The part of an optical fiber through which light rays travel is called the core of the fiber. Light rays can only enter the core if their angle is inside the numerical aperture of the fiber. Likewise, once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called “multimode” fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber.

Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. One fiber carries transmitted data from device A to device B. The second fiber carries data from device B to device A. The fibers are similar to two one-way streets going in opposite directions. This provides a full-duplex communication link. Just as copper twisted-pair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to transmit and one to receive. Typically, these two fiber cables will be in a single outer jacket until they reach the point at which connectors are attached.

Until the connectors are attached, there is no need for twisting or shielding, because no light escapes when it is inside a fiber. This means there are no crosstalk issues with fiber. It is very common to see multiple fiber pairs encased in the same cable. This allows a single cable to be run between data closets, floors, or buildings. One cable can contain 2 to 48 or more separate fibers. With copper, one UTP cable would have to be pulled for each circuit. Fiber can carry many more bits per second and carry them farther than copper can.

Usually, five parts make up each fiber-optic cable. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket.

The core is the light transmission element at the center of the optical fiber. All the light signals travel through the core. A core is typically glass made from a combination of silicon dioxide (silica) and other elements. Multimode uses a type of glass, called graded index glass for its core. This glass has a lower index of refraction towards the outer edge of the core. Therefore, the outer area of the core is less optically dense than the center and light can go faster in the outer part of the core. This design is used because a light ray following a mode that goes straight down the center of the core does not have as far to travel as a ray following a mode that bounces around in the fiber. All rays should arrive at the end of the fiber together. Then the receiver at the end of the fiber receives a strong flash of light rather than a long, dim pulse. 

Surrounding the core is the cladding. Cladding is also made of silica but with a lower index of refraction than the core. Light rays traveling through the fiber core reflect off this core-to-cladding interface as they move through the fiber by total internal reflection. Standard multimode fiber-optic cable is the most common type of fiber-optic cable used in LANs. A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter (1µ).

Surrounding the cladding is a buffer material that is usually plastic. The buffer material helps shield the core and cladding from damage. There are two basic cable designs. They are the loose-tube and the tight-buffered cable designs. Most of the fiber used in LANs is tight-buffered multimode cable. Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding. The most practical difference between the two designs is the applications for which they are used. Loose-tube cable is primarily used for outside-building installations, while tight-buffered cable is used inside buildings.

The strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests.

The final element is the outer jacket. The outer jacket surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. The color of the outer jacket of multimode fiber is usually orange, but occasionally another color.

Infrared Light Emitting Diodes (LEDs) or Vertical Cavity Surface Emitting Lasers (VCSELs) are two types of light source usually used with multimode fiber. Use one or the other. LEDs are a little cheaper to build and require somewhat less safety concerns than lasers. However, LEDs cannot transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6,560 ft).

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