multimode fiber-optic cabling

Definition of multimode fiber-optic cabling in The Network Encyclopedia.

What is Multimode Fiber-Optic Cabling?

A type of fiber-optic cabling that allows multiple signals to be transmitted simultaneously. Line drivers for multimode fiber-optic cabling use light-emitting diodes (LEDs) to generate the light signals that carry the data down the fiber.

How It Works

Multimode fiber, which has a glass core whose index of refraction varies with the distance from the core axis, is implemented in two main forms:

  • Step-index multimode fiber:
    The light rays reflect off the walls of the core by total internal reflection. Depending on the angle at which the rays are incident on the surface of the core, different light paths are created that can carry additional signal bandwidth. In longer cables, these paths can get out of step with each other at the far end of the fiber and degrade signal quality.

     

  • Graded-index multimode fiber:
    The core consists of concentric layers of material. Each successive layer has a lower index of refraction than the one that it envelops. As a result, rays of light travel along curved paths and all arrive in step with each other at the far end of the fiber.

     

Multimode fiber is available with different core diameters, typically 50, 62.5, and 100 microns. Multimode fiber can carry more bandwidth than single-mode fiber, but single-mode fiber can carry signals up to 50 times farther than multimode.

Multimode fiber is not recommended for long cable runs and should generally be restricted to runs of 914 meters. If this limit is exceeded, the light traveling along different paths through the fiber can produce a condition called modal dispersion, which results in parts of the signal arriving at unexpected times at the end station. This can degrade the quality of the signal or cause it to be unrecognizable.

Graphic M-20. Multimode fiber-optic cabling.

TIP

Step-index fiber is cheaper than graded-index fiber and should be used only for shorter cable runs or where less bandwidth is required.