Page 103 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 93

Figure 6.6. Schematic (not to scale) of an edge-emitting LED. The output beam is lambertian in the plane of
the pn junction (θ 120°) and highly directional perpendicular to the pn junction (θ 30°).

into a unit solid angle per unit of emitting surface area. A high radiance is needed to
couple sufficiently high optical power levels into a fiber core. The quantum efficiency
describes the fraction of injected electron-hole pairs that emit photons when they
recombine at a pn junction (not every recombination results in light being emitted).
Conventionally, the electron-hole pairs are called charge carriers or simply carriers.
To achieve a high radiance and a high quantum efficiency, the LED structure must
provide a means of confining the carriers to the active region of the pn junction where
radiative recombination takes place. In addition, the structure needs to confine the
light so that the emitted photons are guided to the fiber end and are not absorbed by
the material surrounding the pn junction.
An effective structure for achieving carrier and optical confinements is the sandwich
configuration shown in Fig. 6.7 for an LED operating around 850nm. This is referred
to as a double-heterostructure (or heterojunction) device because of the two different
alloy layers on each side of the active region. The bandgap differences of adjacent lay-
ers confine the charge carriers, while the differences in the indices of refraction of the
adjoining layers confine the emitted photons to the central active layer. Light sources
at longer wavelengths consist of similar structures but different materials.

6.3. Laser Diodes
Semiconductor-based laser diodes are the most widely used optical sources in
fiber communication systems. The four main laser types are the Fabry-Perot
(FP) laser, the distributed-feedback (DFB) laser, tunable lasers, and the vertical
cavity surface-emitting laser (VCSEL). Key properties of these lasers include
high optical output powers (greater than 1mW), narrow linewidths (a fraction
of a nanometer, except for the FP laser), and highly directional output beams
for efficient coupling of light into fiber cores.

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