What Are Fiber Optic Cables?

Fiber optic cables are transmission media that carry information as modulated light pulses through thin strands of glass or plastic rather than as electrical signals through copper conductors. Each strand consists of a core, where light propagates, and a surrounding cladding with a slightly lower refractive index that confines the light through total internal reflection. Multiple fibers are bundled together with strength members, water-blocking gel or tape, and a protective outer jacket to form the finished cable. Because light experiences far less attenuation than electrical signals over long distances and is immune to electromagnetic interference, fiber optic cables support data rates and reach that copper cabling cannot match.

The first practical low-loss silica fiber, demonstrated by Corning in 1970 with attenuation below 20 dB/km, proved that optical transmission was commercially viable. Modern single-mode fibers achieve attenuation near 0.18 dB/km at the 1550 nm wavelength window, enabling transoceanic cables that span thousands of kilometers with only periodic optical amplification.

Single-Mode and Multimode Fiber

The distinction between single-mode and multimode fiber determines which applications each type serves. Single-mode fiber has a core diameter of approximately 8 to 10 µm, narrow enough that only one spatial mode of light propagates. This eliminates modal dispersion entirely, allowing signals to travel tens to hundreds of kilometers with minimal pulse spreading. Long-haul telecommunications networks, submarine cable systems, and metropolitan core rings rely on single-mode fiber, almost universally conforming to the ITU-T G.652 standard profile. Multimode fiber uses cores of 50 or 62.5 µm diameter that support dozens to hundreds of propagating modes, each traveling at a slightly different group velocity. The resulting modal dispersion limits transmission distances to hundreds of meters, but multimode fiber's larger core simplifies connectorization and allows the use of low-cost vertical-cavity surface-emitting lasers (VCSELs). The OM3 and OM4 grades of 50 µm laser-optimized multimode fiber support 10 Gigabit and 100 Gigabit Ethernet over 100 to 150 m, making them standard in data center interconnects.

Cable Construction and Standards

A fiber optic cable's mechanical design depends on its deployment environment. Loose-tube cables place fibers within gel-filled tubes that allow the fiber to move freely, protecting it from tensile stress during installation and thermal contraction in outdoor aerial or direct-burial installations. Tight-buffered cables apply a polymer layer directly over the fiber coating, producing a more compact structure suited to indoor distribution and patch cords. Armored cables add corrugated steel or aluminum interlocking tape to resist rodent damage and mechanical crushing in direct-buried or industrial settings. Submarine cables wrap the fiber bundle in layers of steel wire armor, copper conductors for remote powering of repeaters, and polyethylene jacket, resulting in a cable weighing 1 to 8 kg/m depending on the water depth it will encounter. The RF Industries guide to fiber optic cable types outlines how each construction variant maps to its intended installation category. Single-mode versus multimode fiber comparison guides describe the standard designations OS1, OS2, OM1 through OM5 and the performance parameters defined for each.

Connectors, Splicing, and Loss Budget

Signal loss in a fiber optic link comes from three sources: intrinsic fiber attenuation, connector insertion loss, and splice loss. Fusion splicing, which melts two fiber ends together using an electric arc, produces joints with loss typically below 0.05 dB and is preferred for permanent field splices. Mechanical connectors for patch cords and equipment interfaces use ceramic ferrules polished to flat (PC), angled (APC), or ultra-physical-contact (UPC) finishes; APC connectors achieve return loss greater than 60 dB, important in systems sensitive to back-reflection such as long-haul coherent transmission links. Link loss budgets account for all these contributions to ensure the received optical power remains above the detector sensitivity.

Applications

Fiber optic cables have applications across a wide range of fields, including:

• Long-haul and submarine telecommunications carrying internet and telephone traffic
• Data center interconnects operating at 100 Gigabit and 400 Gigabit Ethernet
• Cable television hybrid fiber-coax (HFC) distribution networks
• Industrial control and power systems where electrical isolation is required
• Medical endoscopy and surgical imaging using flexible fiber bundles