Fiber Optics Glossary



The absorbing of light energy within an optical fiber due to natural impurities in the glass. Absorption and scattering are the main cause of attenuation (signal loss) in an optical fiber.

Acceptance Angle

The angle at which the core of the fiber will take in light.

Active Device

A device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Examples include controlled power supplies, transistors, LEDs, amplifiers, and transmitters.


A mechanical device designed to align fiber-optic connectors. An adapter contains a split sleeve (coupling sleeve) that holds the two ferrules together.

Adapter Sleeve

A mechanical fixture within an adapter body that aligns, holds two terminated fiber connectors. Now, the adapter sleeve material is typically ceramic.

Add/Drop Multiplexer

A device that includes or removes one or more optical channels to a signal passing through it.

All-Dielectric Cable

Cable made entirely of dielectric (insulating) materials without any metal conductors, armor, or strength members.

All Silica Fiber

Also known as all-glass fiber. A fiber with both a silica core and a silica cladding, regardless of the presence of a polymer overcoat or buffer.


A device, inserted within a transmission path, that boosts the strength of an electronic or optical signal. Amplifiers may be placed just after the transmitter (power booster), at a distance between the transmitter and the receiver (in-line amplifier), or just before the receiver (preamplifier).


A signal that varies continuously (e.g., sound waves). Analog signals have frequency and bandwidth measured in hertz.

APC (Angled Physical Contact)

Abbreviation for angled physical contact. A style of fiber optic connector with a 5°-15° angle on the connector tip for the minimum possible back-reflection.

Aramid Yarn

An ingredient in optical fiber cables that provides support, protection, and tensile strength. Also referred to as KEVLAR, which is a brand of aramid yarn.


A protective layer, usually metal, wrapped around a cable.


The loss of signal strength (optical power) during transmission between two points. It expresses the total loss of an optical system, measured in decibels per kilometer (dB/km) at specific wavelengths. See “intrinsic loss” and “extrinsic loss” in this glossary.

Attenuation Meter

A device used to measure power loss in fiber optic connectors, cables, or systems.


A passive device for reducing the amplitude of a signal without appreciably distorting the waveform. Can be fixed, manually adjustable, or electrically adjustable

Avalanche Photodiode (APD)

A semiconductor photodetector with integral detection and amplification stages. Electrons generated at a p/n junction are accelerated in a region where they free an avalanche of other electrons. APDs can detect faint signals but require higher voltages than other semiconductor electronics.

Average Power

The average level of power in a signal that varies with time.

AWG (Arrayed Waveguide Grating)

An array of curved planar waveguides that separates many optical channels at once. Also called Waveguide Array. A device, built with silicon planar lightwave circuits (PLC), that allows multiple wavelengths to be combined and separated in a dense wavelength-division multiplexing (DWDM) system.


The center of an optical fiber.


Backbone Cabling

The interbuilding and intrabuilding cable connections between entrance facilities, equipment rooms, and the telecommunications closets. It consists of the transmission media, main and intermediate cross-connects, and terminations at these locations.

The highest frequency that can be transmitted by an analog system.. Also, the information-carrying capacity of a system (especially for digital systems). The range of frequencies within which a fiber optic waveguide or terminal device can transmit data or information.

Bandwidth-limited Operation

The condition in a fiber optic link when bandwidth, rather than received optical power, limits performance. This condition is reached when the signal becomes distorted, principally by dispersion, beyond specified limits.


A method of communication in which a signal is transmitted at its original frequency without being impressed on a carrier. The number of signal level transitions per second in digital data. The term is often confused with bits per second. Telecommunications specialists prefer to use “bits-per-second” to provide an accurate description.

Beam Splitter

A device that divides incident light into two separate beams. An optical device, such as a partially reflecting mirror, that splits a beam of light into two or more beams. Used in fiber optics for directional couplers.

Bending Loss

Attenuation caused by high-order modes radiating from the outside of a fiber optic waveguide which occurs when the fiber is bent around a small radius. See also macrobending, microbending.

Bend Radius

The smallest radius an optical fiber or fiber cable can bend before excessive attenuation or breakage occurs.

Biconic Connector

A type of fiber optic connector consisting of two cone-shaped ferrules aligned by a mating sleeve.


Operating in both directions. Bidirectional couplers split or combine light the same way when it passes through them in either direction. The bidirectional transmission sends signals in both directions, sometimes through the same fiber.


Having a refractive index that differs for the light of different polarizations.


An abbreviation for broadband on a passive optical network.

BR (Backreflection)

A term applied to any process in the cable plant that causes light to change directions in fiber and return to the source. Occurs most often at connector interfaces where a glass-air interface causes a reflection.

Bragg Grating

A technique for building optical filtering functions directly into a piece of optical fiber-based on interferometric techniques. Usually, this is accomplished by making the fiber photosensitive and exposing the fiber to deep UV light through a grating. This forms regions of higher and lower refractive indices in the fiber core.

Bragg Scattering

Scattering of light caused by a change in refractive index, as used in Fiber Bragg Gratings and Distributed Bragg Reflectors.

Break Out
To separate the individual fibers or buffer tubes of a fiber-optic cable for the purpose of splicing or installing optical connectors.

Breakout Cable

A type of fiber optic cable containing several fibers, each with its own jacket and all of them surrounded by one common jacket. Breakout cables are designed for convenient installation of fiber optic connectors but tend to have high transmission losses due to bends in the fibers.


In fiber-optic applications, typically a range that will minimize loss variation over a wide range of wavelengths.

Bundle of Fibers

A rigid or flexible group of fibers assembled in a unit. Coherent fiber bundles have fibers arranged in the same way on each end and can transmit images.


Material that is used to protect an optical fiber or cable from physical damage and to provide mechanical isolation or protection. Fabrication techniques include both tight jacket or loose tube buffering, as well as multiple buffer layers.

Buffer Tubes

A protective tubing used to protect the exposed fiber. Commonly used in terminating multi-fiber cable or “fan-out” situations. Also known as furcation tubing.



One or more optical fibers enclosed, with strength members, in a protective covering.

Cable Assembly

An optical fiber cable that has connectors installed on one or both ends. See “pigtail” and “patch cord” in this glossary.

Cable Plant

The cable plant consists of all the optical elements including fiber, connectors, splices, etc. between a transmitter and a receiver.

C Band

A range of wavelengths from 1530 to 1565 nm. In this region, erbium-doped amplifiers (EDFAs) have the highest gain. See EDFA and optical bands.

Center Wavelength

In a laser, the nominal value central operating wavelength. It is the wavelength defined by a peak mode measurement where the effective optical power resides (see illustration). In an LED, the average of the two wavelengths measured at the half amplitude points of the power spectrum.

Central Office

A telephone company facility for switching signals among local telephone circuits; connects to subscriber telephones. Also called a switching office.

Central Member

The center component of a cable that provides strength. Commonly referred to as “Central Strength Member.”

Channel Spacing

The amount of bandwidth allotted to each channel.


Passive three-port devices that couple light from Port 1 to 2 and Port 2 to 3 and have high isolation in other directions.


The material surrounding the core of an optical fiber. The cladding has a lower refractive index (faster speed) in order to keep the light in the core. The cladding and core make up an optical waveguide.


The process of scoring and breaking the optical fiber end in order to terminate a connector.

Coarse WDM (CWDM)

A WDM technology that spaces wavelengths widely apart.


A protective layer applied over the fiber cladding during the drawing process to protect it from the environment.

Coherent Bundle of Fibers

Fibers packaged together in a bundle so they retain a fixed arrangement at the two ends and can transmit an image.

Coherent Communications

In fiber optics, a communication system where the output of a local laser oscillator is mixed with the received signal, and the difference frequency is detected and amplified.

Coherence Length

That length over which energy in two separate waves remains constant. With respect to a laser, the greatest distance between two arms of an interferometric system for which sufficient interferometric effects can be obtained.


A mechanical device used on fiber to provide a means for aligning, attaching, and decoupling the fiber to a transmitter, receiver, or other fiber. Commonly used connections include 568SC (Duplex SC), ST, FDDI, FC, D4, and Biconic.


The central region of an optical fiber through which light is transmitted. It has a higher refractive index (slower speed) than the surrounding cladding.


A device that combines two or more fiber inputs into one fiber output or divides one fiber input into two or more fiber outputs. See “directional coupler” in this glossary.


The transferring of light going into and coming out of a fiber. This term does not imply that a coupler is used.


Connections between terminal blocks on the two sides of a distribution frame or between terminals on a terminal block (also called straps). Also called cross-connection or jumper.

Cutoff Wavelength

The shortest wavelength at which a single-mode fiber transmits only one mode. At shorter wavelengths, it transmits two or more modes.


Dark Fiber

Fiber lines that are supplied without any electronic or optical signaling equipment in its path.

dBm (decibels milliwatt)

A measurement of decibels (dB) at one milliwatt.


Decibels relative to 1µW.


A unit of measure used to express the relative strength of a signal.

Dense Wavelength-Division Multiplexing (DWDM)

Another term for closely spaced WDM. DWDM and WDM are used synonymously.


A material such as a glass fiber, which is not metallic and is not conductive.

Diode Laser

See “laser diode” in this glossary


The bending of light rays as they pass around corners or through holes smaller than their own wavelengths.

Diffraction Grating

A series of scored lines that separates light into its various colors.

Directional Coupler

A coupler in which light is transmitted differently depending on the direction of transmission. See “coupler” in this glossary.


The spreading or broadening of light pulses as they travel through a fiber. The fiber property that causes this effect is also called dispersion. The three principal types are modal dispersion, chromatic dispersion, and polarization mode dispersion.

Dispersion Compensation

Reducing dispersion in fiber in order to reduce total dispersion. Different methods are used for chromatic dispersion and polarization mode dispersion.

Distributed Bragg Reflection

Reflection of light caused by periodic changes in refractive index in a stack of layers of different compositions or-equivalently-by a corrugation at the boundary between two semiconductor layers. The period and the refractive index select one wavelength.

Distributed Feedback Laser (DFB Laser)

A diode laser with corrugation in the electrically pumped part of the laser, which selects the laser wavelength by reflecting that wavelength back into the active layer.

Distribution System

Part of a cable system consisting of trunk and feeder cables used to carry signals from headend to customer terminals.

Dominant Mode

The mode in an optical device spectrum with the most power.


An impurity added to an optical medium to change its optical properties. EDFAs use erbium as a dopant for optical fiber.

Double-window Fiber (Dual Window Fiber)

1) Multimode fibers optimized for 850 nm and 1300 nm operation.
2) Single-mode fibers optimized for 1310 nm and 1550 nm operation.

Doubly Clad Fiber

Optical fiber exhibits wide transmission bandwidth and low bending loss to the reduction of guided modes as a result of the high-refractive-index external cladding and the tight confinement within the core regions.

Drawing Tower

A system for fabricating optical fiber, consisting of a furnace that heats the materials, a polymer coating stage, a capstan-pulling apparatus that free-draws the preform into a fiber and a drum on which the finished product is wound.

Drop Cable

A cable that delivers service to an individual customer.

Duplex Cable

In cables, one that contains two fibers. For connectors, one that connects two pairs of fibers. For data transmission, full-duplex transmitters and receivers simultaneously send and receive signals in both directions, but half-duplex cannot do both at the same time.

Duplex Patch Cord

A two-fiber cable used for bi-directional transmission.

Duplex Transmission

Transmission in both directions, either one direction at a time (half-duplex) or both directions simultaneously (full-duplex).


E2000 Connector

The E2000/LX-5 is like an LC but with a shutter over the end of the fiber.

EDFA (erbium-doped fiber amplifier)

An optical amplifier that boosts all channels in the optical signal at the same time.

EDWA (erbium-doped waveguide amplifier)

An optical amplifier similar to an EDFA, but derives a higher gain through a small waveguide rather than several meters of fiber.

Electro-absorption Modulator

A semiconductor diode that modulates light from a separate laser, but that may be fabricated on the same wafer. Turning current on causes light absorption.

EMI (electromagnetic interference)

The interference in signal transmission or reception resulting from the radiation of electrical or magnetic fields. Optical fibers are not susceptible to EMI.


A cabinet used to organize and enclose cable terminations and splices for use within main equipment rooms, entrance facilities, main or intermediate cross-connects, and telecommunications closets.

End Face

Often used to describe the end of a ferrule. The end face is finished or polished to have a smooth end, which can minimize connector loss or back-reflection. Typical polish types are PC, UPC, and APC.


A thermosetting resin used to secure the fiber with the connector ferrule.

Evanescent Waves 

The light that passes into the cladding from the core.

Extrinsic Loss

The loss that is induced in an optical transmission system by an external source. In a fiber-optic link, this can be caused by improper alignment of connectors or splices.



A multi-fiber cable constructed in a tight-buffered tube design. At a termination point, cable fibers must be separated from the cable to their separate connection positions.

FC Connector

FC stands for Fixed Connection. It is fixed by way of a threaded barrel housing. FC connectors are typical in test environments and for single-mode applications.


See FC. A threaded optical connector that uses a special curved polish on the connector for very low backreflection. Good for single-mode or multimode fiber.


The rigid prong in a fiber-optic plug that aligns the fiber with the socket. Ferrule materials are ceramic, plastic, and stainless steel.


A thin filament of glass or plastic consisting of a core (inner region) and a cladding (outer region) and a protective coating.

Fiber Bragg Grating

A series of periodically spaced zones in a short length of fiber with a higher refractive index used to filter out wavelengths.

Fiber Laser

An alternate way of building a laser. The laser is built into the fiber itself.

Fiber to the Curb (FTTC)

Fiber optic service to a node that is connected by wires to several nearby homes, typically on a block.

Fiber to the Home (FTTH)

A network in which optical fibers bring signals all the way to homes.

Fibre Channel

A standard for transmitting signals at 100 Mbit/s to 4.25 Gbit/s over fiber or (at slower speeds) copper. An industry-standard specification that originated in Great Britain which details computer channel communications over fiber optics at transmission speeds from 132 Mb/s to 1062.5 Mb/s at distances of up to 10 kilometers.

Fiber Tracer

An instrument that couples visible light into the fiber to allow visual checking of continuity and tracing for correct connections.

Fiber Identifier

A device that clamps onto a fiber and couples light from the fiber by bending, to identify the fiber and detect high-speed traffic of an operating link or a 2 kHz tone injected by a test source.


A device that transmits only part of the incident energy and may thereby change the spectral distribution of energy.


Fiber to the Building. This is in reference to fiber optic cable, carrying network data, connected all the way from an Internet service provider to a customer’s physical building.

An abbreviation for fiber to the curb.


An abbreviation for fiber to the desk.


An abbreviation for fiber to the home.


Stands for Fiber to the Premises.


An abbreviation for ‘Fiber to the x’. The ‘x’ is a variable that can mean fiber to the: premises, curb, home, business, or desk, for example.

Full Duplex

In data transmission, transmitters and receivers simultaneously send and receive signals in both directions.

Fundamental Mode

The lowest order mode of a waveguide. Note: In optical fibers, the fundamental mode is designated LP01 or HE11.

Fused Fibers

A bundle of fibers melted together so they maintain a fixed alignment with respect to each other in a rigid rod.

Fused Coupler

A method of making a multimode or single-mode coupler by wrapping fibers together, heating them, and pulling them to form a central unified mass so that light on any input fiber is coupled to all output fibers.

Fusion Splice

The joining of two fiber ends by applying enough heat to fuse or melt the ends together to form a continuous single fiber.

Fusion Splicer
An instrument that permanently bonds two fibers together by heating and fusing them.



Gigabits (billion bits ) per second


Abbreviation for gigahertz. One billion Hertz (cycles per second) or 109 Hertz.

Graded-index Fiber

A multimode fiber designed to compensate for modal dispersion by allowing light to travel increasingly faster from the center of the core to its outer edge.



In data transmission, a system in which transmitters and receivers cannot simultaneously send and receive signals.

HFC – Hybrid Fiber/Coax (HFC)

The use of fiber to distribute cable-television signals to nodes, which in turn distribute them to homes over coaxial cable.

HFC Network

A telecommunication technology in which optical fiber and coaxial cable are used in different sections of the network to carry broadband content. The network allows a CATV company to install fiber from the cable headend to serve nodes located close to business and homes, and then from these fiber nodes, use coaxial cable to individual businesses and homes.

Hybrid Cable
1. A fiber optic cable containing two or more different types of fiber, such as 62.5µm multimode and single-mode.
2. A cable containing both optical fiber and copper wire. Also known as composite cable.



A range of light from approximately 700 to 1000 nm. Fiber-optic systems transmit between 700 and 1700 nm.

Injection Loss, Insertion Loss

The amount of light that leaks out or is otherwise lost after being inserted into a fiber either from a light source or another fiber.


The combination of light waves in which the wave amplitudes add together. Constructive interference produces bright light when the peaks are in phase with each other. Destructive interference produces dark zones when the peaks of one wave align with the valley of the second.

In-line Amplifier

An EDFA or other type of amplifier placed in a transmission line to strengthen the attenuated signal for transmission onto the next, distant site. In-line amplifiers are all-optical devices.


Abbreviation for International Standards Organization. Established in 1947, ISO is a worldwide federation of national standards committees from 140 countries. The organization promotes the development of standardization throughout the world with a focus on facilitating the international exchange of goods and services and developing the cooperation of intellectual, scientific, technological, and economic activities.


Abbreviation for an Internet service provider. A company or organization that provides Internet connections to individuals or companies via dial-up, ISDN, T1, or some other connection.


L Band

A range of wavelengths from 1565 to 1625 nm. In this region, erbium-doped amplifiers (EDFAs) can be used, but have less gain than in the C band.


A device that generates a coherent beam of light all in phase and of a single (or nearly single) wavelength. A cavity with mirrors at each end causes a chain reaction that stimulates the emission of photons.

Laser Diode

A laser made of semiconductor materials widely used to transmit light into optical fibers. It is always used for single-mode fiber and certain high-bandwidth multimode fiber such as used with Gigabit Ethernet.

LED (light-emitting diode) 

A device that produces light with a wide range of wavelengths. LEDs are typically used with lower-bandwidth multimode fiber.

Loose Tube

The protective tube surrounding one or more fibers. This is usually found in cables used for outdoor installations.



The loss due to large scale bending (extrinsic loss). Bending causes imperfect guiding of light which will exceed the critical angle of reflection. Macro bending loss can be reversed once the bend is corrected.

Mechanical Splice

Joining two fiber ends together by a temporary or permanent mechanical method in order to maintain continuous signal transmission.

MEMS (microelectromechanical systems)

Tiny components etched from a semiconductor material that can move under the control of electronic signals. MEMS devices include movable mirrors that can switch or redirect the path of light.


The loss of light due to small distortions in the fiber, not usually visible to the naked eye.


One micrometer or one-millionth of a meter. Used to express the geometric dimension of fibers.


A reflective path that the light takes in a fiber. Each mode has its own pattern of electromagnetic fields as it propagates through the fiber. There is only one mode in single-mode fiber. In multimode fiber, multiple modes are generated, causing pulse dispersion at the receiving end.

Mode Field Diameter

In a single-mode fiber, the diameter of the zone where the single-mode propagates down the center of the fiber. It is slightly larger than the core diameter.


An optical fiber in which light travels in multiple modes. Multimode fiber is used in shorter-distance applications than single-mode fiber.


Combining two or more signals into a single bitstream that can be individually recovered.


One billionth of a meter.

OFNR (Optical Fiber Non-conductive Riser)

A type of fiber-optic cable.

OFNP (Optical Fiber Non-conductive Plenum)

A type of fiber-optic cable.

Optical Channel

A signal transmitted at one wavelength in a fiber-optic system.

Optical Network

A network that processes and switches signals in optical form.

Optical Switch

A device that routes optical signals to their appropriate destination. All-optical switches (OOO) do not have to convert light back to electricity for processing.

Optical Waveguide

An optical fiber, planar waveguide, or other structure that guides light along its length.

OTDR (Optical Time Domain Reflectometer)

An instrument that measures optical transmission characteristics by sending a short pulse of light down a fiber and observing backscattered light. Used to measure fiber attenuation and evaluate optical transmission at splices and connectors.


Passive Optical Network

A fiber-optic system with no active components between its distribution point and remote receiver nodes.


A device that receives optical power and changes it to electrical power.

PC (Physical Contacting)

A type of fiber-optic connector that causes two terminated fiber ends to contact each other, keeping signal losses to a minimum.

Patch Cord

A specific length of optical fiber cable with terminated connectors on each end. Used for connecting patch panels or optoelectronic devices.


A particle of light.


Having to do with light or photons.


A short length of fiber in which one end is attached to a component and the other is free to be spliced to another fiber.

Planar Waveguide

A flat waveguide on the surface of a substrate with a lower refractive index. It confines light similar to an optical fiber. Used in waveguide arrays.


The alignment of the perpendicular electrical and magnetic fields that make up a light wave.

Polarization Mode Dispersion

The dispersion that arises from slight asymmetries in optical fibers. The speed of light varies with polarization.

Polishing Paper

Also known as lapping film, it is a paper with a fine-grit used to remove any imperfections in the fiber end surface that may exist after cleaving. Fiber ends terminated within a connector are polished flush with the end of the ferrule.

Polishing Puck

A device used to hold the connector during the polishing of the fiber.

Population Inversion

The state of atoms that have been excited.


Raman Amplifier

A device that boosts the signal in an optical fiber by transferring energy from a powerful pump beam to a weaker signal beam.

Receiver (RX)

An optoelectronic device that converts optical signals into electrical signals.


The process that occurs when a light ray traveling in one material hits a different material and reflects back into the original material without loss of light.


The bending of light rays as they pass through a transmission medium of one refractive index into a medium with a different refractive index.

Refractive Index

The ratio of the velocity of light in a vacuum to the velocity of light in a specific material. Using 1.0 as the base reference, the higher the number, the slower light travels.


A transceiver that converts optical signals to electronic and backs out to optical.


A pathway for indoor cables that pass between floors.


S Band

A range of wavelengths from 1460 to 1530 nm.

SDH (Synchronous Digital Hierarchy)

A scale of standard data rates for fiber-optic systems defined by the ITU.


An optical fiber in which the signal travels in one mode (path). It typically has an 8-10 µm core within a 125 µm cladding.

SONET (Synchronous Optical Network)

A scale of standard data rates for fiber-optic systems used in North American systems.


A method for joining two optical fiber ends. Fusion splicing and mechanical splicing are the two types.

Splice Closure

A container used to hold and protect splice trays.

Splice Tray

A container used to hold, organize, and protect spliced fibers.

Split Sleeve

The part of a fiber-optic adapter that aligns the ferrules of two terminated connectors.


A device that takes the light from one fiber and injects it into the cores of several other fibers.

Step-index Fiber

A fiber in which the core and cladding each have a uniform, but different, refractive index.


Tight Buffer

A protective coating (typically 900 µm) that is extruded directly over the primary coating of fibers. Provides high tensile strength, durability, ease of handling, and termination.


A transmitter and receiver combined in one device.

Transmitter (TX)

An optoelectronic device that converts an electrical signal to an optical signal. It is usually an LED or laser diode.

Transparent Network

A fiber-optic network that is entirely light-based with optical switches and other optical-only devices.

Tunable Laser

A laser that can change its frequency over a given range.


VCSEL (vertical-cavity surface-emitting laser)

A semiconductor laser that emits a beam from its surface rather than its edge.

VOA (variable optical attenuator)

A device that can be adjusted to block different fractions of light passing through it.



A structure that guides electromagnetic waves. An optical fiber is an optical waveguide.

Waveguide Array

A device that separates wavelengths by passing them through an array of curved waveguides running between a pair of mixing regions.


The length of a wave measured from any point on one wave to the corresponding point on the next. The wavelengths of light used in optical fibers are measured in nanometers. Common wavelengths are 850, 1300, and 1350 nm.

WDM (wavelength division multiplexing)

Transmitting several wavelengths of light (colors) in one fiber.

Fibers And Cables

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