Sistemas de Comunicacao Optica Coerentes com Modulacao de Fase

The fundamental difference between coherent optical communication systems and direct detection optical communications systems lies in the fact that the former ones make use of the coherent properties of the optical carrier. These systems can be based on heterodyne (or homodyne) detection, or alternatively they can use optical demodulation associated with direct detection. In this thesis, analytical methods to evaluate the performance of these two categories of coherent systems are developed, assuming that the information is transmitted at optical level by using phase modulation. These methods combine a detailed modelling of the optical receivers with an analytical formulation appropriate for estimating the average error probability. Special emphasis has been placed on the characterisation of the statistical properties of the non-Gaussian random process associated with the baseband signal disturbed by the action of the phase noise from semiconductor lasers, which is an important source of degradation in coherent optical communication systems. The influence of this noise on the carrier recovery circuits, used for synchronous demodulation, is also studied in detail. The error probability has been evaluated taking into consideration not only the usual noise sources (quantum noise, circuit noise, laser phase noise) but also intersymbol interference and the effects of baseband filtering on the referred non-Gaussian process. The used error probability estimation procedure is based on a Gram-Charlier series, or alternatively on a Gaussian quadrature rules method, involving the calculation of the cumulants of the signal at the baseband filter output. The obtained results show that the baseband filter reduces significantly the degradation induced by laser phase noise, reducing the demands on laser spectral coherenceAvailable from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga