Hi Ranjeet
Correctly mentioned by alistu..that link and power budget are the same.Term power budget is mostly used but u wil find a few papers using the term link budget too but anyway both have the same significance.
Now for calculating the power budeget of a WDM PON system you can use the formula
Power Budget = Transmitted power – Reciever sensitivity
Also calculate the total losses of your system.
Then make sure that the power budget you calculate using the above equation is higher than the total loss of your system.Take the losses due to MUX,Fiber and all other components into consideration to calculate the total system loss.
If your power budget comes out to be low than the total loss you can then make use of amplifiers at the OLT and ONU terminals to improve the reciever sensitivity of your system.