Multimode & Singlemode fiber are the five types of fiber in common use. Both fibers are 125 microns in outside diameter - a micron is one one-millionth of a meter & 125 microns is 0.005 inches- a bit larger than the typical human hair. Multimode fiber has light travelling in the core in lots of rays, called modes. It's a bigger core (always 62.5 microns, but sometimes 50 microns) & is used with LED sources at wavelengths of 850 & 1300 nm for slower local area networks (LANs) & lasers at 850 & 1310 nm for networks jogging at gigabits per second or more. singlemode has a much smaller core, only about 9 microns, so that the light travels in one ray. It is used for telephony & CATV with laser sources at 1300 & 1550 nm. Plastic Optical Fiber (POF) is large core (about 1mm) fiber that can only be used for short, low speed networks. Step index multimode was the first fiber design but is slow for most makes use of, due to the dispersion caused by the different path lengths of the various modes. Step index fiber is rare - only POF makes use of a step index design today. Graded index multimode fiber makes use of variations in the composition of the glass in the core to compensate for the different path lengths of the modes. It offers hundreds of times more bandwidth than step index fiber - up to about 2 gigahertz. Singlemode fiber shrinks the core down so small that the light can only travel in one ray. This increases the bandwidth to infinity - but it is practically limited to about 100,000 gigahertz - that is still a lot! describe the image Fiber comes in five types, singlemode & multimode. Except for fibers used in specialty applications, singlemode fiber can be regarded as one size & type. In case you deal with long haul telecom or submarine cables, you may must work with specialty singlemode fibers. The usual fiber specifications you will notice are size, attenuation & bandwidth. While manufacturers have other specs that concern them, like numerical aperture (the acceptance angle of light in to the fiber), ovality (how round the fiber is), concentricity of the core & cladding, etc., these specs do not affect you. Fiber Optics, as they said, is sending signals down hair-thin strands of glass or plastic fiber. The light is "guided" down the middle of the fiber called the "core". The core is surrounded by a optical material called the "cladding" that traps the light in the core using an optical method called "total internal reflection." The core & cladding are usually made of ultra-pure glass, although some fibers are all plastic or a glass core & plastic cladding. The fiber is coated with a protective plastic covering called the "primary buffer coating" that protects it from moisture & other destroy. More protection is provided by the "cable" which has the fibers & strength members inside an outer covering called a "jacket". describe the image Multimode fibers originally came in several sizes, optimized for various networks & sources, but the information industry standardized on 62.5 core fiber in the mid-80s (62.5/125 fiber has a 62.5 micron core as well as a 125 micron cladding.) Recently, as gigabit & 10 gigabit networks have become widely used, an elderly fiber has been revived. The 50/125 fiber was used from the late 70s with lasers for telecom applications before singlemode fiber became available. It offers higher bandwidth with the laser sources used in the gigabit LANs & can go longer distances. While it still represents a smaller volume than 62.5/125, it is growing.