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Surface Emitting LEDs in Fiber Optic Communication

Light Emitting Diodes are the optical sources that made the fiber optic communication possible. Developments in LEDs 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

The surface-emitting LED is also known as the Burrus LED. The name Burrus came after its developer C. A. Burrus. Surface emitting LEDs are often called “Burrus” LEDs because they were first described by Burrus and Miller in their research paper published in 1971. In Surface emitting LEDs, the size of the primary active region is limited to a small circular area having a diameter of 20 micrometers to 50 micrometers. Photons are generated in the active region from where they emit and can be guided thorugh the medium. The primary active region is below the surface of the semiconductor substrate perpendicular to the fiber’s axis.

A well is engraved into the substrate to enable direct coupling of the emitted light to the optical fiber. The etched well allows the optical fiber to come into close contact with the emitting surface.

In Surface emitting LEDs, epoxy resin is used to bind the optical fiber to the SLED structure. The alignment of optical fiber during the process of binding with epoxy resin determines some of the characteristics of the LED such as coupling efficiency. Coupling efficiency is directly related to the amount light that can be targeted to the fiber from the LED.

Use of epoxy resin reduces the refractive index mismatch in Surface emitting LEDs. The lower refractive index mismatch values helps longer transmission lengths and reduces the reflection related transmission issues In surface emitting LEDs, a combination of insulating materials and junctions is used to focus the current-flow to the active region as much as possible and to guide the light produced in the LED to the optical fiber.

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