Optical fibers are thin strands of glass or plastic that carry data as pulses of light over long distances.
What they are
An optical fiber is a thin strand of glass (or plastic) about as thick as a human hair that carries data as pulses of light by total internal reflection.
Parts
Core, cladding and protective coating.
Types
- Single-mode: small core (about 9 micrometers), long distance.
- Multi-mode: larger core (50 to 62.5 micrometers), shorter distance.
Speed
Lab records exceed 1 petabit per second. Home fiber plans offer 1 to 10 Gbps.
Benefits
High bandwidth, low loss and no electromagnetic interference.
History
Charles Kao proposed glass fibers for communication in 1966, winning the 2009 Nobel Prize in Physics.
Use
Internet backbones, undersea cables (over 95 percent of intercontinental data) and medical endoscopes.
How light stays inside the fiber
Optical fibers carry light by total internal reflection. Light enters the glass core and strikes the boundary with the cladding at a shallow angle. Because the cladding has a lower refractive index than the core, the light reflects back into the core instead of escaping. The core of a single-mode fiber is about 8 to 10 micrometers wide, narrower than a human hair, which is about 70 micrometers.
Single-mode and multimode in detail
Single-mode fiber carries one path of light and is used for long distances such as undersea cables and telecom backbones, with signals traveling 100 kilometers or more before amplification. Multimode fiber has a core of 50 or 62.5 micrometers and carries many light paths. It is cheaper to connect and common inside buildings and data centers, but signals spread out over distance, so reach is limited to about 300 to 550 meters at 10 Gbps depending on grade (OM3, OM4, OM5).
Signal loss
Modern silica fiber loses about 0.2 decibels per kilometer at the 1550 nanometer wavelength, which is why light can travel so far. Loss also comes from bends, dirty connectors and poor splices. Installers clean connector end faces and test with an optical time domain reflectometer, which shows where loss occurs along a cable. Fiber bending tighter than the rated radius, often about 30 millimeters for modern bend-insensitive fiber, increases loss.
Capacity and speed
Light in glass travels at about two thirds of its speed in a vacuum, roughly 200,000 kilometers per second. A single fiber can carry many wavelengths at once through wavelength division multiplexing. Commercial systems carry tens of terabits per second per fiber pair, and research systems have passed one petabit per second over multicore fiber. Home fiber plans typically sell 300 Mbps to 5 Gbps, and the limit is usually the equipment and the plan, not the glass.
Fiber to the home
Fiber to the home (FTTH) usually uses a passive optical network, where one fiber from the provider is split to serve 16 to 64 homes. Common standards are GPON at 2.5 Gbps down and XGS-PON at 10 Gbps symmetrical. Fiber is also described as FTTH, FTTP, FTTB or FTTC depending on how close it ends to the user. Fiber to the curb still uses copper for the last section, so speeds are lower than a full fiber line.
Where fiber is used
- Internet backbones and about 95% of intercontinental data traffic, which runs through roughly 500 undersea cables.
- Medical endoscopes, which use bundles of fibers to carry light and images.
- Sensors for temperature, strain and pressure in pipelines, bridges and oil wells.
- Telephone, cable television and mobile network backhaul.
- Decorative lighting and some industrial laser cutting systems.
History
Charles Kao and George Hockham proposed in 1966 that purified glass could carry light over long distances, and Kao won the Nobel Prize in Physics in 2009. Corning produced the first fiber with loss below 20 decibels per kilometer in 1970. The first transatlantic fiber cable, TAT-8, started service in 1988 and carried about 40,000 phone calls at once.
Advantages and downsides
Fiber is immune to electromagnetic interference, is hard to tap without detection and is lightweight. It uses less power per bit than copper over long distances. It is more fragile than copper and needs special tools and training to splice, and installing it underground is costly, which is the main reason rollout takes years.
Based on data from the Fiber Optic Association, Corning, the FTTH Council and the Nobel Prize organization.
