Hi Ahmad,
The attachment appears for me (it also works when I am logged out). Let me know if you are still having trouble seeing the attachment.
The Free Space Optical Links post gives you a brief description. They are basically two different, yet similar models, that are optimized for short and long free space channels. For further info, take a look at the component help file (double-click the component in the layout and then choose the Help button), this will give you details on how each are modeled.