Installed cables often need to be joined together to complete a cable plant route; the route may be too long to pull or to place a continuous length of cable, since cable can only be manufactured in lengths of around 5 kilometers (km). One cable may be split to feed two or more cables, or even multiple cables will need to be interconnected at a single point. Whatever the reason, the cables need to be joined with low loss and reliability.

(Note: Part 8 is here.)

There are two ways of joining cables, either using connectors on each fiber to mate them or splicing the fibers together. Connectors can be disconnected, which allows rearranging connections among fibers and testing at the junction, but connectors are more expensive, bulkier and less reliable than a permanent splice joint. And connectors typically have much higher loss than splices.

Most outside plant cable joints are made by splicing. Outside cables must endure extremes of temperature, weather, wind and other environmental factors, so the reliability of splicing is preferable. In addition, the junction of two cables may involve hundreds of splices. The smaller sizes of splices make it easier to enclose and seal the splices for convenient mounting or burial, and splices generally cost less than connectors.

Indoors, however, most fibers are joined with connectors, since they allow more flexibility in future usage, and the higher loss and reliability are less important.

The vast majority of outdoor splices are fusion splices, where the two fibers or two fiber ribbons are welded together in an electrical arc, creating a fiber junction with practically no loss or reflectance, high mechanical strength and very low cost. Well, the low cost is true for each individual fiber junction, which only requires a few minutes labor and an inexpensive protective sleeve over the completed splice, but the contractor will have invested in or rented an expensive fusion splicing machine, some of which can even splice 12 fiber ribbons at once, making splicing very productive. The cost must be factored into each splice made.

The alternative to fusion splicing is mechanical splicing. A mechanical splice has a sleeve or bushing that holds the two fibers in alignment with some optical gel between them that matches the fibers’ optical characteristics to reduce the loss and reflectance. Because of the precision of the alignment sleeve, the need for a splice housing to hold the fibers and the cost of the gel, mechanical splices are comparable in cost to a pair of connectors. But the tools needed for mechanical splicing are about the same as those needed for installing connectors, making mechanical splicing more attractive to the contractor who only occasionally has to make splices.

Before choosing a type of splicing, consider the tradeoffs in loss (which favors fusion), reliability (which also favors fusion) and cost (depends on volume) plus the preference of the customer, who usually has the final say in the decision.

Whichever splice type is used, the quality of the splice and the yield of the process will depend on one tool: the fiber cleaver. No splice will have good performance unless the fibers being spliced have square, flat, perfectly cleaved ends. There are several types of cleavers available in two categories, manual and automatic. Manual cleavers, which are supplied in mechanical splicing tool kits, look like staplers or strippers and are relatively inexpensive. All manual cleavers can produce good cleaves if the installer has good technique and is consistent in using the tool. In our experience, that is not common in the field. Better results will be obtained with an automatic cleaver, the type always supplied with fusion splicers. While these splicers are more expensive, their consistency will have an immediate payback by producing higher yield of acceptable splices.

Splices must be protected in weathertight closures appropriate for the location. There are splices for buried, aerial, and underwater cable and for cable placed in pedestals. Cable and hardware manufacturers can help the contractor choose an appropriate closure.

Unlike connectors, splices cannot be directly tested because the two fibers are joined permanently. If a link containing a splice is tested end-to-end for insertion loss, the loss of the splice will be included in the total loss measured. If the measured loss is reasonable, within the calculated loss determined by the loss budget, the link passes. Longer lengths of spliced cables are usually tested with an OTDR to verify splice losses, but one needs to be aware of the inherent uncertainty of the OTDR measurements.

HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.