Expanding the Capacity of Undersea Data Cables: A Strategic Overview

Submarine communications cables form the backbone of the global internet, carrying more than 99 percent of all international data traffic. From streaming video to financial transactions, cloud computing, and scientific research, these fiber-optic arteries connect continents and enable the modern digital economy. As demand for bandwidth grows exponentially—driven by 5G, the Internet of Things, artificial intelligence, and high-definition content—network operators and technology vendors must continuously increase the capacity of undersea infrastructure. This article examines the primary strategies for boosting cable capacity, from fundamental physics in optical fiber to software-based traffic management, and explores emerging technologies that promise to sustain future growth.

The Critical Role of Submarine Cables in Global Connectivity

Undersea cables are not only the most reliable means of transoceanic communication but also the most cost-effective. Unlike satellite links, which suffer from latency and limited throughput, fiber-optic cables offer low latency and virtually unlimited bandwidth potential. According to the Submarine Cable Map, there are over 400 active submarine cable systems worldwide, spanning hundreds of thousands of kilometers. Each new generation of cable technology must support higher capacities to keep pace with data growth, which has historically doubled every two years. Capacity increases depend on improvements in optical fiber design, amplification techniques, and intelligent network management.

Optical Fiber Technology: Beyond Wavelength-Division Multiplexing

The most straightforward way to increase cable capacity is to transmit more data through each fiber. Wavelength-division multiplexing (WDM) enables multiple data channels at different optical wavelengths to travel simultaneously on a single fiber. Commercial systems now support dense WDM (DWDM) with hundreds of channels, each carrying up to 400 Gbps or even 800 Gbps using advanced modulation formats. However, WDM alone is reaching fundamental limits due to the nonlinear properties of glass and the available optical bandwidth (typically the C-band, 1530–1565 nm, and L-band, 1565–1625 nm). To push beyond these limits, engineers are exploring several approaches.

Space-Division Multiplexing (SDM)

Instead of relying solely on wavelengths, SDM increases capacity by using multiple spatial paths within a single cable. This can be achieved through multi-core fibers (MCF), which contain several independent cores within a common cladding, or few-mode fibers (FMF), where multiple spatial modes carry separate data streams. Early field trials have demonstrated capacities exceeding 10 Pb/s (petabits per second) using MCF. While SDM is still in research and development, it represents a promising path for future cable systems, especially when combined with advanced signal processing to manage crosstalk between cores or modes.

Coherent Detection and Advanced Modulation

Modern undersea systems rely on coherent optical receivers that detect both the amplitude and phase of light, enabling complex modulation schemes such as DP-16QAM (dual-polarization 16-state quadrature amplitude modulation). These techniques allow more bits per symbol, but they also require higher optical signal-to-noise ratios (OSNR). Improvements in digital signal processing (DSP) chips, including nonlinear compensation algorithms, have extended the reach of high-order modulation. As DSP technology evolves, it will support higher symbol rates and more bits per symbol, further increasing per-channel capacity.

Wider Optical Bandwidths

Expanding beyond the conventional C+L bands to include the S-band (1460–1530 nm) could triple the usable spectrum. However, this requires new amplifier technologies (discussed below) and fibers with broader low-loss windows. Research into bismuth-doped and other advanced fiber amplifiers aims to cover the S-band efficiently. If successful, multi-band transmission could be a practical upgrade for existing cables by replacing or augmenting repeater modules.

Advanced Cable Design and Deployment

Physical cable construction also influences capacity. Traditional single-mode fibers with a 125-micron cladding are being replaced by fibers with larger effective areas, typically 130–150 square microns, which reduce nonlinear distortion and allow higher launch power. New materials, such as pure silica core fibers, minimize attenuation and improve OSNR. Additionally, cable designs that support multiple fiber pairs—often 8, 16, or more—are becoming standard. The Nokia Submarine Networks division, for example, offers systems with up to 24 fiber pairs, each capable of carrying hundreds of Gbps. Deploying such cables requires careful route planning to avoid underwater hazards and ensure longevity, but the capacity payoff is substantial.

Upgrading Versus Replacing Existing Cables

In many cases, upgrading the terminal equipment (transponders and receivers) at both ends of an existing cable can double or triple capacity without laying new fiber. This approach leverages the optical infrastructure already in place, replacing legacy transponders with modern coherent units that use better modulation and forward error correction (FEC). Because the cable itself and the repeaters remain unchanged, this strategy is both faster and cheaper than building a new system. However, the ultimate limit is set by the original design parameters, including repeater output power and fiber quality. For major capacity jumps, new cable builds are necessary.

Repeater and Amplifier Innovations

Optical repeaters, placed every 50 to 90 kilometers along the cable, compensate for signal attenuation. Their design directly affects how much capacity the system can deliver. Erbium-doped fiber amplifiers (EDFAs) have been the workhorse for decades, amplifying all WDM channels simultaneously. Modern EDFAs operate across the C and L bands with high gain and low noise figure. However, as the number of channels increases and higher-order modulation is used, the noise figure becomes critical.

Raman Amplification

Distributed Raman amplification uses the transmission fiber itself as a gain medium by sending high-power pump light from the terminals. This technique can lower the effective noise figure and improve OSNR, particularly when combined with EDFAs (hybrid amplification). Some newer cable systems incorporate Raman capability in the terminal stations to boost performance without modifying the undersea repeaters. Raman amplification is especially beneficial for extending the reach of high-capacity signals.

Multi-Band and Wideband EDFAs

To support S-band transmission, researchers are developing amplifiers based on bismuth-doped fibers or thulium-doped fibers. These are not yet commercially proven for submarine applications, but prototypes exist. Another approach is to use semiconductor optical amplifiers (SOAs) for the S-band, though they typically have higher noise and polarization sensitivity. As the industry pushes toward multi-band systems, repeaters will need to accommodate broader gain spectra, possibly using multiple amplifier stages tailored for different bands.

Coherent Repeaters and Regeneration

In very long cables (e.g., trans-Pacific), the accumulated noise may require signal regeneration—converting the optical signal back to electrical, cleaning it, and retransmitting. Modern coherent repeaters can perform 3R regeneration (re-amplify, re-shape, re-time) using DSP chips. These repeaters are more complex and expensive but enable higher capacities over extreme distances. Some system operators choose to deploy regeneration at intermediate landing points to boost performance.

Software-Defined Networking and Intelligent Traffic Management

Physical upgrades are costly and time-consuming. Software-defined networking (SDN) offers a way to maximize the use of existing capacity by dynamically allocating bandwidth based on demand. In a submarine cable context, SDN controllers can adjust modulation formats, forward error correction overhead, and even routing paths to avoid congestion or compensate for fiber degradation. For example, if a particular wavelength channel experiences high bit error rates due to aging components, the SDN controller can reduce its modulation order (e.g., from 16QAM to QPSK) to maintain error-free transmission, albeit at lower bit rate. Conversely, when conditions are favorable, the controller can increase the data rate. This adaptive approach can increase overall cable efficiency by 10–30 percent compared to fixed provisioning.

Artificial intelligence (AI) and machine learning are also being integrated into network management systems. AI algorithms can predict failures, optimize power levels, and detect subtle performance changes before they become critical. This proactive management helps prevent outages and ensures that the cable operates at peak capacity. The Submarine Networks World conference has highlighted several SDN deployments on existing systems, demonstrating significant operational savings.

Dynamic Spectrum Allocation

Similar to flexible-grid optical networks, future submarine cables may implement dynamic spectrum allocation, where channel spacing and center frequencies can be adjusted in real-time. This flexibility allows operators to share capacity among multiple users or applications with different service level agreements. For instance, a cloud provider could temporarily lease additional spectrum during a major data migration, then release it. Such as-a-service models are already emerging for terrestrial optical networks and are being studied for submarine links.

Future Directions: From Quantum Key Distribution to Cable as a Service

Looking beyond incremental improvements, several emerging technologies could reshape undersea cable capacity.

Quantum Key Distribution (QKD) Integration

QKD allows two parties to generate a shared cryptographic key with security guaranteed by quantum physics. Researchers have demonstrated QKD over long-haul submarine fibers, but it consumes a portion of the optical spectrum and requires very low noise. Future cables may include dedicated QKD channels to provide secure communications for governments or financial institutions, adding a new service layer without affecting classical data traffic.

Submarine Cable as a Service (SCaaS)

Consortia of telecommunications and internet content providers (ICPs) traditionally finance cable projects. SCaaS is a newer model where a third party builds and operates the cable, then sells capacity on-demand to multiple tenants. This model can increase utilization and lower barriers for smaller players. SDN and dynamic spectrum allocation are essential enablers for SCaaS, allowing fine-grained resource slicing.

Underwater Data Centers and Edge Computing

Microsoft’s Project Natick experiment demonstrated that deploying data centers underwater near cable landing points can reduce latency and energy consumption. While not directly increasing cable throughput, placing compute resources closer to users reduces the volume of data that must traverse long-haul cables, effectively increasing capacity for the most time-sensitive traffic. Future integrated systems may combine submarine cables with submerged data centers to optimize the overall network.

Conclusion

Increasing the capacity of undersea data cables requires a multifaceted engineering approach that spans fiber physics, amplifier technology, cable design, and intelligent software. The strategies currently in play—higher-order modulation, space-division multiplexing, multi-band amplification, SDN-based optimization, and AI-driven management—are each powerful on their own, but their combined impact is transformative. As data demand continues its relentless growth, the submarine cable industry must continue innovating, supported by investment in research and development. The cable systems of the next decade will likely incorporate many of these technologies, delivering capacities measured in hundreds of terabits per second per fiber pair, and ensuring that the world’s digital infrastructure remains robust, scalable, and secure.