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3 Optical Signal Attenuation and Dispersion
This section first discusses the general factors that cause signal dispersion and then
examines the various dispersion mechanisms in more detail. Section 3.2.2 addresses
modal delay and shows how this delay is related to the information capacity of a
multimode fiber in terms of a transmitted bit rate B. Section 3.2.3 examines the
various factors contributing to dispersion in terms of the frequency dependence of
the propagation constant β. The next topics include a discussion of group velocity in
Sect. 3.2.4 and details of the various dispersion mechanisms in Sects. 3.2.5 through
3.2.8.
3.2.1 Origins of Signal Dispersion
Signal dispersion is a consequence of factors such as intermodal delay (also called
intermodal dispersion), intramodal dispersion, polarization-mode dispersion, and
higher-order dispersion effects. These effects can be explained by examining the
behavior of the group velocities of the guided modes, where the group velocity is the
speed at which energy in a particular mode travels along the fiber (see Sect. 3.2.4).
Intermodal delay (or simply modal delay) appears only in multimode fibers. Modal
delay is a result of each mode having a different value of the group velocity at a single
frequency. From this effect one can derive an intuitive picture of the informationcarrying capacity of a multimode fiber.
Intramodal dispersion or chromatic dispersion is pulse spreading that takes place
within a single mode. This spreading arises from the finite spectral emission width
of an optical source. The phenomenon also is known as group velocity dispersion,
because the dispersion is a result of the group velocity being a function of the wavelength. Because intramodal dispersion depends on the wavelength, its effect on signal
distortion increases with the enlarging of spectral width of the light source. The spectral width is the band of wavelengths over which the source emits light. This wavelength band normally is characterized by the root-mean-square (rms) spectral width
σ λ . Depending on the device structure of a light-emitting diode (LED), the spectral width is approximately 4–9% of a central wavelength. For example, as Fig. 3.7
illustrates, if the peak wavelength of an LED is 850 nm, a typical source spectral
width would be 36 nm; that is, such an LED emits most of its light in the 832–868 nm
wavelength band. Laser diode optical sources exhibit much narrower spectral widths,
with typical values being 1–2 nm for multimode lasers and 10
−4 nm for single-mode
lasers (see Chap. 4).
The two main causes of intramodal dispersion are as follows:
1. Material dispersion arises due to the variations of the refractive index of the
core material as a function of wavelength. Material dispersion also is referred
to as chromatic dispersion, because this is the same effect by which a prism
spreads out a spectrum. This refractive index property causes a wavelength
dependence of the group velocity of a given mode, because as indicated by
3 Optical Signal Attenuation and Dispersion
This section first discusses the general factors that cause signal dispersion and then
examines the various dispersion mechanisms in more detail. Section 3.2.2 addresses
modal delay and shows how this delay is related to the information capacity of a
multimode fiber in terms of a transmitted bit rate B. Section 3.2.3 examines the
various factors contributing to dispersion in terms of the frequency dependence of
the propagation constant β. The next topics include a discussion of group velocity in
Sect. 3.2.4 and details of the various dispersion mechanisms in Sects. 3.2.5 through
3.2.8.
3.2.1 Origins of Signal Dispersion
Signal dispersion is a consequence of factors such as intermodal delay (also called
intermodal dispersion), intramodal dispersion, polarization-mode dispersion, and
higher-order dispersion effects. These effects can be explained by examining the
behavior of the group velocities of the guided modes, where the group velocity is the
speed at which energy in a particular mode travels along the fiber (see Sect. 3.2.4).
Intermodal delay (or simply modal delay) appears only in multimode fibers. Modal
delay is a result of each mode having a different value of the group velocity at a single
frequency. From this effect one can derive an intuitive picture of the informationcarrying capacity of a multimode fiber.
Intramodal dispersion or chromatic dispersion is pulse spreading that takes place
within a single mode. This spreading arises from the finite spectral emission width
of an optical source. The phenomenon also is known as group velocity dispersion,
because the dispersion is a result of the group velocity being a function of the wavelength. Because intramodal dispersion depends on the wavelength, its effect on signal
distortion increases with the enlarging of spectral width of the light source. The spectral width is the band of wavelengths over which the source emits light. This wavelength band normally is characterized by the root-mean-square (rms) spectral width
σ λ . Depending on the device structure of a light-emitting diode (LED), the spectral width is approximately 4–9% of a central wavelength. For example, as Fig. 3.7
illustrates, if the peak wavelength of an LED is 850 nm, a typical source spectral
width would be 36 nm; that is, such an LED emits most of its light in the 832–868 nm
wavelength band. Laser diode optical sources exhibit much narrower spectral widths,
with typical values being 1–2 nm for multimode lasers and 10
−4 nm for single-mode
lasers (see Chap. 4).
The two main causes of intramodal dispersion are as follows:
1. Material dispersion arises due to the variations of the refractive index of the
core material as a function of wavelength. Material dispersion also is referred
to as chromatic dispersion, because this is the same effect by which a prism
spreads out a spectrum. This refractive index property causes a wavelength
dependence of the group velocity of a given mode, because as indicated by