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3.3.1.1 Multimode Fiber

Multimode fiber allows multiple modes (paths) of light to propagate through the
fiberoptic core, as compared to single-mode fiber, which allows only one mode. Multiple
modes of light propagating through fiber might travel different distances, depending on their
entry angles. This angle causes them to arrive at the destination (receiving end of the cable)
at slightly different times—a phenomenon called modal dispersion. Multimode uses a type of
glass, called graded index glass, which has a lower index of refraction towards the outer edge
of the core. This glass causes the light to slow down when passing through the center of the
core and accelerate when passing through the outer areas of the core, ensuring that all modes
of light reach the end at approximately the same time. This design is used because a light ray
following a mode that goes straight down the center of the core does not have to go as far 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. A standard multimode fiber-optic cable (the most common type
of fiber-optic cable used in LANs) uses an optical fiber with either a 62.5- or a 50-micron core
and a 125-micron diameter cladding. This cable is commonly designated as 62.5/125 or
50/125-micron optical fiber. A micron is one millionth of a meter. Because the diameter of the
cladding is considerably larger than the wavelength of the light being transmitted, the light
bounces around (reflects) inside the core as it is propagated along the transmission line.
Infrared light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) are
usually the light source used with multimode fiber. 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 (6560 feet). Multimode fiber is mainly used in LAN applications including backbone
cabling.

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