It’s possible to terminate fiber optic cable in two ways––connectors or splices. Connectors install on fiber ends that mate to other fibers creating a temporary joint, or connect the fiber to the transmitter or receiver of a piece of network gear. Splices are used for permanent joints.
Most premises-cabling connections are made by connectors to allow future moves, adds and changes (MACs), while most outside plant joints are made by splicing. Splicing is needed if outside-plant cable runs are too long for one straight pull or if different cable types are mixed, such as splicing a 48-fiber cable to six eight-fiber cables going in different directions. And, of course, we use splices for restoration, usually after a buried cable has been dug up and cut. That’s called “backhoe fade,” for obvious reasons.
There are fusion and mechanical splices, and their use is based on performance, ease of termination, cost or reliability. Fusion splicing is used mainly on long-haul, outside-plant, single-mode cables. Though rare, premises-cabling splices are normally mechanical.
Fusion splices are made by “welding” the two fibers together, usually by an electric arc. Obviously, you don’t do that in an explosive atmosphere, so fusion splicing is usually done aboveground in a truck or trailer set up for the purpose. Good fusion splicers are very expensive, costing $15,000 to $40,000, but the splices themselves cost only a few dollars each. Today's single-mode fusion splicers are automated and hardly ever make a bad splice.
Mechanical splices are simple alignment guides that hold the two fibers ends together with some index-matching gel or glue. There are several types of mechanical splices, such as little glass tubes or V-shaped metal clamps. Tool kits that make mechanical splices are inexpensive, but the splices themselves are more costly, about $5 to $15 each.
Many mechanical splices are used for restoration and, with practice, can work well with both single-mode and multimode fiber.
Which splice do you use?
If highest performance is the major concern, fusion splices provide very low loss and back reflection, and are preferred for single-mode, high-speed digital or CATV networks. They’re also more reliable, making them the choice for direct burial, aerial or hard-to-repair places. However, fusion sometimes doesn't work as well on multimode fiber with all the material layers in the core. Mechanical splices are often used for multimode fiber, unless the more-reliable fusion splice is preferred.
If cost matters, you must know the number of splices to be made. Fusion splicing requires expensive equipment but splices are cheap, while mechanical splices offers cheap equipment but expensive splices. If you make a lot of splices (thousands in an big telecom or CATV network) fusion splices are more cost-effective. If you need just a few, use mechanical splices.
Making good splices
Like everything in fiber optics, you need training and practice to make good splices. Whether it’s a fusion or mechanical splice, the key issues are stripping the fiber carefully to prevent scratches or cracks, cleaving the fiber properly to produce a flat end, and keeping everything spotlessly clean.
Fusion splicing is actually simpler, since the machine does the hard work. Training includes learning how to strip fiber properly and use the machine. Every fusion splicer has an expensive fiber cleaver that gives high-quality cleaves every time, regardless of the user’s skill. And the fusion splicer can remove any dust with its electric arc at the “prefuse” stage.
Mechanical splices are harder to make properly. Most tool kits include an inexpensive cleaver that produces good cleaves, but in the hands of a clumsy or untrained installer, are quite good at breaking fibers. Broken fibers do not make good cleaves. Before using one of these cleavers, you need to practice cleaving until you can get a good cleave every time, inspecting the fibers in a microscope for proof. When cleaving in the field, clean the fiber after stripping and inspect after cleaving. If you plan to do many mechanical splices, invest in a $1,000 to $1,500 cleaver, such as those supplied with fusion splicers. They save money in the long run.
Finally, most mechanical splices can be monitored during the process with a visual fault locator, a bright-red laser source. You can actually see the light lost in the joint between the fibers, so you tweak the fiber until you minimize the light loss and you have a good splice. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.