Light Emitting Diodes are the optical devices that made the fiber optic communication possible. Developments in LED researches are in parallel to the historical developments in fiber optic communication. 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
Earlier LEDs used for telecommunication systems could transmit light for a few kilometers. Fiber optics in telecommunication networks are more beneficial when they can transmit longer distances by avoiding the repeaters and regenerators. longer transmission span means less number of electronics to be deployed that directly contributes to cost saving. If optical sources can support longer distance transmission, telecom operators can save in project cost. The demand for optical sources for longer distance, higher bandwidth systems operating at longer wavelengths led to the development of edge-emitting LEDs.
Different layers of semiconductor material are used to make an edge emitting LED. The primary active region of the ELED is a narrow stripe that lies below the surface of the semiconductor substrate. The semiconductor substrate is cut or polished so that the stripe runs between the front and back of the device. The polished or cut surfaces at each end of the stripe are called facets.
In an ELED the rear facet is highly reflective and the front facet is coated with antireflection material. The rear facet reflects the light propagating toward the rear end-face back toward the front facet. Since there is a coating on the front facet with antireflection material, the front facet reduces optical feedback and allows light emission. ELEDs emit light only through the front facet. ELEDs emit light in a narrow emission angle. This feature allows better coupling efficiency.
ELEDs couple more power into optical fibers that have small Numerical Aperture. Singlemode fibers are lower numerical aperture values. Hence coupling of light into a singlemode fiber is relatively difficult. This task can be better achieved by using an ELED. An ELED can couple optical power to a single mode fiber that is enough to transmit loner distances compared to that of multimode fiber applications. ELEDs emit power over a narrower spectral range than SLEDs. However, ELEDs typically are more sensitive to temperature fluctuations than SLEDs.