EuroWire March 2015

Technical article

2.4 Cutting costs by minimising material in the cable and reducing design margins Many overhead cables are designed with zero per cent strain on the optical fibre. With increased cost pressure, design engineers are challenged to reduce material costs. As strength elements around the optical fibre are removed, the optical fibre starts to take some of the axial strain traditionally taken by the strength members in the cable. The design engineer can look to the various cabling standards and see that the maximum allowable long-term strain is 20 per cent of the proof test level. In effect, for these cables, the industry is progressing from a common design practice where no strain was carried by the optical fibres after installation to one where a strain of up to 20 per cent of the proof test level is allowed. The long history of reliable cable performance at this strain level makes it seem a reasonable decision. 2.5 Higher proof-tested fibres 1.38 GPa (200 kpsi) are now available In the previous section it was shown that material costs can be reduced by allowing strain on optical fibre. For traditional optical fibre that is proof tested at 0.69 GPa (100 kpsi), the maximum allowable strain on the fibre at the 20 per cent limit is 0.14 GPa. A design engineer could choose to use higher proof-tested fibre, such as 1.38 GPa (200 kpsi) fibre, at the 20 per cent limit, and the allowable strain on the fibre after installation would increase to 0.28 GPa. This would allow further material reductions in the optical cable by allowing greater cable strain to impart twice the strain on the optical fibre. The net result could be a lower cost optical cable. 2.6 Combined impact of modified optical cable design criteria Taken together, all these trends can result in a scenario that may not be optimal to the service provider. The strain on the fibres allowed by the usual criteria is higher, but the strain is not impacting the attenuation because of the use of G.657 fibres. The net result could be an optical cable that is deployed with up to 0.28 GPa long-term strain on the optical fibres. Meanwhile, there remains an expectation that the fibres will survive 30+ years without breaking. This situation tests the limits of reliability theory and should be looked at more closely before it is implemented. 3 Origin of the current allowable strain criterion The current rule of thumb used for cable design is a maximum allowable strain of

in characterising the long-term reliability of an optical cable. This region contains flaws closer to the proof-test level that are spaced at a frequency which may be several kilometres apart. Over time, these can become fibre breaks if the cable is left in tension. Understanding this region requires information that can only be gathered by measuring many kilometres of fibre. Higher proof test levels will eliminate some of the larger flaws in the fibre. However, the exact impact to optical fibre reliability in a deployed cable is hard to determine without more information on the overall flaw distribution in the fibre. One way to illustrate this would be to proof-test an optical cable at a level just shy of the intrinsic strength of the fibre, or about 3.8 GPa (550 kpsi). If a 1,000m fibre sample generated from that experiment were left at a constant stress of 110 kpsi, the fibre would likely break in less than a day, or well in advance of the 40-year expected life time. This example is an extreme case, but highlights the importance of understanding the complex equations that govern reliability. 4 Guidance from IEC technical report on reliability One of the currently accepted reliability models has been published by the IEC [4] . One of the equations found in that report is used to predict fibre lifetime – the lifetime equation for optical fibre after proof testing. This can be shown as the following expression:

20 per cent of the proof test level. This criterion comes from the reliability work done in the 1990s [2,3] . In those studies, the authors show that long-term performance can be related to the proof test stress, but this assumes a certain proof test failure probability. They, then, look at various stress corrosion parameters and at 50 kpsi and 100 kpsi proof-tested fibre to show that their approximation is a reasonable, conservative method to ensure long-term reliability. This work was an important step forward for the fibre industry and supported the move for proof-testing fibre at the current levels. Unfortunately, there is a key assumption about the flaw distribution of the optical fibre – specifically the chance of a fibre breaking when proof-tested. This probability is not constant and can vary for fibres manufactured under different conditions or using different raw materials. Figure 1 shows a failure probability curve for silica fibre generated by one of the authors’ facilities using 10m gauge length to illustrate the range of flaws found in optical fibres. The figure shows two regions: region I (intrinsic strength) and region II (extrinsic strength). The curve illustrates the main regions that need to be characterised to predict long-term fibre reliability. Region I is the high strength intrinsic region. The fibre investigated showed the inherent strength of the glass at ~4.6 GPa, which is significantly above the limit of 3.1 GPa recommended in Telcordia GR-20. Short gauge-length strength testing in this region can be used to determine the n value, which is greater than 20 for the fibre investigated. The intrinsic strength and n values are typically specified by end users to ensure long-term reliability of the cable. Unfortunately, the extrinsic portion, shown as region II, plays an important role

▼ ▼ Figure 1 : Failure probability for over 100km of fibre tested at 10m gauge lengths

Region I Intrinsic

Region II Extrinsic

Log (failure probability)

Log (stress)

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March 2015