In this work we have been presented the influence of nonlinear optical effects on the NRZ and RZ modulation signals. The results show the nonlinear changes in different WDM systems, where different dispersion levels and input signals powers were presented. As an engineer tries to reach better linear parameters, and moves zero dispersion level to the third optical window, they got worse nonlinear effects inside the optical fiber. Results present NOE dependency on dispersion and input power levels. As designer try to get smaller dispersion value as SPM occur more in RZ systems. It is dramatically for DSF fibers. If input power level increases XPM can distort both WDM systems performances. FWM effect occurs more in NRZ systems near zero dispersion value. New generated harmonics peak power is two times lower in RZ systems compared to NRZ systems. That is allows us to install more optical channels. FWM influence will be reduced, if channel interval will be at least two times greater than 100 GHz. When signal power values are higher than NOE threshold it is better to use dispersion compensation technique like a dispersion compensation fibers or dispersion compensators not to use DSF. For the next WDM generation, where the bit rates will be more than 40 Gbit/s for each channel new modulation methods are needed, because of ISI and NOE; such research will be provided in our future works.