Introduction

The type of systems OWS deals with are primarily concerned with some form of communication, whether it be audio signals to speakers, wireless networking, or data sent through tangible cables. The medium used for the signal is critical. Fibre optic cable has emerged as the medium of choice for high-speed data connections in the last decade.

The title of this page reflects the fact that different spellings of this cable category may be encountered; the rest of this document uses the ‘fibre’ variation.

Advantages of Fibre-Optic Cable

Whilst copper cabling technologies are continually improving in terms of performance, optical fibre is not subject to the same constraints, and has become the de facto standard for all high-speed backbone and infrastructure connections. For example, the OM4 multimode fibre cables OWS recently installed for a media archiving institution can sustain 10Gb Ethernet over long distances (over 400m).

Cheaper material components
Whilst some fibre networking components may be expensive, the core material constituting the medium is a form of silica, and therefore not subject to the relative scarcity of materials such as copper.

Immune to interference
One of the main limiting factors in copper cable performance derives from the fact that it depends on electrical signals, which are subject to interference from the neighbouring environment. The signal can be affected by the nearby electrical surges caused by motors, for example, but also suffers from interference from adjacent bundled cables. For this reason, measurements of “crosstalk” are an integral part of structured cabling testing procedures. The fact that the signal is not based on electricity also makes it particularly useful in critical environments where sparks are to be avoided at all costs, such as gas installations.

Low signal loss
Another, related, characteristic of fibre is its ability to sustain a signal over long distances, without relay signal boosters. Single-mode fibre can transmit signals over distances measured in kilometres, rather than just hundreds of metres. Clearly, this is a tremendous benefit for uses such as subterranean networks maintained by telephone companies.

Smaller & Lighter
Fibre solutions can be achieved with much narrower and lighter cables than found with some competing technologies, particularly useful in aircraft, for example.

Cable Selection Considerations

Like copper cable, fibre choice is influenced by the potential for damage to the cable coming from crushing, or where a tight bending radius is required. A curve radius of greater than ten to twenty times the diameter of the cable is typically recommended. Being inherently less malleable than copper, however, the fibre strand needs to be well protected for rugged applications, or in locations where it bears a heavy load. Note that load may result from the cable's own weight in a vertical run, just as much as forces impacting upon a horizontal cable from surrounding cables, or earth laid on top, or pressures resulting from its working environment.

A polyurethane sheath is often employed where more flexibility is required. It always has resistance to moisture, and hence is found in outdoor applications. PVC is the commonest material found encasing indoor cables. However, in spaces where fire regulations demand a retardant material (such as "plenum" spaces above false ceilings with air-conditioning ducts), the sheath may be made from polyvinyl difluoride. In locations where safety is a critical consideration (such as confined public transport spaces), halogen-free (non-PVC) sheaths are used.

The illustration shows tight- and loose-buffered cables. Whilst this example shows a single strand on the tight-buffered cable, there may be more. The main distinction is the mobility of the fibre within its housing. Loose-buffered construction allows the fibre to move within a surrounding gel, which allows for thermal expansion and contraction, and movement from crushing forces without resultant fibre damage.

In terms of performance, fibre optic cables fall essentially into two categories: single mode, and multimode. Single-mode fibres are tailored to transmit a single ray of light (a "mode"). This precise engineering of the cable and associated equipment allows for the highest speeds and longest transmission distances. Multimode, by comparison, has a thicker core, and does not preserve so well the fidelity of the signal, leading to lower bandwidth. The light is able to travel various paths through the fibre, as opposed to the more restricted path of single-mode light, resulting in more "modal dispersion" which needs to be reconciled when the light reaches the end of its path. However, the larger diameter allows the use of LEDs (light-emitting diodes) and VCSELs ("vertical-cavity surface-emitting lasers") which are cheaper light sources than the precise laser required for single-mode installations.

The main categories of multi-mode ("MM") fibres used for LANs (Local Area Networks) are illustrated in the the table below, with corresponding speeds and distances recommended for Ethernet networks.

Fibre dimensions are conventionally represented as a combination of core diameter and diameter including the central cladding (this refers to the immediate cladding surrounding the fibre itself, and does not take into consideration the protective layers around it). Thus, '62.5/125' represents a core of 62.5 micrometres, surrounded by cladding of 125-micrometre width. The table below includes typical dimensions for the category of fibre in question, accompanied by maximum cable lengths for given bit rate speeds. The standards do not specify values for other cable/speed combination.

Fibre Optic Cable Connectors

Poor connections between cable segments can result in "signal attenuation" through "back reflection" and "insertion loss"(generally measured in a decibel scale of power loss). The light can easily be deflected or impeded if the interface is not precisely finished. The end of each fibre section may be polished in various ways to improve connectivity, whether it be flat, domed or angled. There is also variation in terms of the way a connector is attached to the fibre. Special adhesives may be used, whether at room temperature or oven-heated; or a polished fibre may be physically spliced and crimped; fusion splicing uses an electric arc to meld two ends together at very high temperature.

Some of the commoner connector types are listed below. These connector types use rigid connector housings referred to as 'ferrules'.

SC Connector
The 'S' in 'SC' is variously interpreted as 'Standard', 'Square' or 'Subscriber'. Originally developed in Japan, it falls into the 'snap-on' category of connection type. The fibre is inserted into a precisely engineered hole in a 2.5mm ferrule. It is then typically held with adhesive and polished at the extremity. This is in turn inserted into sub-assembly and further outer assembly which provides for the protective part of the fibre cable to be crimped on. A mating sleeve then joins two connectors together. Like other connectors, the SC can be found in simplex or duplex form, the latter providing for two adjacent fibres (transmit and receive) to be plugged in together.

Full details can be found in IEC standard 61754-4.

ST Connector
The 'Straight Tip' connector favours a bayonet twist form of coupling, where the connection is held in place by spring-mounting. The ceramic ferrule is not as susceptible as its metal counterparts to temperature-related expansion/contraction. The angled means of polishing the fibre tip (mentioned above) cannot be used with this type of connector.

Its specification is covered by the same IEC standard 61754-4.


LC Connector
The Lucent Connector, orginally named for its developing corporation, is also sometimes referred to as 'Little Connector', its smaller size being a salient characteristic. The overall diameter is around half a centimetre, whereas ST and SC connectors are about twice the width. The 1.25mm ceramic ferrule, and small associated housing, result in a form factor which is convenient for sites where large concentrations of fibres are found, such as large corporate environments. It is often used in single-mode applications.

See IEC 61754-20 for further details.


FC Connector
With a 2.5mm ferrule, the FC ('Ferrule Connector') uses a screw-on mating attachment type. A combination of a key to prevent rotation of the fibre and an internal spring loading protect the fibre as two FCs are mated. The FC may be used with different physically polished fibre ends, being either rounded or angled ("APC", angled physical contact, by convention coloured green). The angled matching facets prevent light being reflected back up the fibre, but may also result in some loss. Whilst it is losing ground to the SC and LC, the FC has a unique "floating ferrule" internal design which makes it suitable for resisting mechanical vibration.

See IEC 61754-13 for more information.

For details of a practical implementation of this technology by Ohms & Watts Services, please see our Media Archiving Centre Fibe-Optic Installation Project page.

Footnote:

For definitive details of specifications and other details, please refer to the relevant standards.

Last updated: 05/2012