LEDs for free-space communication
Free-space communication LEDs are commonly made with GaAs or GaInAs active regions and are grown on GaAs substrates. The GaInAs layer is pseudomorphic, i. e. sufficiently thin that it is coherently strained, and no dislocations are generated. The emission wavelength of GaAs and coherently strained GaInAs LEDs is limited to wavelengths in the IR ranging from 870 nm (for GaAs active regions) to about 950 nm (for GaInAs active regions).
The wavelength of free-space communication LEDs is in the infrared so that the light emitted is invisible to the human eye and does not distract. Since free-space communication usually involves transmission distances of less than 100 m, the transmission medium (air) can be considered, to a good approximation, to be lossless and dispersionless.
The total light power is an important figure of merit in free-space communication LEDs, so that the internal efficiency and the extraction efficiency need to be maximized. The emission pattern (far field) is another important parameter. The emission pattern should be wide to reduce the requirement of aiming the emitter towards the receiver.