Hello jyoti,
Your query seems to be very confusing. As far as i understand you want to compare the performance of one receiver with performance of other receivers and that in terms of power. So for that i suggest you to keep same launching power for every transmitter and check the result for each receiver and proceed in a same way for different power levels using sweep concept as mentioned rightly by Aasif. For varying the fiber length , you can also use the sweep concept and check the performance for different lengths.
You can go through the lesson of which Aasif has provided the link. It would surely help you.
Regards