Laser light
A standard visable laser diode is centered around 630-660 nano meters. Keep in mind the output is pulsed. So you need a photodiode centered at that frequency, filter screen in front of it to block as much of the rest of the spectrum not in use. If it's an IR laser that's a different story. It really depends on the laser you're using. It could be as simple as using a few sheets of red colored plastic to filter out the recieve end, and using a low enough transmit rate not to get interference from the pulsed nature of laser diodes.