Undersea Fiber Optic Cables: The Technology Connecting the World

At the same time, researchers are developing multicore fiber that could dramatically increase the amount of information carried by a single optical fiber.

How Undersea Fiber Optic Cables Work

At the heart of every submarine cable are optical fibers that transmit information as pulses of light. Modern systems use Dense Wavelength Division Multiplexing (DWDM) and coherent optical transmission to send many independent high-speed channels through each fiber pair.

Because optical signals gradually lose power as they travel through thousands of kilometers of fiber, long-distance submarine systems use repeaters containing optical amplifiers. These repeaters are powered electrically through the submarine cable from the cable landing stations.

Unlike terrestrial fiber networks, submarine systems must be designed to operate reliably for decades under extreme environmental conditions. The cable must withstand pressure, temperature variations, seabed movement, and potential damage from fishing activity, anchors, and other hazards.

The Evolution of Submarine Cable Capacity

Early fiber optic submarine cables carried only a few hundred megabits per second. Modern systems operate at capacities measured in hundreds of terabits per second.

A major development has been the transition from increasing capacity primarily by improving the performance of each wavelength to increasing the number of independent optical transmission paths within the cable. This approach is known as Space Division Multiplexing (SDM).

Rather than relying on a small number of fiber pairs operating at extremely high power, SDM distributes transmission across a larger number of fiber pairs. This allows submarine cable designers to optimize the power available to each fiber pair and improve overall system efficiency.

Google’s Dunant cable was an early example of this approach, using 12 fiber pairs and shared pump lasers in its repeaters. Newer systems are extending the concept even further, with 16 and 24 fiber-pair systems now being developed or deployed.

New Generation Submarine Cable Systems

The latest generation of submarine cables is being driven heavily by cloud providers, hyperscalers, telecommunications operators, and governments seeking greater international connectivity and route diversity.

Bifrost, connecting Southeast Asia with the United States, uses 12 fiber pairs and was ready for service in 2025. Its main trunk spans approximately 16,556 kilometers between Singapore, Guam, and California, with system capacity upgraded to approximately 260 Tb/s.

Honomoana, part of Google’s Pacific connectivity strategy, uses 16 fiber pairs to connect California with French Polynesia, New Zealand, and Australia. The system is designed around multiple branching points and diverse landing locations, illustrating the growing emphasis on both capacity and geographic resilience.

Taihei, another Google-led Pacific system, is designed with 16 fiber pairs and connects Japan with Hawaii over approximately 7,000 kilometers. The system is scheduled for commercial operation in 2027.

Other new systems illustrate the same trend. The E2A cable between East Asia and North America is designed with 12 fiber pairs and more than 192 Tb/s of capacity, while the new Synapse system between Brazil and the United States is planned with 16 fiber pairs to support growing cloud and AI traffic.

The trend toward larger fiber-pair counts is even more apparent in newer regional systems. The I-AM Cable, connecting Japan, Malaysia, Singapore and other Asian locations, is designed for up to 16 fiber pairs and approximately 320 Tb/s of initial capacity. It will also incorporate wavelength-selective switching capabilities to provide greater flexibility in managing capacity.

Open Cable Systems and Greater Flexibility

Another important development is the adoption of open cable architecture. Traditional submarine systems often paired the undersea cable with proprietary terminal equipment from the same supplier.

Open cable systems separate the submarine line system from the submarine line terminal equipment (SLTE). This gives network operators greater freedom to select transmission equipment from different vendors and upgrade the terrestrial terminals as optical technology advances.

This is particularly valuable as coherent technology continues to evolve. A cable designed for decades of service can potentially take advantage of improved transponders and higher-capacity modulation formats without replacing the entire undersea cable.

Multicore Fiber: A Potential Breakthrough

Perhaps one of the most important longer-term developments is multicore fiber (MCF).

Conventional single-mode fiber contains one optical core. Multicore fiber places multiple independent or coupled cores within the same standard-diameter fiber. Each core can provide another spatial transmission path, potentially increasing capacity without proportionally increasing the physical size of the cable.

Researchers have already demonstrated remarkable results. In 2024, NEC and NTT successfully transmitted 12 spatially multiplexed optical signals over a 7,280-kilometer transmission experiment using a coupled 12-core fiber. The experiment demonstrated the potential for multicore fiber to eventually be used in transoceanic submarine systems.

The challenge is that adding cores introduces inter-core crosstalk. In coupled multicore systems, signals from neighboring cores interact and must be separated using sophisticated multiple-input multiple-output (MIMO) digital signal processing.

Recent research shows that this technology is progressing beyond laboratory demonstrations. A 2025 field-installed 12-coupled-core fiber experiment achieved approximately 389 Tb/s over more than 1,000 kilometers, demonstrating 12-Tb/s-plus spatial MIMO channels.

Research is also moving toward practical submarine applications. NEC has been developing multicore submarine cable technology and has demonstrated how multicore fibers could potentially increase capacity while maintaining the conventional cable footprint. Future systems could use significantly more than 12 cores as the technology matures.

Toward Petabit-Class Submarine Cables

The combination of SDM, multicore fiber, higher baud rates, advanced modulation, and improved coherent technology points toward a future in which submarine cable capacity could reach the petabit-per-second range.

This is important because simply increasing the transmission rate of individual wavelengths eventually encounters fundamental limitations involving optical power, nonlinear impairments, amplifier efficiency, and available optical spectrum.

Spatial multiplexing provides another dimension for increasing capacity: instead of pushing more information through the same spatial channel, engineers can create additional spatial channels using more fiber pairs, multiple cores, or eventually multiple modes.

Resilience Is Becoming Just as Important as Capacity

Capacity is only one consideration in modern submarine network design. As international connectivity becomes increasingly important to cloud computing, financial services, AI infrastructure, and national economies, route diversity and resilience are becoming equally important.

New systems are increasingly designed with multiple landing stations, branching units, geographically diverse routes, and connections to different terrestrial network hubs. This reduces dependence on individual cable routes and provides alternative paths when a cable is damaged or unavailable.

Submarine cables are also becoming increasingly important as strategic infrastructure. Governments and network operators are paying greater attention to cable protection, monitoring, repair capabilities, and the security of cable landing stations.

The Future of Undersea Fiber Optic Networks

The next generation of submarine networks will be defined by much more than simply putting more fiber on the ocean floor. SDM, open cable architectures, higher fiber-pair counts, advanced coherent transmission, multicore fiber, and intelligent network management are collectively changing how undersea networks are engineered.

For the foreseeable future, conventional single-core fiber will remain the foundation of most commercial submarine systems. However, multicore fiber could provide a fundamentally new path to higher capacity when improvements in crosstalk management, MIMO processing, optical amplification, splicing, and deployment technology make it commercially practical.

The result will be submarine networks capable of supporting the enormous bandwidth requirements of AI, cloud computing, data centers, streaming media, international commerce, and the increasingly interconnected global economy.

Learn More About Optical Networking

Undersea cables are only one part of the much larger optical networking ecosystem. Understanding how submarine systems work also requires knowledge of fiber optics, DWDM, optical amplifiers, coherent transmission, modulation formats, optical signal-to-noise ratio (OSNR), dispersion, nonlinear impairments, and network architecture.

If you would like to develop a deeper understanding of these technologies, explore FiberGuide’s Certified Optical Network Associate (CONA) and Certified Optical Network Engineer (CONE) training programs.

CONA provides a foundation in optical communications, fiber fundamentals, CWDM, DWDM, link budgets, transmission systems, and practical optical network design. CONE builds on that foundation with advanced coherent transmission, digital signal processing, modulation formats, high-capacity DWDM, and next-generation optical networking.

Explore CONA and CONE Optical Network Training