HELLO JYOTI..
You are right there are number of amplifiers in the optical library of amplifiers and as far as ur query is concerned the usage of a particular amplifier depends well on the application of respective amplifiers…there are times when each amplifier can give u same results and then sometimes there is large difference between the results..
I will suggest u to refer to these links to get an idea where when and why we use different amplifiers.. i hope it is helpful to u.
Thanks & regards