In the SDH system, we mainly consider the attenuation coefficient and dispersion coefficient of the fiber. However, in a WDM system, due to the long distance of the regenerator section, the insertion loss of the optical component of the WDM system is relatively large. In order to solve the problem of fiber attenuation, the use of EDFA for amplification compensation, while amplifying the optical power, also greatly increases the nonlinear effect in the optical fiber, and becomes an important factor limiting the regenerative relay distance.
The nonlinear effects in the fiber include:
Scattering effect (Stimulated Brillouin scattering (SBS) and Stimulated Raman scattering (SRS), etc.)
Kerr effect, ie, related to the refractive index of the fiber have serious effects on the system. These effects including, Self-Phase Modulation (SPM), Cross-Phase Modulation (XPM), Four-Wave Mixing effect (FWM), in which four-wave mixing and cross-phase modulation.
In the actual WDM project, there is nominal output power for each amplifiers, and there is also a sporadic error at the receiving end due to the nonlinear effect of the optical fiber (the SNR reaches the requirement). However, the occurrence of nonlinear effects affects the system with a small probability of error, and there is no certain regularity.
To solve the problem caused by non-linear effect, one important act is to lower the transmitting power, and try to optimize the line quality from other components of the line, for example the attenuation of line, the configuration of DCM modules.
