Page 74 - Optical Communications Essentials

Optical Fibers

64 Chapter Four

of the attenuation depends on the wavelength of the light and on the fiber
material. The loss of power is measured in decibels, and the loss within a cable
is described in terms of decibels per kilometer.
The information-carrying capacity of the fiber is limited by various distortion
mechanisms in the fiber, such as signal dispersion factors and nonlinear effects.
The three main dispersion categories are modal, chromatic, and polarization
mode dispersions. These distortion mechanisms cause optical signal pulses to
broaden as they travel along a fiber.
The ITU-T and the TIA/EIA 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 capabil-
ity and consistency. Multimode fibers are used in LAN environments, storage
area networks, and central-office connections, where the distance between
buildings is typically 2km or less. The two principal multimode fiber types have
either 50- or 62.5-µm core diameters, and both have 125-µm cladding diameters.
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.
The ITU-T also has published a series of recommendations for single-mode
fibers. Of these, two key ones for DWDM use are the ITU-T G.655 (nonzero
dispersion-shifted fiber, or NZDSF) and the ITU-T G.655b (advanced nonzero
dispersion-shifted fiber, or A-NZDSF). The G.655 fibers are designed for the
C-band, and the G.655b fibers allow DWDM operation over the entire S-band
and the C-band. In addition, the G.652.C recommendation describes fibers for
CWDM applications.
Whereas telecommunication fibers, such as those described above, are
designed to transmit light over long distances with minimal change in the sig-
nal, specialty fibers are used to manipulate the light signal. Specialty fibers
interact with light and are custom-designed for specific applications such as
optical signal amplification, wavelength selection, wavelength conversion, and
sensing of physical parameters.

Further Reading
1. B. Comycz, Fiber Optic Installer’s Field Manual, McGraw-Hill, New York, 2000.
2. G. Keiser, Optical Fiber Communications, 3d ed., McGraw-Hill, Burr Ridge, Ill., 2000.
3. T. R. Jordal, “How to test laser-based premises networks,” Lightwave, vol. 18, pp. 126, 128, 156,
February 2001 (www.lightwaveonline.com).
4. R. E. Kristiansen, “Holey fibers,” SPIE OE Magazine, vol. 2, pp. 25–27, June 2002 (www.
oemagazine.com).
5. TIA/EIA-568-B.3, Optical Fiber Cabling Components Standard, April 2000.
6. ITU-T Recommendation G.651, Characteristics of a 50/125-µm Multimode Graded-Index
Optical Fiber Cable, February 1998.
7. ITU-T Recommendation G.652, Characteristics of a Single-Mode Optical Fiber Cable, October
2000.
8. ITU-T Recommendation G.653, Characteristics of a Dispersion-Shifted Optical Fiber Cable,
October 2000.

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