Connector Types and Terminology – What's In A Name?
Rarely do FOA instructors present a fiber optic seminar without getting
some questions on what some connector terminology means or how such a
term or connector name originated. Since some of us have been in the business for over 25
years ourselves, we do remember some of the origins of these terms and
find it amusing to share the stories. But we’ll also cover some
important information about fiber optic connector design and
termination along the way.
 The AT&T Biconic
Many of you probably remember the Biconic and SMA connectors which were
the most popular connectors 25+ years ago. (Here is the FOA Tech Topic that identifies most popular connectors from the history of fiber optics.) AT&T named their first
commercial fiber optic connector the “Biconic” since the connector had
a conical molded-plastic ferrule that fitted into a “bi-conical” mating
adapter when connected to another connector. The original Biconic was
actually molded around the fibers to get exact centering, a practice
that stopped when they developed a way to insert a 125 micron ( 5 mil)
pin in the plastic molding cavity to produce a connector ferrule with a
hole into which fibers could be epoxied.
 SMA
The SMA connector was the first connector widely used for data links
using multimode fiber. It was developed by Amphenol using the hardware
from the “SubMiniature A” microwave connector, hence it’s name, with a
precise 1/8 inch machined metal ferrule.
The
next major development in fiber optic connectors came from Japan, with
the development of the ceramic ferrule. Metal ferrules had a problem
with glass fiber. The uneven expansion coefficients sometimes caused
the adhesive to fail, allowing the fiber to extend or retract, a
process called “pistoning.” The ceramic ferrule had a very low
expansion, like glass, so the adhesive held better, the connector could
be polished to a finer finish and performed better over extremes in
temperature.
 NTT FC  AT&T ST
The “FC” or “fiber connector” from NTT (Nippon Telephone and Telegraph)
was the first connector to use the 2.5 mm ceramic ferrule. Shortly
thereafter, AT&T introduced the “ST” or “straight terminus”
(contrasting to the “biconical” – get it?) that used exactly the same
ceramic ferrule as the FC but replaced the screw-on nut of the FC with
a bayonet lock like a BMC coax connector, simplifying and speeding up
connector insertion. NTT responded a couple of years later with the
“SC” or “subscriber connector” and the ST and SC dominated fiber optics
for over a decade. The 2.5 mm ferrule was also used in the FDDI and
ESCON connectors, both named for the systems for which they were
designed.
 FDDI  IBM ESCON
The next confusing nomenclature for fiber optic connectors came from
the polishing of the end of the ferrule, giving us “PC,” “Ultra PC” or
“Super PC” and “APC” connectors. As the long-haul industry moved from
multimode (MM) to singlemode (SM) fiber around 20 years ago, the
reflections at SM connections caused big problems with most laser
sources. The reflections actually interacted with the lasers, causing
instability and noise in the laser itself.
Early connectors
like the biconic and SMA, 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 air gap
between the fibers causes a reflection when the light encounters the
change in refractive index from the glass fiber to air in the gap.
Beginning
with the ST and FC, which had keyed ferrules, connectors
were designed to contact mating ferrules tightly, creating what
we now call “PC” or “physical contact” connectors. Reducing the
air gap reduced both loss and back reflection, since light has a loss
of about 5% (~0.25 dB) at each air gap. While air gap
connectors usually had losses of 0.5 dB or more and return
loss of 20 dB (1%), PC connectors had typical losses of 0.3
dB and a return loss of 30 to 50 dB (0.1 to 0.001%.)
Connectors
use a number of polishing techniques to create a convex end to
the connector ferrule to insure physical contact of the fiber
ends. Hand polishing is done on a rubber pad and machine
polishing uses concave polishing fixtures. “Super PC” or “Ultra PC” are
simply marketing names various manufacturers give to their polishing
processes.
The final 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.
It
seems like more recent connectors, like the “SFF” or “small form
factor” connectors were named by the marketing department instead of
engineering, which is why we have names like “Volition” and “OptiJack.”
But the exception is the "LC" or lightwave connector, “MT” (“mass
termination” or "multiple termination") for 12 or 24 fibers (also
called MTP "multiple termination push-on" or MPO - "multi-fiber push-on") and its duplex cousin, the
“MT-RJ” where “RJ” refers to the “RJ-45” style of copper connector.
 AT&T LC
 MTP  MT-RJ
Fiber Optic Connector Identifier - What Connector is THAT?
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