I am doing a project on Visible Light Communication using LEDs. I need to send serial data from the laptop to LED. Any ideas for implementing this transmitter part?

 

Well, many laptops already have IrDA's in them, or infrared datalinks.

Have a read on this Microsoft article:
http://support.microsoft.com/kb/313296

If you can figure out that protocol, you might be able to replace the existing IR LED and receiver with visible light emitters and receivers. However, you could also easily ruin your laptop.

If your laptop has a USB port, you might consider using that as an interface point, as it could supply a bit of power (maybe 200mA or so), and communication for your interface. You'll need to write driver software for it.

 

Make it big and powerful so it's all the receiver sees.