Dear Ahmed,
please refer to the examples (Lesson 4 Parameter Sweeps – BER x Input power.osd) in OptiSystem Tutorials located at the following link for guidance on IM-DD. You may check the theory of any component by accessing its datasheet through the Help button in the component properties popup window.
C:\Users\USER NAME\Documents\OptiSystem 17.1 Samples\Introductory tutorials
Regards,
Ahmad