Fibre & Data Industries

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 is given greater attention than termination.  Manufacturers have come up with over 80 styles of connectors 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 of these types are used for most applications.   Different connectors are used in singlemode and multimode terminations, so be sure of the type required before you order.

TERMINATION PROCEDURES

Connectors are usually installed on the cables after installing and are usually terminated by splicing a factory-made “pigtail” onto the Fibre.  Cables can be pulled with connectors already on them if, and a big if, you can deal with these two problems: Firstly, the length must be precise.  Too short and you have to pull another longer one (its often 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 with MPO multifibre connectors.  These are tiny & shy; not much bigger than an ST or SC, but has up to 12 fibres. Manufactures sell multifibre cables with MPOs on them that connect to preterminated patch panels with SCs or LCs.  These work 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 ensure 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 your skin causes the fibre to attract dirt.  Before connection and testing, it is necessary to clean connectors with a suitable cleaning device.

SPLICING

Splicing is a “permanent” connection between two fibre cores, and is the preferred method for terminating.  There are two types of splices, fusion and mechanical.

Fusion Splices are made by “welding” the two fibers together by an electric arc.  Field 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.