An Introduction to Fiber Optic Connectors
By Larry Johnson
In all fiber optic systems, it is necessary to join two fibers together with low signal attenuation while
maintaining low reflection levels depending upon the type of system used. Fiber optic connectors are used
to the mechanical and optical means for cross connecting fibers and linking to fiber optic transmission
equipment.
A wide range of connectors are available to fit very specific needs.
Connectors have evolved with the communications industry. Today’s users have a multitude of
connectivity needs and the fiber industry has responded with innovative solutions. The most common
connector in use today is the SC (Subscriber Connector), the ST (Straight Tip) and the FC (Fiber
Connector). In addition the small form factor (SFF) LC* connector is used in high-density optical
transmission products, and also for applications including fiber-to-the-home and dense wavelength
division multiplexing where space is at a premium. Another connector gaining in popularity for use in high
fiber count terminations is the MPO/MTP connector which can handle fiber counts as high as 96 using
ribbonized fibers.
*
LC is not an acronym as many believe. The LC connector was invented by Norm Lampert while at AT&T.
An Introduction to Fiber Optic Connectors
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To understand fiber optic connectors one must understand both the mechanics and the optics
involved. The ideal optical connector holds the fibers in perfect alignment, in three axes. This alignment
must be maintained over hundreds or even thousands of connect-disconnect cycles to provide stable,
repeatable attenuation characteristics.
The most important element of the connector plug is the ferrule, which provides the precise
alignment and centering of the optical fiber. Ferrules can be made of ceramic, metal, glass or plastic.
Zirconia ceramic ferrules or ceramic ferrules with metal inserts are most widely used, providing the best
tolerances and durability. The most common ferrule sixes are the 2.5 mm used in the SC, ST, and the FC
plugs. The 1.25 mm ferrule is used in the small form factor LC plug. For connectors designed for military
and aerospace connectors the “termini” performs the same functions as the ferrule and are provided as both
pin and sockets.
Cutaway of an SC connector plug and adapter
The body of the plug holds the ferrule, the coupling mechanism, and the boot. The body contains
either a threaded, push-pull or bayonet coupling mechanism that mates the plug with the mating adaptor
and also provides a keying function that allows the connector to mate in only one position. Strain relief of
the cable is usually by a crimp sleeve or by adhesives which firmly secures the aramid yard in the cable to
the plug body. At the rear of the plug body is the “boot” which functions as a bend radius limiter for the
cable entering the plug body.
Another critical component is the mating adaptor, and its internal sleeve that aligns the two mating
ferrules. Like the ferrule, sleeves are most often made from ceramic and can be either a split or solid
construction. The alignment sleeve must precisely align the two precision ferrules by means of the ferrules
outside diameters. This alignment must be done while providing a low-friction fit over a wide temperature
range.
Over the years, fiber tolerances have greatly improved along with the mechanical tolerances of
precision parts of the connectors. Early connectors suffered from poor alignment tolerances and
mechanical stability problems due to loose fiber tolerances and mechanical tolerances of the connector
design. The body and mating adapter also provide a keying function allowing the connector to mate in
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An Introduction to Fiber Optic Connectors
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only one rotational position. This, combined with the tight tolerances of today’s optical fiber, ensures
compliance with the Telcordia GR326 generic requirements repeatability standard of less than .2 dB
connection loss over 500 mating cycles for single-mode fibers.
To reduce the negative effects of poor mechanical tolerances, some early connectors incorporated
lenses. The earliest design by the Deutsch Company in the 1970s used a wet lens approach and was a field
terminable connector. Later lens connectors included Radial’s Optiball connector used on OTDRs, the
Lamdek expanded beam connector made by Eastman Kodak.
Today most lensed connectors work by expanding the beam exiting one fiber, then collimating the
beam to the adjacent plug where a second lens is used to collect that light and re-direct it to the core of the
second fiber. The main advantage of this approach is that the connector is less sensitive to alignment
tolerances, so attenuation will remain more constant in the presence of vibration or temperature cycling. In
addition the large area of the beam minimizes the impact of contaminants. The largest supplier of multifiber expanded beam connectors are developed and manufactured by TE Connectivity and are provided in
many variations for harsh environments such as mining, military, and aerospace applications.
As the fiber optic industry evolved, numerous standards have been developed. The role of standards
is to make sure that mechanical and optical considerations are specified for the benefit of all. A key
connector standard is the Fiber Optic Intermateability standard, or “FOCIS” document. Known officially
as the TIA/EIA 604, the standard covers various connector styles, their configuration options including
keys, fiber counts, polishes, mounting options, descriptions and tolerances. The written objective is the
minimum physical attributes of mating connector components so that any combination of plugs and
adaptors will mechanically inter-mate, and when mated will meet a common level of performance.
Even though there have been vast improvements in mechanical tolerances of modern fiber and
connector products, most connectors still have higher optical losses than splices. The ITU-T G.671
standard specifies a maximum attenuation of 0.5 dB per mated pair, while the Telcordia GR326 standard
specifies a 0.4 dB maximum. Both of these standards are focused on single-mode connectors where the
fiber attenuation ranges from 0.2 dB/km to 0.4 dB/km. For premises and other multimode applications, the
TIA/EIA 568 standard specifies a maximum attenuation value of 0.75 dB per mated connection.
Within connectors, fiber separation and misalignments are the chief causes of signal attenuation, but
poor quality fiber, differing fiber types, and the quality of the fiber end-face also contribute to total
connector losses. In addition, stresses placed on the fiber such as over-crimping or macrobending at
connector boots can add attenuation.
Another major attribute of a fiber optic connection is its reflectance. Reflectance, also measured in
decibels, is the percentage of light reflected from the glass-to-glass surface contact of the mated
connection. This reflection can disrupt the operation of laser light sources used in transmitters, especially
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in DFB and Fabry-Perot lasers used in single-mode systems. This affects the quality of the signal and can
result in unacceptable bit-error rates, increased noise in analog systems and increases the latency of data
communications systems.
Fresnel reflections can be minimized in a connector plug by polishing the fiber end-faces to specific
profiles and having the optical surfaces come into physical contact.
Polishes include a domed profile known as a “Physical Contact” or PC finish, and the Ultra Physical
Contact, or UPC finish. Another profile, called the Angled Physical Contact, or APC finish slightly angles
each of the mated fiber ends to 8 degrees.
Connector polish reflection
PC connectors have a reflection value of 40 dB, which means that the connector reflects one
hundredth of one percent of the incident light back towards the transmitter.
The UPC polish are most often rated at 50 dB or better due to their improved spherical profile and is
most often in high speed digital systems.
The APC polish used in amplitude modulation analog, dense wavelength division multiplexing
systems, optical amplifiers and in “fiber-to-the-home” installations is usually rated at 60 dB or better.
Producing PC polishes is a precision process. Even a slight deviation from the correct spherical or
angled profile renders the connector unacceptable. This makes PC connectors more expensive than
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conventional types. For this reason, these polishes are made using automated polishing equipment and are
100 percent visually inspected, followed by optical loss and reflection testing.
Multimode systems are usually much more forgiving of reflections. This is due to the fact that the
optical transmitters use Light Emitting Diodes or the Vertical Cavity Surface Emitting Lasers (VCSEL)
which are insensitive to Fresnel reflections. It is because of this that hand polishing to a higher reflectance
flat finish is adequate for multimode fibers but unacceptable for single-mode terminations. In addition the
9 micron mode field diameter in single-mode fibers is much smaller than the 50 and 62.5 micron cores of
multimode fibers. Even the tolerances of single-mode fiber and ferrules must be much tighter than
multimode to provide lower attenuation and reflection values. This makes field installation of connectors
much more challenging. Never install a lower cost multimode plug on a single-mode fiber as the singlemode ferrules have much better mechanical tolerances to provide proper alignment of the fibers.
Historically, for this reason single-mode terminations are mostly factory-terminated on short lengths
of 900 micron buffered fibers or 3 millimeter cordage known as “pigtails”. The pigtails are then spliced on
to the ends of the optical fibers. Today field installable “no-polish” plugs are available that incorporate a
factory polished fiber stub in the ferrule which can be either bonded using either mechanical or fusion
splice techniques. These terminations provide low attenuation and reflection values but do require a
quality cleaving tool.
Example of a “pigtail” connector
This article was edited from the first chapter of The Light Brigade’s Fiber Optic Connectors DVD
(#W-6D-181), which is available from the Light Brigade at www.lightbrigade.com.
About The Author
Larry Johnson is Director and Founder of The Light Brigade.
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©2013, The Light Brigade, Inc.