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The performances of an optical coherent FDM system based on ASK modulation and optical multiport receiver are analysed. The receiver allows baseband operation without phase-locking and, in presence of an ideal optical hybrid, it completely eliminates the intermodulation noise. The effects of channel crosstalk and of optical hybrid non-idealities are considered to evaluate the feasibility of the system.

In this paper the performance of FDM systems with a capacity of the order of a few hundreds of Gbit/s and a span up to 500 km is analysed by means of an analytical model. These systems can be important in the future high speed digital optical network.The comparison between IM-DD and coherend systems gives quite different results if transoceanic single channel systems or very high capacity FDM systems with a span of a few hundreds of km (300-500 km) are considered. While in the first case it is well known that IM-DD exhibits better performance the situation is different in the second case. Coherent systems exhibit better performance: up to 300 Gbit/s can be transmitted over a distance of 500 km adopting homodyne PSK technique.

A new system architecture is presented that allows to perform multilevel PSK and QAM transmission using coherent optical techniques. It is based on the transmission of a reference carrier on a polarization orthogonal to that of the message in order to attain a high insensitiveness to laser phase noise. The independence of the fiber induced polarization fluctuations is attained by means of a completely electronic tracking of the fiber Jones matrix.The system performance is evaluated using the characteristic function method both at the quantum limit and in the presence of phase noise both for PSK and for QAM modulation.