Hello naazira,
A very good question indeed. I would also like to know about it. Because if we consider a 4 input WDM MUX , incase it is 10 GHz for each channel then i would be using 40 GHz for my system which is very huge bandwidth.
From the discussion i had with Alistu , i guess this Bandwidth would be to accommodate every channel although i am not sure.
Regards