Fiber Optic Terminations 
Fiber Optic Connectors (L) and Splices in Splice Tray (R)
Fiber
optic joints or terminations - where cables are terminated -
are made two ways: 1) connectors that mate
two fibers to create a temporary joint and/or connect the fiber to a
piece of network gear (left) or 2) splices which create a
permanent joint between the two fibers (right). Either termination
method must have two primary characteristics: good optical performance (low loss and minimal reflectance) and high
mechanical strength. Terminations
must also be of the right style to be compatible to the equipment involved and be protected against the
environment in which
they are installed.
Probably
fiber optic coponent has been given greater attention than connectors.
Manufacturers have come up with over 80 styles of connectors and about
a dozen different ways to install them. There are only two types of
splices but numerous ways of implementing them. Fortunately for both
manufacturers and installers, only a few types of either are the ones
used for most applications.
Most
fiber optic connectors are plugs or so-called male connectors with a
protruding ferrule that holds the fibers and aligns two fibers for
mating. They use a mating adapter to mate the two connectors that fits
the securing mechanism of the connectors (bayonet, screw-on or
snap-in.) The ferrule design is also useful as it can be used to
connect directly to active devices like LEDs, VCSELs and detectors.
Different connectors and termination procedures are used
for singlemode and multimode connectors. Multimode fibers are
relatively easy to terminate, so field termination is generally done by
installing connectors directly on tight buffered fibers using the
procedures outlined below. Most field singlemode
terminations are made by splicing a factory-made pigtail onto the
installed cable rather than terminating the fiber directly as is
commonly done with multimode fiber. Singlemode terminations require
extreme care in assembly, especially polishing, to get good performance
(low loss and reflectance), so they are usually done in a clean
manufacturing facility using heat-cured epoxy and machine polishing.
Choosing
a connector type for any installation should consider if the connector
is compatible with the systems planned to utilize the fiber optic cable
plant, if the termination process is familiar to the installer and if
the connector is acceptable to the customer. If the systems are not yet
specified, hybrid patchcords with different connectors on each end may
be necessary. If the installer is not familiar with connector
installation, training may be necessary. And sometimes, the user may
have been sold on a connector type that is not ideal for the
installation, so the installer should discuss the merits of other types
before committing to the project.
Splices
are considered permanent joints. Fusion splicing is most widely used as
it provides for the lowest loss and least reflectance, as well as
providing the 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. Connector and Splice Loss The primary specification for connectors or splices is loss or the amount of light lost in the connection. When we say connector
loss, we really mean "connection" loss - the loss of a mated pair of
connectors, expressed in "dB." Thus, testing connectors requires mating them to reference
connectors which must be high quality connectors themselves to not
adversely affect the measured loss when mated to an unknown connector. This
is an important point often not fully explained. In
order to measure the loss of the connectors you must mate them to a
similar, known good, connector. When a connector being tested is mated
to several different connectors, it may have different losses, because
those losses are dependent on the reference connector it is mated to.
 Connector and splice loss
is caused by a number of factors. Loss is minimized when the
two fiber cores are identical and perfectly aligned (more on the effects of misalignment), the connectors
or splices are properly finished and no dirt is present. Only
the light that is coupled into the receiving fiber's core will
propagate, so all the rest of the light becomes the connector
or splice loss.
End gaps cause two problems,
insertion loss and reflectance. The emerging cone of light from
the connector will spill over the core of the receiving fiber
and be lost. In addition, the air gap in the joint between the fibers causes
a reflection when the light encounters the change n refractive
index from the glass fiber to the air in the gap. This reflection
(called fresnel reflection) amounts to about 5% in typical flat
polished connectors, and means that no connector with an air
gap will have less than about 0.3 dB loss. This reflection is called
to as reflectance or optical return loss, which can be a
problem in laser based systems. Connectors use a number of polishing
techniques to create convex fiber ends that ensure physical contact of the fiber ends to minimize
reflectance. On mechanical splices, it is possible to reduce
back reflection by using non-perpendicular cleaves, which cause
back reflections to be absorbed in the cladding of the fiber.
The end of the fiber must
be properly polished and clean to minimize loss. A rough surface will scatter
light and dirt can scatter and absorb light. Since the optical
fiber is so small, typical airborne dirt can be a major source
of loss. Whenever connectors are not terminated, they should
be covered with dust caps provided by the manufacturer to protect the end of the ferrule from dirt. One should
never touch the end of the ferrule, since the oils on one's skin
causes the fiber to attract dirt. Before connection and testing,
it is advisable to clean connectors with lint-free wipes moistened
with isopropyl alcohol or special dry fiber cleaners.
Two sources of loss caused by mismatched fibers are directional;
numerical aperture (NA) and core diameter differences inherent in the fibers being joined. Differences in these
two will create connections that have different losses depending
on the direction of light propagation. Light from a fiber with
a larger NA will overfill the core of the receiving fiber be more sensitive to angularity and end gap,
so transmission from a fiber of larger NA to one of smaller NA
will be higher loss than the reverse direction. Likewise, light from a
larger core fiber will have high loss coupled to a fiber of smaller
diameter, while one can couple a small diameter fiber to a large
diameter fiber with minimal loss, since it is much less sensitive
to end gap or lateral offset.
These
fiber mismatches occur for two reasons, the occasional need to
interconnect two dissimilar fibers and production variances in fibers
of the same nominal dimensions. Production variances are only a few
microns and contribute only small amounts of loss, but the loss caused
by mismatches will be directional, causing larger losses when
transmitting from larger to smaller core fibers.
With two multimode fibers in common usage today (50/125 and 62.5/125) and two
others which have been used occasionally in the past (100/140 and 85/125) and several
types of singlemode fiber in use, it is possible to sometimes
have to connect dissimilar fibers or use systems designed for
one fiber size on another. If you connect a smaller
fiber to a larger one, the coupling losses will be minimal, often
only the fresnel loss (about 0.3 dB). But connecting larger fibers
to smaller ones results in substantial losses, not only due to
the smaller cores size, but also the smaller NA of most small
core fibers.
More on mismatched fiber losses.
Typical connector losses are generally less than 0.3 dB for factory-polished singlemode or multimode
connectors using adhesive/polish techniques. Few installers tackle
singlemode field termination, generally fusion splicing factory-made
pigtails onto the fibers, since SM polishing is not so easy in the
field, especially in terms of reflectance. Multimode field terminations
are common, since experienced installers can get results comparable to
factory-terminations with adhesive/polish techniques. Field termination
of prepolished/splice connectors using a precision cleaver (those made
for fusion splicing) can produce consistent results around 0.5 dB,
while the simple cleaver produces losses more often in the 0.75 dB
range. Few industry standards put numbers on connector losses, but TIA
568 calls for connection losses of less than 0.75 dB, a high number but
one which will allow use of prepolished/splice connectors.
Connector Reflectance Reflectance
or optical return loss (which has also been called "back reflection")
of the connector is the amount of light that is
reflected back up the fiber toward the source by light reflections off
the interface of the polished end surface of the connector and air. It
is called fresnel reflection and is caused by the light going through
the change in index of refraction at the interface between the fiber
(n=1.5) and air (n=1). Reflectance is primarily a problem with
connectors but may also affect mechanical splices which contain an
index matching gel to prevent reflectance.
Reflectance
one component of the connector's loss, representing
about 0.3 dB loss for a non-contact or air-gap connector where the two
fibers do not make contact. Minimizing the reflectance is necessary to
get maximum performance out of high bit rate laser systems and
especially AM modulated CATV systems. In multimode systems, reflections
are
less of a problem but can add to background noise in the fiber. Since
this is more a problem with singlemode systems, manufacturers have
concentrated on solving the problem for their singlemode components but
multimode connectors benefit also since any reduction in reflectance also reduces loss. Several schemes have been used to
reduce reflectance, mainly using a convex physical contact (PC) polish
on the end of the connector ferrule, which reduces the fresnel
reflection. The technique involves polishing the end surface of the
fiber to a convex surface or even better at a slight angle (APC or
angled physical contact) to prevent reflectance.
See Connector Ferrule Shapes
& Polishes below for more information on connector ferrule endface polish to reduce reflectance.
See Measuring Reflectance to see the methods and issues of measuring reflectance.
Styles of Fiber Optic Connectors
 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.
Of
course, the marketplace eventually
determines which connectors are successful. However several attempts to
standardize connectors have been attempted. Some were unique to systems or networks. FDDI, the first fiber LAN,
and ESCON, the IBM mainframe
peripheral network, required unique connectors. TIA 568
originally called for SC connectors as a standard, but when users continued
to use more STs than SCs and a whole new generation of smaller
connectors were introduced, TIA-568B was changed to say that any
connector standardized by a FOCIS standard document was acceptable. The
four connectors shown at left show how fiber optic connectors have
evolved. The bottom connector is a Deutsch 1000, the first
commercially-available fiber optic connector. It was really a
mechanical splice, where fibers were held inside the connector with a
tiny screw-tightened chuck. The nose piece was spring-loaded, allowing
exposing the fiber for cleaving and mating with a small plastic lens in
a mating adapter. The mating adapter also had index-matching fluid to
reduce loss but it was a dirt problem. The
yellow connector is an AT&T Biconic. It was developed by Jack Cook
at Bell Labs in the late 1970s. The conical ferrule was molded from
glass-filled plastic. The first Biconics had ferrules molded around the
fiber, until a die with a tiny 125 micron pin in the exact center was
developed. When Biconics were adapted to singlemode fiber, the ferrules
were ground on a special grinding machine to center the fiber. The
SC, which was introduced in the mid-1980s, used a new invention, the
molded ceramic ferrule, that revolutionized fiber optic termination.
Ceramic was an ideal ferrule material. It could be made cheaply by
molding, much cheaper than machining metal for example. It was
extremely
stable with temperature, having similar expansion characteristics to
glass which prevented "pistoning" when the ferrule came unglued, a
problem with metal or plastic ferrules. It's hardness was similar
to glass which made polishing much easier. And it readily
adhered to fibers using epoxies or anaerobic adhesives.
Today, virtually all connectors use the ceramic ferrule, usually 2.5 mm
diameter (SC, ST, FC) or 1.25 mm (LC, MU.) The
LC connector was introduced in the late 1990s to miniaturize connectors
for higher density in patch panels or equipment. It uses a smaller
ceramic ferrule, 1.25 mm diameter. The LC is the connector of choice
for telecom and high speed data (>1 Gb/s) networks.
Guide to Identifying Fiber Optic ConnectorsCheck out the "spotters
guide" below and you will see the most common fiber optic
connectors. (All the photos are to the same scale except the MTP, so you can
get an idea of the relative size of these connectors.)
| 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
is a new connector that 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. |  | | 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 is a 12 fiber connector for
ribbon cable. It's main use is for preterminated cable assemblies.
|  |
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.
- Connector Popularity
The
ST is still one of the most popular multimode connectors because it is
inexpensive and easy to install. The SC connector was specified as a
standard by the old EIA/TIA 568A specification, but its higher cost and
difficulty of installation (until recently) limited its popularity in
premises applications at first. However, newer SCs are much better in
both cost and installation ease, so it has been growing in use, but is
now challenged by the LC, which is the connector of choice for
transceivers for systems operating at gigabit speeds because of its
small size and high performance.
Singlemode networks have used FC or SC connectors in about the same
proportion as ST and SC in multimode installations. There are
some D4s out there too. But LCs have become the most popular, again for their performance and small size.
- EIA/TIA 568 now allows any fiber
optic connector as long as it has a FOCIS (Fiber Optic Connector
Intermateability Standard) document behind it. This opened the
way to the development of several new connectors, which we call the "Small
Form Factor" (SFF) connectors, including AT&T LC, the
MT-RJ, the Panduit "Opti-Jack," 3M's Volition, the
E2000/LX-5 and MU. The LC has been particularly successful in
the US.
Specialty Fiber Optic Connectors
 
There
are a number of specialty fiber optic connectors available such as this
multifiber military connector, special underwater or aircraft
connectors, plastic optical fiber (POF) connectors, etc. Most have been
designed for very specific applications and require extremely rigorous
qualification testing. Some like the Mil-C-38999, are copper wiring
connectors adapted to hold fiber optic ferrules. Many of these
connectors require special cable types, termination procedures,
cleaning, handling and test procedures. Refer to manufacturer's
instructions whenever dealing with these types of connectors.
-
- Connector Ferrule Shapes
& Polishes
- Fiber optic connectors can have
several different ferrule shapes or finishes, usually referred
to as end finish or polish types.
- Early
connectors, which did not have keyed ferrules and could rotate in
mating adapters, always had an air gap between the connectors to
prevent them rotating and grinding scratches into the ends of the
fibers. The ends of the ferrules were polished on hard,flat surfaces.
They are sometimes referred to as NC or "Non-Fiber Contact" styles.
- Beginning
with the ST and FC which had keyed ferrules, the connectors were
designed to contact tightly, what we now call physical contact (PC)
connectors. These connectors were still polished flat on the end.
Reducing the air gap reduced the loss and reflectance (very important
to laser-based singlemode systems ), since light has a loss of about 5%
(~0.25 dB) at each air gap and light is reflected back up the fiber.
While air gap connectors usually had losses of 0.5 dB or more and a
reflectance of -20 dB, PC connectors had typical losses of 0.3 dB and
a reflectance
of -30 to -40 dB. PC connectors required polishing on a flat surface
with a soft rubber pad to allow the end to be polished convex.
- Soon thereafter, it was determined
that polishing the connector ferrules to a convex end face would produce an even
better connection. The convex ferrule guaranteed the fiber cores
were in contact. Losses were under 0.3dB and reflectance -40 dB
or better.
- The ultimate solution for singlemode systems extremely
sensitive to reflections, like CATV or high bitrate telco links,
was to angle the end of the ferrule 8 degrees to create what
we call an APC or angled PC connector. Then any reflected light
is at an angle that is absorbed in the cladding of the fiber, resulting in reflectance of >-60 dB.
Connector Color Codes:
Since
the earliest days of fiber optics, orange, black or gray was multimode
and yellow singlemode. However, the advent of metallic connectors like
the FC and ST made color coding difficult, so colored boots were often
used.
The TIA 568 color code for connector bodies and/or boots is
Beige for multimode fiber except aqua for laser-optimized fiber, Blue for singlemode fiber, and Green for APC
(angled) connectors.
- Termination Procedures
Whatever you do, always follow the manufacturer's termination instructions
closely. - Multimode connectors are usually installed in the field on the cables
after pulling, while singlemode connectors are usually installed by
splicing a factory-made "pigtail" onto the fiber. The tolerances on
singlemode terminations are much tighter than multimode and the polishing processes
are more critical, so singlemode termination is better done in a
factory environment using polishing machines (right). You can install
singlemode connectors in the field for low speed data networks, but you
may not be able to get losses lower than 1 dB and reflectance may be a
problem!
- Connectors
can be installed directly on most cable types, including jacketed tight buffer types
like simplex, zipcord and breakout cables, where the where the aramid fiber strength members in the cable are crimped or glued to the connector body to create a strong connector. Connectors can
be attached to the 900 micron buffered fibers in distribution cables,
but the termination is not as rugged as those made to jacketed cables,
so they should be placed in patch panels or boxes for protection. The
250 micron buffered fibers in loose tube cables cannot be easily
terminated unless they have a reinforcement called a breakout kit or
furcation kit installed, where each fiber is covered by a larger
plastic tube. Generally loose tube and ribbon cables are terminated by
splicing on a terminated pigtail.
- Cables can be pulled with connectors already on them if, and a big if,
you can deal with two issues: First, the length must be precise.
Too short and you have to pull another longer one (its not cost
effective to splice), too long and you waste money and have to store
the extra cable length. Secondly, the connectors must be protected.
Some cable and connector manufacturers offer protective sleeves to
cover the connectors, but you must still be much more careful in
pulling cables. You might consider terminating one end and pulling the
unterminated end to not risk the connectors. There is a growing
movement to install preterminated systems with the MTP 12 multifiber
connector. It's tiny not much bigger than a ST or SC, but has up to
12 fibers. Manufacturers sell multifiber cables with MTPs on them that
connect to preterminated patch panels with STs or SCs. (See "Do You Have To Terminate In The Field" below.)
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. 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 are
costly, as much as ten 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.
Do You Have To Terminate In The Field? 
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 prefab cable systems.
More on connectors and termination, including hands-on tutorials. Videos on fiber optic termination on the FOA Channel on Splicing
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.
More on fusion splicing.
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.
- Video on fusion splicing can be found on the FOA Channel on
- Protecting Splices
- Splices
require placement in a protective case. They are generally placed in a
splice tray which is then placed inside a splice closure for OSP
installations or a patch panel box for premises applications. At splice
closures and at each end, cables with metallic shielding or strength
members must be properly grounded and bonded.
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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