Page 102 - Optical Communications Essentials

Light Sources and Transmitters

92 Chapter Six

Figure 6.4. Schematic (not to scale) of a high-radiance surface-emitting LED. The active region is a circular sec-
tion that has an area compatible with the fiber-core end face.

Figure 6.5. Schematic of an optical source coupled to a fiber. Light
outside of the acceptance angle is lost.

6.2.3. Edge emitters
The edge emitter shown in Fig. 6.6 consists of an active pn junction region
where the photons are generated and two light-guiding layers that function in
the same manner as an optical fiber. This structure forms a waveguide channel
that directs the optical radiation toward the edge of the device, where it can be
coupled into an optical fiber. The emission pattern of the edge emitter is more
directional than that of a surface emitter, which allows a greater percentage of
the emitted light to be coupled into a fiber.

Photon and Carrier Confinement To be useful in fiber optic transmission applica-
tions, an LED must have a high radiance output and a high quantum efficiency. Its
radiance (or brightness) is a measure, in watts (W), of how much optical power radiates

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