Fiber Optic Termination With Adhesive/Polish Connectors
Adhesive/Polish Connectors
Terminating optical fibers by attaching connectors with an adhesive and
polishing the ferrules has been used since the beginning of fiber
optics. Dozens of other methods have been developed but most
have not been widely adopted. You can get prepolished/splice
connectors you splice onto the fiber, probably the second most popular termination technique. You can get prefabricated cable assemblies
that need no field termination at all, and their acceptance is growing.
But adhesive polish connectors are preferred because of their
lower loss and higher reliability. Criticism of their termination times
ignore the total process of terminating a fiber optic cable. The total
time involved in termination includes getting to the work site and
setting up, preparing the cable, making terminations, testing, cleaning
up and leaving. Connectors that offer faster termination of an
individual fiber don't save any time on the rest of the processes,
making time savings minimal. Even adhesive curing time becomes
irrelevant when several fibers are being terminated, as each termination
can be allowed to cure while other fibers are being terminated.
Even polishing time is short. In fact, many installers make the
mistake that polishing more is better, when in fact over-polished
connectors will generally show higher loss. Learning to polish the connector the
least amount is best, and using a microscope to inspect the polish is
advised.
Adhesive Choices
There
are three general categories of adhesives used in fiber optics, epoxy,
Hot Melt and anaerobic adhesives. Some epoxies can be cured quickly in
a oven at about 90-100° Celsius while others are designed for room
temperature curing which typically takes 8-12 hours. Hot Melt (a 3M
exclusive) uses an even hotter oven at about 200° Celsius to melt the adhesive so the fiber can be inserted and the allowed to cool to set the adhesive. (The
epoxy and Hot Melt ovens are not interchangeable - epoxy ovens will not
melt Hot Melt adhesive and a Hot Melt oven will ruin epoxies!)
Anaerobic adhesives are fast-setting adhesives that can set instantly
if an accelerator liquid is used, maybe too fast for some installers!
With
all adhesives, one wants to have a reliable adhesion between the fiber
and the ferrule. 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 Termination Process
For all adhesive connectors, the process is similar:
Prepare the cable
Strip the jacket off the cable, exposing an adequate length of buffered fiber for processing
Cut the aramid fibers and other strength members to the proper length
Strip the individual fiber
Prepare the adhesive
Mix epoxy, fill syringe, inject into connector
Apply anaerobic adhesive to fiber or inject into connector
Heat Hot Melt connectors to melt adhesive
Insert fiber into connector
Cure adhesive
Scribe and break (cleave) fiber protruding from connector ferrule
Air polish to grind fiber down to near end of ferrule
Finish
polish with polishing film on top of resilient pad using a polishing
puck to hold the fiber perpendicular to the polishing film
Inspect
Test
Epoxy/Polish Termination
Every
factory-terminated connector uses the epoxy/polish method of
termination. It’s a matter of performance, reliability and economics.
The epoxy/polish type of connector provides the lowest loss, greatest
reliability, highest yield and lowest cost of any termination type.
Another big advantage is the small bead of epoxy on the end of the
ferrule (blue dot in the photo) makes polishing easier - practically
foolproof.
For
singlemode fiber, it is virtually the only method of termination that
can provide the precise end finish necessary for the low loss and
minimal reflectance required for high speed networks. Almost all
singlemode connectors are factory-made with epoxy adhesives and machine
polishing. Field terminations are accomplished by fusion splicing
a factory-made pigtail onto the cable. Field terminations of singlemode
are possible using anaerobic or prepolished/splice connectors, but the
higher reflectance common with field terminations are likely to
be a problem with today’s multi-Gigabit networks.
Termination of
multimode connectors is much less critical, especially where
reflectance is concerned. But low loss is still a big issue, especially
since multimode premises networks often have several patch panels and
patch cords between transmitters and receivers and the maximum
tolerable link loss for high speed networks like gigabit Ethernet and
Fibre Channel is quite low. If an installed network has two
intermediate connections and two patchcords with all connectors
near the maximum loss allowable in the TIA 568 standards – 0.75
dB – the link loss will exceed the maximum loss in some gigabit network
standards.
With typical losses of field-terminated epoxy/polish
connectors around 0.1-0.3 dB, meeting loss budgets is easy. A
well-trained technician using a portable epoxy curing oven can get
almost 100% yield also, reducing the cost of replacing connectors that
fail insertion loss tests. The real secret behind the epoxy/polish
connector is the small bead of epoxy that you create on the end of the
connector ferrule when you inject the adhesive. It creates a hard
extension of the ferrule that holds the fiber perfectly for quick and
easy polishing.
Virtual Hands-On Epoxy/Polish Termination
Hot Melt Adhesive Termination
Another
choice is the 3M Hot Melt. In the Hot Melt connector, the adhesive is
already in the connector. The connector is heated in a very hot oven
(about 200 deg C) so the adhesive melts. The connector is removed from
the oven and the prepared fiber inserted into the connector. The
connector is allowed to cool a few minutes and then it is cleaved and
polished just like a epoxy adhesive connector. The adhesive provides a
nice hard bead for polishing and it has the advantage of reusability -
if you mess up the termination, you can reheat it in the oven and try
again.
Virtual Hands-On Hot Melt Adhesive Termination
Anaerobic Adhesive Termination
Anaerobic
adhesives set much quicker than epoxy and are neater too. Plus these
adhesives are much less expensive than epoxy. A popular adhesive is
Loctite 648. The accellerator for it is Loctite 7471 or 7649.
Several methods are used
for applying an adhesive and some use an “accelerator” or chemical that
makes the adhesive set instantaneously. These methods are:
-Inject adhesive
into the connector and dip the prepared fiber end into the accelerator
before inserting it in the connector. One must work quickly as the
adhesive sets in 30 seconds or so, so if trouble getting the fiber into
the connector is encountered, the adhesive may set before the fiber is
fully inserted.
-Inject adhesive into the connector and then insert
the prepared fiber end into the connector. Spray or drop the
accelerator on the protruding fiber and move the fiber back and forth
to spread the accelerator inside the connector ferrule. It should set in ~30 seconds.
-Wipe the
fiber with the adhesive and insert it in the connector. With no
accelerator, the adhesive will set in 4-5 minutes and be much stronger.
If instant adhesion is required, spray or wipe accelerator on the
protruding fiber as above.
Anaerobic
connectors require more skill
in cleaving and polishing, since they do not have the adhesive bead on
the end of
the ferrule that makes an epoxy/polish or Hot Melt connector so easy to
use. Thus, anaerobic terminations usually have a slightly lower
yield.
Virtual Hands-On Anaerobic Adhesive Termination
Videos on termination on the FOA Channel on 
Termination Time And Cost
What
about the other types of connectors or processes that promise faster
and easier termination? Well, the time it takes to put the connector on
the fiber is not the only time to consider. The installer must set up
at the work site, install the patch panel and any other hardware and
prepare the cable for termination before actually putting the connector
on the fiber itself. After termination, the cable must be connected
into the patch panel and tested then bad terminations fixed and
retested. After all connectors are tested good, the work area must be
cleaned up and all the tools repacked.
Demonstrations of
quick-connect connectors show how easy it is to put a connector on one
fiber in a minute or less. But they forget to mention all the other
activities at the work site, like setup and cleanup time, preparing
cable ends for termination and they only consider one connector. If you
are terminating, for example, a 24 fiber cable with epoxy/polish
connectors using a curing oven in a “production line,” the effective
termination time is only a couple of minutes per connector. You inject
epoxy into several connectors at one time, strip a fiber and attach a
connector, then put it in the oven to cure for 5 minutes or so. While
it cures, you attach more connectors. By the time you fill the oven up,
the first one is cured and ready to polish. If you have more to do, you
replace it in the oven with another ready to cure until all are cured,
then you polish them all at once, at about a minute each. With this
process, the epoxy curing time becomes irrelevant.
Which brings us
to the bottom line. Connectors for epoxy/polish or anareobic
terminations are available today in quantity for not much over $1 each.
Quick connect connectors sell for 5 to 15 times more, depending on the
type and quantities. When the total cost of termination is properly
calculated, including the realistic time necessary to set up before
making terminations and cleaning up afterwards, saving a few minutes
labor per connector may not offset the higher cost of the connector
itself.
So are epoxy/polish connectors obsolete? Not on your life.
Lots of smart contractors still use them and end users still specify
them, knowing they are still the highest quality solution to fiber
optic termination.
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