EoW May 2013

Technical article

In terms of optical fibre cable, we must design products that behave closer to copper insulated wire in fibre cable handling, placement and management. New optical waveguides have made this option viable, but we as cablers need to continue the evolution and design installable “cables” (wires) that meet customer needs and define a new class of optical waveguide product. The design presented here is a geometric core design whereby the optical fibre is located in the centre of the core and loose yarns have been removed in place of geometric strength members. These strength members provide multiple functions, such as outer jacket adhesion (to assist with hand pulling), fibre buffering (against impact and crush loads) and reliable access to the optical fibre for fusion splicing or field connectorisation. As with all communication devices, improved performance must be accom- plished 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.

Aramid ribbon strength member

Aramid yarn

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

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.

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.

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.

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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May 2013