Hello Chihab,
Just to add to what Arjina said, the examples on PAM modulated signal and PON can be found at the following links in OptiSystem Example Library.
C:UsersUSER NAMEDocumentsOptiSystem 18.0 SamplesAdvanced modulation systemsPAM systems.
C:UsersUSER NAMEDocumentsOptiSystem 18.0 SamplesMetro and access systemsPON and CDMA systems
You need to modify the modulation in the PON example using PAM modualtion.
Regards,
Ahmad