TERMINATING
We terminate fibre optic cable two ways – with connectors that can mate two fibres to create a temporary joint and/or connect the fibre to a piece of network gear or with splices which create a permanent joint between the two fibres. These terminations must be of the right style, installed in a manner that makes them have little light loss and protected against dirt or damage in use. No area of fibre optics has been given greater attention than termination. Manufacturers have come up with over 80 styles of connectors and and about a dozen ways to install them. There are two types of splices and many ways of implementing the splice. Fortunately for me and you, only a few types are used most applications. Different connectors and splice termination procedures are used for singlemode and multimode connectors, so make sure you know what the fibre will be before you specify connectors or splices!
Termination Procedures
Multimode connectors are usually installed in the field on the cables after pulling, while singlemode connectors are usually installed by splicing a factory-made “pigtail” onto the Fibre. That is because the tolerances on singlemode terminations are much tighter and the polishing processes are more critical. You can install singlemode connectors in the field for low speed data networks, but you may not be able to get losses lower than 1 dB!
Cables can be pulled with connectors already on them if, and a big if, you can deal with these two problems: First, the length must be precise. Too short and you have to pull another longer one (its not cost effective to splice), too long and you waste money and have to store the extra cable length. Secondly, the connectors must be protected. Some cable and connector manufacturers offer protective sleeves to cover the connectors, but you must still be much more careful in pulling cables. You might consider terminating one end and pulling the unterminated end to not risk the connectors.
There is a growing movement to install preterminated systems but with the MT 12 multifibre connector. It’s tiny ­ not much bigger than a ST or SC, but has up to 12 fibres. Manufactures sell multifibre cables with MTs on them that connect to preterminated patch panels with STs or SCs. Works well if you have a good designer and can live with the higher loss (~1 dB) typical of these connectors.
Connector and Splice Loss Mechanisms
Connector and splice loss is caused by a number of factors. Loss is minimized when the two fibre cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fibre’s core will propagate, so all the rest of the light becomes the connector or splice loss.
End gaps cause two problems, insertion loss and return loss. The emerging cone of light from the connector will spill over the core of the receiving fibre and be lost. Connectors use a number of polishing techniques to insure physical contact of the fibre ends to minimize back reflection. On mechanical splices, it is possible to reduce back reflection by using non-perpendicular cleaves, which cause back reflections to be absorbed in the cladding of the fibre.
The end finish of the fibre must be properly polished to minimize loss. A rough surface will scatter light and dirt can scatter and absorb light. Since the optical fibre is so small, typical airborne dirt can be a major source of loss. Whenever connectors are not terminated, they should be covered to protect the end of the ferrule from dirt. One should never touch the end of the ferrule, since the oils on one’s skin causes the fibre to attract dirt. Before connection and testing, it is advisable to clean connectors with lint-free wipes moistened with isopropyl alcohol.
SPLICING
Splices are “permanent” connections between two fibers, and is the preferred method for terminating. There are two types of splices, fusion and mechanical. Most splicing is on long haul outside plant SM cables, not multimode LANs, so if you do outside plant SM jobs, you will want to learn how to fusion splice. If you do mostly MM LANs, you may never see a splice.
Fusion Splices are made by “welding” the two fibers together usually by an electric arc. Fusion splicing is usually done above ground in a truck or trailer set up for the purpose.
Mechanical Splices are alignment gadgets that hold the ends of two fibers together with some index matching gel or glue between them. There are a number of types of mechanical splices, like little glass tubes or V-shaped metal clamps.
