Fiber optics, like many of today's technologies, has developed its own language, created from technical terms specific to fiber optic technology. Sometimes the fiber optic community cannot-or will not-standardize, and we mean beyond choosing a standard connector. Sometimes the industry cannot even agree on what to call something. Let's review some of the technical terms used in fiber and try to clear up some of the confusion that sometimes makes understanding fiber optics difficult.

What is fiber optics?

Picture-sending signals zip along from one location to another in the form of light guided through thin fibers of glass or plastic. Fiber optic signals are made of photons, or light particles, not electrons. Optical signals can be analog or digital, voice data or video, just like electrical signals in copper wires.

Signals travel faster in fiber, but not like you think

Everybody talks about fiber being faster than copper. This means fiber can transport more information longer distances in less time than any copper wire because it has greater bandwidth, or information-carrying capacity, and lower loss, or signal attenuation, to allow longer spans between repeaters.

Light does not travel from point to point in fiber in less time than it would take in copper wire. Signals travel along twisted-pair copper wire and optical fiber at about the same speed-around two-thirds the speed of light-while signals in coax cable travel much faster-about nine-tenths the speed of light.

Let's start with fiber

Optical fibers are thin strands of highly transparent glass (or sometimes plastic) that guide light. The fiber has both a core, which is the center of the fiber where the light is transmitted, and cladding, which is the outside optical layer of the fiber that traps the light in the core and guides it along-even around curves. The core and cladding are made of different optical materials chosen by the requirements of the fiber design. Since most fibers have a glass core and cladding, we will generally refer to them throughout this article.

The fiber is covered by a buffer coating or primary coating, a hard plastic on the outside of the fiber that protects the glass from moisture or physical damage. This is the coating you strip off the fiber for termination or splicing.

Inside the fiber, light travels in modes, a single electromagnetic field pattern (think of a ray of light) that travels in fiber. It is how light travels in modes that gives the two main types of fiber their names: multimode fiber carries many modes and single-mode fiber carries only one mode of light.

Multimode fiber has a bigger core-almost always 62.5 or 50 microns (a micron is one millionth of a meter)-that carries multiple modes of rays of light. It is used with light-emitting diode (LED) sources at wavelengths of 850 and 1,300 nm for short distances in lower speed networks like LANs. Multimode networks operating at more than 1 gigabit per second speeds (1 gigabit = 1 billion bits) use special 850 nm lasers called VCSELs, a long acronym for vertical cavity surface-emitting laser.

Single-mode fiber has a much smaller core, only about 9 microns, that carries only one mode of light, reducing attenuation and increasing bandwidth. It can go very long distances at very high speeds. Single-mode fiber is used for telephony and CATV with laser sources at 1,300 and 1,550 nm.

Most multimode and single-mode fibers have an outside diameter of 125 microns-about five-thousandths of an inch-just slightly larger than a human hair.

Plastic optical fiber (POF) is a large core (about 1 mm) multimode fiber that can be used for short, low-speed networks. POF is used in consumer hi-fi and in cars as part of a new standard for automotive multimedia communication systems called MOST.

Fiber optic cable

Fiber needs protection to survive all the places it gets installed and it is the cable that provides this shielding. Cables may have from one to hundreds of fibers inside. The tough outer covering on the cable is called the jacket. Cables installed inside buildings must meet fire codes by using special jacketing materials. Cables installed outdoors must be resistant to environmental conditions, including moisture and sunlight, and sometimes even have armor under the jacket to prevent rodent damage.

All cables have internal strength members. Aramid fibers (Kevlar is the DuPont trade name) are generally included to pull the cable. The term “strength member” is also used for the fiberglass rod, which adds stiffness and prevents kinking in some cables.


All fibers must be terminated to allow connection to other fibers or to communications equipment. A connection between two fibers is often called joint. A connector is a nonpermanent device for connecting two fibers to each other or fibers to equipment when they are expected to be disconnected occasionally for testing or rerouting. It also provides protection to both fibers. A splice is a permanent joint between two fibers.

Most connectors use a ferrule to hold the fiber for alignment at the joint. Fibers are generally glued in the ferrule with epoxy or some fast-setting anaerobic adhesive, an adhesive that sets only when spread in a thin film and isolated from outside air.

Some connectors have a short length of polished fiber already glued in the ferrule and use a mechanical splice to connect the fiber being terminated. Most people refer to these connectors by brand names or generically as prepolished/splice connectors.

A mechanical splice is a splice where the fibers are aligned by mechanical means, usually a v-groove or glass capillary, then crimped or glued in place. Most single-mode fiber is spliced with a fusion splicer, a machine that splices the fibers by welding or fusing two fibers together, typically by electrical arc.

Terminations and splices require hardware for protection and management. Patch panels are boxes or racks for connecting multiple fiber optic cables with connectors. Splice closures are sealed protective cases that hold as many as hundreds of splices and can be mounted in the outside environment.

Fiber optic performance specifications

The reduction in optical power as it passes along a fiber, usually expressed in decibels (dB), is called attenuation. Fiber is specified by its attenuation per unit length in dB/km.

The nearly equivalent but more general term “optical loss” refers to the total amount of optical power lost as light is transmitted through the cable plant or any individual component like a connector or splice.

Decibel is the unit of measurement of optical power which is a logarithmic measurement. Decibel measurements are relative, with positive (+) readings meaning larger and negative (-) readings meaning lower power, so loss is always a negative number. Thus, -10 dB means a reduction in power by 10 times, -20 dB means another 10 times or 100 times overall, -30 means another 10 times or 1,000 times overall, and so on.

Optical loss is measured in decibels and means relative optical power, for example, the relative levels of power before and after a fiber joint by splice or connectors.

Absolute optical power is measured in “dBm” or decibels referenced to one miliwatt of power. While loss is a relative reading, optical power is an absolute measurement, referenced to standards. You measure absolute power to test transmitters or receivers and relative power to test loss.

A complete glossary of fiber optic terms is included in the book The Fiber Optic Technicians Manual or on the Internet at EC

HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at