I think it is not good to tell to without doing literature survey regarding this concept.i will try to understand the concept.Till then i suggest you to read all the references given in your base paper .you will definitely get the concept behind it.in general fwm occures when multiple signal fed to fiber ,then there is a grneration of new wavelengths due to sum and subtration.phase matching is a key issue here and in this system they have used phase modulators to mismatch that phase in order to supress the FWM.but i will let you know exact reason and further work you can do on this work.