Fiber Types
This section discusses various MMF and SMF types currently used for premise,
metro, aerial, submarine, and long-haul applications. The International
Telecommunication Union (ITU-T), which is a global standardization body for
telecommunication systems and vendors, has standardized various fiber types.
These include the 50/125-
m graded index fiber (G.651),
Nondispersion-shifted fiber (G.652), dispersion-shifted fiber (G.653), 1550-nm
loss-minimized fiber (G.654), and NZDSF (G.655).
Multimode Fiber with a 50-Micron Core (ITU-T G.651)
The ITU-T G.651 is an MMF with a 50-m nominal core diameter and
a 125-m nominal cladding diameter with a graded refractive index.
The attenuation parameter for G.651 fiber is typically 0.8 dB/km at 1310 nm. The
main application for ITU-T G.651 fiber is for short-reach optical transmission
systems. This fiber is optimized for use in the 1300-nm band. It can also
operate in the 850-nm band.
Nondispersion-Shifted Fiber (ITU-T G.652)
The ITU-T G.652 fiber is also known as standard SMF and is the most commonly deployed fiber. This fiber has a simple step-index structure and is optimized for operation in the 1310-nm band. It has a zero-dispersion wavelength at 1310 nm and can also operate in the 1550-nm band, but it is not optimized for this region. The typical chromatic dispersion at 1550 nm is high at 17 ps/nm-km. Dispersion compensation must be employed for high-bit-rate applications. The attenuation parameter for G.652 fiber is typically 0.2 dB/km at 1550 nm, and the PMD parameter is less than 0.1 ps/ km. An example of this type of fiber is Corning SMF-28.
Low Water Peak Nondispersion-Shifted Fiber (ITU-T G.652.C)
The legacy ITU-T G.652 standard SMFs are not optimized for WDM applications due to the high attenuation around the water peak region. ITU G.652.C-compliant fibers offer extremely low attenuation around the OH peaks. The G.652.C fiber is optimized for networks where transmission occurs across a broad range of wavelengths from 1285 nm to 1625 nm. Although G.652.C-compliant fibers offer excellent capabilities for shorter, unamplified metro and access networks, they do not fully address the needs for 1550-nm transmission. The attenuation parameter for G.652 fiber is typically 0.2 dB/km at 1550 nm, and the PMD parameter is less than 0.1 ps/ km. An example of this type of fiber is Corning SMF-28e.
Dispersion-Shifter Fiber (ITU-T G.653)
Conventional SMF has a zero-dispersion wavelength that falls near the 1310-nm window band. SMF shows high dispersion values over the range between 1500 nm and 1600 nm (third window band). The trend of shifting the operating transmission wavelength from 1310 nm to 1550 nm initiated the development of a fiber type called dispersion-shifted fiber (DSF). DSF exhibits a zero-dispersion value around the 1550-nm wavelength where the attenuation is minimum. The DSFs are optimized for operating in the region between 1500 to 1600 nm. With the introduction of WDM systems, however, channels allocated near 1550 nm in DSF are seriously affected by noise induced as a result of nonlinear effects caused by FWM. This initiated the development of NZDSF. Figure 3-14 illustrates the dispersion slope of DSF with respect to SMF and NZDSF. G.53 fiber is rarely deployed any more and has been superseded by G.655.
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Figure
3-14 Fiber Dispersion Slopes
1550-nm Loss-Minimized Fiber (ITU-T G.654)
The ITU-T G.654 fiber is optimized for operation in the 1500-nm to 1600-nm region. This fiber has a low loss in the 1550-nm band. Low loss is achieved by using a pure silica core. ITU-T G.654 fibers can handle higher power levels and have a larger core area. These fibers have a high chromatic dispersion at 1550 nm. The ITU G.654 fiber has been designed for extended long-haul undersea applications.
Nonzero Dispersion Shifted Fiber (ITU-T G.655)
Using nonzero dispersion-shifted fiber (NZDSF) can mitigate nonlinear characteristics. NZDSF fiber overcomes these effects by moving the zero-dispersion wavelength outside the 1550-nm operating window. The practical effect of this is to have a small but finite amount of chromatic dispersion at 1550 nm, which minimizes nonlinear effects, such as FWM, SPM, and XPM, which are seen in the dense wavelength-division multiplexed (DWDM) systems without the need for costly dispersion compensation. There are two fiber families called nonzero dispersion (NZD+ and NZD–), in which the zero-dispersion value falls before and after the 1550-nm wavelength, respectively. The typical chromatic dispersion for G.655 fiber at 1550 nm is 4.5 ps/nm-km. The attenuation parameter for G.655 fiber is typically 0.2 dB/km at 1550 nm, and the PMD parameter is less than 0.1 ps/ km. The Corning LEAF fiber is an example of an enhanced G.655 fiber with a 32 percent larger effective area. Figure 3-14 illustrates the dispersion slope of NZDSF with respect to SMF and DSF.