[ Back ] [ Home ] [ Up ] [ Next ]
FIBER OPTIC CONNECTORS
A fiber optic connector is a demateable device that permits the coupling of optical
power between two optical fibers or two groups of fibers. Designing a device that allows
for repeated fiber coupling without significant loss of light is difficult. Fiber optic
connectors must maintain fiber alignment and provide repeatable loss measurements during
numerous connections. Fiber optic connectors should be easy to assemble (in a laboratory
or field environment) and should be cost effective. They should also be reliable. Fiber
optic connections using connectors should be insensitive to environmental conditions, such
as temperature, dust, and moisture. Fiber optic connector designs attempt to optimize
connector performance by meeting each of these conditions.
Fiber optic connector coupling loss results from the same loss mechanisms described
earlier in this chapter. Coupling loss results from poor fiber alignment and end
preparation (extrinsic losses), fiber mismatches (intrinsic loss), and Fresnel reflection.
The total amount of insertion loss for fiber optic connectors should remain below 1 dB.
Fiber alignment is the critical parameter in maintaining the total insertion loss below
the required level. There is only a small amount of control over coupling loss resulting
from fiber mismatches, because the loss results from inherent fiber properties. Index
matching gels cannot be used to reduce Fresnel losses, since the index matching gels
attract dust and dirt to the connection.
Fiber optic connectors can also reduce system performance by introducing modal and
reflection noise. The cause of modal noise in fiber optic connectors is the interfering of
the different wavefronts of different modes within the fiber at the connector interface.
Modal noise is eliminated by using only single mode fiber with laser sources and only
low-coherence sources such as light-emitting diodes with multimode fiber. Fiber optic
connectors can introduce reflection noise by reflecting light back into the optical
source. Reflection noise is reduced by index matching gels, physical contact polishes, or
antireflection coatings. Generally, reflection noise is only a problem in high data rate
single mode systems using lasers.
Butt-jointed connectors and expanded-beam connectors are the two basic types
of fiber optic connectors. Fiber optic butt-jointed connectors align and bring the
prepared ends of two fibers into close contact. The end-faces of some butt-jointed
connectors touch, but others do not depending upon the connector design. Types of
butt-jointed connectors include cylindrical ferrule and biconical connectors. Fiber optic expanded-beam
connectors use two lenses to first expand and then refocus the light from the transmitting
fiber into the receiving fiber. Single fiber butt-jointed and expanded beam connectors
normally consist of two plugs and an adapter (coupling device). Figure 4-15 shows how to
configure each plug and adapter when making the connection between two optical fibers.
Figure 4-15. - Plug-adapter-plug configuration.
Ferrule connectors use two cylindrical plugs (referred to as ferrules), an
alignment sleeve, and sometimes axial springs to perform fiber alignment. Figure 4-16
provides an illustration of this basic ferrule connector design. Precision holes drilled
or molded through the center of each ferrule allow for fiber insertion and alignment.
Precise fiber alignment depends on the accuracy of the central hole of each ferrule. When
the fiber ends are inserted, an adhesive (normally an epoxy resin) bonds the fiber inside
the ferrule. The fiber-end faces are polished until they are flush with the end of the
ferrule to achieve a low-loss fiber connection. Fiber alignment occurs when the ferrules
are inserted into the alignment sleeve. The inside diameter of the alignment sleeve aligns
the ferrules, which in turn align the fibers. Ferrule connectors lock the ferrules in the
alignment sleeve using a threaded outer shell or some other type of coupling mechanism.
Figure 4-16. - Basic ferrule connector design.
As stated before, fiber alignment depends on an accurate hole through the center of the
ferrule. Normally, ferrule connectors use ceramic or metal ferrules. The center hole is
generally drilled in a metal ferrule. Drilling an accurate hole through the entire metal
ferrule can be difficult. To improve fiber alignment, some metal ferrule connectors use
precision watch-jeweled centering. In precision watch-jeweled centering, a watch jewel
with a precision centered hole is placed in the tip of the ferrule. The central hole of
the watch jewel centers the fiber with respect to the axis of the cylindrical ferrule. The
watch jewel provides for better fiber alignment, because regulating the hole tolerance of
the watch jewel is easier than maintaining a precise hole diameter when drilling through
an entire ferrule.
The center hole in a ceramic ferrule is created by forming the ferrule around a
precision wire, which is then removed. This method produces holes accurately centered in
the ferrule. Most cylindrical ferrule connectors now use ceramic ferrules. The Straight
Tip (ST®) connector is an example of a ceramic ferrule connector. (ST® is
a registered trademark of AT&T.)
Other cylindrical ferrule connectors have a ferrule that contains both metal and
ceramic. For these connectors a ceramic capillary is placed within the tip of a metal
ferrule to provide for precision fiber alignment. The ceramic capillary is a ceramic tube
with a small inner diameter that is just larger than the diameter of the fiber. Figure
4-17 shows the placement of the ceramic capillary within the metal ferrule.
Figure 4-17. - A ceramic capillary set within a metal ferrule.
Another type of butt-jointed connector is the biconical connector. Biconical connectors
use two conical plugs, a double conical alignment sleeve, and axial springs to perform
fiber alignment. Figure 4-18 is an illustration of this basic biconical connector design.
Formation of the plugs and alignment sleeve involves transfer molding. Transfer molding
uses silica-filled epoxy resin to mold the conical plug directly to the fiber or around a
cast (precision wire). After connecting the conical plugs to the optical fibers, the
fiber-end faces are polished before the plugs are inserted into the molded alignment
sleeve. During fiber insertion, the inside surface of the double conical sleeve performs
fiber alignment, while the axial springs push the fiber ends into close contact. If the
alignment sleeve permits the fibers to actually become in contact, then the axial spring
provides enough force to maintain fiber contact but prevent damage to the fiber-end faces.
Normally, biconical connectors lock the fibers in alignment using a threaded outer shell.
Figure 4-18. - Biconical connector design.
Multifiber connectors join and align multifiber cables to reduce the time it takes to
connect multiple fibers. One type of multifiber connector is the array connector. The
array connector is used to connect individual ribbons of ribbon-type cables. The array
connector is similar to the ribbon splice. In the array connector, the fibers of each
ribbon are epoxied into grooves of a silicon chip so that the fiber ends protrude from the
end of the chip. The chip and the protruding fibers are polished flat for connection. Each
half of the connector is prepared separately before being butt-jointed. A spring clip and
two grooved metal-backed plates are used to align and connect the stacked ribbons of the
two ribbon cables. Array connectors may also use an alignment sleeve with V-grooved
silicon chips and metal springs to align and connect stacked ribbons. Figure 4-19 shows
the spring clip method of array connector alignment. The multifiber array connector is
only one example of a multiple connector. Many types of multiple connectors exist that
connect different types of multifiber cables.
Figure 4-19. - Spring clip method of ribbon connection.
Figure 4-20 shows how an expanded-beam connector uses two lenses to expand and then
refocus the light from the transmitting fiber into the receiving fiber. Expanded-beam
connectors are normally plug-adapter-plug type connections. Fiber separation and lateral
misalignment are less critical in expanded-beam coupling than in butt-jointing. The same
amount of fiber separation and lateral misalignment in expanded beam coupling produces a
lower coupling loss than in butt-jointing. However, angular misalignment is more critical.
The same amount of angular misalignment in expanded-beam coupling produces a higher loss
than in butt-jointing. Expanded-beam connectors are also much harder to produce. Present
applications for expanded-beam connectors include multifiber connections, edge connections
for printed circuit boards, and other applications.
Figure 4-20. - Expanded-beam connector operation.
Q.24 What connection properties result in fiber optic connector coupling loss?
Q.25 Which is the more critical parameter in maintaining total insertion loss below the
required level, fiber alignment or fiber mismatch?
Q.26 Fiber optic connectors can reduce system performance by increasing what two types of
noise?
Q.27 Which type of fiber optic connector (butt-jointed or expanded beam) brings the
prepared ends of two optical fibers into close contact?
Q.28 Is coupling loss from fiber separation and lateral misalignment more critical in
expanded-beam or butt-jointed connectors?
Q.29 Is coupling loss from angular misalignment more critical in expanded beam or
butt-jointed connectors?
[ Back ] [ Home ] [ Up ] [ Next ]
