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LIGHT-EMITTING DIODES
A light-emitting diode (LED) is a semiconductor device that emits incoherent
light, through spontaneous emission, when a current is passed through it. 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 (SLED), the edge-emitting LED (ELED), and the superluminescent diode (SLD). LED
performance differences help link designers decide which device is appropriate for the
intended application. For short-distance (0 to 3 km), low-data-rate fiber optic systems,
SLEDs and ELEDs are the preferred optical source. Typically, SLEDs operate efficiently for
bit rates up to 250 megabits per second (Mb/s). Because SLEDs emit light over a wide area
(wide far-field angle), they are almost exclusively used in multimode systems.
For medium-distance, medium-data-rate systems, ELEDs are preferred.
ELEDs may be modulated at rates up to 400 Mb/s. ELEDs may be used for both single mode
and multimode fiber systems. Both SLDs and ELEDs are used in long-distance, high-data-rate
systems. SLDs are ELED-based diodes designed to operate in the superluminescence mode. A
further discussion on superluminescence is provided later in this chapter. SLDs may be
modulated at bit rates of over 400 Mb/s.
Surface-Emitting LEDs
The surface-emitting LED (shown in figure 6-1) is also known as the Burrus LED in honor
of C. A. Burrus, its developer. In SLEDs, the size of the primary active region is limited
to a small circular area of 20 μm to 50 μm in diameter. The active region is
the portion of the LED where photons are emitted. The primary active region is below the
surface of the semiconductor substrate perpendicular to the axis of the fiber.
A well is etched into the substrate to allow 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 addition, the epoxy resin that binds the optical fiber to the SLED reduces the
refractive index mismatch, increasing coupling efficiency.
Figure 6-1. - Example of the SLED structure.
Edge-Emitting LEDs
The demand for optical sources for longer distance, higher bandwidth systems operating
at longer wavelengths led to the development of edge-emitting
LEDs. Figure 6-2 shows a typical ELED structure. It shows the different layers of
semiconductor material used in the ELED. The primary active region of the ELED is a narrow
stripe, which 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.
Figure 6-2. - Example of the ELED structure.
In an ELED the rear facet is highly reflective and the front facet is
antireflection-coated. The rear facet reflects the light propagating toward the rear
end-face back toward the front facet. By coating 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 allowing
for better source-to-fiber coupling. They couple more power into small NA fibers than
SLEDs. ELEDs can couple enough power into single mode fibers for some applications. ELEDs
emit power over a narrower spectral range than SLEDs. However, ELEDs typically are more
sensitive to temperature fluctuations than SLEDs.
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