Fiber Types: Multimode & Singlemode, Core/Cladding Size

The two types of fiber are multimode and singlemode. Within these categories, fibers are identified by their core and cladding diameters expressed in microns (one millionth of a meter), e.g. 50/125 micron multimode fiber.

Most fibers are 125 microns in outside diameter - a micron is one one-millionth of a meter and 125 microns is 0.005 inches- a bit larger than the typical human hair.

Multimode fiber has light traveling in the core in many rays, called modes. It has a larger core (almost always 50 or 62.5 microns) which supports the transmission of multiple modes (rays) of light. Multimode is generally used with LED sources at wavelengths of 850 and 1300 nm (see below!) for slower local area networks (LANs) and lasers at 850 (VCSELs) and 1310 nm (Fabry-Perot lasers) for networks running at gigabits per second or more.

Singlemode fiber has a much smaller core, only about 9 microns, so that the light travels in only one ray (mode.) It is used for telephony and CATV with laser sources at 1300 and 1550 nm because it has lower loss and virtually infinite bandwidth.

Plastic Optical Fiber (POF) is large core ( about 1mm) fiber, usually step index, that is used for short, low speed networks.

October 25, 2011

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