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Nonlinearity effect manifests in optical fibers as data rates, optical power levels, number of wavelengths and transmission lengths increase. Fiber nonlinearity presents a new realm of obstacle that needs special attention when designing fiber optic transmission systems with solutions to minimize the detrimental effects.
The nonlinearities include stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), four wave mixing (FWM), self-phase modulation (SPM) and cross-phase modulation (XPM).
The figure here shows how a Q-factor, which is a qualitative description of the system performance, changes with respect to the launch power. The higher the Q-factor, the better is the system performance. Firstly, increasing the launch power until an optimum power P_1CH increases the Q-factor. The gradual improvement is seen here because as the signal power is increased the OSNR gets better for an EDFA enabled system.
For launch powers greater than the optimum power, the fiber nonlinearities come into play and degrade the system performance.
For a WDM-enabled system, the optical launch power is clearly lower than for a single channel transmission and therefore is the transmission distance also lowered. The decrease in Q-factor is because of nonlinear effects arising from the WDM neighboring channels.
Fiber nonlinearities are therefore important to be considered during transmission system design and they fundamentally limit the fiber capacity.