EuroWire May 2020

Technical Article

Deployed terrestrial optical cables with ultra-low loss fibres By Nikita Korotkov (Corning SNG, Moscow, Russia) and Vladimir Pigarev (Saranskkabel-Optikа, Saransk, Russia)

Abstract Ultra-low loss fibres allow cable systems to achieve longer transmission distances without optical signal regeneration and reduce the overall network construction costs. At the same time, terrestrial optical cables have to operate over a wide range of temperatures, have medium or high fibre density and maintain their loss performance after installation. In this paper, we show results of the attenuation measurements of terrestrial cables with ultra-low loss fibres after field installation. The mean fibre attenuation used for the deployment discussed in this paper was less than 0.163 dB/km, excluding splices. The mean value for attenuation in the installed optical ground wire (OPGW) cables including splice losses was measured to be 0.165 dB/km at 1,550nm wavelength. Such a low level of cable attenuation enables 100+ G unrepeatered signal transmission at distances longer than 500 km. 1 Introduction Unrepeatered long haul lines are used for terrestrial applications where electrical power supply is limited or prohibitively expensive. In these cases a standard network design that involves the use of EDFAs (erbium doped fibre amplifiers) cannot be realised, which makes them very sensitive to optical power loss. Raman amplifiers from both sides and ROPA (remote optical pump amplifiers) are frequently used for these lines to extend maximum reach. However, the only way to achieve distances longer than 400 km for 100+ G

Aluminium clad steel wire Stainless steel loose tube Optical fibre

▲ ▲ Figure 1 : OPGW cable design

▲ ▲ Figure 2 : The climatic chamber with the cable sample

ITU-T G.652-compliant ultra-low loss fibres with mean attenuation less than 0.163 dB/ km were used to make the tested cable.

unrepeatered signal transmission is to use optical fibres with ultra-low attenuation. A number of hero experiments [1, 2, 3] demonstrated unrepeatered 100 G signal transmission with total optical loss around 90 to 100 dB and reachable distances of 500 to 600 km. However, for terrestrial applications we need to consider additional margin of optical budget to account for end-of-life system performance. To achieve such a performance in the field for 500+ km distances, the use of ultra-low attenuation fibres (average attenuation less than 0.165 dB/km) is essential. The optical cable design should prevent significant attenuation increase during and after cable installation and at all operating temperatures. The desired attenuation increase should be less than 0.01 dB/km to ensure that final cable attenuation is compliant with the required transmission margin. This paper discusses OTDR measurements of OPGW cable during temperature cycling test and after cable deployment.

2 OPGW cable design OPGW cable is a fibre optic cable incorporated in ground wire for suspension on overhead power line support with voltage of 110 kV and higher. It performs two functions: to transmit data and to protect the phase conductors from a lightning strike, which may damage the transmission system. This cable design can also be used in areas with high corrosion activity. The structure of OPGW cable used in this work is shown in Figure 1 . It consists of stranded aluminium clad steel wires and two stainless steel loose tubes filled with hydrophobic gel. The cable has an outer diameter of 12mm and a loose tube diameter of 2.8mm. Each loose tube contains 24 optical fibres, and

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May 2020