Impairments due to nonlinearity and birefringence in optical transmission systems

Fiber nonlinearity provides for a difficult problem in optical fibre communication system design. System designers often treat nonlinearity as a set of penalties resulting from four wave mixing, self-phase modulation, and cross-phase modulation. The Brillouin effect, and the Raman effect can be assessed separately from each other and from noise and dispersion. In fact, the first three nonlinear effects mentioned, all of which stem from the Kerr effect, are tightly coupled to each other, and the separation of the Kerr effect into three separate effects is often ambiguous and can lead to wrong results. Additionally, the nonlinearity is coupled in a complex way to the noise and dispersion, so that a separate assessment of the penalties is usually unreliable