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

---

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.

---

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