hi gokul,
i have my self used this filter.
FBG is the component that has three three in optisystem(but practically two ports),
taking of FBG in optisystem,
it has one as input, in which optical signal acts as input
then two remaining provides the reflection and transmission spectrum,
the frequency you have to suppress in the transmission spectrum is the frequency you have to replect back,
i suggest you to use uniform gratting in the filter library.
the frequency you want to reflect back or suppress in transmission spectrum can be edited.
i hope it will help you in using grating
hope you reply soon
with regards