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FEATURE FIBRE OPTIC SYSTEMS


Optical Fibre’s Migration to a Highly Demanding Technology Clean Sweep By Mike Gilmore, Technical Director, Fibreoptic Industry Association


Records show a disturbing trend - an increase in functional failures where the only resolution is the effective cleaning of ports and optical fibre end-faces on cords and within patch panels and even equipment. What is behind this, and what can be done about it? The need for corrective cleaning


As FIA Technical Director, Mike Gilmore acts in disputes relating to functional failures of installed cabling submitted to the FIA Arbitration Scheme.


programmes should not be surprising - how many users remember to replace the dust caps on bulkhead adaptors? How many installers provide training on inspection and cleaning procedures? How many users would implement such procedures if they had received that training? My point regarding dust caps should be treated with some caution, because the presence of a dust cap does not indicate that the optical fibre end-face is ‘clean’. However, their absence is a warning sign of potential contamination, and is often an indicator of a ‘plug it in, see if it works - if not, try the next port’ philosophy on behalf of the user. We have reached a point where


cleaning has become more than just a reactive measure. As a repair strategy, that cleaning may be rendered ineffective due to demands on connection performance. This article explains what has changed to put so much pressure on connection performance, and why effective inspection and decontamination is so important.


Nothing new


Perhaps we should start with what has not changed. Cabled multimode optical fibres have a specified maximum attenuation of 3.5 dB/km at 850 nm and the connections specified in the structured cabling standards have a maximum loss of 0.75 dB. So what has changed? In the early days of optical fibre


within customer premises, most installations were of campus backbones. The associated networks had maximum specified lengths of 2000m, and the cables installed generally had external grade, loose tube constructions which exhibited attenuation significantly better than the maximum value detailed above. Taking into account


8 NETCOMMS europe Volume II, Issue 1 2011


that networks such as 100BASE-F and FDDI operated at 1300 nm (where the cable attenuation is much lower) and most channel lengths were of the order of 150-400 metres, end-face contamination at connections tended to be ignored as an operational issue, even if it raised the loss of the cabling well beyond its specified value.


In short


As optical fibre cabling began to be used to support these networks within building backbones, the proportion of shorter lengths increased. This further undermined the need for cleanliness in lower speed networks. However, the cables used were more likely to be of tight jacket construction in which the attenuation would be more in line with the maximum specified values indicated above. Optical fibre cabling ‘in-building’ in both backbone and data centre applications served to increase the quantity of spare, unused optical fibres. This acts against good housekeeping, allowing non-functional links to be sidelined without reparative work simply by the selection of new, unused optical fibres. It also encourages a misguided notion that spare optical fibres will be fully functional when required, without any inspection or cleaning.


At length


The advent of 1 Gigabit Ethernet began to reverse this tolerance of poor operational practice by restricting channel length to around 500m, with an associated reduction in maximum allowed cabling loss. Longer channels forced the installation of either higher bandwidth multimode product or the use of singlemode optical fibre, where end-face contamination is much more critical. As campuses and buildings had not changed shape and size, the remaining channel lengths were much closer to the maximum specified for higher speed networks, and the tolerance for high loss connections began to disappear. This process has continued – first with the advent of 10GBASE-S


networks, and now with 40/100 Gigabit Ethernet. These short length networks simply do not have enough margin in the cable attenuation to support any increased connection losses due to contamination. The fibre optic industry has


always recognised the impact of contamination. As long ago as 1991, the FIA Code of Practice for the Installation of Fibre Optic Cabling identified the minimum requirements for end-face quality. These have been enhanced in BS EN 61300-3-35, which not only redefines those requirements, but specifies the criteria for inspection equipment. CCD microscopes are now available for the inspection of plugs and adaptors against these criteria. They are becoming an indispensible component of the quality assurance armoury carried by professional installers.


Resisting temptation


The cleaning of end-faces and ports is obviously time-consuming and it is very tempting not to undertake the appropriate practices. Similarly, it is tempting to assume that end- faces are clean because they have come sealed in a plastic bag and have end-caps fitted. For installers, the greatest risk is during testing. A failure to clean prior to each test can impact all the end-faces under test as they get increasingly contaminated. For users, the greatest risk is gradual build up of contamination that may not be repairable. Nevertheless, installers and users alike


must realise that increased connection attenuation due to contamination cannot be tolerated by high bit rate networks, and that the damage caused by that contamination may not be reversible. Once the damage is done, it is too late. When two fibre optic connectors are mated, the pressure applied across the core-core area is enormous and loose contaminants can be embedded into the core surfaces. This occurs on ‘first mate’ and when this embedded contamination cannot be removed, optical performance can be reduced permanently.


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