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www.read-wca.comWire & Cable ASIA – May/June 2015
Log (failure probability)
Log (stress)
Region II Extrinsic
Region I
Intrinsic
❍
❍
Figure 1
: Failure probability for over 100km of fibre tested at
10m gauge lengths
Together, these two improvements to the optical fibre have
a huge impact in observed cable attenuation, even under
aggressive conditions. The superior fibre and coating
properties can ‘mask’ the impact of a poor cable design or
installation.
When optical cables using traditional G.652 fibres are
deployed with high residual strain on the fibre, higher
attenuation is often observed. By default, the cable
manufacturer is required to control the strain on the fibre to
ensure the cable can meet the qualification requirements.
When G.657 fibres with micro bend-resistant coatings
are used for the same cable design, then the measured
attenuation will improve and the same cable design may
pass this optical requirement. The net result of using G.657
fibres is that the cable will pass this qualification test.
However, after deployment, higher fibre strain could pose a
long-term reliability risk.
In short, if the cable is designed properly, G.657 fibres
and micro bend coatings are a huge benefit to the optical
performance of the deployed cable. But if the cable is
designed poorly, the improved optical fibres can mask
the strain issue from the end user, which could pose a
long-term mechanical reliability risk.
2.4 Cutting costs by minimising material in the cable
and reducing design margins
Many overhead cables are designed with zero per cent
strain on the optical fibre. With increased cost pressure,
design engineers are challenged to reduce material costs. As
strength elements around the optical fibre are removed, the
optical fibre starts to take some of the axial strain traditionally
taken by the strength members in the cable. The design
engineer can look to the various cabling standards and see
that the maximum allowable long-term strain is 20 per cent
of the proof test level.
In effect, for these cables, the industry is progressing from
a common design practice where no strain was carried by
the optical fibres after installation to one where a strain of
up to 20 per cent of the proof test level is allowed. The long
history of reliable cable performance at this strain level
makes it seem a reasonable decision.
2.5 Higher proof-tested fibres 1.38 GPa (200 kpsi) are
now available
In the previous section it was shown that material costs
can be reduced by allowing strain on optical fibre. For
traditional optical fibre that is proof tested at 0.69 GPa (100
kpsi), the maximum allowable strain on the fibre at the 20
per cent limit is 0.14 GPa. A design engineer could choose
to use higher proof-tested fibre, such as 1.38 GPa (200
kpsi) fibre, at the 20 per cent limit, and the allowable strain
on the fibre after installation would increase to 0.28 GPa.
This would allow further material reductions in the optical
cable by allowing greater cable strain to impart twice the
strain on the optical fibre. The net result could be a lower
cost optical cable.
2.6 Combined impact of modified optical cable design
criteria
Taken together, all these trends can result in a scenario
that may not be optimal to the service provider. The strain
on the fibres allowed by the usual criteria is higher, but the
strain is not impacting the attenuation because of the use
of G.657 fibres.
The net result could be an optical cable that is deployed
with up to 0.28 GPa long-term strain on the optical fibres.
Meanwhile, there remains an expectation that the fibres
will survive 30+ years without breaking. This situation tests
the limits of reliability theory and should be looked at more
closely before it is implemented.
3 Origin of the current
allowable strain criterion
The current rule of thumb used for cable design is a
maximum allowable strain of 20 per cent of the proof
test level. This criterion comes from the reliability work
done in the 1990s
[2,3]
. In those studies, the authors show
that long-term performance can be related to the proof
test stress, but this assumes a certain proof test failure
probability. They, then, look at various stress corrosion
parameters and at 50 kpsi and 100 kpsi proof-tested
fibre to show that their approximation is a reasonable,
conservative method to ensure long-term reliability.
This work was an important step forward for the fibre
industry and supported the move for proof-testing fibre at
the current levels.
Unfortunately, there is a key assumption about the flaw
distribution of the optical fibre – specifically the chance
of a fibre breaking when proof-tested. This probability is
not constant and can vary for fibres manufactured under
different conditions or using different raw materials.
Figure 1
shows a failure probability curve for silica fibre
generated by one of the authors’ facilities using 10m gauge
length to illustrate the range of flaws found in optical fibres.
The figure shows two regions: region I (intrinsic strength)
and region II (extrinsic strength). The curve illustrates the
main regions that need to be characterised to predict
long-term fibre reliability. Region I is the high strength
intrinsic region.
The fibre investigated showed the inherent strength of the
glass at ~4.6 GPa, which is significantly above the limit of
3.1 GPa recommended in Telcordia GR-20.
Short gauge-length strength testing in this region can be
used to determine the n value, which is greater than 20 for
the fibre investigated.