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| FIBRE
OPTIC CONNECTOR ENDFACE GEOMETRY |
The
endface geometry of a PC polished connector has long been known
to play an important role in the long-term performance of a singlemode
connection. The primary function of the PC polish design is to insure
core to core contact between mating connectors while taking into
account the effects of time, temperature, vibration and pressure.
The three most important parameters to be controlled in order to
achieve this are: |
1. Radius of curvature
2. Fibre Height
3. Apex Offset
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| 1.
Radius of curvature |
The endface of ferrules are domed to insure that the contact
area between mating connectors is at the centre of the ferrule
where the fibre core will be located. The radius of this dome
is called the "Radius of curvature". Acceptable radius
of curvatures for 2.5mm ferrules is in the range 10mm to 25mm.
If the radius is too low, there will be a smaller contact area
thus putting more force on the fibre during mating. If the radius
is too high, physical contact between the too fibres may not
be achieved because there will be a larger contact area resulting
in less ferrule deformation. Measurement is performed over a
250 micron fitting area, by calculating the best fitting sphere
over this area.
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| 2.
Fibre Height (Spherical) |
Spherical
fibre height is the difference in height between the centre
of the fibre and the
theoretical height of the ferrule where the centre would be
when considered a continuous sphere (Fig. 1).
In the past the allowable spherical fibre height was a fixed
value, e.g. +/- 50nm or
+/-100nm depending on the standard used. This resulted in an
unfairly tight tolerance for connectors with low radius of curvatures
because the projected centre of the ferrule was higher than
for ferrules with low radius of curvatures. To correct this
both EIA/TIA and Bellcore have changed to a variable value for
allowable fibre height based on the measured radius of curvature.
The value is calculated by the following relationship:
| Max.
allowable height = -0.02R³ + 1.3R²+ 31R + 325 |
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Fig.
2
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| 3.
Apex Offset |
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Apex
offset is a measure of the distance between the highest point
of the convex of the
polished endface and the centre of the fibre. The objective is
for the centre of the fibre
to be the highest point on the endface, thus guaranteeing contact
between mating fibres.
The maximum apex offset allowed is 50 microns.
Offset can also be defined as angular offset. Angular offset is
the angle between the
radial line through the fibre centre and the radial line through
the apex of the polish.
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Fig.
3
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100%
inspection of endface geometry is vital. Variations in ferrule length,
hardness, positioning and polishing component differences all have
an influence on endface geometry quality. Nothing
can be taken for granted.
Fibrepulse inspect 100% of all connector endfaces for the parameters
described above. Results
are directly entered and stored under the assembly serial number
and can be traced back from
the cable assembly serial number at any time in the future. Reports
are available showing batch results.
For angled connectors there are two more parameters. These
are: |
| 1.
Angle. |
There is no international standard tolerance for the angle of
an APC connector
because if the apex offset is good when the connector is oriented
at the desired
angle, then it doesn't matter because point of contact will
be at the core for that angle. However, Fibrepulse uses a tolerance
±0.2°.
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| 2.
Key Error. |
This is an indication of the orientation of the angle to the
connector key. If this is too
high it means that the angle will not square up properly to
a mated connector, causing an air-gap between the two. The tolerance
Fibrepulse uses is ±0.25°.
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Castlebar
Technology Park,
Moneen, Castlebar, Co. Mayo, Ireland
Tel: +353 94 9027822 Fax: +353 94 9027811
E-Mail: info@fibrepulse.com
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