Hello,
Which version of OptiSystem are you using?
First, you need to make the transmission condition suitable to receive a good quality signal and good eye. Then you change the conditions slowly to what you need. This can be achieved by making shorter distances, less noise in the system, etc…
All, of these parameters are design parameters, that you need to adjust.
You may go to OptiSystem example library for working examples that you may use to improve your design.