Page 70 - Optical Communications Essentials

Optical Fibers

60 Chapter Four

a nonzero dispersion value over the entire C-band, which is the spectral
operating region for erbium-doped optical fiber amplifiers (see Chap. 11).
This is in contrast to G.653 fibers in which the dispersion varies from
negative values through zero to positive values in this spectral range.
ITU-T G.655b. Advanced nonzero dispersion-shifted fiber (A-NZDSF) was
introduced in October 2000 to extend WDM applications into the S-band. Its
principal characteristic is that it has a nonzero dispersion value over the
entire S-band and the C-band. This is in contrast to G.655 fibers in which the
dispersion varies from negative values through zero to positive values in
the S-band.

4.8. Specialty Fibers
Whereas telecommunication fibers, such as those described above, are designed
to transmit light over long distances with minimal change in the signal, spe-
cialty fibers are used to manipulate the light signal. Specialty fibers interact
with light and are custom-designed for specific applications such as optical sig-
nal amplification, wavelength selection, wavelength conversion, and sensing of
physical parameters.
A number of both passive and active optical devices use specialty fibers to
direct, modify, or strengthen an optical signal as it travels through the device.
Among these optical devices are light transmitters, optical signal modulators,
optical receivers, wavelength multiplexers, couplers, splitters, optical ampli-
fiers, optical switches, wavelength add/drop modules, and light attenuators.
Table 4.5 gives a summary of some specialty fibers and their applications. Later
chapters describe the applications of each of these devices in greater detail.

4.8.1. Erbium-doped fiber
Erbium-doped optical fibers have small amounts of erbium ions added to the
silica material and are used as a basic building block for optical fiber amplifiers.
As described in Chap. 11, a length of Er-doped fiber ranging from 10 to 30m
is used as a gain medium for amplifying optical signals in the C-band (1530
to 1560nm). There are many variations on the doping level, cutoff wavelength,

TABLE 4.5. Summary of Some Specialty Fibers and Their Applications

Specialty fiber type Application
Erbium-doped fiber Gain medium for optical fiber amplifiers
Photosensitive fibers Fabrication of fiber Bragg gratings
Bend-insensitive fibers Tightly looped connections in device packages
High-loss attenuating fiber Termination of open optical fiber ends
Polarization-preserving fibers Pump lasers, polarization-sensitive devices, sensors
High-index fibers Fused couplers, short-λ sources, DWDM devices
Holey (photonic crystal) fibers Switches; dispersion compensation

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