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Limits are obtained to a method which utilizes fiber nonlinearity (self phase modulation) to increase the bit rate capacity of installed single channel non dispersion shifted fiber links at the wavelength of 1550 nm. We focused on a set-up that uses no in-line optical amplipers. Maximum attainable bit rate versus distance are obtained under optimal transmitted powers for different pulse shapes, which may be helpful in designing optical fiber systems. Substantial improvements are possible and better performances have been achieved for smooth pulses. A practical example of bit rate upgrading in an installed link is shown and compared with experimental papers.

The advent of optical amplifiers made possible repeaterless optical fibre links with thousands of kilometres. An important limiting factor to achieve such a distances is the interaction between ASE noise, generated by optical amplification, and the co-propagating signal, consisting in the phenomena of modulational instability and side-band instability. This paper describes briefly the modelling of these phenomena and, through numerical simulations, verifies the influence of modulational instability in the choice of parameters such as power level, fibre group-velocity dispersion and dispersion compensation methods, allowing conclusions with direct application to the design of optical communication systems.Keyword --modulational instability, side-band instability, spontaneous emission noise, optical amplifiers, dispersion compensation.

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