidirectional fiber ring laser is demonstrated with quasi single-frequency ( < 7.5 MHz) output. A method for transverse mode selection in multimode fiber lasers to reduce higher order mode content and stabilize the output beam profile is developed using angular selectivity of reflecting VBGs. By placing the VBG output coupler in a convergent beam, stabilization of the far-field beam profile of a 20 μm core large mode area fiber laser is demonstrated. Beam combining techniques are essential to power scale beyond the limitations of single laser sources. Several beam combining techniques relevant to fiber lasers were compared in this study and found to be lacking in one or more of the following aspects: the coherence of the individual sources is compromised, the far-field beam quality is highly degraded with significant power in iv side lobes, spectrally broad and unstable, and uncertainty over scaling to larger arrays and higher power. Keeping in mind the key requirements of coherence, good far-field beam quality, narrow and stable spectra, and scalability in both array size and power, a new passive coherent beam combining technique using multiplexed volume Bragg gratings (M-VBGs) is proposed. In order to understand the mechanism of radiation exchange between multiple beams via these complex holographic optical elements, the spectral and beam splitting properties a 2nd order reflecting M-VBG recorded in PTR glass is experimentally investigated using a tunable single frequency seed laser. Two single-mode Yb-doped fiber lasers are then coherently combined using reflecting M-VBGs in both linear and unidirectional-ring resonators with > 90% combining efficiency and diffraction-limited beam quality. It is demonstrated that the combining bandwidth can be controlled in the range of 100s of pm to a few pm by angular detuning of the M-VBG. Very narrow-linewidth ( < 210 MHz) operation in a linear cavity and possibility of singlefrequency operation in a unidirectional ring cavity of the coherently combined system is demonstrated using this technique. It is theoretically derived and experimentally demonstrated that high combining efficiency can be achieved even by multiplexing low-efficiency VBGs, with the required diffraction efficiency of individual VBGs decreasing as array size increases. Scaling of passive coherent beam combining to four fiber lasers is demonstrated using a 4th order transmitting M-VBG. Power scaling of this technique to 10 W level combined powers with 88% combining efficiency is demonstrated by passively combining two large mode area fiber lasers using a 2nd order reflecting M-VBG in a unidirectional ring resonator. High energy compact single-frequency sources are highly desired for several applications – one of which is as a seed for high power fiber amplifiers. Towards achieving the goal of a monolithic solid-state laser, a new gain medium having both photosensitive and luminescence properties is investigated – rare-earth doped PTR glass. First lasing is demonstrated in this new gain element in a VBG-stabilized external cavity.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2015-12-16T14:08:06Z No. of bitstreams: 1 michellesoarespereira.pdf: 4441399 bytes, checksum: 19c7a2a01a8d195e44aa21afaacc4de5 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2015-12-16T15:15:37Z (GMT) No. of bitstreams: 1 michellesoarespereira.pdf: 4441399 bytes, checksum: 19c7a2a01a8d195e44aa21afaacc4de5 (MD5) / Made available in DSpace on 2015-12-16T15:15:37Z (GMT). No. of bitstreams: 1 michellesoarespereira.pdf: 4441399 bytes, checksum: 19c7a2a01a8d195e44aa21afaacc4de5 (MD5) Previous issue date: 2015-05-29 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho aborda a análise de protocolos de cooperação para melhorar o desempenho de sistemas powerline communication (PLC). O estudo se baseia em um conjunto de dados de canais PLC in-home. As análises discutidas abordam a capacidade teórica do canal quando protocolos amplify-and-forward (AF) e decode-and-forward (DF) são considerados. Estudos similares são realizados a respeito da taxa máxima de dados adquirida quando Hermitian − symmetric orthogonal frequency division multiplexing (HS-OFDM) juntamente com as técnicas equal gain combining (EGC), selection combining (SC) and maximal ratio combining (MRC) são considerados. Além disso, são analisadas as influências da largura de banda de freqüência e da alocação ótima e uniforme da potência de transmissão. Os resultados obtidos mostram que a distância relativa entre os nós da fonte, relay e destino impactam severamente no desempenho do sistema. Os resultados numéricos revelam uma gama de potência de transmissão ótima e de frequência em que a melhoria pode ser verificada. Tambà c m observa-se que a vantagem do protocolo DF relacionada com a do protocolo AF termina quando o erro de detecção de símbolo no relay atinge um limiar. Adicionalmente, mostra-se que as técnicas de combinação, MRC e SC apresentam resultados semelhantes. Embora MRC ofereça desempenho ligeiramente melhor, considerando a complexidade computacional, recomenda-se a técnica SC. Finalmente, uma análise de máxima taxa de dados adquirida, considerando HS-OFDM com equalização baseada nos critérios zero forcing e minimum mean square error, mostra que o primeiro oferece quase o mesmo desempenho que o último. / This work focuses on analyses of cooperative protocols to enhance the performance of in-home power line communication (PLC) systems based on a data set consisting of measurements of in-home PLC channels. The discussed analyses address theoretical channel capacity when amplify-and-forward (AF) and decode-and-forward (DF) protocols are considered. Similar investigations are performed for the maximum data rate attained by using Hermitian-symmetric orthogonal frequency division multiplexing (HS-OFDM) together with equal gain combining (EGC), selection combining (SC) and maximal ratio combining (MRC) techniques. Also, the influences of optimally and uniformly allocated transmission power and frequency bandwidth are analyzed. The attained results show that the relative distance among source, relay, and destination nodes severely impacts the system performance. Also, numerical results reveal a range of optimal transmission power and frequency bandwidth in which improvement can be verified. Moreover, it is observed that the advantage of the DF protocol related to AF one ends, when the symbol detection error achieves a threshold. Additionally, it is shown that among the combining techniques, MRC and SC present similar results with MRC offering slightly better performance, but considering computational complexity, a decision in favor of SC is recommended. Finally, a maximum data rate analysis of HS-OFDM with frequency domain equalizer based on zero forcing and minimum mean square error shows that the former scheme offers almost the same performance as the latter.