Fiber Optic Fusion Splicing
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.
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.
In addition to the splicer and cleaver, the tech doing the splicing will need a set of cable preparation and fiber stripping tools. Since much fusion splicing is done in the outside plant, the splicing tech should have tools to handle all types of loose tube cable, both gel-filled and dry water-blocked, with various jacket styles, armor, etc.
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.
The Fusion Splicing Process
Prepare the cables to be spliced (VHO on cable preparation)
Strip jacket, removing an adequate amount of jacket, usually 2-3 m, for splicing and dressing the buffer tubes and fibers in the splice closure. Leave the proper amount of strength members to attach the cable to the closure. Refer to the splice closure directions for lengths needed. Clean all water-blocking materials using appropriate cleaners.
Remove buffer tubes exposing fibers for splicing. Generally splice closures will require ~1 m buffer tubes inside the closure to and ~ 1 m fiber inside the splice tray. Clean all water-blocking materials.
Prepare the fibers to be spliced
- The process is the same for all splice types: strip, clean & cleave.
- Each fiber must be cleaned thoroughly before stripping for splicing.
- When ready to splice a fiber, strip off the buffer coating(s) to expose the proper length of bare fiber
- Clean the fiber with appropriate wipes
- Cleave the fiber using the process appropriate to the cleaver being used
- Place the fiber into the guides in the fusion splicing machine and clamp it in place
- Repeat for the other fiber to be spliced
- Running the splicer program
- Choose the proper program for fusion splicing the fiber types being spliced
- The splicer will show the fibers being spliced on the video screen.
- Fiber ends will be inspected for proper cleaves and bad ones like the one on the right above will be rejected.
- Fibers will be moved into position
- Prefuse cycle will remove any dirt on the fiber ends and preheat the fibers for splicing
- The fibers will be aligned using core alignment method for that splicer
- The fibers will be fused by an automatic arc cycle that heats them in an electric arc and feeds the fibers together at a controlled rate
- When fusion is completed, the splicing machine will inspect the splice and estimate the optical loss of the splice. It will tell the operator if a splice needs to be remade.
- The operator will remove the fibers from the guides and attach a permanent splice protector by heat-shrinking or clamping clamshell protectors.
Visually inspect splice after the program has run, using both X and Y views. Some flaws that do not affect optical transmission are acceptable, as shown. Some fibers (e.g. fluorine-doped or titanium coated) may cause white or black lines in splice region that are not faults.
Some flaws are unacceptable and require starting the splicing process over. Some, like black spots or lines, can be improved by repeating the ARC step, but never more than twice. For large core offsets, bubbles or bulging splices, always redo.
Splice Problem Troubleshooting
Here are some common problems and likely causes.
- Not Fused Through
- Fusion current too low
- Prefusion time too short
- Contaminated electrodes
- Fusion current much too high
- Prefusion time much too long
- Prefusion current much too high
- Autofeed too small
- Gap too large
- Current too high
- Feed rate too slow
- Prefusion time too long
- Prefusion current too high
- Gap too wide
- Contaminated electrodes
- Autofeed too fast
- Incorrect current
- Bubble or Inclusion
- Contaminated fiber end faces
- Poor cleave
- Fusion current too high
- Prefusion current or time too low
Fusion splicers generally have stored programs for most fibers and the user can modify those program parameters or create new ones. Refer to the instruction manual or ask the manufacturer is there is any question about using the splicer with the fiber you are installing. It is sometimes necesary to splice older fibers, either in restoration or modifying networks. Older fibers may become brittle and hard to strip
After fibers are spliced, they will be placed in a splice tray which is then placed in an splice closure. Outside plant closures will be carefully sealed to prevent moisture damage to the splices. The closure placed in a designated protected place to complete the installation.
All cables that contain metallic elements like armor or strength members must be grounded and bonded at each splice point. Closures are designed to clamp cable strength members to provide strength to prevent pulling the cable out and seals to prevent moisture damage to the splices.
Fusion splicers are used to create long cable lengths by splicing multiple cable segments. Although the splicer will give an estimate of the splice loss, the only way to test it is with an OTDR. Since OTDRs have directional errors, testing may be required from both directions and averaged. Generally long concatenated cables are tested with an OTDR and traces kept for documentation in case of restoration.
Many high fiber count cables today are made from ribbons of fibers, usually 12 fibers per ribbon. Splitting all those fibers out to splice individually would be time consuming, so ribbon fusion splicers, also called mass fusion splicers, can splice entire ribbons at one time, creating a splice that looks like this.
Ribbon splicers look similar to single fiber splicers and work in much the same way, except the ribbons are treated as one assembly, stripped, cleaved and spliced by special tools while held in a special holder.
Fusion Splicer Maintenance
All fusion splicers have maintenance requirements which should be described in the operating manual. Besides cleaning regularly, they require electrode alignment and occasional replacement. Follow manufacturer’s requirements for servicing.