The usual Fibre specifications you will see are size, attenuation and bandwidth. While manufacturers have other specs that concern them, like numerical aperture (the acceptance angle of light into the Fibre), ovality (how round the Fibre is), concentricity of the core and cladding, etc., these specs do not affect you.

Fibre Itself

Fibre Optics, as we said, is sending signals down hair-thin strands of glass or plastic Fibre. The light is "guided" down the centre of the Fibre called the "core". The core is surrounded by a optical material called the "cladding" that traps the light in the core using an optical technique called "total internal reflection."

Multimode & Singlemode Fibres

Multimode & Singlemode Fibre are the two types of Fibre in common use. Both fibres 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 Fibre has light traveling in the core in many rays, called modes. It has a bigger core (almost always 62.5 microns, but sometimes 50 microns ) and is used with LED sources at wavelengths of 850 and 1300 nm (see below!) for slower local area networks (LANs) and lasers at 850 and 1310 nm for networks running at gigabits per second or more.

Singlemode Fibre has a much smaller core, only about 9 microns, so that the light travels in only one ray. It is used for telephony and CATV with laser sources at 1300 and 1550 nm.

Plastic Optical Fibre (POF) is large core ( about 1mm) Fibre that can only be used for short, low speed networks.

Step index multimode was the first Fibre design but is too slow for most uses, due to the dispersion caused by the different path lengths of the various modes. Step index Fibre is rare - only POF uses a step index design today.

Graded index multimode fibre uses 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 Fibre - up to about 2 gigahertz.

Single mode Fibre shrinks the core down so small that the light can only travel in one ray. This increases the bandwidth to almost infinity - but it's practically limited to about 100,000 gigahertz - that's still a lot!

Size Matters

Fibre, as we said, comes in two types, single mode and multimode. Except for fibres used in specialty applications, single mode fibre can be considered as one size and type. If you deal with long haul telecom or submarine cables, you may have to work with specialty single mode fibres.

Multimode fibres originally came in several sizes, optimized for various networks and sources, but the data industry standardized on 62.5 core fibre in the mid-80s (62.5/125 fibre has a 62.5 micron core and a 125 micron cladding.) Recently, as gigabit and 10 gigabit networks have become widely used, an old fibre has been revived. The 50/125 fibre was used from the late 70s with lasers for telecom applications before single mode fibre became available. It offers higher bandwidth with the laser sources used in the gigabit LANs and can go longer distances. While it still represents a smaller volume than 62.5/125, it is growing.

Fibre Optic Cable

Fibre optic "cable" refers to the complete assembly of fibres, strength members and jacket. Fibre optic cables come in lots of different types, depending on the number of fibres and how and where it will be installed. Choose cable carefully as the choice will affect how easy it is to install, splice or terminate and, most important, what it will cost!

Choosing a cable - what hazards will it face?

Cable's job is to protect the fibres from the hazards encountered in an installation. Will the cables be exposed to chemicals or have to withstand a wide temperature range? What about being gnawed on by a rodent or some animal? Inside buildings, cables don't have to be so strong to protect the fibres, but they have to meet all fire code provisions. Outside the building, it depends on whether the cable is buried directly, pulled in conduit, strung aerially or whatever.

Since the plan will call for a certain number of fibres, consider adding spare fibres to the cable - fibres are cheap! That way, you won't be in trouble if you break a fibre or two when splicing, breaking-out or terminating fibres. And request the end user consider their future expansion needs. Most users install lots more fibres than needed, especially adding singlemode fibre to multimode fibre cables for campus or backbone applications