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2 Optical Fiber Structures and Light Guiding Principles
of cable manufacturers have been that the optical fiber cables should be installable
with the same equipment, installation techniques, and precautions as those used
for conventional wire cables. This requires special cable designs because of the
mechanical properties of glass fibers.
2.9.1 Fiber Optic Cable Structures
One important mechanical property is the maximum allowable axial load on the cable
because this factor determines the length of cable that can be reliably installed. In
copper cables the wires themselves are generally the principal load-bearing members
of the cable, and elongations of more than 20% are possible without fracture. On the
other hand, extremely strong optical fibers tend to break at 4% elongation, whereas
typical good-quality fibers exhibit long-length breaking elongations of about 0.5–
1.0%. Because fiber ruptures occur very quickly at stress levels above 40% of the
permissible elongation and very slowly below 20%, fiber elongations during cable
manufacture and installation should be limited to 0.1–0.2%.
Steel wire has been used extensively for reinforcing electric cables and also can be
used as a strength member for optical fiber cables. For some applications it is desirable
to have a completely nonmetallic construction, either to avoid the effects of electromagnetic induction or to reduce cable weight. In such cases, plastic strength members
and high- tensile-strength synthetic yarns are used. A popular yarn is Kevlar
® , which
is a soft but tough yellow synthetic nylon material belonging to a generic yarn family
known as aramids. Good fabrication practices will isolate the fibers from other cable
components, keep them close to the neutral axis of the cable, and allow the fibers to
move freely when the cable is flexed or stretched.
The generic cable configuration shown in Fig. 2.28 illustrates some common materials that are used in the optical fiber cabling process. Individual fibers or modules
of bundled fiber groupings are wound loosely around the central buffered strength
member. A cable wrapping tape and other strength members such as Kevlar then
encapsulate and bind these fiber groupings together. Surrounding all these components is a tough polyethylene (PE) jacket that provides crush resistance and handles
any tensile stresses applied to the cable so that the fibers inside are not damaged.
The jacket also protects the fibers inside against abrasion, moisture, oil, solvents, and
other contaminants. The jacket type largely defines the application characteristics;
for example, heavy-duty outside-plant cables for direct-burial and aerial applications have much thicker and tougher jackets than indoor cables that have lower stress
environments. Some cable designs might contain optional copper wires for powering
in-line equipment. Other cable components can include steel armoring tapes, waterblocking or water-absorbing materials, optional copper wires for powering in-line
equipment, and rip cords that allow the jacket to be cut back easily without damaging
the components inside the cable.
To distinguish individual fiber strands within a grouping of fibers, each fiber is
designated by a separate and distinct jacket color. The TIA-598-D Optical Fiber
2 Optical Fiber Structures and Light Guiding Principles
of cable manufacturers have been that the optical fiber cables should be installable
with the same equipment, installation techniques, and precautions as those used
for conventional wire cables. This requires special cable designs because of the
mechanical properties of glass fibers.
2.9.1 Fiber Optic Cable Structures
One important mechanical property is the maximum allowable axial load on the cable
because this factor determines the length of cable that can be reliably installed. In
copper cables the wires themselves are generally the principal load-bearing members
of the cable, and elongations of more than 20% are possible without fracture. On the
other hand, extremely strong optical fibers tend to break at 4% elongation, whereas
typical good-quality fibers exhibit long-length breaking elongations of about 0.5–
1.0%. Because fiber ruptures occur very quickly at stress levels above 40% of the
permissible elongation and very slowly below 20%, fiber elongations during cable
manufacture and installation should be limited to 0.1–0.2%.
Steel wire has been used extensively for reinforcing electric cables and also can be
used as a strength member for optical fiber cables. For some applications it is desirable
to have a completely nonmetallic construction, either to avoid the effects of electromagnetic induction or to reduce cable weight. In such cases, plastic strength members
and high- tensile-strength synthetic yarns are used. A popular yarn is Kevlar
® , which
is a soft but tough yellow synthetic nylon material belonging to a generic yarn family
known as aramids. Good fabrication practices will isolate the fibers from other cable
components, keep them close to the neutral axis of the cable, and allow the fibers to
move freely when the cable is flexed or stretched.
The generic cable configuration shown in Fig. 2.28 illustrates some common materials that are used in the optical fiber cabling process. Individual fibers or modules
of bundled fiber groupings are wound loosely around the central buffered strength
member. A cable wrapping tape and other strength members such as Kevlar then
encapsulate and bind these fiber groupings together. Surrounding all these components is a tough polyethylene (PE) jacket that provides crush resistance and handles
any tensile stresses applied to the cable so that the fibers inside are not damaged.
The jacket also protects the fibers inside against abrasion, moisture, oil, solvents, and
other contaminants. The jacket type largely defines the application characteristics;
for example, heavy-duty outside-plant cables for direct-burial and aerial applications have much thicker and tougher jackets than indoor cables that have lower stress
environments. Some cable designs might contain optional copper wires for powering
in-line equipment. Other cable components can include steel armoring tapes, waterblocking or water-absorbing materials, optional copper wires for powering in-line
equipment, and rip cords that allow the jacket to be cut back easily without damaging
the components inside the cable.
To distinguish individual fiber strands within a grouping of fibers, each fiber is
designated by a separate and distinct jacket color. The TIA-598-D Optical Fiber