WHY WE NEED VISIBILITY INTO SUBMARINE CABLE ACTIVITY.

We can’t protect what we can’t measure. Today, we are mostly blind concerning our global submarine communications networks. We cannot state with absolute certainty whether critical parts of this infrastructure are already compromised by capable hostile state actors ready to press the button at an appropriate time. If the global submarine cable network were to break down, so would the world order as we know it. Submarine cables form the “invisible” backbone of the global digital infrastructure, yet they remain highly vulnerable. Over 95% of intercontinental internet and data traffic traverses subsea cables (which is in the order of between 25% of the total internet traffic worldwide), but these critical assets lie largely unguarded on the ocean floor, exposed to environmental events, shipping activities, and increasingly, geopolitical interference.

In 2024 and early 2025, multiple high-profile incidents involving submarine cable damage have occurred, highlighting the fragility of undersea communication infrastructure in an increasingly unstable geopolitical environment. Several disruptions affected strategic submarine cable routes, raising concerns about sabotage, poor seamanship, and hybrid threats, particularly in sensitive maritime corridors (e.g., Baltic Sea, Taiwan Strait, Red Sea, etc.).

As also discussed in my recent article (“What lies beneath“), one of the most prominent cases of subsea cable cuts occurred November 2024 in the Baltic Sea, where two critical submarine cables, the East-West Interlink between Lithuania and Sweden, and the C-Lion1 cable between Finland and Germany, were damaged in close temporal and spatial proximity. The Chinese cargo vessel Yi Peng 3 was identified as having been in the vicinity during both incidents. During a Chinese-led probe, investigators from Sweden, Germany, Finland, and Denmark boarded the ship in early December. By March 2025, European officials expressed growing confidence that the breaks were accidental rather than acts of sabotage. In December 2025, and also in the Baltic Sea, the Estlink 2 submarine power cable and two telecommunications cables operated by Elisa were ruptured. The suspected culprit was the Eagle S, an oil tanker believed to be part of Russia’s “shadow fleet”, a group of poorly maintained vessels that emerged after Russia’s invasion of Ukraine to circumvent sanctions and transport goods covertly. These vessels are frequently operated by opportunists with little maritime training or seamanship, posing a growing risk to maritime-based infrastructure.

These recent incidents further emphasize the need for proactive monitoring or sensing tools applied to the submarine cable infrastructure. Today, more than 100 subsea cable outages are logged each year globally. Most are attributed to natural or unintentional human-related causes, including poor seamanship and even worse vessels. Moreover, Authorities have noted that, since Russia’s full-scale invasion of Ukraine in 2022, the use of a “ghost fleet” of vessels, often in barely seaworthy condition and operated by underqualified or loosely regulated crews, has grown substantially in scope. These ships, appearing also to be used for hybrid operations or covert missions, operate under minimal oversight, raising the risk of both deliberate interference and catastrophic negligence.

As detailed in my article “What lies beneath“, several particular cable break signatures may be “fingerprints” of hybrid or hostile interference signatures. This may include simultaneous localized cuts, unnatural uniform damage profiles, and activity in geostrategic cable chokepoints, traits that appear atypical of commercial maritime incidents. One notable pattern is the lack of conventional warning signals, e.g., no seismic precursors, known trawling vessels in the area, and rapid phase discontinuities captured in coherent signal traces of the few sensing equipment on submarine cables we have. Equally concerning is the geopolitical context. The Baltic Sea is a critical artery connecting Northern Europe’s cloud infrastructure. Taiwan’s subsea cables are vital to the global chip supply chain and financial systems. Disrupting these routes can create outsized geopolitical pressure, allowing the hostile actor to maintain plausible deniability..

Modern sensing technologies now offer a pathway to detect and characterize such disturbances. Research by Mazur et al. (OFC 2024) has demonstrated real-time anomaly detection across transatlantic submarine cable systems. Their methodology could spot small mechanical vibrations and sudden cable stresses that precede an optical cable failure. Such sensing systems can be retrofitted onto existing landing stations, enabling authorities or cable operations to issue early alerts for potential sabotage or environmental threats.

Furthermore, continuous monitoring allows real-time threat classification, differentiating between earthquake-triggered phase drift and artificial localized cuts. Combined with AI-enhanced analytics and (near) real-time AIS (Automatic Identification System) information, these sensing systems can serve as a digital tripwire along the seabed, transforming our ability to monitor and defend strategic infrastructure.

Without these capabilities, the subsea cable infrastructure landscape remains an operational blind spot, susceptible to exploitation in the next phase of global competition or geopolitical conflict. As threats evolve and hybrid tactics and actions increase, visibility into what lies beneath is advantageous and essential.

ISLANDS AT RISK: THE FRAGILE NETWORK BENEATH THE WAVES.

Submarine fiber-optic cables form the “invisible” backbone of global connectivity, silently transmitting over 95% of international data traffic beneath the world’s oceans (note: intercontinental data traffic represents ~25% of the worldwide data traffic). These subsea cables are essential for everyday internet access, cloud services, financial transactions (i.e., over 10 billion euros daily), critical infrastructure operations, emergency response coordination, and national security. Despite their importance, they are physically fragile, vulnerable to natural disruptions such as undersea earthquakes, volcanic activity, and ice movement, as well as to human causes like accidental trawling, ship anchor drags, and even deliberate sabotage. A single cut to a key cable can isolate entire regions or nations from the global network, disrupt trade and governance, and slow or sever international communication for days or weeks.

This fragility becomes even more acute when viewed through the lens of island nations and territories. The figure below presents a comparative snapshot of various islands across the globe, illustrating the number of international subsea cable connections each has (in blue bars), overlaid with the population size in millions (in orange). The disparity is striking: densely populated islands such as Taiwan, Sri Lanka, or Madagascar often rely on only a few cables, while smaller territories like Saint Helena or Gotland may have just a single connection to the rest of the world. These islands inherently depend on subsea infrastructure for access to digital services, economic stability, and international communication, yet many remain poorly connected or dangerously exposed to single points of failure. Some of these Islands may be less important from a global security, geopolitical context and a defense perspective. However, for the inhabitants of those islands, that of course will not matter much, and some islands are of critical importance to a safe and secure world order.

The chart below underscores a critical truth. Island connectivity is not just a matter of bandwidth or speed but a matter of resilience. For many of the world’s islands, a break in the cable doesn’t just slow the internet; it severs the lifeline. Every additional cable significantly reduces systemic risk. For example, going from two to three cables can cut expected unavailability by more than 60–80%, and moving from three to four cables supports near-continuous availability, which is now required for modern economies and national security.

Reducing systemic risk means lowering the chance that a single point of failure, or a small set of failures, can cause a complete system breakdown. In the context of subsea cable infrastructure, systemic risk refers to the vulnerability that arises when a country’s or island’s entire digital connectivity relies on just one or two physical links to the outside world. With only two international submarine cables connecting a given island in parallel, it would mean that it is deemed acceptable to have up to ~13 minutes of (a total service loss) downtime per year (note: for a single cable, that would be ~2 days per year). This should be compared to the time it may take to get the submarine cable repaired and operational again (after a cut), which may take weeks, or even months, depending on the circumstances and location. Adding a third submarine cable (parallel to the other two) reduces the maximum expected total loss of service to ~4 seconds per year. The likelihood that all 3 would be compromised by naturally occurring incidents would be very small (i.e., one in ten million). Relying on only two submarine cables for an island’s entire international connectivity, at bandwidth-critical scale, is a high-stakes gamble. While dual-cable redundancy may offer sufficient availability on paper, it fails to account for real-world risks such as correlated failures, extended repair times, and the escalating strategic value of uninterrupted digital access. This represents a technical fragility and a substantial security liability for an island economy and a digitally reliant society.

Suppose one cable is accidentally or deliberately damaged, with little or no redundancy. In that case, the entire system can collapse, cutting off internet access, disrupting communication, and halting financial and governmental operations. Reducing systemic risk involves increasing resilience through redundancy, ensuring the overall system continues functioning even if one or more cables fail. This also means not relying on only one type of connectivity, e.g., subsea cables or satellite. Still, combinations of different kinds of connectivity are incredibly important to safeguard continuous connectivity to the outside world from the perspective of an Island, even if alternative or backup connectivity does not match the capacity of the primary means of connectivity. Moreover, islands with relatively low populations tend to rely on one central terrestrial-based switching hub (e.g., typically at the central population hub), without much or meshed connectivity, exposing all communication on an island if such a hub becomes compromised.

Submarine cables are increasingly recognized as strategic targets in a hybrid warfare or full-scale military conflict scenario. Deliberate severance of these cables, particularly in chokepoints, near shore landing zones (i.e., landing stations), or cable branching points, can be a high-impact, low-visibility tactic to cripple communications without overt military action.

Going from two to three (or three to four) subsea cables may offer some strategic buffer. If an attacker compromises one or even two links, the third can preserve some level of connectivity, allowing essential communications, coordination, and early warning systems to remain operational. This may reduce the impact window for disruption and provide authorities time to respond or re-route traffic. However, it is unlikely to make a substantial difference in a conflict scenario, where a capable hostile actor may easily compromise a relatively low number of submarine cable connections. Moreover, if the terrestrial network is exposed to a single point of failure via a central switching hub design, having multiple subsea connections may matter very little in a crisis situation.

And, think about it, there is no absolute guarantee that the world’s critical subsea infrastructure has not already been compromised by hostile actors. In fact, given the strategic importance of submarine cables and the increasing sophistication of state and non-state actors in hybrid warfare, it appears entirely plausible that certain physical and cyber vulnerabilities have already been identified, mapped, or even covertly exploited.

In short, the absence of evidence is not evidence of absence. While major nations and alliances like NATO have increased efforts to monitor and secure subsea infrastructure, the sheer scale and opacity of the undersea environment mean that strategic surprise is still possible (maybe even likely). It is also worth remembering that most submarine cables operate in the dark in the historical and even present-day context. We rely on their redundancy and robustness, but we largely lack the sensory systems that allow us to proactively defend or observe them in real time.

This is what makes submarine cable sensing technologies such a strategic frontier today and why resilience, through redundancy, sensing technologies, and international cooperation, is critical. We may not be able to prevent every act of sabotage, but we can reduce the risk of catastrophic failure and improve our ability to detect and respond in real time.

THE LIKELY SUSPECTS – THE CAPABLE HOSTILE ACTOR SEEN FROM A WESTERN PERSPECTIVE.

As observed in the Western context, Russia and China are considered the most capable hostile actors in submarine cable sabotage. China is reportedly advancing its ability to conduct such operations at scale. These developments underscore the growing need for technological defenses and multilateral coordination to safeguard global digital infrastructure.

Several state actors possess the capability and potential intent to compromise or destroy submarine communications networks. Among them, Russia is perhaps the most openly scrutinized. Its specialized naval platforms, such as the Yantar-class intelligence ships and deep-diving submersibles like the AS-12 “Losharik”, can access cables on the ocean floor for tapping or cutting purposes. Western military officials have repeatedly raised concerns about Russia’s activities near undersea infrastructure. For example, NATO has warned of increased Russian naval activity near transatlantic cable routes, viewing this as a serious security risk impacting nearly a billion people across North America and Western Europe.

China is also widely regarded as a capable actor in this domain. The People’s Liberation Army Navy (PLAN) and a vast network of state-linked maritime engineering firms possess sophisticated underwater drones, survey vessels, and cable-laying ships. These assets allow for potential cable mapping, interception, or sabotage operations. Chinese maritime activity around strategic chokepoints such as the South China Sea has raised suspicions of dual-use missions under the guise of oceanographic research.

Furthermore, credible reports and analyses suggest that China is developing methods and technologies that could allow it to compromise subsea cable networks at scale. This includes experimental systems enabling simultaneous disruption or surveillance of multiple cables. According to Newsweek, recent Chinese patents may indicate that China has explored ways to “cut or manipulate undersea cables” as part of its broader strategy for information dominance.

Other states, such as North Korea and Iran, may not possess full deep-sea capabilities but remain threats to regional segments, particularly shallow water cables and landing stations. With its history of asymmetric tactics, North Korea could plausibly disrupt cable links to South Korea or Japan. Meanwhile, Iran may threaten Persian Gulf routes, especially during heightened conflict.

While non-state actors are not typically capable of attacking deep-sea infrastructure directly, they could be used by state proxies or engage in sabotage at cable landing sites. These actors may exploit the relative physical vulnerability of cable infrastructure near shorelines or in countries with less robust monitoring systems.

Finally, it is not unthinkable that NATO countries possess the technical means and operational experience to compromise submarine cables if required. However, their actions are typically constrained by strategic deterrence, international law, and alliance norms. In contrast, Russia and China are perceived as more likely to use these capabilities to project coercive power or achieve geopolitical disruption under a veil of plausible deniability.

WE CAN’T PROTECT WHAT WE CAN’T MEASURE – WHAT IS THE SENSE OF SENSING SUBMARINE CABLES?

In the context of submarine fiber-optic cable connections, it should be clear that we cannot protect this critical infrastructure if we are blind to the environment around it and along the cables themselves.

While traditionally designed for high-capacity telecommunications, submarine optical cables are increasingly recognized as dual-use assets, serving civil and defense purposes. When enhanced with distributed sensing technologies, these cables can act as persistent monitoring platforms, capable of detecting physical disturbances along the cable routes in (near) real time.

From a defense perspective, sensing-enabled subsea cables offer a discreet, infrastructure-integrated solution for maritime situational awareness. Technologies such as Distributed Acoustic Sensing (DAS), Coherent Optical Frequency Domain Reflectometry (C-OFDR), and State of Polarization (SOP) sensing can detect anomalies like trawling activity, anchor dragging, undersea vehicle movement, or cable tampering, especially in coastal zones or strategic chokepoints like the GIUK gap or Arctic straits. When paired with AI-driven classification algorithms, these systems can provide early-warning alerts for hybrid threats, such as sabotage or unregistered diver activity near sensitive installations.

For critical infrastructure protection, these technologies play an essential role in real-time monitoring of cable integrity. They can detect:

• Gradual mechanical strain due to shifting seabed or ocean currents,
• Seismic disturbances that may precede physical breaks,
• Ice loading or iceberg impact events in polar regions.

These sensing systems also enable faster fault localization. While they are not likely to prevent a cable from being compromised, whether by accidental impact or deliberate sabotage, they dramatically reduce the time required to identify the problem’s location. In traditional submarine cable operations, pinpointing a break can take days, especially in deep or remote waters. With distributed sensing, operators can localize disturbances within meters along thousands of kilometers of cable, enabling faster dispatch of repair vessels, route reconfiguration, and traffic rerouting.

Moreover, sensing technologies that operate passively or without interrupting telecom traffic, such as SOP sensing or C-OFDR, are particularly well suited for retrofitting onto existing brownfield infrastructure or deployment on dual-use commercial-defense systems. They offer persistent, covert surveillance without consuming bandwidth or disrupting service, an advantage for national security stakeholders seeking scalable, non-invasive monitoring solutions. As such, they are emerging as a critical layer in the defense of underwater communications infrastructure and the broader maritime domain.

We should remember that no matter how advanced our monitoring systems are, they are unlikely to prevent submarine cables from being compromised by natural events like earthquakes and icebergs or unintentional and deliberate human activity such as trawling, anchor strikes, or sabotage. However, the sensing technologies offer the ability to detect and localize problems faster, enabling quicker response and mitigation.

TECHNOLOGY OVERVIEW: SUBMARINE CABLE SENSING.

Modern optical fiber sensing leverages the cable’s natural backscatter phenomena, such as Rayleigh, Brillouin, and Raman effects, to extract environmental data from a subsea communications cable. The physics of these effects is briefly described at the end of this article.

In the following, I will provide a comparative outline of the major sensing technologies in use today or may be deployed in future greenfield submarine fiber deployments. Each method has trade-offs in spatial or temporal resolution, compatibility with existing infrastructure, cost, and robustness to background noise. We will focus on defense applications in general applied to Arctic coastal environments, such as around Greenland. The relevance of each optical cable sensing technology described below to maritime defense will be summarized.

Some of the most promising sensing technologies today are based on the principles of Rayleigh scattering. For most sensing techniques, Rayleigh scattering is crucial in transforming standard optical cables into powerful sensor arrays without necessarily changing the physical cable structure. This makes it particularly valuable for submarine cable applications in the Arctic and strategic defense settings. By analyzing the light that bounces back from within the fiber, these systems can enable (near) real-time monitoring of intrusions or seismic activity over vast distances, spanning thousands of kilometers. Importantly, promising techniques are leverage Rayleigh scattering to function effectively even on legacy cable infrastructure, where installing additional reflectors would be impractical or uneconomical. Since Rayleigh-based sensing can be performed passively and non-invasively, it does not interfere with active data traffic, making it ideal for dual-use cables for communication and surveillance purposes. This approach offers a uniquely scalable and resilient way to enhance situational awareness and infrastructure defense in harsh or remote environments like the Arctic.

Before we get started on the various relevant sensing technologies let us briefly discuss what we mean by a sensing technology’s performance and its sensing capability, that is how well it can detect, localize, and classify physical disturbances, such as vibration, strain, acoustic pressure, or changes in light polarization, along a fiber-optic cable. The performance is typically judged by parameters like spatial resolution, detection range, sensitivity, signal-to-noise ratio, and the system’s ability to operate in noisy or variable environments. In the context of submarine detection, these disturbances are often caused by acoustic signals generated by vessel propulsion, machinery noise, or pressure waves from movement through the water. While the fiber does not measure sound pressure directly, it can detect the mechanical effects of those acoustic waves, such as tiny vibrations or refractive index changes in the surrounding seabed or cable sheath. The technologies we deploy have to be able to detect these vibrations as phase shifts in backscattered light. In contrast, other technologies may track subtle polarization changes induced by environmental stress on the subsea optical cables (as a result of an event in the proximity of the cable). A sensing system is considered effective when it can capture and resolve these indirect signatures of underwater activity with enough fidelity to enable actionable interpretation, especially in complex environments like coastal Arctic zones or the deep ocean.

In underwater acoustics, sound is measured in units of decibels relative to 1 micro Pascal, expressed as “dB re 1 µPa”, which defines a standard reference pressure level. The notation “dB re 1 µPa @ 1 m” refers to the sound pressure level of an underwater source, expressed in decibels relative to 1 micro Pascal and measured at a standard distance of one meter from the source. This metric quantifies how loud an object, such as a submarine, diver, or vessel, sounds when observed at close range, and is essential for modeling how sound propagates underwater and estimating detection ranges. In contrast, noise floor measurements use “dB re 1 µPa/√Hz,” which describes the distribution of background acoustic energy across frequencies, normalized per unit bandwidth. While source level describes how powerful a sound is at its origin, noise floor values indicate how easily such a sound could be detected in a given underwater environment.

Measurements are often normalized to bandwidth to assess sound or noise frequency characteristics, using “dB re 1 µPa/√Hz”. For example, stating a noise level of 90 dB re 1 µPa/√Hz in the 10 to 1000 Hz band means that within that frequency range, the acoustic energy is distributed at an average pressure level referenced per square root of Hertz. This normalization allows fair comparison of signals or noise across different sensing bandwidths. It helps determine whether a signal, such as a submarine’s acoustic signature, can be detected above the background noise floor. The effectiveness of a sensing technology is ultimately judged by whether it can resolve these types of signals with sufficient clarity and reliability for the specific use case.

In the mid-latitude Atlantic Ocean, typical noise floor levels range between 85 and 105 dB re 1 µPa/√Hz in the 10 to 1000 Hz frequency band. This environment is shaped by intense shipping traffic, consistent wave action, wind-generated surface noise, and biological sources such as whales. The noise levels are generally higher near busy shipping lanes and during storms, which raises the acoustic background and makes it more challenging to detect subtle events such as diver activity or low-signature submersibles (e.g., ballistic missile submarine, SSBN). In such settings, sensing techniques must operate with high signal-to-noise ratio thresholds, often requiring filtering or focusing on specific narrow frequency bands and enhanced by machine learning applications.

On the other hand, the Arctic coastal environment, such as the waters surrounding Greenland, is markedly quieter than, for example, the Atlantic Ocean. Here, the noise floor typically falls between 70 and 95 dB re 1 µPa/√Hz, and in winter, when sea ice covers the surface, it can drop even lower to around 60 dB. In these conditions, noise sources are limited to occasional vessel traffic, wind-driven surface activity, and natural phenomena such as glacial calving or ice cracking. The seasonal nature of Arctic noise patterns means that the acoustic environment is especially quiet and stable during winter, creating ideal conditions for detecting faint mechanical disturbances. This quiet background significantly improves the detectability of low-amplitude events, including the movement of stealth submarines, diver-based tampering, or UUV (i.e., unmanned underwater vehicles) activity.

Distributed Acoustic Sensing (DAS) uses phase-sensitive optical time-domain reflectometry (φ-OTDR) to detect acoustic vibrations and dynamic strain in general. Dynamic strain may arise from seismic waves or mechanical impacts along an optical fiber path. DAS allows for structural monitoring at a resolution of ca. 10 meters and a typical distance with amplification of 10 to 100 kilometers (can be extended by more amplifiers). It is an active sensor technology. DAS can be installed on shorter submarine cables (e.g., less than 100 km), although installing on a brownfield subsea cable is relatively complex. For long submarine cables (e.g., transatlantic), DAS would be greenfield deployed in conjunction with the subsea cable rollout, as retrofitting on an existing fiber installation would be impractical.

Phase-sensitive optical time domain reflectometry is a sensing technique that allows an optical fiber, like those used in subsea cables, to act like a long string of virtual microphones or vibration sensors. The method works by sending short pulses of laser light into the fiber and measuring the tiny reflections that bounce back due to natural imperfections inside the glass. When there is no activity near the cable, the backscattered light has a stable pattern. But when something happens near the cable, like a ship dragging an anchor, seismic shaking, or underwater movement, those vibrations cause tiny changes in the fiber’s shape. This physically stretches or compresses the fiber, changing the phase of the light traveling through it. φ-OTDR is specially designed to be sensitive to these phase changes. What is being detected, then, is not a “sound” per se, but a tiny change in the timing (phase) of the light as it reflects back. These phase shifts happen because mechanical energy from the outside world, like movement, stress, or pressure, slightly changes the length of the fiber or its refractive properties at specific points. φ-OTDR is ideal for detecting vibrations, like footsteps (yes, the technique also works on terra firma), vehicle movement, or anchor dragging. It is best suited for acoustic sensing over relatively long distances with moderate resolution.

So, in simple terms:

• The “event” is not inside the fiber but in sufficient vicinity to cause a reaction in the fiber.
• That external event causes micro-bending or stretching of the fiber.
• The fiber cable’s mechanical deformation changes the phase of light that is then detected.
• The sensing system uses these changes to pinpoint where along the fiber the event happened, often with meter-scale precision.

DAS has emerged as a powerful tool for transforming optical fibers into real-time acoustic sensor arrays, capable of detecting subtle mechanical disturbances such as vibrations, underwater movement, or seismic waves. While this capability is very attractive for defense and critical infrastructure monitoring, its application across existing long-haul subsea cables, particularly transoceanic systems, is severely constrained. The technology requires dark fibers or at least isolated, unused wavelengths, which are generally unavailable in (older) operational submarine systems already carrying high-capacity data traffic. Moreover, most legacy subsea cables were not designed with DAS compatibility in mind, lacking the bidirectional amplification or optical access points required to maintain sufficient signal integrity for acoustic sensing over long distances.

Retrofitting existing transatlantic or pan-Arctic submarine cables for DAS would be technically complex and, in most scenarios, likely economically unfeasible. These systems span thousands of kilometers, are deeply buried or armored along parts of their route, and incorporate in-line repeaters that do not support the backscattering reflection needed for DAS. As a result, implementing DAS across such long-haul infrastructure would entail replacing major cable components or deploying parallel sensing fibers. Both options may likely be inconsistent with the constraints of an already-deployed system. Suppose this kind of sensing capability is deemed strategically necessary. In that case, it may be operationally much less complex and more economical to deploy a greenfield cable with the embedded sensing technology, particularly for submarine cables that are 10 years old or older.

Despite these limitations, DAS offers significant potential for defense applications over shorter submarine segments, particularly near coastal landing points or within exclusive economic zones. One promising use case involves the Arctic and sub-Arctic regions surrounding Greenland. As geopolitical interest in the Arctic intensifies and ice-free seasons expand, the cables that connect Greenland to Iceland, Canada, and northern Europe will increasingly represent strategic infrastructure. DAS could be deployed along these shorter subsea spans, especially within fjords, around sensitive coastal bases, or in narrow straits, to monitor for hybrid threats such as diver incursions, submersible drones, or anchor dragging from unauthorized vessels. Greenland’s coastal cables often traverse relatively short distances without intermediate amplifiers and with accessible routes, making them more amenable to partial DAS coverage, especially if dark fiber pairs or access points exist at the landing stations.

The technology can be integrated into the infrastructure in a greenfield context, where new submarine cables are being designed and laid out. This includes reserving fiber strands exclusively for sensing, installing bidirectional optical amplifiers compatible with DAS, and incorporating coastal and Arctic-specific surveillance requirements into the architecture. For example, new Arctic subsea cables could be designed with DAS-enabled branches that extend into high-risk zones, allowing for passive real-time monitoring of marine activity without deploying sonar arrays or surface patrol assets (e.g., not actively communicate for example a ballistic missile submarine that it has been found as would have been the case with an active sonar).

DAS also supports geophysical and environmental sensing missions relevant to Arctic defense. When deployed along the Greenlandic shelf or near tectonic fault lines, DAS can contribute to early-warning systems for undersea earthquakes, landslides, or ice-shelf collapse events. These capabilities enhance environmental resilience and strengthen military situational awareness in a region where traditional sensing infrastructure is sparse.

DAS is best suited for detecting mid-to-high frequency acoustic energy, such as propeller cavitation or hull vibrations. However, stealth submarines may not produce strong enough vibrations to be detected unless they operate close to the fiber (e.g., <1 km) or in shallow water where coupling to the seabed is enhanced. Detection is plausible under favorable conditions but uncertain in deep-sea environments. However, in shallow Greenlandic coastal waters, DAS may detect a submarine’s acoustic wake, cavitation onset, or low-frequency hull vibrations, especially if the vessel passes within several hundred meters of the fiber.

Deploying φ-OTDR on brownfield submarine cables requires minimal infrastructure changes, as the sensing system can be installed directly at the landing station using a dedicated or wavelength-isolated fiber. However, its effective sensing range is limited to the segment between the landing station and the first in-line optical amplifier, typically around 80 to 100 kilometers. This limitation exists because standard submarine amplifiers are unidirectional and amplify the forward-traveling signal only. They do not support the return of backscattered light required by φ-OTDR, effectively cutting off sensing beyond the first repeater in brownfield systems. Even in a greenfield deployment, φ-OTDR is fundamentally constrained by weak backscatter, incoherent detection, poor long-distance SNR, and amplifier design, making it a technology mainly for coastal environments.

Coherent Optical Frequency Domain Reflectometry (C-OFDR) employs continuous-wave frequency-chirped laser probe signals and measures how the interference pattern (of the reflected light) changes (i.e., coherent detection). It offers high resolution (i.e., 100 -200 meters) and, for telecom-grade implementations, long-range sensing (i.e., 100s km), even over legacy submarine cables without Bragg gratings (i.e., period variation of the refractive index of the fiber). It is an active sensor technology. C-OFDR is one of the most promising techniques for high-resolution distributed sensing over long distances (e.g., transatlantic distances), and it can, in fact, be used on existing operational subsea cables without any special modifications to the cable itself, although with some practical considerations on older systems and limitations due to a reduced dynamic range. However, this sensing technology does require coherent detection systems with narrow-linewidth lasers and advanced DSP, which might make brownfield integration complex without significant upgrades. In contrast, greenfield deployments can seamlessly incorporate C-OFDR by leveraging the coherent optical infrastructure already standard in modern long-haul submarine cables. C-OFDR technique, like φ-OTDR, also relies on sensing changes in lights properties as it is reflected from imperfections in the fiber optical cable (i.e., Rayleigh backscattering). When something (an “event”) happens near the fiber, like the ground shakes from an earthquake, an anchor hits the seabed, or temperature changes, the optical fiber experiences microscopic stretching, squeezing, or vibration. These tiny changes affect how the light reflects back. Specifically, they change the phase and frequency of the returning signal. C-OFDR uses interferometry to measure these small differences very precisely. It is important to understand that the “event” we talk about is not inside the fiber, but its effects are causing changes to the fiber that can be measured by our chosen sensing technique. External forces (like pressure or motion) cause strain or stress in the glass fiber, which changes how the light moves inside. C-OFDR detects those changes and tells you where along the cable these changes happened, sometimes within a few centimeters.

Deploying C-OFDR on brownfield submarine cables is more challenging, as it typically requires more changes to the landing station, such as coherent transceivers with narrow-linewidth lasers and high-speed digital signal processing, which are normally not present in legacy landing stations. Even if such equipment is added at the landing station, like φ-OTDR, sensing may be limited to the segment up to the first in-line amplifier unless modified as shown in the work by Mazur et al.. C-OFDR, compared to φ-OTDR, leverages coherent receivers, DSP, and telecom-grade infrastructure to overcome those barriers, making C-OFDR a very relevant long-haul subsea cable sensing technology.

An interesting paper using a modified C-OFDR technique,  “Continuous Distributed Phase and Polarization Monitoring of Trans-Atlantic Submarine Fiber Optic Cable” by Mazur et al., demonstrates a powerful proof-of-concept for using existing long-haul submarine telecom cables, equipped with more than 70 amplifiers, for real-time environmental sensing without interrupting data transmission. The authors used a prototype system combining a fiber laser, FPGA (Field-Programmable Gate Array), and GPU (Graphical Processing Unit) to perform long-range optical frequency domain reflectometry (C-OFDR) over a 6,500 km transatlantic submarine cable. By measuring phase and polarization changes between repeaters, they successfully detected a 6.4 magnitude earthquake near Ferndale, California, showing the seismic wave propagating in real-time from the West Coast of the USA, across North America, and was eventually observed by Mazur et al. in the Atlantic Ocean. Furthermore, they demonstrated deep-sea temperature measurements by analyzing round-trip time variations along the full cable spans. The system operated for over two months without service interruptions, underscoring the feasibility of repurposing submarine cables as large-scale oceanic sensing arrays for geophysical and defense applications. Their system’s ability to monitor deep-sea environmental variations, such as temperature changes, contributes to situational awareness in remote oceanic regions like the Arctic or the Greenland-Iceland-UK (GIUK) Gap, areas of increasing strategic importance. It is worth noting that while the basic structure of the cable (in terms of span length and repeater placement) is standard for long-haul subsea cable systems, what sets this cable apart is the integration of a non-disruptive monitoring system that leverages existing infrastructure for advanced environmental sensing, a capability not found in most subsea systems deployed purely for telecom.

Furthermore, using C-OFDR and polarization-resolved sensing (SOP) without disrupting live telecommunications traffic provides a discreet means of monitoring infrastructure. This is particularly advantageous for covert surveillance of vital undersea routes. Finally, the system’s fine-grained phase and polarization diagnostics have the potential to detect disturbances such as anchor drags, unauthorized vessel movement, or cable tampering, activities that may indicate hybrid threats or espionage. These features position the technology as a promising enabler for real-time intelligence, surveillance, and reconnaissance (ISR) applications over existing subsea infrastructure.

C-OFDR is very sensitive over long distances and, when optimized with narrowband probing, may detect subtle refractive index changes caused by waterborne pressure variations. While more robust than DAS at long range, its ability to resolve weak, broadband submarine noise signatures remains speculative and would likely require AI-based classification. In Greenland, C-OFDR might be able to detect subtle pressure variations or cable stress caused by passing submarines, but only if the cable is close to the source.

Phase-based sensing, which φ-OTDR belongs to, is an active sensing technique that tracks the phase variation of optical signals for precise mechanical event detection. It requires narrow linewidth lasers and sensitive DSP algorithms. In phase-based sensing, we send very clean, stable light from a narrow-linewidth laser through the fiber cable. We then measure how the phase of that light changes as it travels. These phase shifts are incredibly sensitive to tiny movements, smaller than a wavelength of light. As discussed above, when the fiber is disturbed, even just a little, the light’s phase changes, which is what the system detects. This sensing technology offers a theoretical spatial resolution of 1 meter and is currently expected to be practical over distances less than 10 kilometers. In general, phase-based sensing is a broader class of fiber-optic sensing methods that detect optical phase changes caused by mechanical, thermal, or acoustic disturbances.

Phase-based sensing technologies detect sub-nanometer variations in the phase of light traveling through an optical fiber, offering exceptional sensitivity to mechanical disturbances such as vibrations or pressure waves. However, its practical application over the existing installed base of submarine cable infrastructure remains extremely limited. Some of the more advanced implementations are largely confined to laboratory settings due to the need for narrow-linewidth lasers, high-coherence probe sources, and low-noise environments. These conditions are difficult to achieve across real-world subsea spans, especially those with optical amplifiers and high traffic loads. These technical demands make retrofitting phase-based sensing onto operational subsea cables impractical, particularly given the complexity of accessing in-line repeaters and the susceptibility of phase measurements to environmental noise. Still, as the technology matures and can be adapted to tolerate noisy and lossy environments, it could enable ultra-fine detection of small-scale events such as underwater cutting tools, diver-induced vibrations, or fiber tampering attempts.

In a defense context, phase-based sensing might one day be used to monitor high-risk cable landings or militarized undersea chokepoints where detecting subtle mechanical signatures could provide an early warning of sabotage or surveillance activity. Its extraordinary resolution could also contribute to low-profile detection of seabed motion near sensitive naval installations. While not yet field-deployable at scale, it represents a promising frontier for future submarine sensing systems in strategic environments, typically in proximity to coastal areas.

Coherent MIMO Distributed Fiber Sensing (DFS) is another cutting-edge active sensing technique belonging to the phase-based sensing family that uses polarization-diverse probing for spatially-resolved sensing on deployed multi-core fibers (MCF), enabling robust, high-resolution environmental mapping. This technology remains currently limited to laboratory environments and controlled testbeds, as the widespread installed base of submarine cables does not use MCF and lacks the transceiver infrastructure required to support coherent MIMO interrogation. Retrofitting existing subsea systems with this capability would require complete replacement of the fiber plant, making it infeasible for legacy infrastructure, but potentially interesting for greenfield deployments.

Despite these limitations, the future application of Coherent MIMO DFS in defense contexts is compelling. Greenfield deployments, such as new Arctic cables or secure naval corridors, could enable real-time acoustic and mechanical activity mapping across multiple parallel cores, offering spatial resolution that rivals or exceeds existing sensing platforms. This level of precision could support the detection and classification of complex underwater threats, including stealth submersibles or distributed tampering attempts. With further development, it might also support wide-area surveillance grids embedded directly into the fiber infrastructure of critical sea lanes or military installations. While not deployable on today’s global cable networks, it represents a next-generation tool for submarine situational awareness in future defense-grade fiber systems.

State of Polarization (SOP) sensing technology detects changes in light polarization that allow sensing environmental disturbances to a submarine optical cable. It can be implemented passively using existing coherent transceivers and thus can be used on existing operational submarine cables. The SOP sensing technology does not offer spatial resolution by default. However, it has a very high temporal sensitivity on a millisecond level, allowing it to resolve temporally localized SOP anomalies that may often be precursors for a structurally compromised submarine cable. SOP sensing provides timely and actionable information even without pinpoint spatial resolution for applications like cable break prediction, anomaly detection, and hybrid threat alerts. However, if the temporal information can be mapped back to the compromised physical location within 10s of kilometers. The SOP sensing can cover up to 1000s of kilometers of a submarine system.

SOP sensing provides path-integrated information about mechanical stress or vibration. While it lacks spatial resolution, it could register anomalous polarization disturbances along Arctic cable routes that coincide with suspected submarine activity. Even global SOP anomalies may be suspicious in Greenland’s sparse traffic environment, but localizing the source would remain challenging. It is likely a technique that, combined with C-OFDR, would offer both a spatial and temporal picture that, in combination, could become a promising use case. SOP provides fast, passive temporal detection, while C-OFDR (or DAS) delivers spatial resolution and event classification. The combination may offer a more robust and operationally viable architecture for strategic subsea sensing, suitable for civilian and defense applications across existing and future cable systems.

Deploying SOP-based sensing on brownfield submarine cables requires no changes to the cable infrastructure, such as landing stations. It passively monitors changes in the state of polarization at the transceiver endpoints. However, this method does not provide spatial resolution and cannot localize events along the cable. It also does not rely on backscatter, and therefore its sensing capability is not limited by the presence of amplifiers, unlike φ-OTDR or C-OFDR. The limitation, instead, is that SOP sensing provides only a global, integrated signal over the entire fiber span, making it effective for detecting disturbances but not pinpointing their location.

Beyond the sensing technologies already discussed, such as DAS (including φ-OTDR), C-OFDR, SOP, and Coherent MIMO DFS, several additional, lesser-known sensing modalities can be deployed on or alongside submarine cables. These systems differ in physical mechanisms, deployment feasibility, and sensitivity, and while some remain experimental, others are used in niche environmental or energy-sector applications. Several of these have implications for defense-related detection scenarios, including submarine tracking, sabotage attempts, or unauthorized anchoring, particularly in strategically sensitive Arctic regions like Greenland’s West and East Coasts.

One such system is Brillouin-based distributed sensing, including Brillouin Optical Time Domain Analysis (BOTDA) and Brillouin Optical Time Domain Reflectometry (BOTDR). These methods operate by sending pulses down the fiber and analyzing the Brillouin frequency shift, which varies with temperature and strain. The spatial resolution is typically between 0.5 and 1 meter, and the sensing range can extend to 50 km under optimized conditions. The system’s strength is detecting slow-moving structural changes, such as seafloor deformation, tectonic strain, or sediment pressure buildup. However, because the Brillouin interaction is weak and slow to respond, it is poorly suited for real-time detection of fast or low-amplitude acoustic events like those produced by a stealth submarine or diver. Anchor dragging might be detected, but only if it results in significant, sustained strain in the cable. These systems could be modestly effective in shallow Arctic shelf environments, such as Greenland’s west coast, but they are not viable for real-time defense monitoring.

Another temperature-focused method is Raman-based distributed temperature sensing (DTS). This technique analyzes the ratio of Stokes and anti-Stokes backscatter to detect temperature changes along the fiber, with spatial resolution typically on the order of 1 meter and ranges up to 10–30 km. Raman DTS is widely used in the oil and gas industry for downhole monitoring, but is not optimized for dynamic or mechanical disturbances. It offers little utility in detecting diver activity, submarine motion, or anchor drag unless such events lead to secondary thermal effects. Furthermore, Raman DTS is unsuitable for detecting fast-moving threats like submarines or divers. It can detect slow thermal anomalies caused by prolonged contact, buried tampering devices, or gradual sediment buildup. Thus, it may serve as a background “health monitor” for defense-relevant subsea critical infrastructures. As its enabling mechanism is Raman scattering, which is even weaker than Rayleigh and Brillouin scattering, it is likely to make this sensor technology unsuitable for Arctic defense applications. Moreover, the cold and thermally stable Arctic seabed provides a limited dynamic range for temperature-induced sensing.

A more advanced but experimental method is optical frequency comb (OFC)-based sensing, which uses an ultra-stable frequency comb to probe changes in fiber length and strain with sub-picometer resolution. This offers unparalleled spatial granularity (down to millimeters) and could, in theory, detect subtle refractive index changes induced by acoustic coupling or mechanical perturbation. However, range is limited to short spans (<10 km), and implementation is complex and not yet field-viable. This technology might detect micro-vibrations from nearby submersibles or diver-induced strain signatures in a future defense-grade network, especially greenfield deployments in Arctic coastal corridors. The physical mechanism is interferometric phase detection, amplified by comb coherence and time-of-flight mapping. Frequency comb-based techniques could be the foundation for a next-generation submarine cable monitoring system, especially in greenfield defense-focused coastal deployments requiring excellent spatial resolution under variable environmental conditions. Unlike traditional reflectometry or phase sensing, the laser frequency comb should be able to maintain calibrated performance in fluctuating Arctic environments, where salinity and temperature affect refractive index dramatically, and therefore, a key benefit for Greenlandic and Arctic deployments.

Another emerging direction is Integrated Sensing and Communication (ISAC), where linear frequency-modulated sensing signals are embedded directly into the optical communication waveform. This approach avoids dedicated dark fiber and can achieve moderate spatial resolution (~100–500 meters) with ranges of up to 80 km using coherent receivers. ISAC has been proposed for simultaneous data transmission and distributed vibration sensing. In Arctic coastal areas, where telecom capacity may be underutilized and infrastructure redundancy is limited, ISAC could enable non-invasive monitoring of anchor strikes or structural cable disturbances. It may not detect quiet submarines unless direct coupling occurs, but it could potentially flag diver-based sabotage or hybrid threats that cause physical cable contact.

Lastly, hybrid systems combining external sensor pods, such as tethered hydrophones, magnetometers, or pressure sensors, with submarine cables are deployed in specialized ocean observatories (e.g., NEPTUNE Canada). These use the cable for power and telemetry and offer excellent sensitivity for detecting underwater acoustic and geophysical events. However, they require custom cable interfaces, increased power provisioning, and are not easily retrofitted to commercial or legacy submarine systems. In Arctic settings, such systems could offer unparalleled awareness of glacier calving, seismic activity, or vessel movement in chokepoints like the Kangertittivaq (i.e., Scoresby Sund) or the southern exit of Baffin Bay (i.e., Avannaata Imaa). The main limitation of hybrid systems lies in their cost and the need for local infrastructure support. The economics relative to such systems’ benefits requires careful consideration compared to more conventional maritime sensor architectures.

DEFENSE SCENARIOS OF CRITICAL SUBSEA CABLE INFRASTRUCTURE.

Submarine cable infrastructure is increasingly recognized as a medium for data transmission and a platform for environmental and security monitoring. With the integration of advanced optical sensing technologies, these cables can detect and interpret physical disturbances across vast underwater distances. This capability opens up new opportunities for national defense, situational awareness, and infrastructure resilience, particularly in coastal and Arctic regions where traditional surveillance assets are limited. The following section outlines how different sensing modalities, such as DAS, C-OFDR, SOP, and emerging MIMO DFS, can support key operational objectives ranging from seismic early warning to hybrid threat detection. Each scenario case reflects a unique combination of acoustic signature, environmental setting, and technological suitability.

• Intrusion Detection: Detect tampering, trawling, or vehicle movement near cables in coastal zones.
• Seismic Early Warning: Monitor undersea earthquakes with high fidelity, enabling early warning for tsunami-prone regions.
• Cable Integrity Monitoring: Identify precursor events to fiber breaks and trigger alerts to reroute traffic or dispatch response teams.
• Hybrid Threat Detection: Monitor signs of hybrid warfare activities such as sabotage or unauthorized seabed operations near strategic cables. This also includes anchor-dragging sounds.
• Maritime Domain Awareness: Track vessel movement patterns in sensitive maritime zones using vibrations induced along shore-connected cable infrastructure.

Intrusion Detection involving trawling, tampering, or underwater vehicle movement near the cable is best addressed using Distributed Acoustic Sensing (DAS), especially on coastal Arctic subsea cables where environmental noise is lower and mechanical coupling between the cable and the seafloor is stronger. DAS can detect short-range, high-frequency mechanical disturbances from human activity. However, this is more challenging in the open ocean due to poor acoustic coupling and cable burial. Coherent Optical Frequency Domain Reflectometry (C-OFDR) combined with State of Polarization (SOP) sensing offers a more passive and feasible alternative in such environments. C-OFDR can detect strain anomalies and localized pressure effects, while SOP sensing can identify anomalous polarization drift patterns caused by motion or stress, even on live traffic-carrying fibers.

For Seismic Early Warning, phase-based sensing (including both φ-OTDR and C-OFDR) is well suited across coastal and oceanic deployments. These technologies detect low-frequency ground motion with high sensitivity and temporal resolution. Phase-based methods can sense teleseismic activity or tectonic shifts along the cable route in deep ocean environments. The advantage increases in the Arctic coastal zones due to low background noise and shallow deployment, enabling the detection of smaller regional seismic events. Additionally, SOP sensing, while not a primary seismic tool, can detect long-duration cable strain or polarization shifts during large quakes, offering a redundant sensing layer.

Combining C-OFDR and SOP sensing is most effective for Cable Integrity Monitoring, particularly for early detection of fiber stress, micro-bending, or fatigue before a break occurs. SOP sensing works especially well for long-haul ocean cables with live data traffic, where passive, non-intrusive monitoring is essential. C-OFDR is more sensitive to local strain patterns and can precisely locate deteriorating sections. In Arctic coastal cables, this combination enables operators to detect damage from ice scouring, sediment movement, or thermal stress due to permafrost dynamics.

Hybrid Threat Detection benefits most from high-resolution, multi-modal sensing, such as detecting sabotage or seabed tampering by divers or unmanned vehicles. Along coastal regions, including Greenland’s fjords, Coherent MIMO Distributed Fiber Sensing (DFS), although still in its early stages, shows great promise due to its ability to spatially resolve overlapping disturbance signatures across multiple cores or polarizations. DAS may also contribute to near-shore detection if acoustic coupling is sufficient. On ocean cables, SOP sensing fused with AI-based anomaly detection provides a stealthy, always-on layer of hybrid threat monitoring, especially when other modalities (e.g., sonar, patrols) are absent or infeasible.

Finally, DAS is effective along coastal fiber segments for Maritime Domain Awareness, particularly tracking vessel movement in sensitive Arctic corridors or near military installations. It detects the acoustic and vibrational signatures of passing vessels, anchor deployment, or underwater vehicle operation. These signatures can be classified using spectrogram-based AI models to differentiate between fishing boats, cargo vessels, or small submersibles. While unable to localize the event, SOP sensing can flag cumulative disturbances or repetitive mechanical interactions along the fiber. This use case becomes less practical in oceanic settings unless vessel activity occurs near cable landing zones or shallow fiber stretches.

These scenario considerations have been summarised in the Table below.

LEGACY SUBSEA SENSING NETWORKS: SONOR SYSTEMS AND THEIR EVOLVING ROLE.

The observant reader might at this point feel (rightly) that I am totally ignoring the good old sonar (e.g., sound navigation and ranging), which has been around since World War I and is thus approximately 110 years old as a technology. In the Cold War era, at its height from the 1950s to the 1980s, sonar technology advanced further into the strategic domain. The United States and its allies developed large-scale systems like SOSUS (Sound Surveillance System) and SURTASS (Surveillance Towed Array Sensor System) to detect and monitor the growing fleet of Soviet nuclear submarines. These systems enabled long-range, continuous underwater surveillance, establishing sonar as a tactical tool and a key component of strategic deterrence and early warning architectures.

So, let us briefly look at Sonar as a defensive (and offensive) technology.

Undersea sensing as a cornerstone of naval strategy and maritime situational awareness; for example, see the account “66 Years of Undersea Surveillance” by Taddiken et al. Throughout the Cold War, the world’s major powers invested heavily in long-range underwater surveillance systems, especially passive and active sonar networks. These systems remain relevant today, providing persistent monitoring for submarine detection, anti-access/area denial operations, and undersea infrastructure protection.

Passive sonar systems detect acoustic signatures emitted by ships, submarines, and underwater seismic activity. These systems rely on the natural propagation of sound through water and are often favored for their stealth since they do not emit signals. Their operation is inherently covert. In contrast, active sonar transmits acoustic pulses and measures reflected signals to detect and range objects that might not produce detectable noise, such as quiet submarines or inert objects on the seafloor.

The most iconic example of a passive sonar network is the U.S. Navy’s Sound Surveillance System (SOSUS), initially deployed in the 1950s. SOSUS comprises a series of hydrophone arrays fixed to the ocean floor and connected by undersea cables to onshore processing stations. While much of SOSUS remains classified, its operational role continues today with mobile and advanced fixed networks under the Integrated Undersea Surveillance System (IUSS). Other nations have developed analogous capabilities, including Russia’s MGK-series networks, China’s emerging Great Undersea Wall system, and France’s SLAMS network. These systems offer broad area acoustic coverage, especially in strategic chokepoints like the GIUK (Greenland-Iceland-UK) gap and the South China Sea.

Despite sonar’s historical and operational value, traditional sonar networks have significant limitations. Passive sonar is susceptible to acoustic masking by oceanic noise and may struggle to detect vessels employing acoustic stealth technologies. Active sonar, while more precise, risks disclosing its location to adversaries due to its emitted signals. Furthermore, sonar performance is constrained by water conditions, salinity, temperature gradients, and depth, affecting acoustic propagation. Additionally, sonar coverage is inherently sparse and highly dependent on the geographical layout of sensor arrays and underwater topology. Furthermore, deployment and maintenance of sonar arrays are logistically complex and costly, often requiring naval support or undersea construction assets. These limitations suggest a decreasing standalone effectiveness of sonar systems in high-resolution detection, mainly as adversaries develop quieter and more agile underwater vehicles.

Think of sonar as a radar for the sea, sensing outward into the subsea environment. Due to sound propagation characteristics (i.e., in water sound travels more than 4 times faster and attenuates very slowly compared to sound waves in air), sonar is an ideal technology for submarine detection and seismic monitoring. In contrast, optical sensing in subsea cables is like a tripwire or seismograph, detecting anything that physically touches, moves, or perturbs the cable along its length. The emergence of distributed sensing over fiber optics has introduced a transformative approach to undersea and terrestrial monitoring. Distributed Acoustic Sensing (DAS), Distributed Fiber Sensing (DFS), and Coherent Optical Frequency Domain Reflectometry (C-OFDR) leverage the existing footprint of submarine telecommunications infrastructure to detect environmental disturbances, including vibrations, seismic activity, and human interaction with cables, at high spatial and temporal resolution. Unlike traditional sonar, these fiber-based systems do not rely on acoustic wave propagation in water but instead monitor the optical fiber’s phase, strain, or polarization variations. So, very simple sonar uses acoustics to sense sound waves in water, while fiber-based sensing is based on optics and how lights travel in an optical fiber. When embedded in submarine cables, such sensing techniques allow for continuous, covert, and high-resolution surveillance of the cable’s immediate environment, including detection of trawler interactions, anchor dragging, subsea landslides, and localized mechanical disturbances. They operate within the optical transmission spectrum without interrupting the core data service. While sonar systems excel at broad ocean surveillance and object tracking, their coverage is limited to specific regions and depths where arrays are installed. Conversely, fiber-based sensing offers persistent surveillance along entire transoceanic links, albeit restricted to the immediate vicinity of the cable path. Together, these systems should not be seen as competitors but very much complementary tools. Sonar covers the strategic expanse, while fiber-optic sensing provides fine-grained visibility where infrastructure resides.

The future of sonar sensing lies in hybridization and adaptive intelligence. Ongoing research explores networks that combine passive sonar arrays with intelligent edge processing using AI/ML to discriminate between ambient and threat signatures. There is also a push to integrate mobile platforms, such as Unmanned Underwater Vehicles (UUVs), into sonar meshes, expanding spatial coverage dynamically based on threat assessments. Material advances may also lead to miniaturized or modular hydrophone systems that can be ad hoc or embedded into multipurpose seafloor assets. Some navies are exploring Acoustic Vector Sensors (AVS), which can detect the pressure and direction of incoming sound waves, offering a richer data set for tracking and identification. Coupled with improvements in real-time ocean modeling and environmental acoustics, these future sonar systems may offer higher fidelity detection even in shallow and complex coastal waters where passive sensors are less effective. Moreover, integration with optical fiber systems is an area of active development. Some proposals suggest co-locating acoustic sensors with fiber sensing nodes or utilizing fiber backhaul for sonar telemetry in real-time, thereby merging the benefits of both approaches into a coherent undersea surveillance architecture.

THE ARCTIC DEPLOYMENT CONCEPT.

As global power competition extends into the Arctic, military planners and analysts are increasingly concerned about the growing strategic role of Greenland’s coastal waters, particularly in the context of Russian nuclear submarine operations. For decades, Russia has maintained a doctrine of deploying ballistic missile submarines (SSBNs) capable of launching nuclear retaliation strikes from stealth positions in remote ocean zones. Once naturally shielded by persistent sea ice, the Arctic has become more navigable due to climate change, creating new opportunities for submerged access to maritime corridors and concealment zones.

Historically, Russian submarines seeking proximity to U.S. and NATO targets would patrol areas along the Greenland-Iceland-UK (GIUK) gap and the eastern coast of Greenland, using the remoteness and challenging acoustic environment to remain hidden. However, strategic speculation and evolving threat assessments now suggest a westward shift, toward the sparsely monitored Greenlandic West Coast. This region offers even greater stealth potential due to limited surveillance infrastructure, complex fjord geography, and weaker sensor coverage than traditional GIUK chokepoints. Submarines could strike the U.S. East Coast from these waters in under 15 minutes, leveraging geographic proximity and acoustic ambiguity. Even if the difference in warning time would be no more than about 2–4 minutes depending on launch angle, trajectory, and detection latency, in the context of strategic warning systems and nuclear command and control, the loss of several minutes of additional reaction time can matter significantly, especially for early-warning systems, evacuation orders, or launch-on-warning decisions.

U.S. and Canadian defense communities have increasingly voiced concern over this evolving threat. U.S. Navy leadership, including Vice Admiral Andrew Lewis, has warned that the U.S. East Coast is “no longer a sanctuary,” underscoring the return of great power maritime competition and the pressing need for situational awareness even in home waters. As Russia modernizes its submarine fleet with quieter propulsion and longer-range missiles, its ability to hide near strategic seams like Greenland becomes a direct vulnerability to North American security.

This emerging risk makes the case for integrating advanced sensing capabilities into subsea cable infrastructure across Greenland and the broader Arctic theatre. Cable-based sensing technologies, such as Distributed Acoustic Sensing (DAS) and State of Polarization (SOP) monitoring, could dramatically enhance NATO’s ability to detect anomalous underwater activity, particularly in the fjords and shallow coastal regions of Greenland’s western seaboard. In a region where traditional sonar and surface surveillance are limited by ice, darkness, and remoteness, the subsea cable system could become an invisible tripwire, transforming Greenland’s digital arteries into dual-use defense assets.

Therefore, advanced sensing technologies should not be treated as optional add-ons but as foundational elements of Greenland’s Arctic defense architecture. Particular technologies that can work well and are relatively uncomplicated to operationalize on brownfield subsea cable installations. These would offer a critical layer of redundancy, early warning, and environmental insight, capabilities uniquely suited to the high north’s emerging strategic and climatic realities.

The Arctic Deployment Concept outlines a forward-looking strategy to integrate submarine cable sensing technologies into the defense and intelligence infrastructure of the Arctic region, particularly Greenland, as geopolitical tensions and environmental instability intensify. Greenland’s strategic location at the North Atlantic and Arctic Ocean intersection makes it a critical node in transatlantic communications and military situational awareness. As climate change opens new maritime passages and exposes previously ice-locked areas, the region becomes increasingly vulnerable, not only to environmental hazards like shifting ice masses and undersea seismic activity, but also to the growing risks of geopolitical friction, cyber operations, and hybrid threats targeting critical infrastructure.

In this context, sensing-enhanced submarine cables offer a dual-use advantage: they carry data traffic and serve as real-time monitoring assets, effectively transforming passive infrastructure into a distributed sensor network. These capabilities are especially vital in Greenland, where terrestrial sensing is sparse, the weather is extreme, and response times are long due to the remoteness of the terrain. By embedding Distributed Acoustic Sensing (DAS), Coherent Optical Frequency Domain Reflectometry (C-OFDR), and State of Polarization (SOP) sensing along cable routes, operators can monitor for ice scouring, tectonic activity, tampering, or submarine presence in near real time.

As emphasized in the article “Greenland: Navigating Security and Critical Infrastructure in the Arctic”, Greenland is not only a logistical hub for NATO but also home to increasingly digitalized civilian systems. This dual-use nature of Arctic subsea cables underscores the need for resilient, secure, and monitored communications infrastructure. Given the proximity of Greenland to the GIUK gap, a historic naval choke point between Greenland, Iceland, and the UK, any interruption or undetected breach in subsea connectivity here could undermine both civilian continuity and allied military posture in the region.

Moreover, the cable infrastructure along Greenland’s coastline, connecting remote settlements, research stations, and defense assets, is highly linear and often exposed to physical threats from shifting icebergs, seabed movement, or vessel anchoring. These shallow, coastal environments are ideally suited for sensing deployments, where good coupling between the fiber and the seabed enables effective detection of local activity. Integrating sensing technologies here supports ISR (i.e., Intelligence, Surveillance, and Reconnaissance) and predictive maintenance. It extends domain awareness into remote fjords and ice-prone straits where traditional radar or sonar systems may be ineffective or cost-prohibitive.

The map of Greenland’s telecommunications infrastructure provides a powerful visual framework for understanding how sensing capabilities could be integrated into the nation’s subsea cable system to enhance strategic awareness and defense. The western coastline, where the majority of Greenland’s population resides (~35%) and where the main subsea cable infrastructure runs, offers an ideal geographic setting for deploying cable-integrated sensing technologies. The submarine cable routes from Nanortalik in the south to Upernavik in the north connect critical civilian hubs such as Nuuk, Ilulissat, and Qaqortoq, while simultaneously passing near U.S. military installations like Pituffik Space Base. While essential for digital connectivity, this infrastructure also represents a strategic vulnerability if left unsensed and unprotected.

Given that Russian nuclear-powered submarines (e.g., SSBMs) are suspected of operating closer to the Greenlandic coastline, shifting from the historical GIUK gap to potentially less monitored regions along the west, Greenland’s cable network could be transformed into an invisible perimeter sensor array. Technologies such as Distributed Acoustic Sensing (DAS) and State of Polarization (SOP) monitoring could be layered onto the existing fiber without disrupting data traffic. These technologies would allow authorities to detect minute vibrations from nearby vessel movement or unauthorized subsea activity, and to monitor for seismic shifts or environmental anomalies like iceberg scouring.

The map above shows the submarine cable backbone, microwave-chain sites, and satellite ground stations. If integrated, these components could act as hybrid communication-and-sensing relay points, particularly in remote locations like Qaanaaq or Tasiilaq, further extending domain awareness into previously unmonitored fjords and inlets. The location of the new international airport in Nuuk, combined with Nuuk’s proximity to hydropower and a local datacenter, also suggests that the capital could serve as a national hub for submarine cable-based surveillance and anomaly detection processing.

Much of this could be operationalized using existing infrastructure with minimal intrusion (at least in the proximity of Greenland’s coastline). Brownfield sensing upgrades, mainly using coherent transceiver-based SOP methods or in-line C-OFDR reflectometry, may be implemented on live cable systems, allowing Greenland’s existing communications network to become a passive tripwire for submarine activity and other hybrid threats. This way, the infrastructure shown on the map could evolve into a dual-use defense asset, vital in securing Greenland’s civilian connectivity and NATO’s northern maritime flank.

POLICY AND OPERATIONAL CONSIDERATIONS.

As discussed previously, today, we are essentially blind to what happens to our submarine infrastructure, which carries over 95% of the world’s intercontinental internet traffic and supports more than 10 trillion euros daily in financial transactions. This incredibly important global submarine communications network was taken for granted for a long time, almost like a deploy-and-forget infrastructure. It is worthwhile to remember that we cannot protect what we cannot measure.

Arctic submarine cable sensing is as much a policy and sourcing question as a technical one. The integration of sensing platforms should follow a modular, standards-aligned approach, supported by international cooperation, robust cybersecurity measures, and operational readiness for Arctic conditions. If implemented strategically, these systems can offer enhanced resilience and a model for dual-use infrastructure governance in the digital age.

As Arctic geostrategic relevance increases due to climate change, geopolitical power rivalry, and the expansion of digital critical infrastructure, submarine cable sensing has emerged as both a technological opportunity and a governance challenge. The deployment of sensing techniques such as State of Polarization (SOP) monitoring and Coherent Optical Frequency Domain Reflectometry (C-OFDR) offers the potential to transform traditionally passive infrastructure into active, real-time monitoring platforms. However, realizing this vision in the Arctic, particularly for Greenlandic and trans-Arctic cable systems, requires a careful approach to policy, interoperability, sourcing, and operational governance.

One of the key operational advantages of SOP-based sensing is that it allows for continuous, passive monitoring of subsea cables without consuming bandwidth or disrupting live traffic​. When analyzed using AI-enhanced models, SOP fluctuations provide a low-impact way to detect seismic activity, cable tampering, or trawling events. This makes SOP a highly viable candidate for brownfield deployments in the Arctic, where live traffic-carrying cables traverse vulnerable and logistically challenging environments. Similarly, C-OFDR, while slightly more complex in deployment, has been demonstrated in real-world conditions on transatlantic cables, offering precise localization of environmental disturbances using coherent interferometry without the need for added reflectors​.

From a policy standpoint, Arctic submarine sensing intersects with civil, commercial, and defense domains, making multinational coordination essential. Organizations such as NATO, NORDEFCO (Nordic Defence Cooperation), and the Arctic Council must harmonize protocols for sensor data sharing, event attribution, and incident response. While SOP and C-OFDR generate valuable geophysical and security-relevant data, questions remain about how such data can be lawfully shared across borders, especially when detected anomalies may involve classified infrastructure or foreign-flagged vessels.

Moreover, integration with software-defined networking and centralized control planes can enable rapid traffic rerouting when anomalies are detected, improving resilience against natural or intentional disruptions. This also requires technical readiness in Greenlandic and Nordic telecom systems, many of which are evolving toward open architectures but may still depend on legacy switching hubs vulnerable to single points of failure.

Sensory compatibility and strategic trust must guide the acquisition and sourcing of sensing systems. Vendors like Nokia Bell Labs, which developed AI-based SOP anomaly detection models, have demonstrated in-band sensing on submarine networks without service degradation. A sourcing team may want to ensure that due diligence is done on the foundational models and that high-risk countries or vendors have not compromised their origin. I would recommend that sourcing teams follow the European Union’s 5G security framework as guidance in selecting the algorithmic solution, ensuring that no high-risk vendor/country has been involved at any point in the model development, training, or operational aspects of inferences and updates that are involved in applications of such models. By the way, it might be a very good and safe idea to extend this principle to the submarine cable construction and repair industry (just saying!).

When sourcing such systems, governments and operators should prioritize:

• Proven compatibility with coherent transceiver infrastructure (i.e., brownfield submarine cable installations). Needless to say, solutions are tested before final sourcing (e.g., PoC).
• Supplier alignment with NATO or Nordic/Arctic security frameworks. At a minimum, guidance should be taken from the EU 5G security framework and its approach to high-risk vendors and countries.
• Firmware and AI models need clear IP ownership and cybersecurity compliance. Needless to say, the foundational models must originate from trusted companies and markets.
• Inclusion of post-deployment support in Arctic (and beyond Arctic) operational conditions.

It cannot be emphasized enough that not all sensing systems are equally suitable for long-haul submarine cable stretches, such as transatlantic routes. Different sensing strategies may be required for the same subsea cable at different cable parts or spans (e.g., the bottom of the Atlantic Ocean vs coastal areas or proximity). A hybrid sensing approach is often more effective than a single solution. The physical length, signal attenuation, repeater spacing, and bandwidth constraints inherent to long-haul cables introduce technical limitations that influence which sensing techniques are viable and scalable.

For example, φ-OTDR (phase-sensitive OTDR) and standard DAS techniques, while powerful for acoustic sensing on terrestrial or coastal cables, face significant challenges over ultra-long distances due to signal loss and diminishing signal-to-noise ratio. These methods typically require access to dark fiber and may struggle to operate effectively across repeated links or when deployed mid-span across thousands of kilometers without amplification. Contrastingly, techniques like State of Polarization (SOP) sensing and Coherent Optical Frequency Domain Reflectometry (C-OFDR) have demonstrated strong potential for brownfield integration on transoceanic cables. SOP sensing can operate passively on live, traffic-carrying fibers and has been successfully demonstrated over 6,500 km transatlantic spans without an invasive retrofit​. Similarly, C-OFDR, particularly in its in-line coherent implementation, can leverage existing coherent transceivers and loop-back paths to perform long-range distributed sensing across legacy infrastructure..

This leads to the reasonable conclusion that a mix of sensing technologies tailored to cable type, length, environment, and use case is appropriate and necessary. For example, coastal or Arctic shelf cables may benefit more from high-resolution φ-OTDR/DAS deployments. In contrast, transoceanic cables call for SOP, or C-OFDR-based systems compatible with repeated, live traffic environments. This modular, multi-modal approach ensures maximum coverage, resilience, and relevance, especially as sensing is extended across greenfield and brownfield deployments.

Thus, hybrid sensing architectures are emerging as a best practice, with each technique contributing unique strengths toward a comprehensive monitoring and defense capability for critical submarine infrastructure.

Last but not least, cybersecurity and signal integrity protections are critical. Sensor platforms that generate real-time alerts must include spoofing detection, data authentication, and secured telemetry channels to prevent manipulation or false alarms. SOP sensing, for instance, may be vulnerable to polarization spoofing unless validated against multi-parameter baselines, such as concurrent C-OFDR strain signatures or external ISR (i.e., Intelligence, Surveillance, and Reconnaissance) inputs.

CONCLUSION AND RECOMMENDATION.

Submarine cables are indispensable for global connectivity, transmitting over 95% of international internet traffic, yet they remain primarily unmonitored and physically vulnerable. Recent events and geopolitical tensions reveal that hostile actors could target this infrastructure with plausible deniability, especially in regions with low surveillance like the Arctic. As described in this article, enhanced sensing technologies, such as DAS, SOP, and C-OFDR, can provide real-time awareness and threat detection, transforming passive infrastructure into active security assets. This is particularly urgent for islands and Arctic regions like Greenland, where fragile cable networks (in the sense of few independent international connections) represent single points of failure.

Key Considerations:

• Submarine cables are strategic, yet “blind & deaf” infrastructures.
  Despite carrying the majority of global internet and financial data, most cables lack embedded sensing capabilities, leaving them vulnerable to natural and hybrid threats. This is especially true in the Arctic and island regions with minimal redundancy.
• Recent hybrid threat patterns reinforce the need for monitoring.
  Cases like the 2024–2025 Baltic and Taiwan cable incidents show patterns (e.g., clean cuts, sudden phase shifts) that may be consistent with deliberate interference. These events demonstrate how undetected tampering can have immediate national and global impacts.
• The Arctic is both a strategic and environmental hotspot.
  Melting sea ice has made the region more accessible to submarines and sabotage, while Greenland’s cables are often shallow, unprotected, and linked to critical NATO and civilian installations. Integrating sensing capabilities here is urgent.
• Sensing systems enable early warning and reduce repair times.
  Technologies like SOP and C-OFDR can be applied to existing (brownfield) subsea systems without disrupting live traffic. This allows for anomaly detection, seismic monitoring, and rapid localization of cable faults, cutting response times from days to minutes.
• Hybrid sensing systems and international cooperation are essential.
  No single sensing technology fits all submarine environments. The most effective strategy for resilience and defense involves combining multiple modalities tailored to cable type, geography, and threat level while ensuring trusted procurement and governance.
• Relying on only one or two submarine cables for an island’s entire international connectivity at a bandwidth-critical scale is a high-stakes gamble. For example, a dual-cable redundancy may offer sufficient availability on paper. However, it fails to account for real-world risks such as correlated failures, extended repair times, and the escalating strategic value of uninterrupted digital access.
• Quantity doesn’t matter for capable hostile actors: for a capable hostile actor, whether a country or region has two, three, or a handful of international submarine cables is unlikely to matter in terms of compromising those critical infrastructure assets.

In addition to the key conclusions above, there is a common belief that expanding the number of international submarine cables from two to three or three to four offers meaningful protection against deliberate sabotage by hostile state actors. While intuitively appealing, this notion underestimates a determined adversary’s intent and capability. For a capable actor, targeting an additional one or two cables is unlikely to pose a serious operational challenge. If the goal is disruption or coercion, a capable adversary will likely plan for multi-point compromise from the outset (including landing station considerations).

However, what cannot be overstated is the resilience gained through additional, physically distinct (parallel) cable systems. Moving from two to three truly diverse and independently repairable cables improves system availability by a factor of roughly 200, reducing expected downtime from over hours per year to under one minute. Expanding to four cables can reduce expected downtime to mere seconds annually. These figures reflect statistical robustness and operational continuity in the face of failure. Yet availability alone is not enough. Submarine cable repair timelines remain long, stretching from weeks to months, even under favorable conditions. And while natural disruptions are significant, they are no longer our only concern. Undersea infrastructure has become a deliberate target in hybrid and kinetic conflict scenarios in today’s geopolitical climate. The most pressing threat is not that these cables might be compromised, but that they may already be; we are simply unaware. The undersea domain is poorly monitored, poorly defended, and rich in asymmetric leverage.

Submarine cable infrastructure is not just the backbone of global digital connectivity. It is also a strategic asset with profound implications for civil society and national defense. The reliance on subsea cables for internet access, financial transactions, and governmental coordination is absolute. Satellite-based communications networks can only carry an infinitesimal amount of the traffic carried by subsea cable networks. If the global submarine cable network were to break down, so would the world order as we know it. Integrating advanced sensing technologies such as SOP, DAS, and C-OFDR into these networks transforms them from passive conduits into dynamic surveillance and monitoring systems. This dual-use capability enables faster fault detection and enhanced resilience for civilian communication systems, but also supports situational awareness, early-warning detection, and hybrid threat monitoring in contested or strategically sensitive areas like the Arctic. Ensuring submarine cable systems are robust, observable, and secured must therefore be seen as a shared priority, bridging commercial, civil, and military domains.

THE PHYSICS BEHIND SENSING – A BIT OF BACKUP.

Rayleigh Scattering: Imagine shining a flashlight through a long glass tunnel. Even though the glass tunnel looks super smooth, it has tiny bumps and little specks you can not see. When the light hits those tiny bumps, some bounce back, like a ball bounces off a wall. That bouncing light is called Rayleigh scattering.

Rayleigh scattering is a fundamental optical phenomenon in which light is scattered by small-scale variations in the refractive index of a medium, such as microscopic imperfections or density fluctuations within an optical fiber. It occurs naturally in all standard single-mode fibers and results in a portion of the transmitted light being scattered in all directions, including backward toward the transmitter. The intensity of Rayleigh backscattered light is typically very weak, but it can be detected and analyzed using highly sensitive receivers. The scattering is elastic, meaning there is no change in wavelength between the incident and scattered light.

In distributed fiber optic sensing (DFOS), Rayleigh backscatter forms the basis for several techniques:

• Distributed Acoustic Sensing (DAS):
  The DAS sensing solution uses phase-sensitive optical time-domain reflectometry (i.e., φ-OTDR) to measure minute changes in the backscattered phase caused by vibrations. These changes indicate environmental disturbances such as seismic waves, intrusions, or cable movement.
• Coherent Optical Frequency Domain Reflectometry (C-OFDR):
  C-OFDR leverages Rayleigh backscatter to measure changes in the fiber over distance with high resolution. By sweeping a narrow-linewidth laser over a frequency range and detecting interference from the backscatter, C-OFDR enables continuous distributed sensing along submarine cables. Unlike earlier methods requiring Bragg gratings, recent innovations allow this technique to work even over legacy subsea cables without them.
• Coherent Receiver Sensing:
  This technique monitors Rayleigh backscatter and polarization changes using existing telecom equipment’s DSP (digital signal processing) capabilities. This allows for passive sensing with no additional probes, and the sensing does not interfere with data traffic.

Brillouin Scattering: Imagine you are talking through a long string tied between two cups, like a string telephone most of us played with as kids (before everyone got a smartphone when they turned 3 years old). Now, picture that the string is not still. It shakes a little, like shivering or wiggling in the wind or the strain of the hands holding the cups. When your voice travels down that string, it bumps into those little wiggles. That bumping makes the sound of your voice change a tiny bit. Brillouin scattering is like that. When light travels through our string (that could be a glass fiber), the tiny wiggles inside the string make the light change direction, and the way that light and cable “wiggles” work together can tell our engineers stories about what happens inside the cable.

Brillouin scattering is a nonlinear optical effect that occurs when light interacts with acoustic (sound) waves within the optical fiber. When a continuous wave or pulsed laser signal travels through the fiber, it can generate small pressure waves due to a phenomenon known as electrostriction. These pressure waves slightly change the optical fiber’s refractive index and act like a moving grating, scattering some of the light backward. This backward-scattered light experiences a frequency shift, known as the Brillouin shift, which is directly related to the temperature and strain in the fiber at the scattering point.

Commercial Brillouin-based systems are technically capable of monitoring subsea communications cables, especially for strain and temperature sensing. However, they are not yet standard in the submarine communications cable industry, and integration typically requires dedicated or dark fibers, as the sensing cannot share the same fiber with active data traffic.

Raman Scattering: Imagine you are shining a flashlight through a glass of water. Most of the light goes straight through, like cars driving down a road without turning. But sometimes, a tiny bit of light bumps into something inside the water, like a little water molecule, and bounces off differently. It’s like the car suddenly makes a tiny turn and changes its color. This little bump and color change is what we call Raman scattering. It is a special effect as it helps scientists figure out what’s inside things, like what water is made of, by looking at how the light changes when it bounces off.

Raman scattering is primarily used in submarine fiber cable sensing for Distributed Temperature Sensing (DTS). This technique exploits the temperature-dependent nature of Raman scattering to measure the temperature along the entire length of an optical fiber, which can be embedded within or run alongside a submarine cable. Raman scattering has several applications in submarine cables. It is used for environmental monitoring by detecting gradual thermal changes caused by ocean currents or geothermal activity. Regarding cable integrity, it can identify hotspots that might indicate electrical faults or compromised insulation in power cables. In Arctic environments, Raman-based Distributed Temperature Sensing (DTS) can help infer changes in surrounding ice or seawater temperatures, aiding in ice detection. Additionally, it supports early warning systems in the energy and offshore sectors by identifying overheating and other thermal anomalies before they lead to critical failures.

However, Raman scattering has notable limitations. Because it is a weak optical effect, DTS systems based on Raman scattering require high-powered lasers and highly sensitive detectors. It is also unsuitable for detecting dynamic events such as vibrations or acoustic signals, better sensed using Rayleigh or Brillouin scattering. Furthermore, Raman-based DTS typically offers spatial resolutions of one meter or more and has a slow response time, making it less effective for identifying rapid or short-lived events like submarine activity or tampering.

Commercial Raman-DTS solutions exist and are actively deployed in subsea power cable monitoring. Their use in telecom submarine cables is less common but technically feasible, particularly for infrastructure integrity monitoring rather than data-layer diagnostics.

FURTHER READING.

• Global Submarine Cable Map is a free and regularly updated resource from TeleGeography data. It is imo a very useful tool for visualizing where submarine cables are located.
•  MaritineTraffic is a global maritime tracking platform that uses Automatic Identification System (AIS) data to provide real-time and historical location information for commercial and private vessels worldwide. It can monitor ship movements near submarine cable routes and landing sites, enabling analysts to identify unusual behavior, loitering patterns, or the presence of non-reporting vessels in sensitive maritime zones, which is important for detecting potential risks to undersea infrastructure. I tend to use this in combination with the Submarine Cable Map above.
• Mauldin, A., “Do Submarine Cables Account For Over 99% of Intercontinental Data Traffic?”, (2023), Telegeography.
• Larsen, K.K., “What Lies Beneath”, (2024), Techneconomyblog.com. The article highlights the vulnerability of submarine cables and proposes using Automatic Identification System (AIS) data to trace vessels near the site and time of a cable break. By analyzing ship trajectories, investigators can identify potentially responsible vessels, distinguishing between accidental damage and deliberate interference. It also introduces a dual-risk framework: a baseline risk score to assess natural or accidental causes, and a sabotage risk score based on vessel behavior, cable location, and geopolitical context. This method enhances attribution, supports early warnings, and protects critical subsea infrastructure.
• Larsen, K.K., “Greenland: Navigating Security and Critical Infrastructure in the Arctic“, (2024), Techneconomyblog.com. The article explores Greenland’s growing strategic role in Arctic security and infrastructure resilience amid rising geopolitical tensions. It emphasizes the need for robust digital and energy infrastructure, including resilient submarine cables and satellite links, to support civilian connectivity and defense readiness. The piece also highlights Greenland’s value as a technological gateway between North America and Europe, underscoring its relevance in future-proofing NATO and allied operations in the High North.
• Martin, A., “European officials increasingly certain Baltic Sea cable breaks are accidental, not sabotage“, The Record, (March 2025).
• Magnuson, S., “NATO to Open Center Focusing on Undersea Cable Sabotage Threat“, National Defense Magazine, (March 2025).
• Koshinom Y., “The changing submarine cable landscape”, (2024), European Union Institute for Security Studies.
• Insikt Group, “The escalating Global Risk Environment for Submarine Cables“, Recorded Future, (2023).
• Hartog, A. H., “An Introduction to Distributed Optical Fibre Sensors“, (2017), CRC Press.
•  Seismology (SM) Division of the European Geosciences Union (EGU), “What is Distributed Acoustic Sensing“, (2023), Blog. A good introductory hands-on article that explains what DAS is about and why its versatility and scalability make it a powerful tool for real-time, large-scale sensing without the need for additional hardware deployment (in short to medium distances <100 km).
• Guerrier, S. et al., “Introducing coherent MIMO sensing, a fading-resilient, polarization-independent approach to φ-OTDR“, (2020), Optics Express, 28(14), 21081–21094. This work introduces the concept of Coherent MIMO Distributed Fiber Sensing. It demonstrates how dual-polarization probing with coherent detection works. It lays the groundwork for future high-performance sensing over optical fibers, with clear implications for subsea and defense infrastructure monitoring.
• Abdelli, K. et al., “Anomaly Detection and Localization in Optical Networks Using Vision Transformer and SOP Monitoring”, OFC 2024, paper Tu2J.4. The paper demonstrates high-accuracy event detection using state-of-polarization (SOP) data and Vision Transformer-based AI for proactive fiber monitoring.
• Mazur, M. et al., “Real-Time Monitoring of Cable Break in a Live Network using a Coherent Transceiver Prototype“, OFC 2024, paper Tu3J.6. The paper uses SOP data from a coherent transceiver prototype to detect precursors to an actual cable break event on a 524 km live network.
• Mazur, M. et al., “Continuous Distributed Phase and Polarization Monitoring of Trans-Atlantic Submarine Fiber Optic Cable“, OFC 2024, paper Tu3J.1.The paper demonstrates coherent OFDR-based phase and polarization sensing over 6,500 km of subsea cable, capturing deep-sea seismic and thermal signals.
• Guerrier, S. et al., “Digital Coherent Sensing over Deployed Fibers for Advanced Network Telemetry“, OFC 2024, paper Tu2J.1.The paper presents coherent MIMO distributed fiber sensing (DFS) with polarization diversity for high-resolution sensing on multi-core fibers.
• Zhang, C. et al., “Field Test of Communication Cable for Environmental Monitoring“, OFC 2024, paper Tu2J.7. This work conducts real-world vibration sensing trials using deployed cables and backscattered light to monitor environmental and traffic events.
• Yan, Y. et al., “High-Efficiency ISAC to Enable Sub-Meter Level Vibration Sensing for Coherent Fiber Networks“, OFC 2024, paper Tu2J.3. This paper introduces an ISAC architecture inserting linear frequency modulated (LFM) probes into telecom frames for simultaneous DAS and communication.
• Mazur, M. et al., “Real-Time In-Line Coherent Distributed Sensing over a Legacy Submarine Cable”, OFC 2024, paper Th4B.8. This work implements coherent OFDR techniques for long-range sensing over legacy cables using high-speed transceivers and GPU/FPGA platforms.
• Nikles, M. et al., P. A., “Simple distributed fiber sensor based on Brillouin gain spectrum analysis“, (1996), Optics Letters, 21(10), 758–760. This work introduces the concept of distributed sensing using the Brillouin gain spectrum, laying the groundwork for Brillouin Optical Time Domain Analysis. It demonstrates how the Brillouin frequency shift can map strain and temperature along standard optical fibers with meter-scale resolution and kilometer-scale range.
• Dai, M., et al., “A Survey on Integrated Sensing, Communication, and Computing Networks for Smart Oceans”, J. Sens. Actuator Netw. (2022). This article explores the rapidly advancing field of ISAC, which combines radar and communication technologies. It comprehensively reviews key enablers, signal processing techniques, system architectures, and application scenarios as they apply to maritime domains (as well as other non-maritime domains).
• Coddington, I. et al., “Dual-comb spectroscopy“, (2016), Optica, 3(4), 414–426. This work presents the foundational principles of dual-comb spectroscopy, a technique that uses two optical frequency combs to perform high-resolution, broadband, and rapid measurements without moving parts. It demonstrates how this method enables precise, real-time sensing of distance, spectral properties, and refractive index changes, with applications in metrology, ranging, and environmental monitoring. For subsea cable defense applications, the Coddington paper provides the theoretical basis for how frequency combs can achieve sub-millimeter resolution in long optical paths (e.g., submarine cables) under various mechanical forces in an underwater environment.
• Konstantino, Y., “DLP 2022: Optical Frequency Domain Reflectometry: Metrology & Sensing Applications“, IEEE Photonics Society, (2022).
• Recorded Future, “The Escalating Global Risk Environment for Submarine Cables”, (July 2023), Threat Analysis by Insikt Group.
• Trevithick, J., “Admiral Warns America’s East Coast Is No Longer A “Safe Haven” Thanks To Russian Subs“, (February 2020).
• Congressional Research Service, “Changes in the Arctic: Background and Issues for Congress.“, (2024).
• Chatham House, “Undersea Cables: Indispensable, Insecure.“, (2021).
• Center for Strategic and International Studies (CSIS), “America’s Arctic Moment: Great Power Competition in the Arctic to 2050.”, (2020).
• Center for European Policy Analysis (CEPA), “Up North – Confronting Arctic Insecurity Implications for the United States and NATO“, (2024).

ACKNOWLEDGEMENT.

I greatly acknowledge my wife, Eva Varadi, for her support, patience, and understanding during the creative process of writing this article. I am furthermore indebted to Andreas Gladisch, VP Emerging Technologies – Deutsche Telekom AG, for sharing his expertise on fiber-optical sensing technologies with me and providing some of the foundational papers on which my article and research have been based. I always come away wiser from our conversations.