EoW January 2013

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Technical article
January 2013 53
standards for conventional fibre. These
existing standards should be carefully
reviewed and appropriate criteria
added to specifically include the unique
characteristics of reduced bend radius
fibres.
New testing
considerations
There is a need for the addition of several
new test criteria to GR-409 and GR-20
in light of the unique characteristics of
reduced bend radius fibre types. For
example, a means of measuring fibre strain
should be added to existing test criteria.
Strain or stress should be measurable
on both indoor and outdoor fibre cables
during tensile FOTP-33b, aging and other
mechanical testing processes where this
type of testing is not currently conducted.
This may be difficult without introducing
a new family of qualification tests for fibre
strain, but the new reduced bend radius
fibres demand it.
A second consideration for changing
testing methods may be to measure delta
excess fibre length, in loose tube type
cables, before and after ageing, and also in
individual tubes. For instance, attenuation
and excess length could be measured
before ageing and temperature cycling
processes, and then again following these
processes. They would then be compared
to established pass/fail criteria. Current
specifications do not require this type of
testing, nor do they require testing in a
loose configuration. All testing is currently
done on spools or coils. In a loose-tube
configuration in a coil, you can have a
great deal more excess length and relaxed
length than in a straight line. Attenuation
increases would be less evident without
the ability to measure excess length as a
mechanical test.
It is worth noting that new measurements
should include the very long wavelength
of 1,625nm. These new measurements
would propose additional qualifications
for that wavelength where the microbend
edge moves in as the fibre is strained.
Although this is a requirement of some
customers in their own standards, it is
not part of the existing generic fibre
standards. Cable shrinkage testing needs
to move to a higher level of repeatability
and gauges for this purpose have been
designed. The range of testing and the
effect of fibre extrusion from the cable
core need to be determined as well. This is
only a secondary effect of GR 326 the test
standard for cable connectors and cable
assemblies.
Conclusions
The introduction of reduced bend radius
fibres, and their emerging popularity in
fibre-to-the-premise (FTTP) architectures,
is cause for concern when it comes to
the current GR-409, GR-20, GR 326 and
other specification standards written for
conventional fibres. New tests should be
proposed to accurately define their unique
characteristics to better ensure long-term
reliability.
A cable design that takes advantage of
the tighter bend radius fibre would most
certainly show much higher attenuation
using conventional single mode fibre. In
other words, a reduced bend radius fibre
would survive very well in an environment
where conventional fibre would not.
The cable to connector interface may
create new aging models where cable
shrinkage can lead to unacceptable
fibre bend radii at the cable/connector
interface. The results of which will only
show up after loose cable assemblies are
aged and then moved. This alone indicates
the need for a set of revised test standards
and requirements for reduced bend radius
types of fibre.
Cable designs tested to GR 409 or GR 20
are requirements for GR 326 testing. The
need exists to use fibre strain and cable
shrinkage as well as fibre extrusion from
cables after aging as a more complete
precursor to GR326 testing.
Also with increased environmental
operating ranges of cables becoming
the norm, thermal co-efficient of linear
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expansion values need to be incorporated
into specification performance precursor
requirements.
This article proposes to update existing
standards, particularly the GR-409
specifications for required tolerance for
shrinkage and fibre strain. Otherwise, it is
possible for sub-par cable designs to pass
existing standards as they are written and
be deployed in the field.
It should be recognised that conventional
fibre and reduced bend radius fibre exhibit
different properties and characteristics,
and test criteria should be written to
address the requirements of both.
Therefore, the proposal is to consider
adding more focused test criteria to
existing standards specifications. This in
effect creating a new class of GGR-409
and GGR-20 qualifications specifically
identifying the fibres used and qualified in
any specific cable designs.
Re-purposing the current standards with
new reduced bend radius fibres in mind
will enable service providers to take full
advantage of the unique characteristics
these fibres bring to the table, particularly
in today’s FTTP deployments. n
Acknowledgments
The author would like to acknowledge
the help of Wagner Aguiar, Ken Nardone,
Henry Rice, and Bill Jacobsen in obtaining
data and test information for this paper.
References
TIA-455A Fiber Optic test procedures
Telcordia GR-220-core Issue 3
Telcordia GR-4409-core issue 22
Telcordia GR-3326-core issue 44
ITU 652 .A-D 22009-11
ITU 657.A 200 9-11
Reliability of bend Insensitive Fibres; Willem Griffoem
Draka Communications Proceedings of the 58th IWCS
pages 251- 257 2009
Macrobending Loss in Bend Insensitive Fibres: A
Statistical Parameter? Susanna Cattelan, Prysmian SpA,
Proceedings of the 58th IWCS pages 258-263
Cutting
Measuring
Master
Rod
Cleaning tool
▲
▲ Figure 6: Fibre strain vs tensile loading ▲
▲ Figure 7: High precision shrinkage gauge