Simple :-)They're commercially available (http://www.totalcommsystems.co.uk/shop/customer/home.php?cat=137), so it's possible. If you're not looking for ultra-high speeds or long distances, it should be pretty simple. At low speeds, you could probably turn off and on the beam rather than modulate it's intensity. That would make the electronics at each end easier. Off the top of my head, Computer->serial port-->microcontroller-->laser diode/laser pointer--->photodiode-->microcontroller--serial port.