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In this paper the error probability of a singlechannel coherent optical DPSK transmission system based on repeterless links in the presence of fiber chromatic dispersion and Kerr effect is evaluated. An accurate model for both the optical signal propagation and the probability distribution of the receiver decision variable is adopted by using a numerical solution of the non-linear Shrodinger equation and the characteristic function method. The obtained results show that the selection of an optimized IF filter bandwidth is crucial in order to obtain the best system performance. When chromatic dispersion dominates its combination with Kerr effect causes best performances to be achieved in normal dispersion region while, when Kerr effect is the most relevant limitation factor for the system performances, the lowest error probability is attained in anomalous regime. As a matter of fact in this regime Kerr effect and chromatic dispersion tends to partially compensate each other. In any case the error probability curves shows a relative minimum that is always attained in the anomalous region. Finally the maximum link length is limited by the presence of Kerr effect independently from the amount of transmitted optical power to be shorter than a threshold length if an error probability of is to be achieved.

The limitation to the amplification of femtosecond pulses by erbium-doped fibers owing to the soliton self-frequency shift may be overcome by using a dispersion-shifted fiber. We show that this result holds irrespective of the relatively large residual frequency-dependent chromatic dispersion that is associated with the pumping of the doping ions. Nevertheless, resonant dispersion may lead to temporal broadening and distortion of the amplified pulse.

The dynamics of a nonlinear resonant tapered coupler is analyzed by using coupled-mode equations. The constituent waveguides are made of a linear host material doped with resonant impurities. Such a system allows a mechanism for all-optical switching. It is shown that nonlinear exchange between coupled waveguides originates from the group-velocity dependence on the propagating pulse power and/or area. The transmitted signal is always a multiple of the self-induced-transparency fundamental soliton.The dynamics of the pulse tuned or detuned from the resonance and with the inclusion of the material relaxation terms is discussed. In the last section the problem of soliton collision in the coupling region is considered; in this case an initial interpulse phase difference may determine a power-dependent outport switching.PACS number(s): 42.50. Rh, 42.65.Pc, 42.79.Gn