PETER DYKES FIBRE SENSING
Many cables are buried up to two metres in the seabed, however due to currents and movements of the seabed, cables can become ‘de-buried’ causing them to move from side to side, which weakens the protective coverings, eventually leading to a break. This too can be detected using DAS, and the movement shows clearly on the sensing device (see image). GOING FURTHER Most damage events happen because of the shallowness of the ocean near the coast, however DAS is at present only effective as far as the first 60 kilometres of the cable, i.e. the distance to the first repeater. Danskin explained that it is possible to have a dedicated fibre running
selective switches. He says, “One of the big challenges on the terrestrial side is to find quiet spectrum. We are using the C and L band, plus all sorts of other wavelengths and it’s necessary to minimise the noise coming from all of these sources. Obviously, terrestrial systems are noisier than a subsea, but still, I believe there’s a fair number of applications on the terrestrial side. Some of the capabilities of DAS are unbelievable. One I’ve seen on the terrestrial side was identifying fibres. Without opening the splice box, you just tap a fibre and you can immediately see which one is moving. You can imagine how big a problem this is when you are in a manhole or somewhere with endless fibres. You don’t want to open up the splice box because you’ll probably break something by even touching it.” He added that this application is still at the proof-of-concept stage and wasn’t aware of any products in the market, however a lot of companies are looking at the application, including Nokia and ASN. Another application for DAS that is in development is network assurance. Advanced monitoring, sensing, and analytics become the focus of High Speed Electro-Optics. The enhanced monitoring functionalities inside coherent ASICs enable a completely new way of understanding the network behavior. Zsigmond said, “There is a very clear use case for network robustness in order to improve service availability. Operators and cloud providers want to know exactly what’s happening in the fibre, minutes before a break happens so they can move their traffic over to spare fibre pairs, so that’s a very clear use case.” Zsigmond also alluded to other possible applications such as road traffic monitoring and as a viable and cheaper alternative to the radar proposed for detecting broken down vehicles on the UKL’s ‘smart’ motorways, given that fibre backbones generally run parallel to such roadways. It is clear that while some aspects of DAS and transponder-based analytics are still in development, the technology will have many uses in the future, some of which are yet to be discovered.
an ordinary distributed acoustic sensing system, which means we can go up to 150 kilometres, and that’s just with a standard system.” He continued, “We’ve got incredibly low noise, so we can pick up faint signals when we’re operating around 50 kilometres and we’ve dropped the noise down on our system. When we’re listening to anything subsea, we’re only three dB above the baseline, so we can pick up very quiet things. So what’s the future hold? Very simply, amplification. That will take us up to 250 kilometres using remote pumped amplifiers, and repeaters will take us to 1,000 kilometres, 4,000 kilometres or maybe more. The future is certainly long range for us.”
DAS trace showing cable movement. Image: Nokia/ASN
FINDING FAULT The uses to which DAS technology can be put are still being explored, but perhaps one of the most pressing is damage from shipping. Danskin estimates that around 70% of cable breaks are caused by bottom-trawling ships or anchors being dragged across a cable. The other 30% are largely down to cables moving to and fro on the seabed, which causes the protective armour casing to crack and break, but even that is detectable. Danskin explained that a cable between Lowestoft on the east coast of the UK and an oil rig in the North Sea, is being monitored and while it should be buried, it has been hit and damaged many times by the large number of trawlers that fish in the area. Most, if not all cables are marked on mariner’s maps, but not every skipper pays attention to them. However, by using DAS, it is possible to detect a trawl as it hits the seabed and by turning the sensor into an array, the exact position of the trawl can be triangulated. With repeated interrogations, movement of the trawl can be tracked as far out as 2 kilometres from the cable and if it comes too close, the ship can be alerted.
the length of a cable to provide end-to- end sensing, by bypassing the repeaters, without going into the amplifiers. On long distance cables however, such as those stretching across the Atlantic Ocean, every fibre is valuable and operators are reluctant to lose any capacity, but he predicts that by the end of 2022, it will be possible to run the technology over lit fibre. TERRESTRIAL SENSING Szilard Zsigmond, explained that ASN has also been engaged in interworking testing with DAS systems over terrestrial networks, using lit fibres over wavelength
Daniel Danskin, Commercial Manager, DAS, Alcatel Submarine Networks
Szilard Zsigmond, Senior Director, High-Speed Electro-Optics subsystems, Nokia
ISSUE 29 | Q2 2022
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