148
4 Light Sources for Fiber Links
injected into the p and n regions, respectively. These injected minority carriers can
recombine either radiatively, in which case a photon of energy hν is emitted, or nonradiatively, whereupon the recombination energy is dissipated in the form of heat. This
pn junction is thus known as the active region or recombination region.
A major difference between LEDs and laser diodes is that the optical output from
an LED is incoherent, whereas that from a laser diode is coherent. In a coherent
source, the optical energy is produced in an optical resonant cavity. The optical
energy released from this cavity has spatial and temporal coherence, which means it
is highly monochromatic and the output beam is very directional. In an incoherent
LED source, no optical cavity exists for wavelength selectivity. The output radiation
has a broad spectral width, because the emitted photon energies range over the energy
distribution of the recombining electrons and holes, which usually lie between 1
and 2k B T (k B is Boltzmann’s constant and T is the absolute temperature at the pn
junction). In addition, the incoherent optical energy is emitted into a broad elliptical
region according to a cosine power distribution and thus has a large beam divergence.
In choosing an optical source compatible with the optical waveguide, various
characteristics of the fiber, such as its geometry, its attenuation as a function of
wavelength, its group delay distortion, and its modal characteristics, must be taken
into account. The interplay of these factors with the optical source power, spectral
width, radiation pattern, and modulation capability needs to be considered. The
spatially directed coherent optical output from a laser diode can be coupled into
either single-mode or multimode fibers. In general, LEDs are used with multimode
fibers, because normally it is only into a multimode fiber that the incoherent optical
power from an LED can be coupled in sufficient quantities to be useful. However,
LEDs have been employed in high-speed local-area applications in which one wants
to transmit several wavelengths on the same fiber. Here, a technique called spectral
slicing is used. This entails using a passive device such as a waveguide grating array
(see Chap. 10) to split the broad spectral emission of the LED into narrow spectral
slices. Because these slices are each centered at a different wavelength, they can be
individually modulated externally with independent data streams and simultaneously
sent on the same fiber.
4.1 Basic Concepts of Semiconductor Physics
Because the material in this chapter assumes a rudimentary knowledge of semiconductor physics, various relevant definitions are given here for semiconductor material
properties, including the concepts of energy bands, intrinsic and extrinsic materials,
pn junctions, and direct and indirect bandgaps. Further details can be found in Refs.
[4–6].
4 Light Sources for Fiber Links
injected into the p and n regions, respectively. These injected minority carriers can
recombine either radiatively, in which case a photon of energy hν is emitted, or nonradiatively, whereupon the recombination energy is dissipated in the form of heat. This
pn junction is thus known as the active region or recombination region.
A major difference between LEDs and laser diodes is that the optical output from
an LED is incoherent, whereas that from a laser diode is coherent. In a coherent
source, the optical energy is produced in an optical resonant cavity. The optical
energy released from this cavity has spatial and temporal coherence, which means it
is highly monochromatic and the output beam is very directional. In an incoherent
LED source, no optical cavity exists for wavelength selectivity. The output radiation
has a broad spectral width, because the emitted photon energies range over the energy
distribution of the recombining electrons and holes, which usually lie between 1
and 2k B T (k B is Boltzmann’s constant and T is the absolute temperature at the pn
junction). In addition, the incoherent optical energy is emitted into a broad elliptical
region according to a cosine power distribution and thus has a large beam divergence.
In choosing an optical source compatible with the optical waveguide, various
characteristics of the fiber, such as its geometry, its attenuation as a function of
wavelength, its group delay distortion, and its modal characteristics, must be taken
into account. The interplay of these factors with the optical source power, spectral
width, radiation pattern, and modulation capability needs to be considered. The
spatially directed coherent optical output from a laser diode can be coupled into
either single-mode or multimode fibers. In general, LEDs are used with multimode
fibers, because normally it is only into a multimode fiber that the incoherent optical
power from an LED can be coupled in sufficient quantities to be useful. However,
LEDs have been employed in high-speed local-area applications in which one wants
to transmit several wavelengths on the same fiber. Here, a technique called spectral
slicing is used. This entails using a passive device such as a waveguide grating array
(see Chap. 10) to split the broad spectral emission of the LED into narrow spectral
slices. Because these slices are each centered at a different wavelength, they can be
individually modulated externally with independent data streams and simultaneously
sent on the same fiber.
4.1 Basic Concepts of Semiconductor Physics
Because the material in this chapter assumes a rudimentary knowledge of semiconductor physics, various relevant definitions are given here for semiconductor material
properties, including the concepts of energy bands, intrinsic and extrinsic materials,
pn junctions, and direct and indirect bandgaps. Further details can be found in Refs.
[4–6].