3.4 ITU-T Standards for Fibers
135
cutoff wavelength of 1500 nm, this fiber is restricted to operation in the 1500–1600 nm
region. It typically is used only in long-distance undersea applications.
3.4.5 Recommendation G.655
Nonzero-dispersion-shifted fiber (NZDSF) was introduced in the mid-1990s for
WDM applications. The recommendation has five different versions ranging from
G.655.A to G.655.E. Each of these categories has a slightly different value of the
dispersion coefficient at 1550 nm. For example, the original G.655.A had a positive
dispersion coefficient for wavelengths greater than 1460 nm. This enabled operation
of WDM systems in the C-band, which is the spectral operating region for erbiumdoped optical fiber amplifiers (see Chap. 11). However, the negative value for lower
wavelengths did not allow the use of this fiber in the S-band. Thus, Version G.655.B
was introduced to extend WDM applications into the S-band. As shown in Fig. 3.18,
the principal characteristic of a G.655.B fiber is that it has a nonzero dispersion value
over the entire S-band and the C-band. Version G.655.C specifies a lower PMD value
of 0.2 ps/
√
km than the 0.5 ps/
√
km value of G.655.A/B. Recommendations G.655.D
and G.655.E have slightly different values of the dispersion coefficient and dispersion slope in order to optimize various tradeoffs among the different G.655 versions
in power, channel spacing, amplifier separation, link length and bit rate.
3.4.6 Recommendation G.656
This recommendation describes the characteristics of a single-mode optical fiber that
has a positive chromatic dispersion value ranging from 2 to 14 ps/(nm km) in the
1460–1625 nm wavelength band. Whereas the dispersion slope is lower than in G.655
fibers, most G.656 attributes are similar to those of G.655 fibers. For example, the
mode-field diameter ranges from 7 to 11 μm (compared to 8–11 μm for G.655 fibers),
the maximum PMD value of cabled fiber is 0.2 ps/
√
km, and the cutoff wavelength
is 1310 nm (the same as for G.655). Thus, because of the close similarity of these
two fiber types, a number of optical fiber manufacturers do not bother to distinguish
between G.655 and G.656 and simply refer to their offerings as G.655/656 compliant.
3.4.7 Recommendation G.657
The rapidly growing worldwide demand for broadband services in high-capacity
access networks and enterprise networks put new demands on the performance characteristics of single-mode fibers that are different from telecom applications. These
135
cutoff wavelength of 1500 nm, this fiber is restricted to operation in the 1500–1600 nm
region. It typically is used only in long-distance undersea applications.
3.4.5 Recommendation G.655
Nonzero-dispersion-shifted fiber (NZDSF) was introduced in the mid-1990s for
WDM applications. The recommendation has five different versions ranging from
G.655.A to G.655.E. Each of these categories has a slightly different value of the
dispersion coefficient at 1550 nm. For example, the original G.655.A had a positive
dispersion coefficient for wavelengths greater than 1460 nm. This enabled operation
of WDM systems in the C-band, which is the spectral operating region for erbiumdoped optical fiber amplifiers (see Chap. 11). However, the negative value for lower
wavelengths did not allow the use of this fiber in the S-band. Thus, Version G.655.B
was introduced to extend WDM applications into the S-band. As shown in Fig. 3.18,
the principal characteristic of a G.655.B fiber is that it has a nonzero dispersion value
over the entire S-band and the C-band. Version G.655.C specifies a lower PMD value
of 0.2 ps/
√
km than the 0.5 ps/
√
km value of G.655.A/B. Recommendations G.655.D
and G.655.E have slightly different values of the dispersion coefficient and dispersion slope in order to optimize various tradeoffs among the different G.655 versions
in power, channel spacing, amplifier separation, link length and bit rate.
3.4.6 Recommendation G.656
This recommendation describes the characteristics of a single-mode optical fiber that
has a positive chromatic dispersion value ranging from 2 to 14 ps/(nm km) in the
1460–1625 nm wavelength band. Whereas the dispersion slope is lower than in G.655
fibers, most G.656 attributes are similar to those of G.655 fibers. For example, the
mode-field diameter ranges from 7 to 11 μm (compared to 8–11 μm for G.655 fibers),
the maximum PMD value of cabled fiber is 0.2 ps/
√
km, and the cutoff wavelength
is 1310 nm (the same as for G.655). Thus, because of the close similarity of these
two fiber types, a number of optical fiber manufacturers do not bother to distinguish
between G.655 and G.656 and simply refer to their offerings as G.655/656 compliant.
3.4.7 Recommendation G.657
The rapidly growing worldwide demand for broadband services in high-capacity
access networks and enterprise networks put new demands on the performance characteristics of single-mode fibers that are different from telecom applications. These