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COURSE DEMO Page 11 of 12

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MULTIMODE FIBER

Multimode fiber generally uses inexpensive LED light sources, although lasers can also be employed. These LED’s are unfocused, and will emit light rays at many angles.

The core of a multimode fiber is large enough, in relative terms, that when the LED blinks, multiple rays enter the core and travel the length of the fiber. Some of the light rays will travel close to the center of the core, and some will bounce from the core/cladding boundary on one side to the other, as illustrated.

“Multi mode” means multiple paths. There are many paths that a light ray can take in a multimode core to reach the other end.

Multimode fiber functions like a shotgun- multiple pellets are sent down the barrel, and come out the other end.

As can be seen in the illustration, light rays traveling near the center will arrive at the receiving end first, followed by the light rays that are bouncing from cladding to cladding. Because the shortest distance between two points is a straight line, the center or “close coupled mode” light rays will arrive before the outer or “loose coupled” rays.

Because of this difference in arrival times of the light rays, the delivery of a light ray to the receiver can be spread out over time, causing slowing of communication. If the blink of the LED is 3 nanoseconds at launch, the same blink may lengthen to 6 or 7 nanoseconds at the receiving end of the multimode fiber. As the receiver can only count one blink at a time, the communication slows. This is called “pulse spreading”, and is the primary factor that limits multimode fiber to approximately 8 kilometers or 5 miles in usable link length.

LESSON REVIEW

Key Points:

1. There are many paths that a light ray can take when travelling in a multimode fiber core.

2. "Multimode" means multiple paths.

3. "Pulse Spreading" limits the distance that multimode fiber can be successfully used.

Pop Quiz:
True or False

When many light rays are sent into a multimode fiber core at the same time, they will all reach the receiver at the same instant.