Ethernet by sa

Author:sa [sa]
Language: fra
Format: epub, pdf
Tags: Informatique
Publisher: O'Reilly
Published: 0101-01-01T00:00:00+00:00

Chapter 14, Structured Cabling.

Fiber optic Ethernet transceivers for use on multimode fiber are based on LED transmitters which emit a form of light that is not dangerous to the eye. However, Ethernet single-mode fiber optic equipment and other network devices based on single-mode fiber typically use laser light.

Sufficiently powerful laser light can damage the retina in your eye without causing any feeling of pain. Treat all fiber optic cables with caution. Beware of looking directly into any fiber optic cable, and observe safety precautions when working around fiber optic cable systems.

Fiber Optic Core Diameters

The thickness of the core optical fiber used in fiber optic cables is very small, and is measured in millionths of a meter, called micrometers (µm) or microns. One frequently used type of multimode fiber optic cable consists of a graded-index cable, in which the individual pieces of fiber have a 62.5 µm fiber optic core and 125 µm outer cladding (62.5/125). Multimode cable is also available with a 50 µm core and 125 µm cladding.

Single-mode fiber has a much smaller core with the same 125 µm outer cladding. Commercial single-mode fibers have core diameters that can vary over the range of approximately 8–10µm.

They are collectively referred to as 10 µm fiber in the standard. By way of comparison, a human hair has a diameter of about 100 µm.

Fiber Optic Modes

A fiber optic "mode" is a path that light can follow in traveling down a fiber. As the name implies, multimode cable has a larger core designed to support multiple modes, or paths, of light propagation. When an incoherent light source such as an LED is coupled to multimode fiber, then multiple paths of light from the LED are transmitted over the cable as shown in Figure 16-1.

An advantage of the larger core is easier coupling of the light source to the cable. A disadvantage is that the wider corridor for light transmission allows the multiple paths of LED light to bounce off the sides of the fiber. When this happens, these

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