Multimode Plastic | Fiber-Optic Cables

Plastic optical fibers (POF) consist of a plastic core of anywhere from 50 microns on up, surrounded by a plastic cladding of a different index of refraction. Generally speaking, these are the lowest-quality optical fibers and are seldom sufficient to transmit light over long distances. Plastic optical cables are used for very short-distance data transmissions or for transmission of visible light in decorations or other specialty lighting purposes not related to data transmission. Recently, POF has been promoted as a horizontal cable in LAN applications for residential systems. However, the difficulty in manufacturing a graded-index POF, combined with a low bandwidth-for-dollar value, has kept POF from being accepted as a horizontal medium in commercial applications.

Buffer

The buffer, the second-most distinguishing characteristic of the cable, is the component that provides additional protection for the optical fibers inside the cable. The buffer does just what its name implies: it buffers, or cushions, the optical fiber from the stresses and forces of the outside world. Optical fiber buffers are categorized as either tight or loose tube.

With tight buffers, a protective layer (usually a 900 micron PVC or Nylon covering) is applied directly over the coating of each optical fiber in the cable. Tight buffers make the entire cable more durable, easier to handle, and easier to terminate. Figure 1 shows tight buffering in a single-fiber (simplex) construction. Tight-buffered cables are most often used indoors because expansion and contraction caused by outdoor temperature swings can exert great force on a cable. Tight-buffered designs tend to transmit the force to the fiber strand, which can damage the strand or inhibit its transmission ability, so thermal expansion and contraction from temperature extremes is to be avoided. However, there are some specially designed tight-buffered designs for either exclusive outdoor use or a combination of indoor/outdoor installation.

Figure 1: A simplex fiber-optic cable using tight buffering

A loose-tube buffer, on the other hand, is essentially a tough plastic pipe about 0.125″ in diameter. One or several coated fibers can be placed inside the tube, depending on the cable design. The tube can then be filled with a protective substance, usually a water-blocking gel, to provide cushioning, strength, and protection from the elements if the cabling is used outdoors. More commonly, water-blocking powders and tapes are used to waterproof the cable. A loose-tube design is very effective at absorbing forces exerted on the cable so that the fiber strands are isolated from the damaging stress. For this reason, loose-tube designs are almost always seen in outdoor installations.

Multiple tubes can be placed in a cable to accommodate a large fiber count for high-density communication areas such as large cities. They can also be used as trunk lines for long-distance telecommunications.

Figure 2 shows a loose-buffered fiber-optic cable. Notice that the cable shown uses water-blocking materials.

Figure 2: A fiber-optic cable using loose buffering with water-blocking materials

Strength Members

Fiber-optic cables require additional support to prevent breakage of the delicate optical fibers within the cable while pulling them into place. That's where the strength memberscome in. The strength member of a fiber-optic cable is the part that provides additional tensile (pull) strength. Strength elements can also provide compression resistance. Compression is encountered when the temperature drops below room temperature.

The most common strength member in tight-buffered cables is aramid yarn, the same material found in bulletproof vests. Thousands of strands of this material are placed in a layer, called a serving, around all the tight-buffered fibers in the cable. When pulling on the cable, tensile force is transferred to the aramid yarn and not to the fibers.

Tip

Aramid yarn is extremely durable, so cables that use it require a special cutting tool, called aramid scissors. Aramid yarn cannot be cut with ordinary cutting tools.

Loose-tube fiber-optic cables sometimes have a strand of either fiberglass or steel wire as a strength member. These strands can be placed around the perimeter of a bundle of optical fibers within a single cable, or the strength member can be located in the center of the cable with the individual optical fibers clustered around it. As with aramid yarn in tight-buffered cable, tensile force is borne by the strength member(s), not the buffer tubes or fiber strands. Unlike aramid yarns, glass or steel strength members also have the ability to prevent compression-induced microbending caused by temperatures as low as −40°C.

Shield Materials

In fiber-optic cables designed for outdoor use, or for indoor environments with the potential for mechanical damage, metallic shields are often applied over the inner components but under the jacket. The shield is often referred to as armor. A common armoring material is 0.006″ steel with a special coating that adheres to the cable jacket. This shield should not be confused with shielding to protect against EMI. However, when present, the shield must be properly grounded at both ends of the cable in order to avoid an electrical-shock hazard should it inadvertently come into contact with a voltage source such as a lightning strike or a power cable.

Cable Jacket

The cable jacket of a fiber-optic cable is the outer coating of the cable that protects all the inner components from the environment. It is usually made of a durable plastic material and comes in various colors. As with copper cables, fiber-optic cables designed for indoor applications must meet fire-resistance requirements of the NEC.

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Exterior Protection of Fiber-Optic Cables

If you ever need to install fiber-optic cabling outdoors, the cable should be rated for an exterior installation. An exterior rating means that the cable is specifically designed for outdoor use. It will have features such as UV protection, superior crush and abrasion protection, protection against the extremes of temperature, and an extremely durable strength member. If you use standard indoor cable in an outdoor installation, the cables could get damaged and stop functioning properly.

Multimode Graded-Index Glass | Fiber-Optic Cables

A graded-index glass-fiber core is made of core of silica glass (SiO₂) and a small amount of Germania glass (GeO₂) in order to increase the index of refraction relative to the all-silica cladding. Graded-index multimode fibers have an index of refraction that changes gradually from the center outward to the cladding. The center of the core has the highest index of refraction. The most commonly used multimode graded-index glass fibers have a core that is either 50 microns or 62.5 microns in diameter. Figure 1 shows a graded-index glass core. Notice that the core is bigger than the single-mode core. The ANSI/TIA-568-C standard recommends the use of an 850nm laser-optimized 50/125 micron multimode fiber. This is commonly referred to OM3, as per the ISO 11801 cabling standard. This class of 50 micron fiber has higher bandwidth than 62.5 micron fiber and other versions of 50 micron fiber. It is recommended for the laser-driven 1–100Gbps applications.

 
Figure 1: A graded-index glass-fiber core

Single-Mode Step-Index Glass | Fiber-Optic Cables

A single-mode glass fiber core is very narrow (usually around 8.3 microns) and made of a core of silica glass (SiO₂) with a small amount of Germania glass (GeO₂) to increase the index of refraction relative to the all-silica cladding. To keep the cable size manageable, the cladding for a single-mode glass core is usually about 15 times the size of the core (around 125 microns). Single-mode fibers systems are expensive, but because of the lack of attenuation (less than 0.35dB per kilometer), very high speeds are possible over very long distances. Figure 1 shows a single-mode glass-fiber core. The latest class of single-mode fibers have very low loss at the 1385nm (water peak) region and are insensitive to bending. These are known as ITU-T G.652D and G.657, respectively.

 
Figure 1: An example of a single-mode glass-fiber core

Composition of a Fiber-Optic Cable

A typical fiber-optic cable consists of several components:

• Optical-fiber strand
• Buffer
• Strength members
• Optional shield materials for mechanical protection
• Outer jacket

Each of these components has a specific function within the cable to help ensure that the data gets transmitted reliably.

Optical Fiber

An optical-fiber strand (also called an optical waveguide) is the basic element of a fiber-optic cable. All fiber strands have at least three components to their cross sections: the core, the cladding, and the coating. Figure 1 depicts the three layers of the strand.

 
Figure 1: Elemental layers in a fiber-optic strand

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Optical Fiber Is Tiny—and Hazardous

Fiber strands have elements so small that it is hard to imagine the scale. You're just not used to dealing with such tiny elements in everyday life. The components of a fiber strand are measured in microns. A micron is one thousandth of a millimeter, or about 0.00004″. A typical single-mode fiber strand has a core only 8.3 microns, or 0.0003″, in diameter. A human hair is huge by comparison. The core of multimode fiber is typically either 50 or 62.5 microns, or 0.002″ in diameter. For both single-mode and multimode, the cladding usually has a diameter of 125 microns, or 0.005″. And finally, commonly used single-mode and multimode fiber strands have a coating layer that is 250 microns, or 0.01″, in diameter. Now we're getting somewhere, huh? That's all the way to one hundredth of an inch.

The tiny diameter of fiber strands makes them extremely dangerous. When stripped of their coating layer, as must be done for some splicing and connectorizing techniques, the strands can easily penetrate the skin. Shards, or broken pieces of strand, can even be carried by blood vessels to other parts of the body (or the brain), where they could wreak serious havoc. They are especially dangerous to the eye because small pieces can pierce the eyeball, doing damage to the eye's surface and possibly getting trapped inside. Safety glasses and special shard-disposal containers are a must when connecting or splicing fibers.

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The fiber core and cladding is usually made of some type of plastic or glass. Several types of materials make up the glass or plastic composition of the optical-fiber core and cladding. Each material differs in its chemical makeup and cost as well as its index of refraction, which is a number that indicates how much light will bend when passing through a particular material. The number also indicates how fast light will travel through a particular material. The refractive index of the core is higher than the cladding.

A fiber-optic strand's cladding is a layer around the central core that has a lower refractive index. The index difference between the core and cladding is what allows the light inside the core to stay in the core and not escape into the cladding. The cladding thus permits the signal to travel in angles from source to destination—it's like shining a flashlight onto one mirror and having it reflect into another, then another, and so on.

The protective coating around the cladding protects the fiber core and cladding from mechanical damage. It does not participate in the transmission of light but is simply a protective material against fracture. It protects the cladding from abrasion damage, adds additional strength to the core, and builds up the diameter of the strand.

The most basic differentiation of fiber-optic cables is whether the fiber strands they contain are single mode or multimode. A mode is a path for the light to take through the cable. The wavelength of the light transmitted, the acceptance angle, and the numerical aperture interact in such a way that only certain paths are available for the light. Single-mode fibers have a lower numerical aperture and cores that are so small that only a single pathway, or mode, for the light is possible. Multimode fibers have larger numerical apertures and cores; the options for the angles at which the light can enter the cable are greater, and so multiple pathways, modes, are possible. (Note that these ray-trace explanations are simplifications of what is actually occurring.)

Using its single pathway, single-mode fibers can transfer light over great distances with high data-throughput rates. Concentrated (and expensive) laser light sources are required to send data down single-mode fibers, and the small core diameters make connections expensive. This is because the mechanical tolerances required to focus the lasers into the core and to hold the fiber in connectors without moving the core away from the laser are extremely precise and require expensive manufacturing methods.

Multimode fibers can accept light from less intense and less expensive sources, usually LEDs or 850nm vertical cavity surface-emitting lasers (VCSELs). In addition, connections are easier to align properly due to larger core diameters. Since the core diameters are larger than single-mode fiber, the tolerances required to manufacture these parts are less precise and less expensive as a result. This is why lasers that are used to operate over only multimode fiber—that is, 850nm sources—are less expensive than single-mode sources. However, distance and bandwidth are more limited than with single-mode fibers. Since multimode cabling and electronics are generally a less expensive solution, multimode is the preferred cabling for short distances found in buildings and on campuses.

Single-mode fibers are usually used in long-distance transmissions or in backbone cables, so you find them mostly in outdoor cables. These applications take advantage of the extended distance and high-bandwidth properties of single-mode fiber.

Multimode fibers are usually used in an indoor LAN environment in the building backbone and horizontal cables. They are also often used in the backbone cabling where great distances are not a problem.

Single-mode and multimode fibers come in a variety of flavors. Some of the types of optical fibers, listed from highest bandwidth and distance potential to least, include the following:

• Single-mode step-index glass
• Multimode graded-index glass
• Multimode plastic