Hi Sarath Chandran…
Actually in a bidirectional optical fiber both backward and forward signals exist at same time and in simulation it is not possible.So we have to introduce some delay in the system such that signals are hold for a while ,so that both signals can travel at the same time like the real time scenario.
Also you can go through the lesson 5 of the optiwave tutorial as already told by fayiqa.
Also i am attaching the link of optiwave forum in which it was discussed by Alistu sir.
Hope you will get some benefit from this.
with regards