PETER DYKES FIBRE SENSING
SENSE AND SENSITIVITY SUBSEA SENSING WITH FIBRE
When we think of subsea fibre cables, we automatically think of ultra-high-speed telecommunications links stretching between continents. However, those same cables have another capability beyond transporting huge amounts of data, they can be used to sense a wide range of events, including but not exclusively, ones which might culminate in damage to the cable itself. To find out more about this intriguing and innovative use of fibre technology, Optical Connections editor Peter Dykes spoke with Daniel Danskin , commercial manager, DAS, at Nokia’s vertical company Alcatel Submarine Networks, and Szilard Zsigmond , senior director, High- Speed Electro-Optics subsystems, at Nokia.
F ibre sensing has its roots back in 1985 with the Norwegian company Optoplan, which began developing the technology for oil and gas installations. Following a series of buyouts, it was acquired by Nokia as part of the Alcatel-Lucent purchase in 2016. Now called Alcatel Submarine Networks (ASN), the company recently released its Distributed Acoustic Sensing (DAS) technology, for use in fibre optic submarine telecoms cables, which can detect a variety of subsea events including nearby bottom trawlers, ships dropping anchors, cable movements and various seismic events. Daniel Danskin, who works specifically on stand-alone DAS systems says, “Essentially, DAS turns the fibre into a microphone every metre using current technology which has a range of 150 kilometres, that’s 150,000 microphones under the sea. With future technology, we’ll be able to reach 4,000 kilometres or more. What we’re looking at now is what can we do with those microphones, and they are essentially microphones. We can
split out and Fourier transforms are used to get the frequency spread, essentially creating a microphone. Individual points on the fibre can then be looked at using time-domain reflectometry. As Danskin says, the OptoDAS Interrogator unit is essentially a phase-based time domain reflectometer. He adds, “ASN do that in a very different way to most other companies. There are four or five companies doing this, but most companies send a pulse out, wait for it to travel the full length of the fibre and repeat the process. The problem with that is there’s not much light in the fibre. These pulses are 10 nanoseconds long and in order to travel longer distances, say up to 50 kilometres, you need to increase the amount of light. You can’t increase it in the pulse because you have to make it bigger and that changes the way you measure and means you can only measure big events. It also means you don’t get a lot of granularity in the data. With ASN’s coherent pulse, we send a frequency sweep and so we get tens of thousands of times more light into the fibre than
pick up frequencies from DC - almost zero hertz, right up to around 600 hertz, so you might wonder that if you can listen to a line of microphones that span from, say, across the English Channel, or from Trondheim to the UK, (if you measure for both sides), what can you hear? But we can do a lot more than just ‘what can you hear’.” HOW IT WORKS ASN’s main sensing product for fibre telecoms cables is it’s 3U, rack-mounted, OptoDAS Interrogator unit, which is also available as a portable field rack. To operate, it requires a power connection, a single fibre connection, an Ethernet connection and a computer to record the data. Danskin explained that whenever a fibre cable is put under strain, the state of polarisation of the light traveling through it, changes. When pulses of light are sent into the fibre, the polarisation change of the returned scattered light can be used to indicate the dynamic of strain in the fibre. If this is done thousands of times a second and integrated over the whole of the fibre, a phase pattern can be built up for the whole cable. The phase is then
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| ISSUE 29 | Q2 2022
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