Fiber-optic cable
A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable is used. Different types of cable are used for fiber-optic communication in different applications, for example long-distance telecommunication or providing a high-speed data connection between different parts of a building.
Design
Optical fiber consists of a core and a cladding layer, selected for total internal reflection due to the difference in the refractive index between the two. In practical fibers, the cladding is usually coated with a layer of acrylate polymer or polyimide. This coating protects the fiber from damage but does not contribute to its optical waveguide properties. Individual coated fibers then have a tough resin buffer layer or core tube extruded around them to form the cable core. Several layers of protective sheathing, depending on the application, are added to form the cable. Rigid fiber assemblies sometimes put light-absorbing glass between the fibers to prevent light that leaks out of one fiber from entering another. This reduces crosstalk between the fibers, or reduces flare in fiber bundle imaging applications.Image:Lc-sc-fiber-connectors.jpg|thumb|Left: LC/PC connectors
Right: SC/PC connectors
All four connectors have white caps covering the ferrules.
For indoor applications, the jacketed fiber is generally enclosed, together with a bundle of flexible fibrous polymer strength members like aramid, in a lightweight plastic cover to form a simple cable. Each end of the cable may be terminated with a specialized optical fiber connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.
Image:Optical breakout cable.jpg|thumb|right|An optical fiber breakout cable
For use in more strenuous environments, a much more robust cable construction is required. In loose-tube construction the fiber is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fiber itself. This protects the fiber from tension during laying and due to temperature changes. Loose-tube fiber may be dry block or gel-filled. Dry block offers less protection to the fibers than gel-filled, but costs considerably less. Instead of a loose tube, the fiber may be embedded in a heavy polymer jacket, commonly called tight buffer construction. Tight buffer cables are offered for a variety of applications, but the two most common are breakout and distribution. Breakout cables normally contain a ripcord, two non-conductive dielectric strengthening members, an aramid yarn, and 3 mm buffer tubing with an additional layer of Kevlar surrounding each fiber. The ripcord is a parallel cord of strong yarn that is situated under the jacket of the cable for jacket removal. Distribution cables have an overall Kevlar wrapping, a ripcord, and a 900 micrometer buffer coating surrounding each fiber. These fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching.
A critical concern in outdoor cabling is to protect the fiber from damage by water. This is accomplished by use of solid barriers such as copper tubes, and water-repellent jelly or water-absorbing powder surrounding the fiber.
Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power that is carried to power amplifiers or repeaters in the cable.
Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, dual use as power lines, installation in conduit, lashing to aerial telephone poles, submarine installation, and insertion in paved streets.
Jacket material
The jacket material is application-specific. The material determines the mechanical robustness, chemical and UV radiation resistance, and so on. Some common jacket materials are LSZH, polyvinyl chloride, polyethylene, polyurethane, polybutylene terephthalate, and polyamide.Fiber material
There are two main types of material used for optical fibers: glass and plastic. They offer widely different characteristics and find uses in very different applications. Generally, plastic fiber is used for very short-range and consumer applications, whereas glass fiber is used for short/medium-range and long-range telecommunications.Hollow-core fiber
In hollow-core optical fibers, light travels through air rather than solid glass. In 2025, a double-nested antiresonant nodeless fiber achieved a record transmission loss of 0.091 dB/km at 1,550 nm, lower than the best solid-core silica fibers. Field trials in China demonstrated an 800 Gbit/s hollow-core link over 20 km with fusion losses as low as 0.05 dB and average cable loss of 0.6 dB/km. Hollow-core fibers reduce latency because light propagates faster in air than in glass, and they also suppress nonlinear effects and dispersion.Performance
Capacity
In September 2012, NTT Japan demonstrated a single fiber cable that was able to transfer 1 petabit per second over a distance of 50 kilometers.Although larger cables are available, the highest strand-count single-mode fiber cable commonly manufactured is the 864-count, consisting of 36 ribbons each containing 24 strands of fiber. These high fiber count cables are used in data centers, and as distribution cables in HFC and PON networks.
In some cases, only a small fraction of the fibers in a cable may actually be in use. Companies can lease or sell the unused fiber to other providers who are looking for service in or through an area. Depending on specific local regulations, companies may overbuild their networks for the specific purpose of having a large network of dark fiber for sale, reducing the overall need for trenching and municipal permitting. Alternatively, they may deliberately under-invest to prevent their rivals from profiting from their investment.
Reliability and quality
Optical fibers are very strong, but the strength is drastically reduced by unavoidable microscopic surface flaws inherent in the manufacturing process. The initial fiber strength, as well as its change with time, must be considered relative to the stress imposed on the fiber during handling, cabling, and installation for a given set of environmental conditions. There are three basic scenarios that can lead to strength degradation and failure by inducing flaw growth: dynamic fatigue, static fatigue, and zero-stress aging.Telcordia GR-20, Generic Requirements for Optical Fiber and Optical Fiber Cable, contains reliability and quality criteria to protect optical fiber in all operating conditions. The criteria concentrate on conditions in an outside plant environment. For the indoor plant, similar criteria are in Telcordia GR-409, Generic Requirements for Indoor Fiber Optic Cable.
Propagation speed and delay
Optical cables transfer data at the speed of light in glass. This is the speed of light in vacuum divided by the refractive index of the glass used, typically around 180,000 to, resulting in 5.0 to 5.5 microseconds of latency per km. Thus, the round-trip delay time for 1000 km is around 11 milliseconds.Losses
Signal loss in optical fiber is measured in decibels. A loss of 3 dB across a link means the light at the far end is only half the intensity of the light that was sent into the fiber. A 6 dB loss means only one quarter of the light made it through the fiber. Once too much light has been lost, the signal is too weak to recover and the link becomes unreliable and eventually ceases to function entirely. The exact point at which this happens depends on the transmitter power and the sensitivity of the receiver.Typical modern multimode graded-index fibers have 3 dB per kilometre of attenuation at a wavelength of 850 nm, and at 1300 nm. Single-mode loses at 1310 nm and at 1550 nm. Very high quality single-mode fiber intended for long-distance applications is specified at a loss of at 1550 nm. Plastic optical fiber loses much more: 1 dB/m at 650 nm. POF is large core fiber suitable only for short, low-speed networks such as TOSLINK optical audio or for use within cars.
Each connection between cables adds about 0.6 dB of average loss, and each joint adds about 0.1 dB. Many fiber optic cable connections have a loss budget, which is the maximum amount of loss that is allowed.
Invisible infrared light is used in commercial glass fiber communications because it has lower attenuation in such materials than visible light. However, the glass fibers will transmit visible light somewhat, which is convenient for simple testing of the fibers without requiring expensive equipment. Splices can be inspected visually and adjusted for minimal light leakage at the joint, which maximizes light transmission between the ends of the fibers being joined.
The charts Understanding wavelengths in fiber optics and Optical power loss in fiber illustrate the relationship of visible light to the infrared frequencies used, and show the absorption water bands between 850, 1300 and 1550 nm.