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PC Connectors
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PC (Physical
Contact) connectors are designed to do just that, bring the cores of the
optical fibers into intimate physical contact. The purpose for
this physical contact is to eliminate the Fresnel reflection that
occurs when light travels from a medium with one index of refraction to a medium
with a different index of refraction. In the case of optical fibers, the light
travels from glass with an index of 1.48 to air having an index of 1. This
results in approximately 4% of the light being reflected back into the fiber in
the reverse direction. When traveling from one fiber to another with an air gap
in between this can result in 8% being reflected back into the launch fiber. The
result is an additional 0.35 dB loss in the fiber-to-fiber connection. This does
not take into account the additional loss due to end gap separation between the
two fibers, which can add another 0.1-0.3dB of loss depending on the amount of
separation and the core diameters. Therefore to achieve a low loss
connection, intimate physical contact between the fiber cores and part of the
cladding is required.
Aside from the additional dB loss incurred by a Fresnel reflection, there is
another important reason that we need to eliminate reflections in optical fiber
systems. High Speed single mode fiber systems most often utilize a semiconductor
laser diode as the optical source. These devices are extremely intolerant of
back reflections and therefore require very high return loss (low back
reflection) connectors throughout the entire system.
Another problem associated with back reflections is optical noise.
This occurs when several connectors are present within the span from the
transmitter to the receiver. Such as often found in patch panels, with short
patch chords interconnecting longer spans. The back reflections from successive
connectors continue to go back and forth between these connectors and can add
upon one another until there is sufficient energy to be detected at the receiver
end. In the case of a digital system you can have extra bits where you should
have none. These extra bits can corrupt data transmission. In analog systems the
reflections are more continuous and add upon the signal, distorting the
linearity. This type of optical noise can plague both singlemode and
multimode systems.
In a PC connector the intimate contact is achieved by starting with a
spherically pre-radiused ceramic ferrule with a radius between 10 and 25
millimeters. After the fiber is bonded into the ferrule, it is then polished on
a rubber polishing pad having the proper durometer (softness). In theory,
if properly polished with the right amount of pressure, the rubber polishing pad
assumes the radius on the ferrule. The radius is then transferred to the fiber’s
end face, producing a spherical surface. The correct polishing time is also
essential, when done properly the outer edge of the fiber is perfectly flush
with the inner edge of the ceramic ferrule face and the center of the fiber core
is the highest point. When the two connector faces are brought into contact, the
first points to touch are the high points at the center of the cores. Under
spring pressure the spherical shape of each connector is caused to flatten out
against the other over the region forming the core and part of the cladding.
This is how the intimate contact is achieved and the air gap is effectively
eliminated, If both connectors have been polished correctly.
To ensure that the connector is polished correctly the process should be
monitored carefully with an interferometric microscope. Without
monitoring it is very easy to over-polish the connector. At the very least
over-polishing produces a flat fiber face that is flush with the ferrule
surface. A more severe consequence of over-polishing causes the glass fiber
surface to drop below the surface of the ceramic ferrule and can take on a
concave or dished surface. This occurs because the glass fiber is much softer
than the surrounding ceramic ferrule and easily polishes away. The result is
called fiber undercut. Both of these conditions result in high
back reflections and an increase in dB loss. If a connector is not polished
sufficiently, the fiber surface may remain higher than the ceramic ferrule
surface. This is called fiber protrusion and can result in the fiber breaking
and damaging the other connector half. This can be serious if the
other mating half is in the transmitter or receiver end. Fiber undercut and
protrusion must be held below 0.3 micron.
When polishing a PC connector, a precision polishing tool must be used to insure
that the highest point of the spherical surface is on the center of the fiber
core. Inexpensive polishing tools allow the connector ferrule to tilt to one
side, resulting in the highest point of the spherical surface being off center,
this is known as apex offset. Current standards require that apex
offset must be held less to than 50 microns.
Each and every connector must be checked for radius of curvature, apex offset,
fiber undercut and fiber protrusion to ensure the lowest possible dB loss and
highest possible return loss (lowest back reflection).
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