Digital fiber-optic networks are the basis of todays communication infrastructure. The maximum capacity that can be transmitted over a single optical fiber is ultimately limited by the nonlinear properties of the fibre. The parameters of the link design are usually optimised by numeric simulations. Analytic descriptions of the nonlinear perturbation of the received signal can help to reduce the parameter space which has to be simulated. In order to expose the fundamental laws governing the nonlinear perturbation of the received signal, a simple analytic model is derived. A...