There are two different ways to join two optical fibers: splices or connectors. Splices are permanent joints, while connectors allow the two fibers to be disconnected at the joint. There are obvious advantages with connectors, allowing for changes in network connections or accessing the network for testing. Splices, however, offer lower optical loss at the joint and higher reliability-an advantage for long-distance networks installed outdoors. Sometimes the decision whether to use splices or connectors is made based on less obvious factors.
There are other subtle differences between connectors and splices. Take size, for instance. A patch panel of connectors takes up a lot of space, especially when you consider the extra space needed to get a human hand into the patch panel to grab a connector, connect or disconnect it.
Even the so-called “small-form factor” connectors take up a lot of space on a panel. The same number of splices will fit in a small, thin splice tray that takes only a fraction of the space required by the same number of connectors.
Splices have less back reflection, an important specification for single-mode networks, especially where high-speed networks are used on short campus or building backbones. But splices may adversely affect the bandwidth of multimode fiber by mode mixing at the splice.
Some splices can be much less expensive if you are doing a lot of fiber joints. This is another big advantage for long distance networks where cables contain many fibers and require splicing every few kilometers. A fusion splice uses a machine to weld fibers together in an electric arc, making the lowest possible loss joint.
Each fusion splice has very low cost, but unless you are splicing many fibers, the extremely high cost of a fusion splicing machine (up to $50,000) makes fusion splicing unaffordable. Smaller numbers of splices can use mechanical splices, but the individual splices are generally as expensive as connectors and take at least as much time to install.
Most of these factors do not overcome the big disadvantage that splices have in most premises networks-the inability to allow easy access to the network for testing or moves, adds and changes. Even long-distance telephone networks use connectors at each end of the cable to allow connections to patch panels or network equipment that are often accessed for changing hardware or testing. Testing an individual splice is impossible without using an OTDR, another expensive tool that may not be useful in a premises network with short cable runs. Connectors can easily be tested individually, verifying performance before the final network is connected.
I am aware of only one large premises network that uses splices instead of connectors- a college campus that runs fiber to every dorm room for connecting students to the campus network. The trade-off was made because the network was not intended to be changed once installed. Each dorm room was to be connected to a main computer room where the electronics for the network resided. Little space was available in each dorm for fiber optic patch panels, making the small size of the splice closures a necessity.
That same reasoning could be used on many fiber optic backbones. If the cabling is simply connecting point A to point B and those are permanent locations, the backbone could be spliced in place and forgotten. If plans changed and modifications were necessary, it is possible to cut out splices and resplice as necessary, but that is much more expensive than simply moving a couple of connectors on a patch panel.
There is one mandatory application for fusion splicing in premises networks, terminating single-mode fibers. All single-mode fiber termination in telco and CATV networks is done by fusion-splicing pigtails on the end of the cable. The reason for this is single-mode termination is a much more critical process than multimode terminations. It requires special techniques with diamond lapping film and a polishing slurry to get a smooth finish on the end of the fiber that will produce low loss and back reflection.
This process is extremely difficult to control in the field, especially when hand polishing, which is why most single-mode terminations are made in factories using polishing machines. Single-mode connectors in a premises network must be high quality, therefore it is recommended that all single-mode terminations be made by an experienced splicer with factory-made pigtails.
It is wise to understand splices and consider their applications even in typical premises applications where connectors are routinely used, making the final decision on the usual factors: cost, installation ease and performance. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.