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Reference Guide To Fiber Optics

Topic: Mechanical Splices Table of Contents: The FOA Reference Guide To Fiber Optics

Mechanical Splices

Splices, from left, fusion splice, Elastomeric, Ultrasplice, Camlock, FiberLok, AT&T Rotary Splice

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
capillary splice

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 spllice
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.

Fiberlok splice
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.
fused glass rods
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.

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
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.

cleaver - cheap
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.

cleave examples

Mechanical Splicing Process
Cable and fiber preparation is practically the same as for fusion splicing.

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 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.


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.

VFL splice optimization

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.

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.

Virtual Hands On, Mechanical Splicing  

Videos on mechancial splicing on the FOA Channel on FOA videos on You Tube

Table of Contents: The FOA Reference Guide To Fiber Optics


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