HI IMENE
You can download optisystem from the optiwave website itself if you wish to use it. For that matter it would be a trial version and you will be having a 30 day evaluation period and after 30 days your tool would get terminated if you do not buy a license which is very necessary to keep using the tool. It is a licensed software. You can further consult the optiwave support team for further inquiries. I hope you find it helpful.
Thanks and regards