Subsea fibre cables can 'listen out' for sabotage

A diver discovered a fiber optic cable on the seabed of the North Sea. He swam closer and closer until it was within reach.

He reached out. But someone noticed him lingering there; someone was watching him.

"He stops, he touches the cable gently, and you can clearly see the signal," said Daniel Gewehr, global sales manager at German tech company AP Sensing. "The acoustic energy transmitted through the fiber optic cable basically interferes with our signal, and we can measure that interference." Recent months have seen multiple incidents of damaged telecommunications cables in the Baltic Sea, raising alarms.

These cables are so important, carrying vast amounts of internet data between nations, that NATO launched an operation called "Baltic Sea Sentinel," patrolling the Baltic Sea with aircraft, warships, and drones. The EU is also strengthening measures to monitor and protect the cables. Despite these efforts, authorities cannot be everywhere.

Therefore, some companies are trying to monitor any conditions near the cables by using fiber optic signals to listen for lurking underwater drones or malicious ships dragging anchors along the seabed. AP Sensing tested its system last year – not a real sabotage attempt – when a diver slapped the seabed cable beneath him, which the company monitored. The company also deployed boats, drones, and divers equipped with underwater scooters to see how accurately its software could identify and recognize the presence of these vehicles.

The team also tested whether their cables could "hear" a ship dropping anchor into the water. When light pulses travel along the fiber, sometimes tiny reflections bounce back along the line. These reflections are affected by factors such as temperature, vibration, or physical disturbance of the cable itself. For example, noticing a temperature change in a section of buried cable could indicate that the section has been exposed.

AP Sensing showed me a video of a man walking across a lawn before raising a rifle and firing it in a test. A fiber optic cable buried a few meters underground recorded the entire process. CEO Clemens Pohl pointed to a graph showing the disturbance in the fiber optic signal, saying, "You see every footstep." Footsteps appeared as brief blips or lines, and the gunshot appeared as a larger smudge.

Using this technology, it is even possible to calculate the approximate size of a ship passing over a submarine cable, as well as its location, and in some cases its direction of travel. This can be correlated with satellite imagery, or even Automatic Identification System (AIS) records, which most ships constantly broadcast.

The company added that monitoring capabilities could be added to existing fiber optic cables if an unused "dark" fiber is available, or a lit fiber with enough spare channels. However, David Webb of Aston University said that fiber optic sensing technology cannot capture disturbances from afar, and you need to install signal listening devices, or interrogators, every 100 kilometers (62 miles) or so along the cable.

AP Sensing says it can pick up vibrations from hundreds of meters away, but "generally not from kilometers away." The company confirmed that its technology is currently deployed on some cable installations in the North Sea, but declined to comment further. Dutch company Optics11, which also produces fiber optic acoustic sensing systems, says its CEO Paul Heijden, "People really need early warning in order to figure out what to do."

Heijden believes that cables installed specifically for monitoring marine activity could be particularly useful – for example, such listening cables could be placed 100 kilometers from important ports, or near critical gas pipelines or telecommunications cables, rather than inside these assets. This would give operators a comprehensive view of vessel traffic in the area and potentially provide advance notice of vessels heading towards critical assets.

Heijden added that Optics11's fiber optic listening technology could be deployed on military submarines, and he also said that the company is about to begin testing a monitoring cable installed on the seabed somewhere in the Baltic Sea. Douglas Clague of network test and measurement company Viavi Solutions said that demand for fiber optic sensing technology is growing: "We are definitely seeing an increase in the number of requests."

Kristian Prytz, head of Central Europe, Middle East, Africa and Subsea Cable Business at Swedish cable company Hexatronic, said that some of the cables damaged in recent incidents were manufactured by his company. Prytz believes that acoustic sensing is an emerging technology that will become more common in the future. But in terms of physical hardening, there is little that can be done to protect cables from damage.

He said that today's fiber optic cables already have a metal sheath folded and welded around the fiber. There are also "armoring wires," which are thick metal wires that run along the outside of the cable, and in some cases, there are two layers of such wires. "On the UK side of the English Channel, where there are a lot of rocks and a lot of fishing activity, you want it to be double armored," Prytz said.

But Prytz said that if a ship deliberately drags its heavy anchor across even a double-armored cable, it will almost certainly still damage it – such is the force of the collision or pulling action. While cables can be buried under the seabed for added protection, this can become very expensive for long distances and depths below a few tens of meters.

TeleGeography research analyst Lane Burdette said, "Cables are always going to break." She added that "the number of cable faults per year has actually remained stable over the last few years," explaining that despite the installation of more and more submarine cables during this period, the 1-200 faults that typically occur each year have not increased.

Burdette also pointed out that even when cables break, telecommunications networks typically have a lot of redundancy built in, meaning that end users usually don't notice too much disruption to their services. Thorsten Benner, co-founder and director of the Global Public Policy Institute, said that despite this, the visible military response to the Baltic Sea cable breaks was welcome: "It's good that NATO and the EU have woken up."

While cable sensing technology may be useful, its effectiveness in preventing damage depends on how quickly coast guard or military patrols receive alerts about potential sabotage and respond. "The question is how quickly you can get in contact with a ship," Benner said.