| Choosing the Right Mechanical Splice
by Larry Johnson---published in Outside Plant April 1997
The selection of a mechanical splice for fiberoptics is important for matching the product to
the application. Like many fiberoptic products, present technology and past history have
evolved to provide the end user with both quality splices and affordable pricing that
continues to decline.
The use of mechanical splices evolves from the early years of fiber installations in the late
1970s. Fusion splicers were primitive at best, and fibers had not yet been standardized. In
fact, until 1982, all fibers used in telephony installation used the multimode 50/125 fiber.
Fiber tolerances at this time were on the scale of 125+/-6 microns and issues such as
centering of the core, ovality, and coatings were still being addressed as to what
techniques were used in manufacturing and what test equipment would be required to
measure these parameters. Fiber splicing is critical for singlemode fibers due to the small
core and mode field diameter through which the optical energy is transmitted. Early fibers
exhibited problems caused by small fiber cores and poor fiber tolerances.
The first generation of mechanical splices were able to be manually "tuned" to align the
cores of the fibers to obtain the lowest loss. Many of the "old time" fiber splicers still
remember the skills required to tune a mechanical splice with the OTDR operator working in
a "real time" function. In fact, much of present fiber terminology reflects back on these
splicing and testing techniques.
Today's optical fibers have extremely tight tolerances of less than 1 micron on outside
diameter ((O.D.).), concentricity, ovality and centering. Measurement techniques and
equipment have evolved to measure these parameters over the years. This has made the
use of the mechanical splice a good, low-cost product in many fiber applications. Fiber
manufacturers can accurately measure fiber core and cladding diameters to 0.05 microns,
core non-circularity of less than 1%, cladding non-circularity of less than 0.1%, and
core/cladding concentricity of less than 0.04 microns. With equipment capable of measuring
these tolerances, fiber manufacturers can produce quality fibers, allowing mechanical
splices to easily splice single-mode fibers.
Roles of the Mechanical Splice
Today, the need for splicing actually applies to four different applications. These are span
splicing, pigtail splicing, acceptance testing, and emergency restoration. Each of these is
unique in its own nature and may require different types of splices. Let's look at what
issues one needs to address when selecting a mechanical splice.
1. What is the insertion loss?
Probably the most basic question, but one that needs to be asked. Interpret the loss and
how it is measured. For example, is the loss specified the best loss or an average or
"mean" loss? What is the maximum value? Remember that factory splice measurements are
made with similar fibers measured through an optical test bench in a controlled
environment. In the field, the fibers will have different tolerances and are not identical.
2. What is the technique used for holding, bonding or fixing the fibers in place?
Through the years, many techniques have been developed to align and fix fibers together.
In the early years, this was primarily through the use of "elastomers, ultraviolet (UV)
adhesives and V-grooves. Today's tight fiber tolerances use unique variations of V-groove
technology to align the fibers and clamping methods to hold the fibers in place. If UV
adhesives are used, be sure to always check the expiration date code prior to use.
3. What is the reflection specification?
With many analog (CATV) applications, reflections can cause degradation of the circuit
quality. All mechanical splices are reflective. How reflective are they and what value will the
system be able to tolerate? Manufacturers use index matching fluids or gels to reduce the
reflection value in their splices; however, only a check against system performance
specifications will give you a reflection value.
Tip of the day:
The higher the back reflection number, the lower the actual reflection. This is measured in
dB.
Specification EIA 455-107, the standard for measuring back reflections for fiberoptics, is
being updated to resolve many of the questions surrounding component (Fresnel) and
system (Fresnel and Rayleigh) reflections. A Standards Proposal EIA-SP 2961* Revision A,
dated May 22, 1996, is available in draft format from Global Engineering Documents, (800)
854-7179. The title is "Determination of Component Reflectance of Link/System Return Loss
Using a Loss Test Set." Since this document is in the draft stage, users must beware of
these equipment manufacturers who are measuring reflections without traceability to an
agreed-upon EIA Fiber Optic Test Procedure (FOTP).
4. What is the physical size of the splice?
The splice you choose must be mounted in splice trays, which are themselves mounted in
either splice closures or in various types of panels. You need to match the dimensions with
the tray manufacturer you intend to use. Most splices are the same physical sizes, but a
few require very specific trays. The tray manufacturers usually have all types, but it is
better to check for compatibility prior to your purchase.
5. Does the splice require a special cleave length?
Most cleaving tools are adjustable to allow for use with most splices, whether fusion or
mechanical. It is best to check prior to your first use. Depending on whether you are
splicing 250-micron outdoor coated fiber or 900- micron coatings used on most pigtails, this
length may differ.
6. What tooling is required?
First, a cleave tool that matches your performance require- meets. For exam- pie, if you are
splicing single-mode fibers, you will want a splice tool with a diamond blade with
adjustable length settings. Remember that the performance of the splice depends on the
quality of the cleave. If you're using the splice for acceptance testing and even emergency
restorations, you may be able to use a simpler and less expensive tool. Secondly, does the
splice require any additional tool fixtures or tools? Evaluate these and see what is required.
Kits are usually available with the basic splicing tools for the splice type to be used.
7. What about consumables?
Some splices require adhesives, index matching fluids, and polishing supplies as well. See
if there are any of these items. Are there any minimum requirements?
8. Can the splice be reused?
Are you going to use the splice for acceptance testing or benchtop work? If so, you should
ask how many times the splice can be used. With most splices this is limited by the index
matching gel or fluid being gradually depleted after each fiber test. My experience in
acceptance testing is Norland's UVC splice, which can be refilled easily with fluid. Norland
Products manufactures a fiber acceptance kit for OTDRs.
9. What is the cost?
Mechanical splices now cost as low as $6.00 each. Depending on the application and splice
count, this may prove to be a low-cost solution for many applications.
10. What skills are required to work with this splice?
Is the tooling simple and easy to understand and use? Does the splice require any special
handling procedures or techniques? If you have a problem, can you fix it in the field?
Emergency Splicing With Mechanical Splices
Today, the mechanical splice is considered the standard for emergency restorations due to
its low signal attenuation and simplicity. Present mechanical splices perform so well that
"blind splicing" during an emergency is considered a normal practice. Blind splicing means
not monitoring with the use of the OTDR. This is critical to many users who may not own a
OTDR but still have fiber networks. Note: If a OTDR is available, we still recommend its use
during any type of splicing scenario for proof of performance documentation.
Emergency restoration is one area in which mechanical splices dominate the industry. Their
simplicity and performance levels demonstrate that users are quite familiar with the product
in high performance and critical circuit applications. In emergency restorations, splice time
is critical. Mechanical splices win because of their simplicity, low loss and no need for
external power supply.
"Today, the mechanical splice is considered the standard for emergency restorations due to
its low signal attenuation and simplicity."
In addition, if you are splicing two or more splice points simultaneously, you can easily
have two splice tool sets at a negligible cost. This really pays off when you have a cable
restoration without retrievable slack. With two splice kits, cleaving tools, cable preparation
tools and trained technicians, today's users can easily perform emergency restorations at a
low cost when compared to the use of fusion splicing.
Cost Issues
Cost tradeoffs in mechanical vs. fusion splicing techniques occur from between 1,000 and
1,600 splices when comparing low-end, manual style fusion splicers and 4,000 splices when
comparing to Profile Alignment System (PAS) and Local Injection-Detection (LID) fusion
splicers.
The cost of the installation, closure and panel, and splice tray preparation is the same for
either splice technique.
Mechanical splices are one of the most cost-effective solutions for many fiberoptic users.
Their universal acceptance for emergency restorations demonstrates the faith that end
users put in today's generation of of mechanical splices. Simplified tooling, processes and
low cost make mechanical splices a viable splice technique in many applications. |
