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Introduction
This section discusses the important characteristics of fiber optic cable structures, the properties of some of the main types of cable construction and particular uses of each type.
1. Basic cable construction objectives
The cable structure is needed to protect the optical fiber from the rigors of the outside world during its installation and throughout its operational life. The cable construction needs to provide some, or all of the following features:
• Mechanical protection for the fibers
• Make them easier to handle
• Provide protection against environmental hazards
• A non-conductive cable for special environments
1.1 Mechanical protection
The cable structure needs to give protection to the fibers against tensile stress, abrasion, cutting, flexing, bending and crushing experienced during the installation and subsequent operational life of the fibers. Stresses imposed on the fibers can cause microbending losses, as discussed in Section 3, resulting in greater attenuation. Higher stress levels could lead to eventual fiber failure.
1.2 Easier handling
Optical fibers are much easier to handle when combined in cable structures. The plastic coated fibers are typically between a quarter- and one-millimeter in diameter (250-900 µm) making them difficult to pick up etc. Most cables contain many fibers, catering to future requirements. These fibers need to be handled together in a single, common structure. The cable structure also acts as the mounting point for the cable. For example, aerial cables are attached to support structures at the end of each span. In addition, the cable connectors are attached to the cable structure, keeping the strain off the individual fibers.
1.3 Environmental protection
The cable structure needs to protect fibers from the environmental hazards of moisture, chemicals, and other conditions.
Moisture
The cable structures are designed to prevent ingress of moisture into the fibers. Petroleum jelly is often utilized in various forms, to act as a water barrier. The long-term exposure to water vapor can result in hydroxyl-ion (OH- found in water) penetration of the fibers, causing loss of strength and increased attenuation.
In cold environments, any water vapor in the cables can freeze, expanding as it does so, causing stresses on fibers.
Hydrogen diffusion
It has been found that when fibers are kept for long periods in an atmosphere with a high hydrogen content, the hydrogen molecules can diffuse into the fiber and cause increased attenuation at longer wavelengths (1300 and especially 1500 nm). Fibers containing phosphorous dopants are most susceptible to this problem. While the hydrogen concentration in air is very low, it can be produced within some cables by the decomposition of plastic materials. Similarly, electrolysis of moisture may be produced within submarine cables by electric currents used to power-feed the regenerators. Another source of hydrogen is from lead acid batteries, which can damage cables in common, equipment rooms.
1.4 All dielectric (galvanic isolation)
All-dielectric cables do not contain any metallic components. Such non-conducting cables are inherently safe for use in hazardous situations where the presence of explosive gas makes metallic cables unsafe (because of their potential to produce sparks). These sparks are capable of causing explosions. All-dielectric cables are not affected by any form of electromagnetic fields and are therefore immune to lightning and high voltages (with respect to earth) that can be caused by power fault conditions. It is necessary to eliminate all metallic components, including aluminum laminate moisture barriers and steel wire armoring, in order to achieve all dielectric construction.
2. Fiber tensile ratings
Optical fibers are potentially stronger than steel, with unblemished fibers having theoretical tensile strength per unit area of over a giga newton per square meter.
However, in practice, fibers break at lower tensions, with cracks that begin from small surface defects. Although the force per unit area appears impressive, remember, we are dealing with a very small cross-sectional area. A standard fiber of 125 microns diameter has a cross-sectional area of 1.2 × 10^-8 square meters, so a one-kilogram force applied to the fiber produces 8 giga newtons per square meter. When longitudinal tensile or transverse forces are applied to an optical fiber, it can cause minute surface defects. These potential weak points can develop into microcracks, which may cause breakage of the fiber when it is later subjected to an equal or greater tension. Optical fibers have some elasticity, stretching under light loads before returning to their original length when the load is removed. Theoretically, the fiber may stretch as much as 9% before breaking under heavy loads. However, it is considered advisable to limit permanent strains to less than 0.2%. This limitation will avoid development of premature failures. Soft copper, on the other hand, is inelastic. It may permanently stretch up to 25% under light loads, and will remain stretched once the load is removed. When a conventional cable with copper conductors is subjected to extra heavy tension, the conductors are able to stretch; whereas extra heavy tension could break the fibers in a cable if stretched to more than 5%. This has an important bearing on how the fibers are installed within the cable. The fibers need to be isolated from the tensile and bending forces to prevent breakage and prolong their life. The following section describes how the cables are constructed to provide necessary isolation.
3. Cable structural elements
Fiber optic cables are constructed in different ways and some of the main types will be discussed in the following sections. The basic structural elements used in cables are a central member, strength members, a fiber housing, water barrier, and a cable sheath.
Some cables also require additional protective armor. A typical structure is depicted in FIG. 1.
FIG. 1 Basic cable structure
4. Central member
Many cables are built around a central member. As the name implies, a central member of the cable is a structure at the center of the cable. It is usually made of fiberglass, possibly incorporating steel. It provides rigidity to the cable, thus preventing the fibers from being bent too sharply, even to the extent of microbending. It contributes to the tensile strength of the cable and is the core about which the cable is built up and the fibers are supported.
However, small indoor cables often do not use central members.
5. Strength members
The fiber cable needs to incorporate a relatively inelastic strength member, running the length of the cable, to withstand any tensile forces and prevent excessive stretch of the fibers. This can be metallic or non-metallic depending on the application. The mechanical properties of the commonly used strength member materials are shown in Table 1.
Table 1 Strength member load table
5.1 Metallic strength members
Steel wire is the obvious choice where metallic strength members can be used. The advantages of steel wire are low cost, relatively small size, good rigidity. In addition, with its thermal mass providing good temperature stability, the steel wire can be depended on for very low temperature performance. The disadvantages are greater weight, loss of galvanic isolation, corrodibility, and the safety considerations associated with metallic components. Steel strength members are often incorporated in the center of a plastic extruded central member, as in the slotted core construction shown in FIG. 4.
5.2 Non-metallic strength members
The common non-metallic strength member materials are Aramid yarn (commonly referred to under the Dupont trade name of Kevlar) and fiberglass reinforced plastic rods (FRP). Other materials include plastic monofilaments such as processed polyester, other textile fibers (nylon, Terylene and Dacron), carbon and glass fibers. Aramid yarn has a high breaking strain, and when comparing the equivalent weights, it is five times stronger than steel. Advantages of non-metallic strength members are the low weight and all dielectric construction. The disadvantages are greater cost and higher elongation. FRP strength members have greater rigidity and contribute to the bending resistance of the cable. FRP is also better suited to extreme low temperature operation because its thermal mass provides better temperature stability.
6. Finer housing
In Section 3, it was shown that fibers are given a primary coating during production, to protect the pristine fiber surface from mechanical and chemical attack. The fiber is then given a secondary or buffer coating for further protection. This is either of a loose buffer construction, usually for outdoor use, or of a tight buffer for indoor use. These fiber housings are shown in FIG. 2.
FIG. 2 Fiber housing types
6.1 Loose buffer construction
Loose buffering, using either a loose tube or the slotted core construction, is used for most outdoor applications where fibers are likely to be subjected to external stresses such as tensile forces or extreme temperature change. Loose buffer construction is used in aerial cables, duct cables, and direct buried cables.
6.2 Loose tube construction
Loose tube construction makes use of a hard, smooth flexible tube, whose inner dimensions are much greater than the fiber diameter. One or more fibers can be installed in a loose helical fashion inside this tube so that they can move freely inside it. The tube isolates the fibers from any external stresses, such as temperature changes or bending forces, applied to the external structure. Even if the tube is stretched, the fibers remain unstressed because they have excess length and are free to move relative to the tube. The tube is usually filled with a moisture resistant compound as a water barrier for outside applications. FIG. 3 illustrates the use of loose tube construction.
FIG. 3 Loose tube cable construction
Loose tube construction can accommodate up to about 12 fibers per tube, using colored tubes and colored fiber coatings to facilitate identification.
6.3 Slotted core cable construction
The slotted core type of cable construction uses an extruded plastic structural member surrounding a central strength member, as shown in FIG. 4. The slots in the core accommodate the fibers, allowing radial movement and isolating them from external forces. This minimizes residual fiber strain and its resulting microbending losses.
FIG. 4 Slotted core cable construction
6.4 Tightly buffered fibers
Tightly buffered fibers are encased in a soft plastic, which reduces the forces applied to the fiber, and then have a further external buffer of harder plastic, such as nylon, Hytrel or Tefzel providing physical protection. The total buffer coatings increase the diameter to the extent of between 250 and 1000 microns and provide stiffening to the fiber, protecting against microbending. 900-micron fibers are the most common in use today.
Advantages for indoor cables
• Single fiber tight buffered cables, such as those used in patch cords, do not need to have their buffer removed to install connectors, and predictably align the fiber in the connector.
• Individual tightly buffered fibers within a cable can be distributed to different devices without the use of patch panels (this is not necessarily good practice though).
• Greater crush and impact resistance than loose buffered cables.
• Riser cables utilize tightly buffered fibers to control the longitudinal stress on the fibers.
• Tightly buffered cables are physically smaller than loose tube cables for low fiber counts.
Disadvantages
• The tight buffer tends to introduce some microbending losses, so fibers often have higher losses than in loose tube cables. Generally, this is not a problem, as indoor distances are usually short.
• There is lower isolation of the fiber from the stresses caused by temperature variations.
6.5 Ribbon cable construction
A variation of the tight buffered construction is the use of ribbon cable. A group of coated fibers is arranged in parallel and then encapsulated in plastic to form a multi-fiber ribbon cable. Individual ribbons may contain 5 to 12 fibers, and multiple ribbons can be stacked together to form the core of the cable.
Advantages of ribbon cables
• Very high density of fibers
• Effective splicing techniques for connecting multiple fiber ribbons in a single splice
• Multi-fiber connectors for joining ribbons
Disadvantages
Installation forces are generally not uniformly distributed over the ribbon width, so individual fibers can have uneven strains, resulting in excessive losses and fiber damage.
FIG. 5 Ribbon cable construction
7. Moisture barrier
We have already seen that fibers are susceptible to long-term degradation caused by hydroxyl ion ( OH- ) contamination. An appropriate moisture barrier is required to prevent the penetration of water into cables. This is obviously of paramount importance for submarine cables, which may be subjected to the static pressure of several kilometers of seawater! Common moisture barriers for ordinary cables include; an axially laid aluminum foil/polyethylene laminated film immediately inside the polyurethane or polyethylene plastic sheaths (which are permeable to moisture); and/or the use of moisture resistant compounds around the fibers. Moisture resistant compounds include petroleum jelly and silicone rubber. These filling compounds need to be soft, self-healing, easily removed, provide corrosion protection for metallic cable components and do not degrade over time or harm the other components, particularly the fiber itself. These materials have another useful property in cushioning the fibers from vibration.
8. Cable sheaths
The cable is usually covered with a heavy plastic sheath, which provides primary protection against abrasion, cut resistance, crushing resistance and additional resistance to excessive bending. Light duty cables can utilize polyvinyl chloride (PVC), or polyurethane sheath materials. The more durable polyethylene (PE) materials are used on outdoor and heavier duty cables. It should be reiterated that plastic sheath materials provide limited protection against penetration of water into cables, and require moisture barriers as discussed in section 7. Thin layers of hard plastic materials such as nylon can be utilized as over-sheaths to provide greater abrasion and cut resistance. Sometimes, Teflon, because of its very slippery surface, provides protection against rodents such as termites. Special cable sheath materials are sometimes used for cables installed in air-handling spaces or 'plenum' areas. These are defined as the spaces above suspended ceilings, below elevated floors and in heating and ventilation ducts. In some countries, cables that are used in plenum areas must have flame retardant properties producing little smoke. For further information on plenum cable requirements in your country, check the relevant fire codes applicable in your area. Sheaths normally incorporate a ripcord to facilitate the sheath removal without facing the risk of damage to the fibers when cutting into the sheath material.
9. Cable armoring
Direct buried cables require armoring on them if they are buried in areas where physical damage to them is likely. Such cables include shallow shore ends of undersea cables where anchoring and fishing activities pose constant dangers. Direct buried cables need protection against crushing damage caused by rocks, rodent attacks and disturbance caused by road works or farming operations. Steel wire armoring is generally utilized, with the steel wound over the inner plastic sheath then surrounded by a further plastic sheath to prevent corrosion.
10. Class of fiber optic cables
There are four broad application areas into which fiber optic cables can be classified: aerial cable, underground cable, subaqueous cable and indoor cable. The special properties required for each of these applications will now be considered. Note that the list is not all encompassing, and that some specialized cables need to combine the features of several of these categories.
10.1 Aerial cables
Aerial cables are literally exposed to the elements more than any other application, and as such, are exposed to many external forces and hazards. Aerial cables are installed between poles with the weight of the cable continuously supported by usually a steel messenger wire to which the cable can be directly lashed, or by the strength members integral to the cable construction. The effects of combined wind and ice loadings can produce greatly increased tensile forces. Other considerations are the wide variations in temperature to which the cable may be subjected, affecting the physical properties of the fibers and the attenuation of the fibers. The longitudinal cable profile is important for reducing the wind and ice loadings of such cables. Moisture barriers are essential, with jelly filled, loose buffered fiber cable configurations predominating. Any water freezing within the fiber housings would expand and could produce excessive bending of fibers. The cable sheath material is required to withstand the extremes of temperature and the intense ultraviolet light exposure from continuous exposure to sunlight. UV stabilized polyethylene is frequently used for this purpose. The installed span length and sag requirements are important design parameters affecting the maximum cable tension. They also dictate the type of cable construction to be used. Short span cables have less stringent tension requirements, which can be met by the use of integral Kevlar layers, whereas long span cables may need to utilize multiple stranded FRP rods to meet the required maximum tensions. Installation aspects for aerial cables are discussed in section 7.5.1.
Cable types
Self-supporting cables
These cables have a separate messenger wire, usually steel, which can be clamped to the support structures on the poles, to carry the tensile forces on the cable. The construction of the cable is illustrated in FIG. 6. The separate messenger wire facilitates installation.
FIG. 6 Self supporting cable (messengered)
Short span dielectric
These cables use non-metallic strength elements. They have peripheral Kevlar layers and a slotted core that use FRP strength members. Such cables allow spans up to 150 m with maximum allowable tensions of around 300 kg. The construction of this type of cable is shown in FIG. 7.
FIG. 7 Short span dielectric cable
Long span dielectric
These cables also use non-metallic strength elements. They consist of a peripheral layer of stranded FRP rods and a slotted core with FRP strength members. Such cables allow spans up to 1000 m with maximum permissible tensions of around 1200 kg. The construction of this type of cable is shown in FIG. 8.
FIG. 8 Long span dielectric cable
Advantages of aerial cable
• Useful in areas where it may be very difficult or too expensive to bury the cable or install it in ducts
• Also useful where temporary installations are required
Disadvantages
• System availability is not as high as for underground cables. Storms can disrupt these communication bearers, with cables damaged by falling trees, storm damage and blown debris. Roadside poles can be hit by vehicles and frustrated shooters seem unable to miss aerial cables!
10.2 Underground cables
Underground cables experience less environmental extremes than aerial cables.
Underground cables are usually pulled into ducts or buried directly in the ground, with the cable being placed in a deep narrow trench, which is backfilled with dirt or else ploughed directly into the ground.
Cable type
Loose buffering, using loose tube or slotted core construction, is generally used to isolate fibers from external forces including temperature variations.
Installation aspects of these cables are discussed in detail in Section 7.
Metallic moisture barrier
This incorporates a longitudinally applied polymer coated aluminum laminate, which is bonded to the inside of the polyethylene sheath. The tape is closely formed around the cable core and the overlap welded by melting the polymer with hot air. This hermetically seals the core against water entry through the sheath.
FIG. 9 Metallic moisture barrier cable
Advantages
• Usually, the most cost-effective method of installing cables outdoors.
• Greater environmental protection compared to aerial cable.
• Usually, more secure than aerial cable.
Disadvantages
• Can be disrupted by earthworks, farming, flooding etc.
• Rodents biting cables can be a problem in some areas. This is overcome with the use of steel tape armor or steel braid, or the use of a plastic duct of more than 38 mm OD for all dielectric cable installations. In addition, the use of Teflon coatings on the sheath makes the cable too slippery for a rodent to grip it between its teeth.
10.3 Subaqueous cables
Subaqueous cables are outdoor cables designed for continuous immersion in water. While the most sophisticated cables are used for deep ocean communications by international telecommunications carriers, there are practical applications for subaqueous cables for smaller users. These include cabling along or across rivers, lakes, water races, or channels where alternatives are not cost effective. Subaqueous cable is a preferred option for direct buried cabling in areas subjected to flooding or with a high water table, where for example, if the cable were buried at say 1 meter depth, it would be permanently immersed in water.
The installation aspects of subaqueous cables are discussed in Section 7.
Cable types
The basic elements of a subaqueous cable are shown in FIG. 10. These are essentially outdoor cable constructions incorporating a hermetically sealed unit, using a welded metallic layer encasing the fiber core.
FIG. 10 Subaqueous cable
Advantages
• Cheaper installation in some circumstances
Disadvantages
• Unit cost of cable is higher
10.4 Indoor cables
Indoor cables are used inside buildings and have properties dictated by fire codes. Such cables need to minimize the spread of fires, and must comply with your relevant local fire codes, such as those outlined in the National Electrical Codes (NEC) in USA. Outdoor cables generally contain oil-based moisture blocking compounds like petroleum jelly.
These support combustion and so, their use inside buildings is strictly controlled in some countries. Outdoor cables are sometimes spliced to appropriate indoor cables close to the building entry points. This would avoid the expense of encasing long runs of outdoor cable inside metallic conduit as is required in some countries. The fibers in indoor cables and the indoor cable itself are (usually) tightly buffered as was discussed in section 5.2. The tight buffer provides adequate water resistance for indoor applications but such cables should not be used for long outdoor cable runs. The buffered fibers can be given sufficient strength to enable them to be directly connected to equipment from the fiber structure without slicing to patch cords.
Installation of indoor cables is discussed in detail in Section 7.
Cable types
Patch cords
Designed for repeated handling at patch panels, these use internal Aramid fiber layers to provide tensile strength while allowing considerable flexibility. Outdoor cables and loose tube cables that contain fibers, which are not tightly buffered, need to be spliced to tightly buffered internal cables or patch cords for termination to equipment.
FIG. 11 Patch cord
Distribution breakout cables
These have tightly buffered fibers in subunits, which have sufficient protection to enable them to be broken out of the cable structure and run direct to equipment.
FIG. 12 Distribution or breakout cables (tight buffered subunits)
Riser cables
These have multiple tightly buffered fibers in a structure designed to withstand long vertical runs without support.
FIG. 13 Riser cables
Plenum cables
Plenum cables are required by the fire safety codes used in some countries, in areas where the cables are installed directly in the air-return path of an air-conditioning system. This can often include cable installations above suspended ceilings. In some cases, these cables may need to be installed within conduit or have sheath materials complying with the UL 910 standard, which, unless under very intense heat, do not support the spread of fires. Unless the cables are actually burning they will not emit poisonous gases and will have low smoke emission. They do emit very poisonous gases if they eventually burn.
Such materials include MEGOLON, which is a filled polyolefin, a copolymer of ethylene vinyl acetate filled with hydrated aluminum oxide. This has no significant halogen content and therefore, cannot emit any halogenated acids when burned.
FIG. 14 Duplex plenum cable
Zip cord cables
These are small diameter cords having internal Kevlar reinforcement, making them easy to handle with good tension, compression and bending properties. These are used with computer data links, terminal links and remote instrumentation connections. They are also used for patch cords since connectors can be attached firmly to the cord structure.
FIG. 15 Zip cord cable
Advantages of indoor cables
• Ability to connect tight buffered fibers direct to equipment without patch panels
• Compliance with fire codes
Disadvantages of indoor cables
• Unsuitable for long external runs
• Higher attenuation than loose buffer fibers
• Unsuitable for extreme temperature variations