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Wire & Cable ASIA – September/October 2007 54 July/August 2013
As with all communication devices, improved performance
must be accomplished while ensuring affordability.
Designs that meet these new requirements but are costly
and hard to produce will not succeed. The cable must also
be able to be mass-produced on typical cable equipment
with acceptable yields and quality performance.
2 Challenges to “optical wire”
Traditional simplex/duplex optical fibre cables, developed
over the past 30 years or more, consist of a loose tube
design with Aramid yarns for strength. The glass fibre is
embedded in the centre of the yarns with a polymer tight
buffer coating to prevent severe bending or impact. Aramid
yarns are deployed so that both ends can securely attach
the connectors. Thus, if a connector is pulled, it is the
non-stretching yarns that are actually being pulled and not
the fibre or jacket itself.
The challenge in strengthening the fibre cables this way is
that if we pull them by the insulation as if they were copper
wires, we’re actually pulling on a piece of polymer plastic
with very little strength. Pulling the fibre jacket temporarily
stretches the polymer while the glass length remains
constant. This causes a mechanical decoupling of the fibre
from the strength members and polymer jacket, allows a
bunching of the outer jacket and allows an unplanned
movement of the buffered fibre to cause excess length on
one side of the pull and a tensile condition to occur on the
other.
This typically results in large macro bend losses as well as
possibly exceeding the minimum bend radius of the optical
fibre. This can shorten the life of the cable significantly.
When developing 3mm fibre cables, the jackets were
relatively thick – in some cases almost a millimetre thick.
This provided a bit more intrinsic strength in the plastic
polymer before it was stretched. And early installers were
more concerned with handling characteristics.
Today, the demand is for density, so fibre cables are
becoming as small as possible. This has two results. First,
cable jacket thickness is becoming as small as possible
and second, the cables are being pulled with more
strength to fill raceways and conduits with more fibre. Both
of these issues can affect reliability and performance of
the fibre. As the smaller fibre cables are pulled, the jackets
are stretched. As they shrink back over time, enough
friction is generated to push the buffered fibres back. This
action results in a localised area of excess fibre, known as
a microbend, as the jacket shrinks.
As optical cable sizes were reduced to 1.6mm, this
phenomenon was caused by as little as a few ounces of
force instead of pounds. Thus, as optical cables became
smaller, more delicate handling was required during
installations. This new category of cables became known
as “small form factor” cables because they could no longer
pass the same testing as their larger counterparts. Tensile
gradings went from 22 pounds to nine pounds, allowing
minimal amounts of Aramid yarn and decreased jacket
thickness. But it also resulted in products that required
much more care in handling than any copper wire.
The challenge was to develop a new fibre cable design
for small form factor products that could meet the
requirements for more density while providing wire-like
strength that would allow it to be handled and pulled
without causing attenuation and other performance issues.
The challenges were met by solving three major issues –
strength, connectivity and thermal balancing.
3 Achieving copper-like strength
To provide the strength of copper in a 1.6mm fibre optic
cable was the first challenge. Installers should be able to
pull the cable in a straight line like copper wire without
needing to wrap it around a mandrel to keep from
damaging the jacket. At the same time, the jacket must be
about one-third the size of conventional jackets. The free
space around the glass needed to be reduced to make the
cable as small as possible. Yet, the cable had to meet all
impact, resistance and crush strength testing.
As small form factor cables are handled, the fibre can
actually migrate to one side or the other of the jacket as
the loose yarns give way. Once that occurs, the fibre is
less protected on one axis and no longer provides the
protection that was conceptually designed into it. By using
a tape with an adhesive matrix material, a custom tooling
was designed to wrap several times around the fibre in
a longitudinal fashion. The longitudinal tape wrapping
ensures the centring of the fibre while only a very thin outer
jacket bonds to the tape. This bonding allows installers
to perform reasonable hand pulling or hand setting of the
cable without stretching the jacket.
Aramid ribbon
strength
member
Aramid yarn
Minimum grip
to lift cable
with 5lb load
1.6mm stan-
dard cable
5lb (2.25kg)
weight
Conventional
optical patch cord
cable after release
of pulling tension.
Note: significant
cable jacket
deformation
❍
❍ Figure 2: Experimental
fixture to simulate 5lb
(2.25kg) hand pull
on cable jacket of
conventional patch cord
❍
❍ Figure 3: New geometric strength member vs old loose yarn
strength members