Hi Gherbi,
In the latest versions of OptiSystem we have expanded our technical background for this area. Please check out the component help file found by double-clicking the component and clicking “Help”. There are detailed instructions found within that will explain how to bias the modulator at different points.
Furthermore, there is a sample about these settings found in the “OptiSystem 13 Samples\Optical transmitter design and analysis\LiNb_Modulator_Settings”.
Regards