Fibre optic cables have become the backbone of digital communication across the oceans. These cables are not just useful for transmitting information from one location to another – they can also be used as an incredibly long sensor array in themselves. Distributed acoustic sensing (DAS) is a powerful way of measuring sound and vibrations in the sea floor. Dr Ole Henrik Waagaard, Dr Erlend Rønnekleiv, and Dr Jan Petter Morten at Alcatel Submarine Networks (ASN) in Norway have improved the sensing range of DAS and are finding innovative ways to employ DAS in submarine applications.
Using sensors, we can translate observations about the world into data. A sensor is a device that responds to a physical change and converts the information into a signal that can be interpreted by a computer. Data can then either be logged for longer-term monitoring or further analysed, for example as part of a feedback or warning system.
An ideal sensor will be capable of real-time monitoring and sufficiently sensitive for the process of interest. While real-time monitoring is often desirable, it results in a large amount of data being transferred – this can be time consuming and potentially challenging.
One type of sensor that offers excellent performance is fibre optic cables. Fibre optic cables now make up the majority of telecommunications infrastructure. The ability to transmit information in the form of light pulses along long fibres has many advantages over traditional conductive metal wires that use changing electrical currents to achieve the same result. Fibre optics allow for the faster transfer of large amounts of data with fewer losses. Some of these properties can also be used for sensor applications with fibre optics.
Distributed acoustic sensing
Distributed acoustic sensing (DAS) uses fibre optic cables to probe changes in the environment. Laser light pulses are passed down the fibre. Small parts of the light can be reflected or scattered due to minor irregularities in the silica glass that makes up the fibre structure. Along the full length of an optical fibre, which may span thousands of kilometres across the oceans, there are many of these small imperfections from which this scattering process, known as Rayleigh scattering, can occur.
DAS systems are capable of detecting changes in the fibre length of just a nanometre.
The time between the transmission and the arrival of reflected signals can be measured accurately. Any slight change in the position of a Rayleigh scattering point will change this delay. This means that each of these small imperfections acts as a sensing point. Any change in temperature or stress on the fibre will cause minuscule changes in the positions of the scattering points and subsequently changes in the optical delay. The optical delay is thus a measure of the magnitude of the deformation or stress. Such fibre stresses may be caused by sound waves moving in the water column or the sea floor that apply stresses in the fibre optic cable material. This effect allows for detecting sound sources several kilometres away from the cable.
With DAS systems, it is possible to create sensors with a range along the cable that is up to 50km, capable of detecting for each metre a change in the fibre length of just a nanometre. While this is already an impressive achievement and such systems are already in use for many real-world applications, Dr Ole Henrik Waagaard, and Dr Erlend Rønnekleiv at Alcatel Submarine Networks (ASN) in Norway have found a way to get DAS systems to work over even longer ranges by sweeping the optical frequency in the transmitted pulse. By extending the feasible range of DAS to over 171km, the team has made an essential development in creating a sensor for applications that need the longest measurement distance possible. This is important for detecting oceanic earthquakes, monitoring coast environments and activity, and cable integrity.
A standard DAS system uses a pulsed interrogation mode. This mode of operation simply involves the measurement of the reflected pulse from the fibre. The reason that pulsed interrogation DAS systems have been limited to around 50km range is because of the signal losses down the fibre. Even the highest-quality fibres suffer some propagation loss, which attenuates the pulse as a function of distance.
The researchers’ approach involves a different technique for measuring the reflected DAS signal. Rather than using pulses with a fixed central frequency, their new approach involves sweeping the optical frequency of the transmitted pulse by stretching it in time, so each frequency component arrives at a different time. In this way, the pulse energy can be greatly increased, while the spatial resolution can be maintained and is determined by the inverse of the bandwidth of the pulses used.
This approach to measuring the reflected DAS signal has meant Waagaard and Rønnekleiv have been able to triple the length over which DAS measurements can be performed. Another advantage of this method is that it does not require improving the transmission properties of the fibre or generating more intense input pulses to achieve this longer sensing range.
The research team hope these extended-range DAS fibre optics can be used for monitoring entire coastlines. ASN are now routinely incorporating this technology into their DAS systems for a number of different applications, including earthquake and submarine environmental sensing.
A vast amount of our communication cables traverses remote, underwater regions that are challenging to access. Internet traffic is shuttled along huge intercontinental, underwater cables that make up the international network we have all come to rely on. Many monitoring stations of ocean conditions, earthquakes, and wildlife are in similarly remote places as gas and oil stations for natural resource extraction.
The challenge with building infrastructure under the oceans is that maintenance can be very costly. It is crucial that, in the event of cable damage or a break, the fault can be diagnosed accurately and quickly to avoid wasting a large amount of expense. DAS networks can help with this – any cable damage will affect the reflected signals, changes in the local pressure will also lead to deformation of the fibre optic cable that is detectable, and the position of the damage can be calculated with high accuracy.
The researchers have been using their DAS sensor instruments for long-term underwater monitoring. They have found that the fibre optic cables can be sufficiently sensitive to detect the very weak oceanic earthquakes that may be precursors for larger earthquakes, and to identify and localise various whale species based on their singing.
During the summer of 2020, one of their instruments was set up for DAS recordings with 120km sensing range on a fibre cable at the Svalbard archipelago. Researchers were able to detect the sound of several hundreds of whale calls. The location of the whales can be calculated based on the difference in the delay to the different portions of the cable. This enables continuous surveillance of whales and protection of the whales from boat traffic.
Waagaard and Rønnekleiv have been able to triple the length over which DAS measurements can be performed.
The fibre optic cable can also be used to identify human activities, such as boats. As fibre optic cables transfer information so quickly, the DAS network can monitor the position of the boat in real time and track the specific positions. As there are so many ‘sensing points’ along the fibre that it essentially acts as one long continuous sensor that spans kilometres, this means there are no blind spots. Based on the information on how the different sections of the cable sense the generated acoustic waves that move through the water from the boat to the cable, both the position along the cable and the distance from the cable can be obtained.
The team believes that DAS technologies could be a powerful asset in the fight to protect marine life and underwater cables. Acoustic waves are also created by anchors or fishing gear that come into contact with the sea floor and would also be propagated along the seabed. The DAS cables could detect these threats that might damage the fibre optic at a distance of kilometres from the cable. In this way it is possible to prevent cable damage and check the amount of human activity in some underwater regions.
DAS is an excellent tool for sensing. Thanks to these innovations, we can now make sensors that can reach different coastal regions for monitoring ship and underwater activity with great accuracy. It is likely, both offshore and onshore, that DAS measurements on cable networks will enable full, online monitoring and early warning systems for faults and natural disasters.
What inspired you to conduct this research?
Both Waagaard and Rønnekleiv have been working with fibre optic sensing for more than 20 years. They understood early the current concept for DAS had limitations in the pulse energy that can be transmitted into the fibre. A new type of DAS was conceived based on first principle understanding of the physical limitations. Later, Morten joined to explore new applications for DAS.
What are the next technical challenges that need to be overcome for the future development of DAS systems?
DAS systems currently have the range to monitor coastal regions; however, to monitor thousands of kilometres across the oceans will require cascaded light amplification of the DAS signals.