Page 283 - Optical Communications Essentials

Optical Link Design

Optical Link Design 273

Amplitude

90%

10%
Time
10-to-90%
rise time
Figure 16.6. Illustration of the 10 to 90
percent rise time of a pulse.

In practice, an optical fiber link seldom consists of a uniform, continuous,
jointless fiber. Instead, a transmission link nominally is formed from several
concatenated (tandemly joined) fibers which may have different dispersion
characteristics. This is especially true for dispersion-compensated links operat-
ing at 10Gbps and higher. In addition, multimode fibers experience modal dis-
tributions at fiber-to-fiber joints owing to mechanical misalignments, different
core index profiles in each fiber, and/or different degrees of mode mixing in indi-
vidual fibers. Determining the fiber rise times resulting from chromatic and
modal dispersion then becomes more complex than for the case of a single uni-
form fiber.
The fiber rise time t CD resulting from chromatic dispersion over a length L
can be approximated by

t CD |D CD |L∆λ (16.8)

where ∆λ is the half-power spectral width of the light source and D CD is the fiber
chromatic dispersion. Since the chromatic dispersion value may change from
one section of fiber to another in a long link, an average value should be used
for D CD in Eq. (16.8).
For a multimode fiber the bandwidth, or information-carrying capacity, is
specified as a bandwidth-distance relationship with units of megahertz times
kilometers. Thus the bandwidth needed to support an application depends on
the data rate of transmission; that is, as the data rate goes up (MHz), the dis-
tance (km) over which signals can be transmitted at that rate goes down.
Multimode fibers with a 50-µm core diameter have about 3 times more band-
width (500MHz km) than 62.5-µm fibers (160MHz km) at 850nm. If B mod is
the modal dispersion bandwidth (in MHz km), then the modal rise time t mod
(in nanoseconds) over a fiber of length L km is given by

440L
t mod (16.9)
B mod

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