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Wire & Cable ASIA – September/October 2007 53
Wir & Cable ASIA – March/April 13
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
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❍ Figure 5: Optical Fibre Strain Gauge Measurement System
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❍ Figure 4: FOTP-33 Long Gauge Tensile Test Fixture