Nonlinear intrachannel distortion in high-speed optical transmission systems

Doktorsavhandling, 2006

The Kerr nonlinearity in optical fibres tends to generate signal distortion in dispersion-managed fibre-optic communication systems. Within a wavelength channel, the resulting intrachannel effects are timing jitter, i.e., random temporal shifting of the signal pulses, amplitude jitter, and generation of noise pulses in empty bit slots, referred to as ghost pulses. An investigation of these effects is carried out using both a variational and a perturbation analysis of the nonlinear Schrödinger equation. This allows the nonlinear interaction to be described analytically and numerical simulations are presented that support and extend the obtained results. The interaction range, i.e., the number of bit slots over which the nonlinear interaction is significant, is an important concept when designing and interpreting system simulations. Analytical and numerical results are given which allow a priori estimations valid for realistic systems. The growth rate of the nonlinear distortion has been examined in order to investigate the possibilities for suppressing the nonlinear interaction. Methods for counteracting the nonlinear effects are proposed, and special emphasis is given to the suppressive effects obtained from phase shifting of consecutive signal pulses. All-optical regeneration is discussed as an alternative method of reducing the signal distortion and in particular two types of regenerators are studied. The first type is based on nonlinear spectral widening and bandpass filtering and is suitable for return-to-zero data. The second type can reduce the phase fluctuations in differential phase-shift keying data and the analysis provides a description of an already experimentally implemented device.