23 - Communication LEDs

Summary

LEDs can be used for either free-space communication or for fiber communication applications. Free-space communication applications include the remote control of appliances such as television sets and stereos, and data communication between a computer and peripheral devices. LEDs used in optical fiber communication applications are suited for distances of a few km and bit rates up to about 1 Gbit/s. Most fibers used with LEDs are multimode (step-index and graded-index) fibers. However, some applications employ single-mode fibers.

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.