**Technical article**

March 2015

**87**

*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

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

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:

Log (failure probability)

Log (stress)

Region II Extrinsic

Region I Intrinsic

**▼**

▼

**Figure 1**

:

*Failure probability for over 100km of fibre tested at 10m gauge lengths*