Types of Fibre Cabling

Fibres were originally made from very high purity glass, any impurities or irregularities will scatter the light and increase signal loss, reducing the bandwidth and distance. Today newer and cheaper plastic technologies are beginning to appear but for high bandwidth and long distance application glass remains by far the most commonly used material.

There are three types of fibre optic cable: single mode, multimode and plastic optical fiber (POF). If a cable has a (relatively) wide diameter the light can travel along several different paths, known as modes. For example a beam can travel straight down the centre of the fibre, or can bounce repeatedly off the sides. The narrower the fibre and the more narrowly focused the beam of light the more limited will be the number of possible modes. In a perfect world you would want all the light to travel in a single mode down the centre of the fibre. In the real world fibre is constructed of either Step Index or Graded Index glass. They use different technologies to keep the signal in the fibre.

Step Index fibre consists of two types of glass with different refractive indexes. The glasses run coaxially along the length of the fibre. A beam of light hitting the boundary between the layers is refracted back into the core. Core diameter tends to be small, typically 10 microns. Step index fibres are expensive but losses are very low, typically 2dB per kilometre or less.

Graded glass fibre has a refractive index that varies progressively from the centre to the edge so stray beams are again directed back to the centre. Typically graded glass fibres are larger diameter, 62 or 50 microns.

• Single Mode is the smallest diameter, most expensive, but also the fastest and lowest loss type of fibre. The thinness of the fibre constrains the light to follow a single path or mode, or a very small number of paths, resulting in lower distortion and less spreading of the signal over longer distances. Typically around 8.3 to 10 microns wide, single mode fibre also requires a light source with a very narrow spectral range. Typically frequencies, usually 1310 and 1550 nanometers are used, selected because they produce the lowest losses in single mode fibre materials. Joins and splices in the fibre need to be highly accurate and to a very fine tolerance to minimize losses.
• Multimode fibre is a thicker type of cable, typically between 50 and 100 microns diameter, common sizes are 50, 62.5 and 100 microns. Multimode fibre allows multiple propagation modes, up to 1000 light paths, and yet permits high data rates, up to 10Gigabit and beyond over medium distances. The actual number of modes depends on the characteristics of the fibre, chiefly the diameter and the frequency of the injected light. Typical frequencies used are 850 and 1300 nanometres, selected because they produce the lowest losses in multimode fibre materials. Since the various paths have different lengths the original signal becomes spread out over longer distances, say 1km, causing distortion and degradation of the signal.
• Plastic Optical Fibre (POF) is a low cost fibre suitable for short cable runs. It supports a data transfer speed of 2.5GB/s.

Cable Buffer Types

Clearly with such thin fibres it would be impossible to pull or bend them if they were unprotected. The naked fibre is enclosed in a tougher casing, known as a buffer. The two main types are tight tube buffer and loose tube buffer. Tight tube buffer is usually a protective plastic moulded over each fibre, making them easier to handle and to terminate. Used mostly in indoor applications

In loose tube buffer a bundle of coated fibres is run inside a larger diameter tube, usually filled with waterproof gel to protect from water and to cushion shocks to the cable. Used mostly in harsh outdoor applications.

Being so small it is common for a number of fibres to be bundled into a larger diameter cable and for the cable to include strength members, usually of materials such as Kevlar. These allow you to pull the cable without stressing the individual fibres.

For outdoor cables the whole cable may be armoured with a protective metal sheath.

All cables are finished with a durable plastic jacket, available in a number of colours.

Cable Designations

When specifying cables you will need to understand the various ratings and suitability for your applications. Typically two numbers are used to specify the ratio of core size to cladding size. The first figure is the core diameter and the second is the cladding diameter, both are quoted in microns. So a cable marked 50/125 has a core of 50 microns and a cladding diameter of 125 microns. Sometimes a third digit is specified, this indicates the outside diameter of the whole cable. Common sizes, and typical applications are as follows:

• 8/125 Single Mode fibre for very high speed applications and for long cable runs
• 50/125 Multimode fibre for structured wiring applications, modern replacement for the older 62.5/125 standard
• 62.5/125 the original Multimode cable size, still very commonly encountered in the field

In addition cables are available with a single fibre, a duplex fibre, i.e. a pair of fibres inside a common jacket, and cables containing large bundles of fibres, sometimes hundreds. In WAN/LAN applications duplex or cables with an even number of fibres are most common, since the network needs a cable for sending and one for receiving data.