Topic: Fiber Optic Terminations |
Table of Contents: The FOA Reference Guide To Fiber Optics |
Fiber Optic Terminations |
| ST (an AT&T Trademark) is the one of the most popular connectors for multimode networks, like most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. And because they are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference. |  |
| FC/PC has been one of the most popular singlemode connectors for many years. It screws on firmly, but make sure you have the key aligned in the slot properly before tightening. It's being replaced by SCs and LCs. |  |
| SC is a snap-in connector that is widely used in singlemode systems for it's excellent performance and multimode systems because it was the first connector chosen as the standard connector for TIA-568 (now any connector with a FOCIS standard is acceptable.) It's a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration. |  |
| The
ST/SC/FC/FDDI/ESON connectors have the same ferrule size - 2.5 mm or
about 0.1 inch - so they can be mixed and matched to each other using
hybrid mating adapters. This makes it convenient to test, since you can
have a set of multimode reference test cables with ST or SC connectors
and adapt to all these connectors. |
 From the top: ST>FC SC>FC SC>ST |
| LC uses a 1.25 mm ferrule, half the size of the ST. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode and the connector of choice for multimode transceivers for gigabit speeds and above, including multimode Ethernet and Fibre Channel. |  |
| MT-RJ is a duplex connector with both fibers in a single polymer ferrule. It uses pins for alignment and has male and female versions. Multimode only, field terminated only by prepolished/splice method. Basically obsolete. |
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| Opti-Jack is a neat, rugged duplex connector cleverly designed around two ST-type ferrules in a package the size of a RJ-45. It has male and female (plug and jack) versions. |  |
| Volition is a simple, inexpensive duplex connector that uses no ferrule at all. It aligns fibers in a V-groove like a splice. Plug and jack versions, but one can field terminate jacks only. |  |
| MU looks a miniature SC with a 1.25 mm ferrule. It's more popular in Japan. |  |
|
MTP/MPO is a 12 fiber connector for ribbon cable. It's main use is for preterminated cable assemblies.
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Here is an even more comprehensive guide to fiber optic connectors, including obsolete ones.
The ST/SC/FC/FDDI/ESCON connectors have the same ferrule size - 2.5 mm or about 0.1 inch diameter- so they can be mixed and matched to each other using hybrid mating adapters. This makes it convenient to test, since you can have a set of multimode reference test cables with ST connectors and adapt to all these connectors. Likewise, the LC and MU use the same ferrule (1.25 mm diameter) so mating is possible.
Multimode Terminations: Several different types of terminations are available for multimode fibers. Each version has its advantages and disadvantages, so learning more about how each works helps decide which one to use.
Singlemode Terminations:
Singlemode fiber requires different connectors and polishing techniques
that are best done in a factory environment. Consequently most SM fiber
is field terminated by splicing on a factory-terminated pigtail or using prepolished/splice connectors. Singlemode
termination requires special connectors with much tighter tolerances on
the ferrule, especially the hole for the fiber. Polishing requires
special diamond polishing film on a soft rubber pad and a polishing
slurry to get low reflectance. But you can put SM connectors on
in the field if you know what you are doing. Expect higher loss
and high reflectance.
Adhesive Terminations:
Most connectors use epoxies or other adhesives to hold the fiber in the
connector ferrule and polish the end of the fiber to a smooth finish. Follow
termination procedures carefully, as they have been developed to
produce the lowest loss and most reliable terminations. Use only the
specified adhesives, as the fiber to ferrule bond is critical for low
loss
and long term reliability! We've seen people use hardware store
epoxies, Crazy Glue, you name it! And they regretted doing it. Only
adhesives approved by manufacturers or other distributors of connectors
should be used. If the adhesive fails, not unusual when connector ferrules
were made of metal, the fiber will "piston" - sticking out or pulling
back into the ferrule - causing high loss and potential damage to a
mated connector.
The
polishing process involves three steps which only takes a minute: "air
polishing" to grind down the protruding fiber, polishing on a soft pad
with the fiber held perpendicular to the polishing surface with a
polishing puck and a quick final fine polish.
Epoxy/Polish: Most connectors, including virtually all factory made terminations, are the simple "epoxy/polish" type where the fiber is glued into the connector with epoxy and the end polished with special polishing film. These provide the most reliable connection, lowest losses (less than 0.5 dB) and lowest costs, especially if you are doing a lot of connectors. The small bead of hardened epoxy that surrounds the fiber on the end of the ferrule even makes the cleaving and polishing processes much easier - practically foolproof. The epoxy can be allowed to set overnight or cured in an inexpensive oven. A "heat gun" should never be used to try to cure the epoxy faster as the uneven heat may not cure all the epoxy or may overheat some of it which will prevent it ever curing. Don't use "Hot Melt" ovens either, as they use a much higher temperature and will ruin the epoxy.
"Hot Melt" Adhesive/Polish: This
is a 3M trade name for a connector that already has the epoxy (actually
a heat set glue) inside the connector. You insert the connector in a special oven. In a few
minutes, the glue is melted, so you remove the connector, insert the stripped fiber, let it cool
and it is ready to polish. Fast and easy, low loss, but not as cheap as
the epoxy type, it has become the favorite of lots of contractors who
install relatively small quantities of connectors. Remember you need a
special Hot Melt oven, as it needs a much higher temperature than is
used for curing epoxy.
Anaerobic Adhesive/Polish:
These connectors use a quick setting "anaerobic" adhesive to replace
the epoxy or Hot Melt adhesive that cures faster than other types of
adhesives. They work well if your technique is good, but some do not
have the wide temperature range of epoxies. A lot of installers are
using Loctite 648, with or without the accelerator solution (Loctite 7471 or 7649), that is
neat and easy to use.
More on processes used for adhesive/polish connectors.
Video on adhesive/polish termination can be found on the FOA Channel on 
Crimp/Polish: Rather than glue the fiber in the connector, these connectors use a crimp on the fiber to hold it in. Early types offered "iffy" performance, but today they are pretty good, if you practice a lot. Expect to trade higher losses for the faster termination speed. And they are more costly than epoxy polish types. A good choice if you only install small quantities and your customer will accept them.
Prepolished/splice: (also called "cleave & crimp"):
Some manufacturers offer connectors that have a short stub fiber
already epoxied into the ferrule and polished and a mechanical splice
in the back of the connector, so you just
cleave a fiber and insert it like a splice, a process which can be done
very quickly. (See next section for splicing info.) Several connectors
use a fusion splice instead of a mechanical splice to attach the
connector.
This
method has
both good and bad points. The manufacturing process is complex so these
connectors cost several times as much as an adhesive/polish
type, since they require careful manufacturing. Some of that extra cost
can be recovered in lower labor costs for installation since they
install very fast. You must
have a
good cleave on the fiber you are terminating to make them low loss, as
the fiber cleave is a major factor in the loss of a mechanical splice.
Using a high quality
cleaver like those used for fusion splicing, available from some
manufacturers as part of their termination kits, is recommended. Even
if you do
everything correctly, loss will be somewhat higher, because you
have a connection loss plus a splice loss in every connector. The
best way to terminate them is to verify the loss of the splice with a
visual fault
locator and "tweak" them. Some kits now have both a quality cleaver and
a tool with VFL that verifies the termination. (Photo: Corning, Unicam)
More on prepolished/splice connectors.
Read more about termination processes and view the actual processes involved in termination with "Virtual Hands-On" tutorials. See the Table of Contents for listings of termination types under Components.
Hints for doing field terminations
Here are a few things to remember when you are terminating connectors in the field. Following these guidelines will save you time, money and frustration.
Choose the connector carefully and clear it with the customer if it is anything other than an epoxy/polish type. Some customers have strong opinions on the types or brands of connectors used in their job. Find out first, not later!
Never, never, NEVER take a new connector in the field until you have installed enough of them in the office that you can put them on in your sleep. The field is no place to experiment or learn! It'll cost you big time!
Have the right tools for the job. Make sure you have the proper tools and they are in good shape before you head out for the job. This includes all the termination tools, cable tools and test equipment. Do you know your test cables are good? Without that, you will test good terminations as bad every time. More and more installers are owning their own tools like auto mechanics, saying that is the only way to make sure the tools are properly cared for.
Dust and dirt are your enemies. It's very hard to terminate or splice in a dusty place. Try to work in the cleanest possible location. Use lint-free wipes (not cotton swaps or rags made from old T-shirts!) to clean every connector before connecting or testing it. Don't work under heating vents, as they are blowing dirt down on you continuously.
Don't overpolish. Contrary to common sense, too much polishing is just as bad as too little. The ceramic ferrule in most of today's connector is much harder than the glass fiber. Polish too much will cause undercutting of the fiber and you create a concave fiber surface not convex as it should be, increasing the loss. A few swipes is all it takes.
Change polishing film regularly. Polishing builds up residue and dirt on the film that can cause problems after too many connectors and cause poor end finish. Check the manufacturers' specs.
Put covers on connectors and patch panels when not in use. Keep them covered to keep them clean.
Inspect and test, then document. It is very hard to troubleshoot cables when you don't know how long they are, where they go or how they tested originally! So keep good records, smart users require it and expect to pay extra for good records.
Not necessarily. Many manufacturers offer prefabricated fiber optic cabling systems for both premises and outside plant systems. In fact, the largest application for prefabricated systems is fiber to the home (FTTH) where it saves a tremendous amount of time in installation and cost. Using prefab systems requires knowing precisely where the cable will be run so cable lengths can be specified. Using CAD systems and design drawings, a complete fiber optic cabling system is designed to the customer's specifications and built in a factory using standard components. Early prefabricated systems (some are still available) simply terminated cables with standard connectors like STs or SCs and put them inside a plastic pulling boot with a pulling loop attached to the fiber strength members. The cable would be placed with the boot in place then removed to connect into patch panels.
Today, it's more common to use backbone cables terminated in small multifiber MTP connectors that are pulled from room to room and connected to rack-mounted modules that have MTP connectors on the back and single fiber connectors on the front (see photo of Fiberware system.) Like everything else, there are tradeoffs. The factory-assembled connectors usually have lower loss than field terminations but the MTP connectors, even factory assembled, are not low loss, so the total loss may be more than field terminated systems. Costs also involve tradeoffs, as factory systems are more expensive for the components but installation time is much less.
In new facilities, considering prefabricated systems is always a good idea, but all factors
should be considered before making a decision.
More on connectors and termination, including hands-on tutorials.
Videos on fiber optic termination on the FOA Channel on
Splicing
is more common in outside plant (OSP) applications than premises
cabling, where most cables are pulled in one piece and directly terminated. Splicing is only
needed if the cable runs are too long for one straight pull or you need
to mix a number of different types of cables (like bringing a 48 fiber
cable in and splicing it to six 8 fiber cables.) And of course, we
often use splices for OSP restoration, after the number one problem of
outside plant cables, a dig-up and cut of a buried cable, usually
referred to as "backhoe fade" for obvious reasons!
Splices: fusion on the far left, other types of mechanical splices.
Splices are "permanent" connections between two fibers. There are two types of splices, fusion and mechanical, and the choice is usually based on cost or location. Most splicing is on long haul outside plant SM cables, not multimode LANs, so if you do outside plant SM jobs, you will want to learn how to fusion splice. If you do mostly premises cabling like MM LANs, you may never see a splice.
Fusion splicing is most widely used as it provides for the lowest loss and least reflectance, as well as providing the strongest and most reliable joint. Virtually all singlemode splices are fusion. Mechanical splicing is used for temporary restoration and for most multimode splicing. Read more on splicing below.
Fusion Splices
are made by "welding" the two fibers together usually by an electric
arc. To be safe, you should not do that in an enclosed space like a manhole
or an explosive atmosphere, and the equipment is too bulky for most
aerial applications, so fusion splicing is usually done above ground in
a truck or trailer set up for the purpose. (photo above) Today's singlemode
fusion splicers are automated and you have a hard time making a bad
splice as long as you cleave the fiber properly. The biggest application is singlemode fibers in outside plant
installations. Fusion splices are so good today that splice points may
not be detectable in OTDR traces. Some splicing machines can do one fiber at a time but Mass Fusion Splicers can do all 12 fibers in a ribbon at once. Fusion splicers cost $15,000 to $40,000, but the splices only cost a few dollars each.
Video on fusion splicing can be found on the FOA Channel on
Mechanical Splices are alignment gadgets that hold the ends of two fibers together with some index matching gel or glue between them. There are a number of types of mechanical splices, like little glass tubes or V-shaped metal clamps. The tools to make mechanical splices are cheap, but the splices themselves are more expensive. Many mechanical splices are used for restoration, but they can work well with both singlemode and multimode fiber, with practice - and using a quality cleaver such as those used for fusion splicing.
More on mechanical splicing.
Which Splice ?
If cost is the issue, we've given you the clues to make a choice: fusion requires expensive equipment and but makes cheap splices, while mechanical splices require inexpensive equipment and expensive splice hardware. So if you make a lot of splices (like thousands in an big telco or CATV network) use fusion splices. If you need just a few, use mechanical splices.
Fusion splices give very low back reflections and are preferred for singlemode high speed digital or CATV networks. However, they may not work well some multimode fibers, so mechanical splices may be preferred for MM, unless it is an underwater or aerial application, where the greater reliability of the fusion splice is preferred.
Making Good Splices
Making
consistently low loss splices depends on proper techniques and keeping
equipment in good shape. Cleanliness is a big issue, of course. Fiber
strippers should be kept clean and in good condition and be replaced
when nicked or worn. Cleavers are most important, as the secret to good
splices - either fusion or mechanical - is having good cleaves on both
fibers. Keep cleavers clean and have the scribing blades aligned and
replaced regularly. Fusion splicers should be properly maintained and
fusing parameters set for the fibers being spliced. For mechanical
splices, light pressure on the fiber to keep the ends together while
crimping is important. Use a visual fault locator (VFL) to optimize the splice before crimping if possible.
More on splices, including hands-on tutorials
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Connector and splice loss is caused by a number of factors. Loss is minimized when the two fiber cores are identical and perfectly aligned (
Since
fiber optic technology was introduced in the late 70s, numerous
connector styles have been developed - probably over 100 designs. Each new design was meant to
offer better performance (less light loss and reflectance) and easier,
faster and/or more inexpensive termination.