EoW January 2013

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Technical article
January 2013 52
They include ‘trench-assisted’ varieties,
‘voids-assisted’ fibre, photonic-crystal
or ‘holey fibres,’ and several other types
and technology combinations. When
compared with conventional fibre, each
of these new innovations has improved
the characteristics and mechanical
performance of today’s optical fibre.
However, during the same time frame,
the existing test regimes have remained
basically unchanged, continuing to
rely on attenuation change based on
physical, mechanical and environmental
testing. Attenuation continues to be the
preferred methodology for determining
a fibre’s performance. However, testing
reduced bend radius fibres using the same
methods for conventional single mode
and multi-mode fibre does not take into
consideration the unique properties of
these new fibres. With that in mind, let’s
look at how attenuation is induced in
conventional fibres and reduced bend
radius fibres.
Macrobends and
Microbends
So what exactly changed with the
introduction of reduced bend radius
fibres? The most obvious improvement
was the fibre’s ability to bend more tightly,
that is, its bend sensitivity was reduced.
These fibres can be bent to a 10, 7.5 or
even 5mm radius with no noticeable
increase in attenuation or damage to
the glass in a long-term environment.
Resistance to macrobend and microbend
loss was also significantly increased. In
fibre optic transmissions, a macrobend
refers to a large visible bend in the optical
fibre that can cause extrinsic attenuation,
a reduction of optical power in the glass.
Microbends are defined as nearly invisible
imperfections in the optical fibre, usually
created during the manufacturing process.
These tiny imperfections can also cause a
reduction in optical power, or increased
attenuation. However, microbends may
also occur from the stress compression
of the plastics placed on the glass due to
polymer shrinkage on the fibre.
In conventional fibre, attenuation
increases indicate when a microbend
has occurred in the fibre. However, in a
reduced bend radius fibre, attenuation
changes are typically minimal and
the same microbend may not be
discovered until an extreme failure in the
performance of the cable. Therefore, the
failure is going to occur over time as the
cable is handled, installed or ages. Modern
aging techniques used for testing, such as
extreme heat exposure, may not exhibit
a failure on today’s new reduced bend
radius fibres.
Insufficient test
methods
The existing test methods for conventional
optical fibre are based on mechanical
testing and attenuation changes, but they
do not specify the cable design being
tested. Therefore, if a reduced bend radius
fibre is undergoing the same tests, its
minimal sensitivity to microbending may
allow it to pass the test while a microbend
could still cause the fibre to stress over
time. That means some cable designs
could still be created with inherent failures
in design, yet they could pass existing
testing standards based solely on what
is contained in GR-409 for tight-buffered
fibres.
In loose-tube outdoor fibre cables,
covered by the GR-20 standard, there are
a number of tests that may determine
whether the fibres are under some stress
or strain. Currently, the only requirement
for strain testing is contained in
TIA-455-33B section FOTP-33a. This covers
tensile testing for these cables using a
component for measuring fibre strain.
The question becomes whether less
than five per cent shrinkage, as stated in
this specification, is still an acceptable
standard or benchmark. It could be too
broad a measurement based on the
fact that new bend insensitive fibres
will not show the same sensitivity. If
any flaw or defect in the fibre could
possibly be missed by current testing
standards, yet could have a significant
impact in deployed fibres over time,
then new criteria such as fibre strain
should be added to current test methods,
specifications and standards.
What might work in bulk cable may not
work in cable connector interfaces, and
what may pass testing today might not
work over the expected life of the fibre.
The existing aging cycle was developed
using high temperature only to detect
changes in the jacket and buffering
compounds, such as hardening, cracking
or shrinkage over the aging process. Today,
it may be wise to consider whether those
compounds will fail or not when testing
is based on different parameters. One
such area is thermal coefficient of linear
expansion. This is the rate of expansion
and contraction of a material over a given
temperature profile. The rate of polymer
change is typically an order of magnitude
compared to glass.
For example, if continuous shrinkage
occurs beyond the normal shrinkage tests
and is identified by increased attenuation,
how do you detect it in reduced bend
radius fibres where no or minimal
increased attenuation is detected? The
answer is that you would not – until
perhaps the fibre reaches a pivot point
where it is no longer a viable long-term
communications medium.
In the loose-tube cable environment,
the opposite can potentially occur. That
is, there could be too much excess fibre
length and the fibre would bunch up –
not due to shrinkage, but because an
attenuation increase was not detected
in the reduced bend radius fibre. The
individual tube is not tested for shrinkage
separately but may be coiled for several
metres in a transition housing and not
have the design of the overall cable
to control shrinkage in the individual
loose tube. The bottom line is that since
attenuation resistance is increased in
reduced bend radius fibres, microbends
and other stresses on the fibre may
not be detectable with today’s testing
▲ ▲ Figure 5: Optical fibre strain gauge measurement
system
▼ ▼ Figure 4: FOTP-33 long gauge tensile test fixture