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.
Automated Splicing
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 clam shell protectors.
Evaluating Splices
Good Splices
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. (Graphic from Sumitomo manual)
Bad Splices
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. (Graphic from Sumitomo manual)
Splice Problem Troubleshooting
Here are some common problems and likely causes.
Not Fused Through
Fusion current too low
Prefusion time too short
Matchheads
Contaminated electrodes
Fusion current much too high
Prefusion time much too long
Prefusion current much too high
Autofeed too small
Gap too large
Constriction
Current too high
Feed rate too slow
Prefusion time too long
Prefusion current too high
Gap too wide
Contaminated electrodes
Enlargement
Autofeed too fast
Incorrect current
Bubble or Inclusion
Contaminated fiber end faces
Poor cleave
Fusion current too high
Prefusion current or time too low
Additional Problems
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
Splice Closures
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.
Testing
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.
Ribbon Splicing
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.
Below is the special holder used by the Corning ribbon splicer shown.
The holder is inserted in a special stripper that uses heat to make stripping easier.
After stripping, the holder is placed in a special cleaver that will cleave all 12 fibers at once.
The fixture with all the cleaved fibers is placed in the splicing machine.
When the second ribbon is prepared, the unit is set for automated splicing. The splices are shown being made below.
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.
Virtual Hands On, Fusion Splicing
Virtual Hands On, Ribbon Splicing
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