LEDs for fiber-optic communication
Light-emitting diodes are the light source of choice for local area low and medium bit rate optical communication. Owing to the spontaneous emission lifetime of about 1 ns in highly excited semiconductors, the maximum bit rates attainable with LEDs are limited to rates < 1 Gbit/s. Thus, multi-Gbit/s transmission rates are not feasible with LED sources. Transmission rates of several hundred Mbit/s are fully sufficient for many local-area communication applications.
LEDs used for fiber communication applications are very different from LEDs used in lamp applications. In communication LEDs, high coupling efficiency of the light emanating from the LED to the fiber is essential. Only the light emanating from one surface, namely the surface of the LED abutting the optical fiber, can be coupled into the fiber. Therefore it is essential to maximize the light emission from one surface of the LED. In LEDs used for fiber communication, the power emitted per unit area is a useful figure of merit. This is in contrast to free-space communication LEDs where the total power emitted by the LED is the appropriate figure of merit.
In order to maximize the LED-fiber coupling efficiency, the light-emitting spot should be smaller than the core diameter of the optical fiber. Typical are circular emission regions with diameters of 20-50 ^m for devices used with multimode fibers. Silica multimode fibers have typical core diameters of 50-100 |j, m.
Plastic optical fibers, on the other hand, can have core diameters as large as 1 mm. Accordingly, LEDs with larger light-emitting areas can be used with plastic fibers.