Page 68 - Optical Communications Essentials

Optical Fibers

58 Chapter Four

Example Consider a single-mode fiber for which D CD 2ps/(km nm) and D PMD
0.1ps/ km . If a transmission link has a length L 500km and uses a laser source
with a spectral emission width of ∆λ 0.01nm (we will see more details about what
this means in Chap. 6), then we have t mod 0, t CD D CD L ∆λ 10ps, and t PMD
D PMD L 2.24ps. Thus

2 2
t (10 ps)
(2.24 ps) 10.2ps
T
If t T can be no more than 10 percent of a pulse width, then the maximum data rate
R max that can be sent over this 500-km link is R max 0.1/t T 9.8Gbps (gigabits per
second).

4.7. Optical Fiber Standards
The International Telecommunications Union (ITU-T) and the Telecommuni-
cations Industry Association (TIA/EIA) are the main organizations that have
published standards for both multimode and single-mode optical fibers used in
telecommunications. The recommended bounds on fiber parameters (e.g.,
attenuation, cutoff wavelength, and chromatic dispersion) designated in these
standards ensure the users of product capability and consistency. In addition,
the standards allow fiber manufacturers to have a reasonable degree of flexibil-
ity to improve products and develop new ones.
Multimode fibers are used widely in LAN environments, storage area net-
works, and central-office connections, where the distance between buildings is
typically 2km or less. The two principal multimode fiber types for these appli-
cations have either 50- or 62.5-µm core diameters, and both have 125-µm
cladding diameters. To meet the demands for short-reach low-cost transmission
of high-speed Ethernet signals, a 50-µm multimode fiber is available for
10-Gbps operation at 850nm over distances up to 300m. Table 4.3 shows the
operating ranges of various multimode fibers for applications up to 10GigE. The
standards document TIA/EIA-568 lists the specifications for 10GigE fiber. The
ITU-T recommendation G.651 describes other multimode fiber specifications
for LAN applications using 850-nm optical sources.
The ITU-T has published a series of recommendations for single-mode fibers.
The characteristics of these fibers are given in the following listing. They are
summarized in Table 4.4.

TABLE 4.3. Operating Ranges of Various Multimode Fibers for
Applications up to 10GigE
Parameter 62.5-µm fiber 50-µm fiber Unit
Bandwidth 160 200 400 500 2000 MHz-km
Range 26 33 66 82 300 m
Attenuation 2.5 2.5 2.5 2.5 2.5 dB/km

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