Multimode--Specifies a light signal containing more than one light ray (mode)
	Single mode--Specifies a light signal containing only one light ray

Multimode Fiber Optic Cable

Each ray of light that passes through a cable is called a mode. Multimode fiber optic cable is wider than single-mode cable, and because of this relative thickness, allows enough room for more than one light ray to be passed through the cable. These rays will have different angles of incidence and reflection as they are reflected off the cladding. The Multimode Fiber Optic Cable Diagram illustrates this type of cable.

When using multimode signaling, not all light rays travel the same distance. Some light rays will travel straight through the core, while others continuously bounce off the cladding prior to reaching the far end of the fiber. With modes traveling different distances, but at the same speed, the spread of the signal increases over time, and can cause data errors because of the overlapping of light pulses. This problem is known as dispersion. The construction of the multimode fiber has a significant effect on this problem. There are two types of multimode fiber:

	Step-index--Consists of only two transparent materials, the core and the cladding. Step-index fiber does not compensate for signal dispersion.
	Graded-index--Specifies a fiber cladding that has several transparent layers. This planned inconsistency causes the multiple light rays to reach the far end of the fiber in a more uniform fashion.

Graded-index fiber allows light to travel at different speeds through the core. The speed at which the modes travel depends upon the part of the core through which they are traveling. Modes traveling down the center of the core do so at a slower speed than those refracting off the cladding. Thus, all modes reach the far end of the fiber more uniformly.

The most commonly specified fiber optic cable is 62.5/125 µm multimode graded-index.

Single-Mode Fiber Optic Cable

Single-mode fibers have diameters sized to the wavelength they are designed to carry. A typical single-mode fiber core diameter is 8 µm. Only one mode will propagate through fiber with this core diameter, if the specified wavelength (for example, 1.3 µm) is sent through the fiber. Because of having a more coherent light source, single-mode fiber optic cable can support longer transmission distances than multimode fiber. It is also much narrower, which makes it more difficult and expensive to install correctly. Single-mode fibers are generally step-index fibers. Because only one mode travels along the fiber, the problem of diffusion or "spreading" into one another does not occur in single-mode fibers. The single-mode fiber shown on the Single-Mode Fiber Diagram is a step-index fiber consisting of a core and a cladding.

   Activities

See the Activities and Extended Activities section in Unit 2 Lesson 2 in your textbook Introduction to Networking to test what you have learned so far.

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