Hi Ajay,
Since I’m traveling, I don’t have OptiSystem “by the hand” and not able to open your file and give you an advice…
Can you, please, send your scheme as a screen shot so I can take a look and try to help you the way I can. Taking a guess, I can assume that stability of modulated laser should play an important role, especially for complex modulation formats. If you have a chance attach your eye diagram as well.