HI ZAKARYA..
As the theoretical perspective has been already explained by aasif bashir dar..i would like to add that as far as i have observed if u compare the results between using EDFA and raman amplifier the results are better with raman amplifier and the reason maybe the gain flattering phenomena in raman amplifier and usage of external pump…but i have no idea about YDPA as i have not used it..i hope it will be helpful..
Thanks & regards