HP 86037B, Instruction Manual

Page: 178 / 204

7-8
Reference
Chromatic Dispersion–theory and management
Chromatic Dispersion–theory and management
Chromatic dispersion results from a variation in propagation delay with wave-
length that is shaped, in turn, by the interplay of fiber materials and dimen-
sions. The result is that different frequencies of light travel down optical fiber
at different speeds. The optical source in a high-speed communication system
is typically a single-line diode laser with a nonzero spectral width. Pulse mod-
ulation increases the spectral width.
Signal dispersion is a factor in fiber-optic systems. As more information is sent
on a wider frequency bandwidth, and for greater distances, the importance of
signal dispersion will become greater.
Each wavelength component of the signal travels at a slightly different speed,
resulting in the pulse broadening illustrated in
Figure 7-6 .
Figure 7-6. Dispersion Effect on a Transmitted Pulse
Chromatic dispersion is the primary dispersive mechanism in singlemode
fiber. In singlemode fiber, chromatic dispersion results from the interplay of
the following two underlying effects:
• Material dispersion results from the dependence upon wavelength of the re-
fractive index (and the corresponding group velocity) of doped silica.
• Waveguide dispersion is rooted in the wavelength-dependent relationships of
the group velocity to the core diameter and the difference in index between the
core and the cladding.
A third component, called second-order PMD (polarization-mode dispersion)
or differential group delay dispersion, arises because the velocity of propaga-
tion depends on the polarization state of the lightwave signal. The PMD of the
In p u t
P u ls e
O u tp u t
P u ls e