Deep-sea lifeline: Applications and technical challenges of active fiber optic cables in marine environments
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Beneath the azure waters lies a world that is both mysterious and crucial to humanity. This realm serves as the backbone of global communication, with over 95% of international data transmitted via submarine optical cables. It is also the frontier of resource exploration, a sentinel for climate monitoring, and a pivotal area for national security. One of the core technologies supporting these deep-sea activities is the active optical fiber cable. This composite cable, integrating power transmission and optical fiber communication functions, acts as a "lifeline" in the deep sea, ensuring energy supply, data exchange, and real-time monitoring in extreme environments.
Structural design of active optical fiber cables
The marine environment poses stringent requirements on active optical fiber cables. Unlike ordinary optical cables used on land, marine-grade active optical fiber cables must withstand seawater corrosion, immense water pressure, continuous ocean current impact, and potential mechanical drag. Therefore, their structural design embodies the concept of multi-level protection.
The core of a typical shallow-water waterproof and tensile-resistant optical fiber composite cable consists of a twisted structure comprising optical fiber units, power line cores, and water-blocking materials. To prevent longitudinal penetration of seawater, the cable is filled with water-blocking strips and wrapped with water-blocking tapes. In terms of sheath design, a multi-layer metal and polymer composite structure is often adopted: for example, a low-density polyethylene inner sheath provides basic waterproofing, a thick steel wire armor layer bears significant mechanical tensile loads, while a corrugated aluminum sheath or copper sheath serves as a crucial waterproof barrier, often coated with asphalt to prevent corrosion. The outermost medium-density polyethylene outer sheath provides the final physical protection.
For applications in deeper sea areas (such as depths of 5000 meters), like deep-sea mining systems, the cables must withstand external water pressure of up to 550kg/cm². These "umbilical cables" not only need to exhibit excellent resistance to repeated bending and twisting, but also require specially designed internal electrical connectors and fiber optic feedthroughs to ensure the stability of optoelectronic transmission under extreme high pressure.
Addressing the core challenges of the marine environment
The main challenges faced by active fiber optic cables in the ocean can be summarized as "water, pressure, corrosion, and tension", which modern technology is addressing through the integration of material science and precision engineering.
Firstly, it is about waterproofing and water blocking. The conductivity and corrosiveness of seawater mean that once water molecules penetrate into the interior of the cable, it will not only cause microbending loss of optical fibers, but also potentially trigger electrical short circuits. Therefore, in addition to using a dense metal sheath as an impermeable barrier, the interior of the cable is filled with water-blocking powder or tape. These materials will rapidly expand when exposed to water, blocking possible water infiltration paths and achieving "longitudinal water tightness".
Next comes resistance to mechanical stress. During deployment and retrieval, the cable can withstand tensile forces up to several tons. The thick steel wire or spiral steel wire armor layer is the main load-bearing component. At the same time, to cope with the undulations of the seabed terrain and the disturbance of ocean currents, the optical fiber itself also needs to have sufficient excess length and buffer layer to avoid direct microstrain that may lead to fracture or signal attenuation when subjected to stress.
Next, corrosion resistance is crucial. Metal components that are immersed in seawater for extended periods are highly susceptible to electrochemical corrosion. Therefore, the metallic materials used in cables, such as steel wires and aluminum sheaths, often require special treatment or are directly made of corrosion-resistant alloys. Furthermore, in critical components like connectors, high-grade anti-corrosion materials and high IP ratings (such as IP68) are employed to seal the enclosure, ensuring that salt spray and moisture cannot infiltrate the internal precision optical interfaces.
Lastly, there is the exemption for electromagnetic interference. For ships, submarines, and exploration platforms in the marine environment, radar, sonar, and high-power power electronics devices generate a complex electromagnetic environment. Fiber-optic communication inherently possesses the ability to resist electromagnetic interference (EMI), ensuring clear and uninterrupted signal transmission. This is also one of the core advantages of active fiber-optic cables over traditional copper cables in military and high-end exploration fields.
Cutting-edge applications: from communication to ubiquitous perception
Traditionally, the role of active fiber optic cables has been as passive "conduits" responsible for transmitting electrical energy and data. However, in recent years, a revolutionary technology has been changing this status quo - utilizing the fiber itself as a sensor to achieve Distributed Acoustic Sensing (DAS) and State of Polarization (SoP) detection.
By integrating or upgrading sensing capabilities on existing submarine communication cables, the originally data-transmitting optical fibers have been transformed into an ultra-long, passive, distributed sensor array. When seismic waves, changes in wave pressure, or sound waves from passing ships act on the submarine cables, the weak strain or polarization state changes generated inside the optical fibers are captured and analyzed by high-precision equipment. For example, during a marine survey conducted in Yingri Bay, South Korea, researchers successfully recorded clear waveforms generated by an air gun source using fiber optic cables laid on the seabed, demonstrating its great potential in high-resolution seismic exploration in shallow waters.
In Europe, multiple large-scale research projects are pushing for the practical application of this technology. Portugal plans to lay a 3,700-kilometer-long "smart cable" connecting Lisbon to the Madeira and Azores Islands, integrating seabed motion, water pressure, and temperature sensors within it, aiming to provide a valuable time window for tsunami early warning. Similarly, the ECSTATIC project is leveraging the seabed cable system in the Mediterranean Sea to develop vibration and acoustic sensing technologies based on interference and polarization, combined with artificial intelligence for real-time data processing, to achieve earthquake early warning and active protection of network infrastructure. The SUBMERSE project goes a step further, not only focusing on earthquakes and tsunamis but also attempting to use these data to track whale activities and monitor changes in ocean conditions, while also facing the challenge of balancing massive data processing with national security data, such as submarine activity detection.
Future Prospects
With the development of deep-sea mining, marine renewable energy (such as offshore wind farms), and the Global Ocean Observing System, the demand for active optical fiber cables will continue to grow. Future cables will not only be stronger and more pressure-resistant, but will also become more "intelligent". They will be capable of transmitting terabit-scale communication data while simultaneously returning vast amounts of information about geophysical fields, marine environments, and even biological activities in real time.
This transformation from "communication lines" to "Earth's stethoscope" not only requires breakthroughs in materials science and optical technology, but also necessitates deep collaboration between telecom operators, scientific research institutions, and national security departments. As a Portuguese telecommunications regulatory official stated, "In the future, I believe many cables will become intelligent, providing people with additional security guarantees in their daily lives." In the deep and dark ocean, these active fiber optic cables are gradually becoming a crucial link connecting human civilization with the understanding of the deep mysteries of the Earth.