Fusion splicing is the process of fusing or welding two fibers together usually by an electric arc. Fusion splicing is the most widely used method of splicing as it provides for the lowest loss and least reflectance, as well as providing the strongest and most reliable joint between two fibers.
Virtually all singlemode splices are fusion. Multimode fibers can be harder to fusion splice as the larger core with many layers of glass that produces the graded-index profile are sometimes harder to match up, especially with fibers of different types or manufacturers.
Fusion splicing may be done one fiber at a time or a complete fiber ribbon from ribbon cable at one time. First we'll look at single fiber splicing and then ribbon splicing.
Fusion splicing machines are mostly automated tools that require you preset the splicing parameters or choose factory recommended settings that will control the splicing process itself. All require the use of a precision fiber cleaver that scribes and breaks (cleaves) the fibers to be spliced precisely, as the quality of the splice will depend on the quality of the cleave. Most splicing machines come with a recommended cleaver.
Proper use of both the splicing machine and the cleaver require carefully following the manufacturer's directions. Each manufacturer's product is slightly different and requires somewhat different procedures. Reading the manuals and practice with the machine are important, especially if the operator has not been trained on the particular splicer in use.
Automatic Fiber Alignment
The ends of the fibers are on moveable stages which are used to align the fibers and set the end gap automatically. During the automated process, the splicer will align the fibers using one of two methods:
Optical Core or Profile Alignment Systems (PAS)
Optical Core Alignment (also called “Profile Alignment”), an optical alignment technique, is used by many models of fusion splicers. The two fibers are illuminated from two directions, 90 degrees apart. From the images in a video camera, software recognizes the core of the fibers and aligns them automatically using movable stages. The software also estimates splice loss after the fusion splicing is complete. Ribbon splicers typically use profile alignment.
Local Injection and Detection (LID System)
LID Core Alignment uses “Local Injection and Detection” of light. Light is coupled into the fiber by bending the fiber and shining a light source (LED or laser) on the outside of one fiber, so some light is coupled into the core. On the other fiber, the bend causes macrobending losses that are measured by a photodetector, providing a relative indication of light transmission through the splice. The splicer measures light coupling through fiber while moving fibers on actuators to get best transmission which means the fibers are optimally aligned. The LID system also checks transmission after splicing to estimate splice loss.
Both techniques work well with most fibers. Refer to the instruction manual or ask the manufacturer is there is any question about using the splicer with the fiber you are installing.
Fusion Splicing Machines
Fusion splicing requires stripping a longer length of bare fiber than termination, so the choice of stripper is important. There are three types of fiber strippers available, known as (from Left) the Miller Stripper, No-Nik and Micro-Strip. All three can work equally well, and most techs choose the one they are most familiar with. The Miller, perhaps the most rugged, has the disadvantage of being "right-handed." The Micro-Strip allows setting strip length for consistent strips. The No-Nik is careful with the fiber but requires careful cleaning. Most strippers are "sized" for the fiber coatings to be removed, so ensure you have the proper stripper for the fiber being stripped. Whichever stripper is used, care must be taken to not nick the fiber during the stripping process as it can cause cracks that may lead to fiber failure sometime in the future. Strippers require careful cleaning and immediate replacement if they become damaged or worn. Splicing machines also generally have a heating device for heat shrinking a protective sleeve over the finished splice to protect it from moisture or other environmental hazards. An alternate method using clamp-on protectors.