EuroWire May 2019

Technical Article

Bottom Profile

DA SA LWS

Cable Tension

Repeater Touch Down

Ship’s Heading Change

Transition (SA >LWS)

Transition (DA >SA)

Repeater Launch

Water Depth [m]

Cable Tension [KN]

Repeater

Distance from Beach [km]

▲ ▲ Figure 5 : Cable tension

▲ ▲ Figure 6 : Bottom profile

Optical spectral attenuation curves at different points of the cable installation are shown in Figure 7 . A small variation (attenuation increase) during laying was confirmed. This change is presumed to be caused by laying tension being applied to the submarine cable section between seabed touch down point and cable ship. In the section where tension/elongation is applied, the condition of optical fibre temporarily changes and sensitively reacts, resulting in the optical attenuation increasing due to microbending occurring when fibres move and compress against each other. After the laying operation, optical spectral attenuation decreased to the same level measured at the factory. Measurements were continually taken for five days after the repeater, where the “loop back splice” is stored, touched the seabed and confirmed that optical spectral attenuation along the entire measurement window was very stable. In the optical attenuation during/after laying operation by OTDR at 1,550nm (C-Band) and 1,625nm (L-Band) was also measured. Measurement results at 1,550nm are shown in Figure 8 . system measurement, approximately one year passed since cable manufacturing completion, with no increase in optical attenuation confirmed by pre-shipment measurement. Results shown in Figure 7 and Figure 8 were consistent with the ones confirmed at the factory, and no additional reflection point or irregular section appeared during handling of the cable. 5 Conclusion The authors obtained excellent optical attenuation results for submarine cable equipped with optical fibres with an Before shipment of the

Optical Attenuation (dB/km)

Remarks

@1,550nm

@1,550nm

At Factory

0.153 0.154 0.152 0.152

0.154 0.156 0.154 0.154

Reference

During Laying After Laying After 5 days

including JB-SpL including JB-SpL including JB-SpL

▲ ▲ Table 1 : Optical attenuation results

Optical attenuation measurements were conducted by cut-back method and two-way OTDR during and after the cable laying operation.

The submarine cable was laid from the landing site towards open seas (direct landing installation) within specified pulling tension ( Figure 5 ). Water depth is 0 to 3,800m as shown on the Bottom Profile of Figure 6 . The cable tension is carefully monitored during installation to control the touch down point as to follow the bottom profile. Therefore, cable tension values change due to the various parameters (cable weight in water, catenary length, laying angle, ship speed, sea swell, etc) that are measured to drive cable-handling equipment on board. The cable tension has its highest value on top of the sheath, where also cable elongation is large and the risk of affecting the transmission properties of the fibre is a concern.

4 Measurement results

Optical attenuation results are shown in Table 1 . Optical attenuation at the factory, as reference, was measured with the cable coiled and without on-board JB. During/ after laying values include extra optical attenuation due to on-board JB inserted to rebuild span (two splices added). The on-board JB splice loss (two splices) was 0.09dB, evaluated from two-way OTDR traces.

▼ ▼ Figure 3 : Submarine cable configuration at factory (before loading)

Repeater

Shore-Side

Sea-Side

Loop Splice

▼ ▼ Figure 4 : Submarine cable configuration as laid

Repeater

Shore-Side

Sea-Side

Loop Splice

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May 2019