How Submarine Fiber-Optic Cables Work

Submarine fiber-optic cables are engineered communications systems that carry optical signals between landing stations across seas and oceans. A complete system includes terminal equipment, cable, fibers, repeaters or amplifiers where needed, power feed equipment, branching units, route protection, monitoring, and operational arrangements for maintenance and repair.

A submarine cable is a system, not a simple wire

A submarine cable is often described as an undersea “internet cable,” but the cable itself is only one part of the system. The full cable system connects terrestrial networks on one side of an ocean or sea to terrestrial networks on another side through landing stations, terminal equipment, and the submerged “wet plant.”

The main system elements are:

  • Submarine line terminal equipment, usually located in or near a landing station, which transmits and receives optical signals.
  • Power feed equipment, which supplies electrical power to submerged repeaters and other powered components.
  • The wet plant, meaning the submerged cable, optical fibers, repeaters, branching units, joints, and associated underwater equipment.
  • The landing interface, including the beach landing, ducts, beach manhole, and landing station; terrestrial backhaul connects the landing station to networks on land but is outside the submerged wet-plant boundary.
  • Operations and monitoring systems, which help operators supervise signal performance, locate faults, and coordinate maintenance.

The exact design depends on distance, route, water depth, capacity objectives, seabed conditions, hazards, regulatory requirements, and commercial structure. A regional unrepeatered system may look very different from a long-haul transoceanic cable. The submarine cable lifecycle explains how those choices move from planning into operations.

From data center to landing station

Most international submarine capacity begins and ends in terrestrial networks. Traffic may originate in a data center, internet exchange, carrier network, enterprise network, mobile core, or cloud region. It reaches the cable landing station through terrestrial backhaul.

The landing station is the controlled facility where submarine terminal equipment, power feed equipment, network management equipment, and interconnection equipment are housed. Landing functions may occupy a dedicated cable landing station or be colocated with, and connected to, a data center, carrier hotel, or other network hub.

At the shore, the submarine cable transitions through a beach landing. This may include a horizontal directional drilled conduit, a beach manhole, ducts, shore-end protection, and route markers depending on local conditions. The goal is to bring the cable safely from the marine environment into terrestrial infrastructure while reducing exposure to anchors, fishing gear, surf, abrasion, and coastal development.

Optical fibers carry signals as light

Inside the cable are optical fibers: thin strands of glass that carry information using light. Digital data is encoded onto optical signals by terminal equipment. Modern systems use advanced modulation and coherent transmission, a technique that measures both the amplitude and phase of light to improve spectral efficiency and performance over long distances.

A submarine cable may contain multiple fiber pairs. A fiber pair normally uses one fiber for transmission in one direction and another fiber for the opposite direction. Capacity depends on the number of fiber pairs, the usable optical spectrum, the terminal equipment installed, modulation formats, signal quality, and operational margins.

Capacity should not be treated as a single permanent number. Operators may refer to:

  • Design capacity, the theoretical or planned maximum under specified assumptions.
  • Equipped or lit capacity, the capacity enabled by installed terminal equipment.
  • Traffic carried, the actual data traffic using the system at a given time.

These numbers are not interchangeable. The capacity, ownership and economics guide explains why public capacity figures require context.

The cable structure: protection and power

A submarine fiber-optic cable must protect fragile glass fibers in a harsh environment. Cable designs vary, but may include optical fibers, strength members, insulation, a metallic power conductor, water-blocking materials, and protective sheathing.

Many repeatered systems include a power conductor so that electrical current can be sent from shore to power repeaters and other active submerged equipment. The conductor is part of a carefully engineered electrical system, not a general-purpose power cable.

Cable protection changes along the route. Near shore and in areas with fishing, anchoring, rocky seabed, or other hazards, the cable may use armoring—steel wires around the cable core—to improve mechanical protection. In deep ocean areas with fewer external threats, systems may use lightweight cable, which is easier to deploy and appropriate for stable deepwater environments.

Cable selection is route-specific. A single system may use several cable types: double-armored, single-armored, lightweight protected, or lightweight cable, depending on depth and risk.

Repeaters and optical amplification

Light weakens as it travels through optical fiber. On long systems, optical amplifiers are integrated into pressure-resistant housings called repeaters and placed along the route. In modern optical submarine systems, repeaters typically amplify optical signals rather than fully converting them into electrical signals at every stage.

Repeaters are spaced according to system design, fiber characteristics, power budget, target performance, and route requirements. They must operate reliably for long periods in deep water, under high pressure, and without routine physical access.

Not every submarine cable uses repeaters. Unrepeatered systems are used over shorter distances where the optical signal can travel from end to end without submerged amplification, sometimes assisted by high-performance terminal equipment and specialized amplification at the ends. Repeatered systems are used for longer routes where submerged amplification is necessary.

Branching units and multi-point systems

A branching unit is submerged equipment that allows a cable system to split toward more than two landing points. Some branching units are passive, while others may include powered switching or optical functions depending on the system design.

Branching units allow a system to connect multiple countries, islands, or network hubs. They can improve commercial reach and route flexibility, but they also add engineering, operational, and ownership complexity. The design must account for power distribution, signal routing, repair procedures, and contractual arrangements among owners or users.

Route depth, burial, and protection

Cable routes are selected through planning, desktop study, marine survey, engineering, and permitting. The goal is to find a route that is technically feasible, commercially useful, legally permissible, and maintainable.

Water depth affects cable design and repair methods. In deep ocean, cables may lie on the seabed without burial because external human activity is limited. In shallower water, burial may be used to reduce risk from fishing gear, anchors, dredging, and other seabed activity. Burial is not always possible; hard seabed, steep slopes, coral, environmental restrictions, or congested corridors may require other protection methods.

Protection is a layered approach. It may include route selection, cable armoring, burial, charting, coordination with maritime users, authority-established protection measures where available, and operational monitoring. Readers can explore route context through the Submarine Cable Map and identify systems in the Cable System Index.

Wavelengths, spectrum, and coherent transmission

Modern submarine systems carry many optical channels over each fiber pair. These channels are often called wavelengths, referring to different colors or frequencies of light within the usable optical spectrum.

Coherent transmission and digital signal processing help operators send more information over a fiber pair than older technologies allowed. Terminal upgrades can improve equipped capacity without replacing the wet plant, although the final result depends on fiber characteristics, repeater design, spectrum, noise, and system margin.

This is why a cable’s public capacity figure can change over time. A system may launch with one level of lit capacity and later support more through terminal upgrades, additional wavelengths, or improved modulation.

Monitoring and fault detection

Submarine cable operators monitor optical performance, electrical power, alarms, and network conditions. Monitoring can identify degradation, outages, or changes that suggest a fault. Fault localization may use electrical measurements, optical test methods, network telemetry, and comparison with route data.

A cable fault does not always mean the entire system is unusable. Impact depends on where the fault occurs, whether multiple fiber pairs or power paths are affected, whether the system has branches, and whether traffic can be restored through other routes. Operators usually plan restoration through both physical maintenance arrangements and network-level routing. The repair and resilience guide follows that process from detection through relay.

Key takeaways

  • A submarine cable is an end-to-end system, not only an underwater wire.
  • Optical fibers carry data as light; terminal equipment determines how signals are encoded and received.
  • Repeaters amplify signals on long routes; shorter systems may be unrepeatered.
  • Cable type, burial, and armoring vary by route depth, seabed conditions, and external risks.
  • Capacity figures must distinguish design capacity, equipped capacity, and actual traffic carried.

Sources and further reading

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FAQs

What is a submarine fiber-optic cable?

A submarine fiber-optic cable is an undersea communications cable that carries data as light through optical fibers. It is part of a larger system that includes landing stations, terminal equipment, power equipment, monitoring, and maintenance arrangements.

Do all submarine cables have repeaters?

No. Long-haul systems usually use repeaters to amplify optical signals along the route. Shorter systems may be unrepeatered if signals can travel end to end without submerged amplification.

Why do capacity numbers change?

Capacity can change when operators install new terminal equipment, light additional wavelengths, use improved coherent modems, or change operating assumptions. Design capacity, lit capacity, and actual traffic are different measures.

Why are some cables buried and others not?

Burial depends on water depth, seabed conditions, external risks, environmental constraints, and technical feasibility. Deep-ocean cable is often laid on the seabed, while shallow-water sections may be buried or armored for protection.