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4 Light Sources for Fiber Links
electric contacts. A technique of crystal growth by chemical reaction is then used
to grow thin layers of semiconductor materials on the substrate. These materials
must have lattice structures that are identical to those of the substrate crystal. In
particular, the lattice spacings of adjacent materials should be closely matched to
avoid temperature-induced stresses and strains at the material interfaces. This type
of growth is called epitaxial, which is derived from the Greek words epi meaning
on and taxis meaning arrangement; that is, it is an arrangement of atoms from one
material on another material. An important characteristic of epitaxial growth is that
it is relatively simple to change the impurity concentration of successive material
layers, so that a layered semiconductor device can be fabricated in a continuous
process. Epitaxial layers can be formed by vapor phase, liquid phase, or molecular
beam growth techniques [4–6].
4.2 Principles of Light-Emitting Diodes (LEDs)
For optical communication systems requiring bit rates less than approximately
100−200 Mb/s together with multimode fiber-coupled optical power in the tens
of microwatts, semiconductor light-emitting diodes (LEDs) are usually the appropriate light source choice. These LEDs require less complex drive circuitry than laser
diodes as no thermal or optical stabilization circuits are needed (see Sect. 4.3.6), and
they can be fabricated less expensively with higher yields.
4.2.1 LED Structures
To be useful in fiber transmission applications, an LED must have a high radiance
output, a fast emission response time, and high quantum efficiency. Its radiance is
a measure, in watts, of the optical power radiated into a unit solid angle per unit
area of the emitting surface. High radiances are necessary to couple sufficiently high
optical power levels into a fiber, as shown in detail in Chap. 5. The emission response
time is the time delay between the application of a current pulse and the onset of
optical emission. As is discussed in Sects. 4.2.4 and 4.3.7, this time delay is the factor
limiting the bandwidth (i.e., the data rate) with which the source can be modulated
directly by varying the injected current. The quantum efficiency is related to the
fraction of injected electron−hole pairs that recombine radiatively. This is defined
and described in detail in Sect. 4.2.3.
To achieve a high radiance and high quantum efficiency, the LED structure must
provide a means of confining the charge carriers and the stimulated optical emission
to the active region of the pn junction where radiative recombination takes place.
Carrier confinement is used to achieve a high level of radiative recombination in
the active region of the device, which yields a high quantum efficiency. Optical
4 Light Sources for Fiber Links
electric contacts. A technique of crystal growth by chemical reaction is then used
to grow thin layers of semiconductor materials on the substrate. These materials
must have lattice structures that are identical to those of the substrate crystal. In
particular, the lattice spacings of adjacent materials should be closely matched to
avoid temperature-induced stresses and strains at the material interfaces. This type
of growth is called epitaxial, which is derived from the Greek words epi meaning
on and taxis meaning arrangement; that is, it is an arrangement of atoms from one
material on another material. An important characteristic of epitaxial growth is that
it is relatively simple to change the impurity concentration of successive material
layers, so that a layered semiconductor device can be fabricated in a continuous
process. Epitaxial layers can be formed by vapor phase, liquid phase, or molecular
beam growth techniques [4–6].
4.2 Principles of Light-Emitting Diodes (LEDs)
For optical communication systems requiring bit rates less than approximately
100−200 Mb/s together with multimode fiber-coupled optical power in the tens
of microwatts, semiconductor light-emitting diodes (LEDs) are usually the appropriate light source choice. These LEDs require less complex drive circuitry than laser
diodes as no thermal or optical stabilization circuits are needed (see Sect. 4.3.6), and
they can be fabricated less expensively with higher yields.
4.2.1 LED Structures
To be useful in fiber transmission applications, an LED must have a high radiance
output, a fast emission response time, and high quantum efficiency. Its radiance is
a measure, in watts, of the optical power radiated into a unit solid angle per unit
area of the emitting surface. High radiances are necessary to couple sufficiently high
optical power levels into a fiber, as shown in detail in Chap. 5. The emission response
time is the time delay between the application of a current pulse and the onset of
optical emission. As is discussed in Sects. 4.2.4 and 4.3.7, this time delay is the factor
limiting the bandwidth (i.e., the data rate) with which the source can be modulated
directly by varying the injected current. The quantum efficiency is related to the
fraction of injected electron−hole pairs that recombine radiatively. This is defined
and described in detail in Sect. 4.2.3.
To achieve a high radiance and high quantum efficiency, the LED structure must
provide a means of confining the charge carriers and the stimulated optical emission
to the active region of the pn junction where radiative recombination takes place.
Carrier confinement is used to achieve a high level of radiative recombination in
the active region of the device, which yields a high quantum efficiency. Optical