H! Shahbaz,well you got the correct plot finally
Alistu pltted the plot between launched power and ber and looks correct and effects of non linearities also can be ssen.Now if you got error by increasing the length of fiber then i think you need to check the system again.Try to place filter for all the four channels and do not let any port free.Also received power in your plotted curve is less so ter to boost signal using EDFA.But i am also amazed that by only increasing fiber length why do you get this error.I will try to access the problem .