Hi lavanya and welcome to Optiwave forums. Here are some papers which might be useful on the subject:
http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6459732&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6459732
(The paper suggests how optical amlifiers could be combined for the benefit of long reach PONs)
http://web.stanford.edu/~shyin/Courses_files/Wideband%20Optical%20Amplifier%20Technology_Shuang%20Yin_EE247%20Term%20Paper.pdf
(The paper contains some hybrid optical amplifier simulations that have been done using Optisystem)
I suggest first taking a look at optical amplifier samples in Optisystem sample files. Let me know if these help.
Regards