136
3 Optical Signal Attenuation and Dispersion
performance differences are due mainly to the high density of localized distribution and drop cables in the access and enterprise networks compared to metro and
long-haul telecom networks. In particular, in building applications the cables can be
installed inside moldings that run along the walls and around corners and doors, and
in equipment racks where the cables often are tightly coiled or bent. These conditions and the many installation manipulations imposed on the cabling system call for
optical fibers with low bending loss sensitivity.
Thus the aim of ITU-T Recommendation G.657 is to describe requirements for a
fiber type that exhibits improved bending loss performance compared with existing
G.652 single-mode fiber and cables. Two categories of single-mode fibers are specified. Fibers in Category A are fully compliant with the G.652 single-mode fibers and
also can be used in other parts of the network. Single-mode fibers in Category B are
not necessarily compliant with G.652 but exhibit low values of losses at very small
bend radii. Fibers in Category B are predominantly intended for in-building use.
3.5 Designs and Use of Specialty Fibers
Telecommunication fibers, such as those described in Sect. 3.4, are designed to
transmit light with minimal change in the signal fidelity. In contrast, specialty fibers
are designed to interact with light and thereby manipulate or control some characteristics of an optical signal. The light manipulation applications include optical
signal amplification, optical power coupling, dispersion compensation, wavelength
conversion, and sensing of physical parameters such as temperature, stress, pressure, vibration, and fluid levels. For light-control applications a specialty fiber can
be insensitive to bends, maintain polarization states, redirect specific wavelengths,
or provide a very high attenuation for fiber terminations.
Specialty fibers can be of either a multimode or a single-mode design. Among
the optical devices that may use a specialty fiber are light transmitters, light signal
modulators, optical receivers, wavelength multiplexers, light couplers and splitters,
optical amplifiers, optical switches, wavelength add/drop modules, and optical power
attenuators. Table 3.3 gives a summary of some specialty fibers and their applications.
Rare-Earth Doped Fiber These fibers have small amounts of rare-earth ions (for
example, 1000 parts per million weight) added to the silica material to form a basic
building block for optical fiber amplifiers. The rare-earth elements could be erbium
(Er), ytterbium (Yb), thulium (Tm), or praseodymium (Pr). As described in Chap. 11,
a length of such fiber ranging from 10 to 30 m serves as a gain medium for amplifying
optical signals in the 1.0 μm region, the C-band (1530–1560 nm), or the L-band
(1560–1625 nm). There are many variations on the doping level, cutoff wavelength,
mode-field diameter, numerical aperture, and cladding diameter for these fibers.
Erbium is the main material used for optical fiber amplifiers operating in the C-band.
Specific erbium-doped fiber configurations will yield a variety of optical amplifier
designs that can be selected according to pump laser power requirement, noise figure,
3 Optical Signal Attenuation and Dispersion
performance differences are due mainly to the high density of localized distribution and drop cables in the access and enterprise networks compared to metro and
long-haul telecom networks. In particular, in building applications the cables can be
installed inside moldings that run along the walls and around corners and doors, and
in equipment racks where the cables often are tightly coiled or bent. These conditions and the many installation manipulations imposed on the cabling system call for
optical fibers with low bending loss sensitivity.
Thus the aim of ITU-T Recommendation G.657 is to describe requirements for a
fiber type that exhibits improved bending loss performance compared with existing
G.652 single-mode fiber and cables. Two categories of single-mode fibers are specified. Fibers in Category A are fully compliant with the G.652 single-mode fibers and
also can be used in other parts of the network. Single-mode fibers in Category B are
not necessarily compliant with G.652 but exhibit low values of losses at very small
bend radii. Fibers in Category B are predominantly intended for in-building use.
3.5 Designs and Use of Specialty Fibers
Telecommunication fibers, such as those described in Sect. 3.4, are designed to
transmit light with minimal change in the signal fidelity. In contrast, specialty fibers
are designed to interact with light and thereby manipulate or control some characteristics of an optical signal. The light manipulation applications include optical
signal amplification, optical power coupling, dispersion compensation, wavelength
conversion, and sensing of physical parameters such as temperature, stress, pressure, vibration, and fluid levels. For light-control applications a specialty fiber can
be insensitive to bends, maintain polarization states, redirect specific wavelengths,
or provide a very high attenuation for fiber terminations.
Specialty fibers can be of either a multimode or a single-mode design. Among
the optical devices that may use a specialty fiber are light transmitters, light signal
modulators, optical receivers, wavelength multiplexers, light couplers and splitters,
optical amplifiers, optical switches, wavelength add/drop modules, and optical power
attenuators. Table 3.3 gives a summary of some specialty fibers and their applications.
Rare-Earth Doped Fiber These fibers have small amounts of rare-earth ions (for
example, 1000 parts per million weight) added to the silica material to form a basic
building block for optical fiber amplifiers. The rare-earth elements could be erbium
(Er), ytterbium (Yb), thulium (Tm), or praseodymium (Pr). As described in Chap. 11,
a length of such fiber ranging from 10 to 30 m serves as a gain medium for amplifying
optical signals in the 1.0 μm region, the C-band (1530–1560 nm), or the L-band
(1560–1625 nm). There are many variations on the doping level, cutoff wavelength,
mode-field diameter, numerical aperture, and cladding diameter for these fibers.
Erbium is the main material used for optical fiber amplifiers operating in the C-band.
Specific erbium-doped fiber configurations will yield a variety of optical amplifier
designs that can be selected according to pump laser power requirement, noise figure,