Light Emitting Diodes in Optical Communication

LED or Light Emitting Diode in fiber optic communication is an opto-electronic device that revolutionized the optical transmission in the early stages of fiber optic telecommunications. When an electric current is passed through, an LED emits incoherent light by using the spontaneous emission characteristics. The first generation fiber optic transmission systems used LEDs that emitted light at 850 nanometers wavelength. This allowed the use of multimode fibers that were developed at the time.

ITU-T has categorized the multimode fiber having 50 micrometers core diameter as the first generation optical fibers used for telecommunication networks. Those multimode fibers are generally used in LAN networks these days. Currently listed under ITU-T G.651.1, multimode fibers with 50 micrometer core diameter not preferred for long haul telecommunication networks. They were replaced with low loss singlemode fibers. Typically LEDs for the 850 nm region are fabricated using GaAs and AlGaAs. LEDs for the 1300 nm and 1550 nm regions are fabricated using InGaAsP and InP.

The basic LED types used for fiber optic communication systems are the Surface Emitting LED known as SLED, the Edge Emitting LED known as ELED, and the Superluminescent Diode known as SLD. All these LEDs differ in their performance. These performance differences help network link designers decide which device is appropriate for the intended application. SLEDs and ELEDs are preferredĀ  for distances ranging up to 3 kilometers. Such short length transmission systems generally employ low data rate transmission. Usually, SLEDs operate efficiently for bit rates up to 250 megabits per second. SLEDs emit light over a wide area. Sine multimode fibers have large core diameters, SLEDs are suitable for injecting light into multimode fibers. Also, multimode fibers are suitable for short distance low data rate transmission. Hence SLEDs are almost exclusively used in multimode systems.

ELEDs are preferred for medium distance and medium data rate transmission systems. ELEDs may be modulated at rates up to 400 Mb/s. ELEDs can be employed in transmission systems that use single mode and or multimode fibers. SLDs are ELED-based diodes designed to operate in the superluminescence mode. SLDs and ELEDs are used in long-distance, high data rate systems. SLDs can be modulated at bit rates of over 400 Mb/s.


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