WCA MAY 2015

❍ ❍ Figure 1 : Failure probability for over 100km of fibre tested at 10m gauge lengths 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 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. allowable strain criterion The current rule of thumb used for cable design is a maximum allowable strain of 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. 3 Origin of the current

Together, these two improvements to the optical fibre have a huge impact in observed cable attenuation, even under aggressive conditions. The superior fibre and coating properties can ‘mask’ the impact of a poor cable design or installation. When optical cables using traditional G.652 fibres are deployed with high residual strain on the fibre, higher attenuation is often observed. By default, the cable manufacturer is required to control the strain on the fibre to ensure the cable can meet the qualification requirements. When G.657 fibres with micro bend-resistant coatings are used for the same cable design, then the measured attenuation will improve and the same cable design may pass this optical requirement. The net result of using G.657 fibres is that the cable will pass this qualification test. However, after deployment, higher fibre strain could pose a long-term reliability risk. In short, if the cable is designed properly, G.657 fibres and micro bend coatings are a huge benefit to the optical performance of the deployed cable. But if the cable is designed poorly, the improved optical fibres can mask the strain issue from the end user, which could pose a long-term mechanical reliability risk. 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.

Region I Intrinsic

Region II Extrinsic

Log (failure probability)

Log (stress)

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Wire & Cable ASIA – May/June 2015