i think you can use  a CW plus a small perturbation as the input condition in your matlab code instead of the gaussian beam used  useally and you will see if you choose the right fiber parametres how this small perturbation will grow in term of distance up into a bit stream.

Suppose we are using the GP Agrawal book program. The time grid contains points from -T/2 to T/2 via 0. 

Now, frequency grid contains points -f/2 to f/2 via 0 which is not absolute frequency grid. So how to give a continuous CW wave there? the central frequency is simply 0. absolute carrier frequency will be w0 which wont come in the program. So will that still work?