Hi Imene Khris
you can download the sogtware from this site only by clicking the downloads link and you would get the trial version of the latest optisystem version that is optisystem 14. As fayiqa said it will be terminated after about 30 days and hence we would suggest you to buy the license in order the optisystem software with no time limit . i would attach a link to download the trial version and if you find it useful you can switch to license version of this software.
https://optiwave.com/members/burhan1409/evaluation-license-downloads/#user-evaluation-tab
Regards
Burhan