Mechanical
splices are used to create permanent joints between two fibers by
holding the fibers in an alignment fixture and reducing loss and
reflectance with a transparent gel or optical adhesive between the
fibers that matches the optical properties of the glass. Mechanical
splices generally have higher loss and greater reflectance than fusion
splices, and because the fibers are crimped to hold them in place, do
not have as good fiber retention or pull-out strength. The splice
component itself, which includes a precision alignment mechanism, is
more expensive than the simple protection sleeve needed by a fusion
splice. Mechanical splices are most popular for fast, temporary
restoration or for splicing multimode fibers in a premises
installation. They are also used - without crimping the fibers - as
temporary splices for testing bare fibers with OTDRs or OLTSs. Of
course most prepolished splice connectors
use an internal mechanical splice (several actually have fusion
splices) so the mechanisms and techniques described here apply to those
also. The advantage of mechanical splices is they do not need an
expensive machine to make the splices. A relatively simple cleaver and
some cable preparation tools are all that's needed, although a visual
fault locator (VFL) is useful to optimize some types of splices.
Alignment Mechanisms The biggest difference between mechanical splices is the way the fibers are aligned. Here are some typical methods.
Capillary Tube
The
simplest method of making a mechanical splice is to align two fibers in
a small glass tube with a hole just slightly larger than the outside
diameter of the fibers. This type of splice works well with UV-cured
adhesive as well as index-matching gel between the fibers. The
Ultrasplice is a capillary splice.
V-Groove V-groove
splices are quite simple and work well. They work for single fibers or
even for fiber ribbons as shown here. The Grooved alignment plates can
be made of many types of materials and are quite inexpensive.
The
3M Fiberlok is a version of a V-groove splice that uses a metal
stamping inside a plastic case to both align fibers and crimp them.
It's elegant design and good performance has made it one of the most
popular mechanical splices. This
method has a more complex alignment mechanism, made from four small
glass rods fused together with a bend in the middle. The fibers follow
the grooves made by the joint of two rods. The complexity and expense
of this, especially compared to a simple V-groove, limited its use.
Elastomeric The
GTE Elastomeric splice (still available from Corning) uses soft
elastomers to hold the fibers in position. It's similar to a v-groove,
but the grooves are soft so they accomodate slight variations in fiber
diameter easily.
Rotary Splice The
AT&T Rotary splice was more like a connector. The fibers were glued
into glass ferrules and polished. They were then inserted into an
alignment sleeve and rotated until the lowest loss was obtained. Again,
complexity and cost, plus labor required, limited their popularity.
Cleaving Is Important The
most important step in mechanical splicing is cleaving the fiber
properly. Most mechanical splicing kits come with an inexpensive
cleaver that looks like a stapler.
While
this cleaver can produce acceptable results, its operation requires
some practice and consistent use. The same can be said of all
inexpensive hand-held cleavers. A better choice is one of the more
expensive cleavers used for fusion splicers. It is more expensive but
will usually pay back its cost quickly in higher yield. It is
helpful to have a microscope capable of inspecting fiber ends after
cleaving to determine if the cleave will yield good splices. Here are
examples of good and bad cleaves.
Mechanical Splicing Process Cable and fiber preparation is practically the same as for fusion splicing.
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 lenths 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.
Splicing
Insert
the first fiber into the mechanical splice. Most splices are designed
to limit the depth of the fiber insertion by the buffer coating on the
fiber.
Clamp the fiber in place if fibers are held separately. FiberLok splices clamp both fibers at once.
Repeat these steps for the second fiber.
Optimizing Splices Using A Visual Fault Locator
You
can sometimes improve the loss of a mechanical splice by gently
withdrawing one of the fibers a slight amount, rotating it slightly and
reinserting it. It works best with a VFL (visual fault locator) if the
fiber ends that are being spliced are visible.
Shine a visual fault locator into the fiber and note the light loss at the splice (Left in photo). Pull one fiber out by 1-2 mm (about 1/16 inch.) Rotate the fiber slightly and reinsert fully. Keep trying and watch for minimal light (Right in photo.) Crimp fiber in place.
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. In premises applications, some
patch panels have provision for splices in the back, simplifying their
storage. 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 Splices can be used to create long
cable lengths by splicing multiple cable segments. After splicing, 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.
Be aware of the OTDR distance resolution as a limitation of testing
short premises cables.
