Intensity fluctuations - driving force of nonlinearity in optical fibers

In long-haul optical communication systems, the Kerr-induced fiber nonlinearities affect the signal fidelity and are driven by the intensity fluctuations along the fiber. In dispersion-unmanaged systems, these intensity fluctuations grow and spectrally evolve along the fiber due to beating between the increasingly overlapping symbols. A semi-analytical model for the intensity fluctuation spectra is presented and validated using Monte Carlo based computer simulations. The model is used to illustrate the composition of intensity fluctuation spectra that provide insight into the mechanisms that lead to the growth of the intensity fluctuations. A key shortcoming of existing cross-phase modulation and four-wave mixing models is the lack of consideration of intensity fluctuation growth along the link, which the presented model aims to address. It is also used to explain the origins of an optimal symbol rate for subcarriers - the growth of low-frequency intensity fluctuations along a dispersive fiber. The analysis reveals that lower-frequency intensity fluctuations are predominantly influenced by symbol overlap within individual subcarriers. It also shows that the inter-subcarrier beating has no significant impact on the growth of the intensity fluctuations, so it is not a factor in explaining an optimal symbol rate. The new understanding in this work could help the development of design rules for long-haul system.