WCA July 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

Aramid ribbon strength member

Aramid yarn

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. ❍ ❍ Figure 3 : New geometric strength member vs old loose yarn strength members

Minimum grip to lift cable with 5lb load

Conventional optical patch cord cable after release of pulling tension. Note: significant

cable jacket deformation

1.6mm stan- dard cable

5lb (2.25kg) weight

❍ ❍ Figure 2 : Experimental fixture to simulate 5lb (2.25kg) hand pull on cable jacket of conventional patch cord

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Wire & Cable ASIA – September/October 2007 July/August 2013