Hello atul Sharma,
I agree with fayiqa that we can easily replace a power combiner with a circulator but i think there maybe difference in results though because using a circulator does affect your results. Normally it is true that the circulator is used for adding and dropping wavelengths and in that process the results do get affected and moreover it you check the power levels after the circulator you will find the power level drops just in case of the WDM Mux. fayiqa has suggested you the right thing.
Regards