Oh, and I have to add, nonlinearities exist in optical fibers and if you disable the optical fiber line nonlinearities, then the results you get are not reliable. So is the case with the paper you are trying to reproduce whose results. So I suggest that instead, you lower the power of all the lasers to, say, 0 dBm, lower the bit rate and shorten the length of the fiber and get more realistic results instead.