The radiation and reception patterns of the optical components have to comply with the "spatial" requirements,i.e. if both are stationary they should be narrow-beam devices to improve the received energy.
The other thing is bandwidth;if data bitrate is more than let's say 100kb/s you should chose the receiver carefully.
Another snag could be the demand to handle both very weak and very strong signals.Check if the input stage(s) is in linear mode even when the signal is strong.
Apply some filtering on the received signal to improve S/N ratio when the signal strength is not enough.