The portion of optical attenuation in optical fiber resulting from the conversion of optical power to heat. Caused by impurities in the fiber such as hydroxyl ions. Due to impurities inherent to the fiber’s manufacture and internal molecular structure, a small percentage of the signal strength is absorbed and released as heat within the optical fiber. Hydroxyl ions are known as one the main contributors of this effect.


A course grinding material used to figure, shape, or finish optical elements. These materials can be silicon carbide, aluminum oxide, silica, cerium oxide, emery or rouge. Abrasives differ from polishing materials mainly in particle size and are often referred to as a measured “grit”. See Polishing

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. Commonly used in fiber optics to refer to the transmitter/transceiver and/or detector of the optical network, these devices can be known as either electro-optical or opto-electrical devices and consist of laser or LED diode transmitters, amplifiers, APD (avalanche photodiode) or PIN (positive intrinsic negative) diode detectors. These devices are responsible for converting both analog and digital signals to and from optical pulses transported over the optical fiber network.

Acceptance Angle

The angle at which the core of an optical fiber accepts incoming light; usually measured from the fiber axis. Light entering at an angle greater than the acceptance angle will be reflected away. Related to numerical aperture (NA).


An adapter is a mechanical fixture designed to interlock fiber-optic connectors. The adapter contains a split sleeve, also known as the interconnect sleeve, that holds the mated connector ferrule endfaces together and in proper alignment. Depending upon the level of alignment accuracy required, this sleeve can be made of a polymer, phosphor bronze, stainless steel or ceramic materials.

Add-drop multiplexer

A device that drops and/or adds one or more optical channels to a signal. Add/drop multiplexers can be manufactured using gratings burned into the core of an optical fiber using masks and UV light, a splitter/coupler having two types of wavelength-specific fibers twisted into them or on a planar waveguide grating manufactured on photonic bandgap type materials.

All-Dielectric Cable

A cable made entirely of non-metallic components. Optical cable assemblies that are considered all-dielectric utilize fiberglass strength members, polymer optical fiber containments, water-blocking element(s) (either in the form of a viscous gel or dry tape), aramid yarn(s), and an environment-suitable casing.

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 signal that varies continuously (e.g., sound waves). Analog signals rely upon clarity of the signal based upon wavelength, amplitude and frequency and are commonly measured in hertz.

Angle of Incidence

The angle at which a light ray either reflects or refracts at an optical surface, in reference to an assumed perpendicular line, called the “normal”, An example of this would be a light ray striking the optical surface at 45 degrees to the “normal” would refract through the optical element. A light ray less than 45 degrees to the plane of the optic would reflect off of the optical surface.


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).

Semi-conductor optical amplifiers (SOAs), doped fiber amplifiers (DFAs) such as erbium doped fiber amplifiers (EDFAs) or erbium doped waveguide amplifiers (EDWAs) are examples of optical amplifiers.

Amplified Spontaneous Emission (ASE)

A background noise mechanism common to all types of erbium-doped fiber amplifiers (EDFAs).

It contributes to the noise figure of the EDFA which causes loss of signal-to-noise ratio (SNR).

Angular Misalignment

Attenuation created Loss at a connector due to the ferrule and/or fiber end face angles being misaligned. Can be caused by damaged alignment interconnect sleeves, contamination or mis-polishing. See Extrinsic Loss

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 backreflection.

AR Coating

Antireflection coating. A thin, dielectric or metallic film applied to an optical surface to reduce its reflectance and thereby increase its transmittance.


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

A flexible, interlocking spiral wrap of corrosion resistant metal, used to protect the cable’s interior fiber components in harsh environmental installations. Common configurations include aerial installations, which have the armor wrapped around the outside of the cable jacket for support, underground cable includes armor just under the outer cable jacket and undersea cables where the armor is within the cable jacket. Armored cables are generally used in direct buried installations where sharp rocks can abrade the outer jacket or in electrical tunnel or sewer installations where the potential for rodent damage can occur. This type of cable is considered conductive and must be grounded after installation per the National Electrical Code (See NEC)


Reduction of signal magnitude, or loss, normally measured in decibels. Fiber attenuation is normally measured per unit length in decibels per kilometer. The decrease in signal strength along a fiber optic waveguide caused by absorption and scattering. Attenuation is usually expressed in dB/km.

The reduction of the optical signal intensity over a length of optical fiber or components. Attenuation (or “loss”) is normally measured in two instances: 1) Optical fiber distance, due to absorption and scattering, and represented by decibels per kilometer (dB/km) accumulative; 2) Individual component attenuation (in dB loss) for splitters/couplers, WDMs, connectors, both mechanical and fusion splices, etc. (See Decibel)

Attenuation Meter

A device used to measure power loss in fiber optic connectors, cables, or systems. Commonly known as a “power meter”, this device is used to measure power loss (in dB) across fiber optic connectors, cables or systems. (See Decibel)


A passive device for reducing the amplitude (or attenuation) of a signal without appreciably distorting the waveform. An optical attenuator is commonly placed between two connectors or between a transmitter or detector port and the mating connector. Attenuators can be fixed, manually adjustable, or electrically adjustable using filters, gratings or separation of the optical components.

Avalanche Photodiode (APD)

A semiconductor photodetector with integral detection and amplification stages, that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region. 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. APDs are used as highly responsive detectors in both higher data-rate, longer distant optical fiber networks as well as local fiber to the x (FTTx) installations.

Average Power

The average level of power in a signal that varies with time. The averaged level of variance in the power within a signal measured over time.

AWG (Arrayed Waveguide Grating)

An array of curved planar waveguides that separates many optical channels (wavelengths) 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. Varying wavelengths exit from an input fiber into the AWG region an due to their differing angles reflect off of the grating and access optical fibers opposite to the grating and within each fibers refractive angle of incidence.



Scattering of light in the direction opposite to that in which it was originally traveling. The return of a portion of scattered light to the input end of a fiber; the scattering of light in the direction opposite to its original propagation. Also known as “Rayleigh backscatter” are the result of physical makeup (impurities, dopants) and interruptions in the optical fiber including mechanical and fusion-type splices, cleaved fiber ends, connectors, impurities included in the manufacture of the optical fiber can all create backscatter. Backscatter is relied upon by optical timed domain interferometers (OTDRs) to measure and chart-display the distance, attenuation and backreflection of either the uninstalled optical fiber reel or installed network (and its components) by marking the time encountered, relating to the distance, based upon the percentage of backscatter sensitivity inserted. The term was named after its discoverer, John William Strutt a.k.a.3rd Baron Rayleigh (or Lord Rayleigh). (See OTDR)

BR (Backreflection)

A term applied to any physical inconsistency in the cable plant that causes a percentage of the transmitted light to change directions in a fiber and return to the source. Measured in decibels (dB) as a form of attenuation, backreflection occurs most often at unterminated or cleaved fiber ends, connectors and improperly installed mechanical splice interfaces where a glass-air separation causes a reflection. A low number in measured dBs across a reflective interface indicates a high level of back-reflected light energy towards the source. Contrary to this, a high measurement of backreflection dBs at a mechanical interface indicates a lower percentage of the transmitted energy returning to the source. Backreflection is considered damaging to laser light sources used in single-mode fiber optic networks and some laser enhanced, high datarate multimode networks. Most OTDRs now measure backreflection for individual components (i.e. connectors and splices) and the over-all network in optical return loss (ORL). (See ORL, OTDR)


The highest frequency that can be transmitted by an analog system. Bandwidth was commonly used to represent highest frequency and information-carrying capacity over multiple wavelengths in older analog telephone circuits. This bandwidth term transitioned when multimode optical fiber, using LED transmitters, began transmitting analog telephony signals over fiber. The term “hertz” (Hz) is used as a measurement of system’s bandwidth capacity and has continued its usage in digital networks over legacy and the newer “laser-enhanced” multimode fibers (and was discontinued for telephony when their systems began using single-mode fiber and laser transmitters). Since multimode fiber has a larger optical core, this allows multiple modes to enter

and refract down the waveguide. Each one of these multiple modes can easily be a wavelength carrying digital (or analog) data. A measurement of both legacy and laser-enhanced multimode optical fibers, known as Effective Modal Bandwidth (EMB), is now performed as minEMBc (minimum EMB calculation) to determine their information carrying capacity limit for dispersion over distance. This test determines the bandwidth limit for all multimode networks in relation to distance in MHz•km (MHz per km). (See , Bandwidth-limited Operation, EMB, Hertz, VCSEL) The range of frequencies within which a fiber optic waveguide or terminal device can transmit data or information.

Bandwidth-limited Operation

A term used to describe the data transport capacity limitations of a network. Exceeding the bandwidth limits with increased data-rates beyond the network’s capability, causes dropped bits and over-lapping data over several pulses of the bandwidth, resulting in an increase in bit error rate (BER). Exceeding the bandwidth distance limitations (in km) can cause dispersion (pulse-spreading) and distortion of the optical signal transmitting over a fiber optic network, once again causing bit error rates to rise. Bandwidth-limited operation is used synonymously with distortion-limited operation. Ethernet data-rates are a good example of bandwidth-limited operation over multimode optical networks (See Bit-Error Rate, Dispersion)


A method of communication in which an analog signal is transmitted at its original frequency without being impressed on a carrier. Examples of this would be a radio broadcast or cell phone communications prior to raising their frequencies for transmission or in an analog to digital data transmission, multiplexing or demultiplexing the data back to its original baseband analog signal. The number of signal level transitions per second in digital data. In Ethernet communications, the term “BASE” is short for baseband and is used to indicate the level of data transport capacity over either wire or optical fiber. 10/1001000BASE-T and 10GBASE-T are examples of baseband datarates used in Ethernet communications. The term is often confused with bits per second. Telecommunications specialists prefer to use “bits-per-second” to provide an accurate description.


An optical device that divides incident light into two separate beams. Two equilateral prisms, mounted hypotenuse to hypotenuse, form a beamsplitter. Incident light entering one side of one prism is first reflected through its adjacent side and then refracted through the hypotenuse, and out of the side of the mated prism. Another type of beamsplitter can be an optically flat glass plate, made to minimally reflect by using a thin, semi-transparent, metallic- based film (aluminum, silver or gold). A percentage of the incident light reflects off of the plate and a percentage of the incident light passes through the plate. 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 through the cladding from the outside of a fiber optic waveguide when the minimum bend radius (in diameter) for the optical fiber or optical cable is exceeded. which occur when the fiber is bent around a small radius. Common

causes for this are known as macrobending and microbending. (See Macrobending, Microbending)

Bend Radius

The minimum bend radius of the optical fiber/cable before excessive attenuation or fiber breakage occurs. Under the set standards, fiber/cable bend radius is limited to twenty-times (20X) the diameter for dynamic (installation) and ten-times (10X) the diameter in static (storage).The smallest radius an optical fiber or fiber cable can bend before excessive attenuation or breakage occurs.

Biconic Connector

One of the first optical connectors used in single-mode and later multimode markets, the biconic was a threaded, non-keyed connector manufactured from either stainless steel or plastic. The mating ferrule was manufactured in the shape of a truncated cone. The mating sleeve was manufactured with opposed, corresponding cones to provide alignment of the two connectors. Attenuation level repeatability issues and mated ferrule endface/fiber damage made this connector obsolete. 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. In optical fiber, bidirectional transmission sends data signals in both directions, over different wavelengths through the same fiber. For example, fiber to the “x” (FTTx) networks utilize bi-directional data streams in the form of “upstream and downstream” communications between the central office and the customer over different wavelengths on the same fiber.


Having a refractive index that differs for light of different polarizations The unplanned separation of a single light path into two-separate light paths due to a transparent material’s imperfections. An example of this would be a viewing an image through a crystal of calcite. The calcite shifts the image and creates two images, one offset slightly (birefringent) from the other. In fiber optics, the fundamental mode has two orthogonal axis represented by the polarized values of X and Y set in a perpendicular field. Due to imperfections in the core’s cylindrical consistency (ovality, for example) over distance, one of the transmitted signal’s polarization axis (X. for example) slows down as its perpendicular axis (Y) speeds up. The X and Y axis become shifted and separate into two birefringent waves. The digital light pulsing the data becomes distorted due to dispersion. (See Polarization Mode Dispersion)


Broadband Passive Optical Network. A common term used for all Fiber to the “X” networks currently operating over single-mode optical fiber using a 1 by 32 splitter to support 32 customers. Distances limited to 20 kilometers. Data speeds limited to 10/100/1000 MBs (Megabits per second). Signals multiplexed bidirectionally, utilizing 1310nm, 1490nm and 1550nm wavelengths for customer request, video and digital returned services respectively. An abbreviation for broadband on passive optical network.

Bragg Grating

Fiber Bragg gratings (FBGs) are a form of optical filter. Based upon interferometric technology, these filters are manufactured by altering the index of refraction in the form of a grating pattern within the core of the optical fiber. This grating pattern allows certain multiplexed wavelengths to pass through, while others will be blocked or reflected elsewhere. These grating patterns are burned into the fiber’s core with the use of high intensity UV light through a grating mask placed over the fiber. within the optical fiber’s core. allowing some multiplexed wavelengths to pass through, while reflecting others 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 exposed fiber. Commonly used in terminating multi-fiber cable or “fan-out” situations. Also known as furcation tubing.

Burn In

The operation of a laser diode or other component prior to its use in its intended application, as a means of testing and stabilizing it.

Butt Splice

A joining of two fibers without optical connectors arranged end-to-end by means of a coupling.

Fusion splicing is an example.



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

Cable Assembly

A cable that is connector terminated and ready for installation.

Cable Plant

The cable plant consists of all the optical elements including fiber, connectors, splices, etc.

between a transmitter and a receiver.


Wavelengths of about 1530 to 1565 nm, where erbium-doped fiber amplifiers have their strongest gain. Normally erbium-fiber amplifiers operate in either C- or L-band. The wavelength range between 1530 nm and 1562 nm used in some CWDM and DWDM applications.

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 Memeber

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


A communications path or the signal sent over that path. Through multiplexing several channels, voice channels can be transmitted over an optical channel.

Channel Spacing

The amount of bandwidth allocated per channel. An example is DWDM components with 100GHz or 200GHz.


In laser diodes, the shift of the laser’s center wavelength during single pulse durations.

Chromatic Dispersion

Wavelength-dependent pulse spreading in optical fibers, measured in pico seconds (of pulse spreading) per nanometer (of source bandwidth) per kilometer (of fiber length). It is the sum of waveguide and material dispersion. Reduced fiber bandwidth caused by different wavelengths of light traveling at different speeds down the optical fiber. Chromatic dispersion occurs because the speed at which an optical pulse travels depends on its wavelength, a property inherent to all optical fiber. May be caused by material dispersion, waveguide dispersion, and profile dispersion.


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 layer of glass or other transparent material surrounding the light-carrying core of an optical fiber. It has a lower refractive index than the core and thus confines light in the core. Coatings may be applied over the cladding. Material that surrounds the core of an optical fiber. Its lower index of refraction, compared to that of the core, causes the transmitted light to travel down the core. This is glass or plastic, having a low refractive index, that surrounds the core of a fiber. Optical cladding promotes total internal reflection for the propagation of light in a fiber.

Cladding Mode

A mode confined to the cladding; a light ray that propagates in the cladding.


The process of separating an optical fiber by a controlled fracture of the glass, for the purpose of obtaining a fiber end, which is flat, smooth, and perpendicular to the fiber axis.

Coarse Wavelength-Division Multiplexing (CWDM)

Transmitting signals at multiple wavelengths through the same fiber with wide spacing between optical channels. Typical spacing is several nanometers or more. Also called wide wavelength multiplexing. CWDM allows eight or fewer channels to be stacked in the 1550 nm region of optical fiber, the C-Band.


An outer plastic layer applied over the cladding of a fiber for mechanical protection. The material surrounding the cladding of a fiber. Generally a soft plastic material that protects the fiber from damage.

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 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 suficient interferometric effects can be obtained.


  1. The process of aligning the optical axes of optical systems to the reference mechanical axes or surfaces of an instrument.
  2. The adjustment of two or more optical axes with respect to each other.


An optical instrument consisting of a well-corrected objective lens with an illuminated slit or reticle at its focal plane. Collimators are used in lens testing to determine focal lengths, and in other metrological applications where a distant object at a known location is required.

Composite Cable

A cable containing both fiber and copper conductors. Also known as hybrid cable.


The process of connecting pieces of fiber together.


A device mounted on the end of a fiber-optic cable, light source, receiver, or housing that mates to a similar device to couple light into and out of optical fibers. A connector joins two fiber ends, or one fiber end and a light source or detector. A mechanical or optical device that provides a demountable connection between two fibers or a fiber and a source or detector.

Connector Variation

The maximum value in dB of the difference in insertion loss between mating optical connectors (e.g., with remating, temperature cycling, etc.). Also called optical connector variation.


The measurement of how well-centered the core is within the cladding.

Constructive Interference

Any interference that increases amplitude of the resultant signal. For example, when the wave forms are in phase, they can create a resultant wave equal to the sum of multiple light waves.


The central part of an optical fiber that carries light. The light-conducting portion of a fiber, defined by its higher refraction index. The core is the center of a fiber, surrounded by concentric cladding of lower refractive index.

Coupled Modes

In fiber optics, a mode that shares energy among one or more other modes, all of which

propagate together. Note: The distribution of energy among the coupled modes changes with propagation distance.


A device that connectors three or more fiber ends, dividing one input between two or more outputs or combining two or more inputs into one output.


Transfer of light into or out of an optical fiber. (Note that coupling does not require a coupler).

Coupling Efficiency

The fraction of available output from a radiant source that is coupled and transmitted by an optical fiber.

Critical Angle

The angle at which light in a high-refractive-index material undergoes total internal reflection. In geometric optics, at a refractive boundary, the smallest angle of incidence at which total internal reflection occurs.


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.

Crosstalk (XT)) Any phenomenon by which a signal transmitted on one channel of a transmission system creates and undesired effect in another channel.

Cut-Back Measurements

Measurement of optical loss made by cutting a fiber to compare loss of a short segment with loss of a longer one.

Cutback Technique

A destructive technique for determining certain optical fiber transmission characteristics, such as attenuation and bandwidth, by (a) performing the desired measurements on a long length of the fiber under test, (b) cutting the fiber under test at a point near the launching end, (c) repeating the measurements on the short length of fiber, and (d) subtracting the results obtained on the short length to determine the results for the residual long length.

Cutoff Mode

The highest order mode that will propagate in a given waveguide at a given frequency.

Cutoff Wavelength

The longest wavelength at which a single mode fiber can transmit two modes, or (equivalently) the shortest wavelength at which a single mode fiber carries only one more.


Abbreviation for continuous wave. Usually refers to the constant optical output from an optical

source when it is biased (i.e., turned on) but not modulated with a signal.

Cycles per Second

The frequency of a wave, or number of oscillations it makes per seconds. One cycle per second equals one hertz.


Dark Current

The noise current generated by a photodiode in the dark.

Dark Fiber

Optical fiber installed without transmitter and receiver, usually to provide expansion capacity. Some carries lease dark fibers to other companies that add equipment to transmit signals through them.

Data Rate

The number of bits of information in a transmission system, expressed in bits per second (b/s or bps), and which may or may not be equal to the signal or baud rate.


Decibels relative to 1mW.


Decibels relative to 1 µW.

Decibel (dB)

A logarithmic comparison of power levels, defined as ten times the base-10 logarithm of the ratio of the two power levels. One-tenth of a bel.

Delay Line

A device used to delay transmission of a signal for functions such as memory loops, sequential processing or built-in testing. The delay can be achieved by coiling long lengths of coaxial cable or optical fiber.


A device that separates a multiplexed signal into its original components; the inverse of a multiplexer.

Dense Wavelength-Division Multiplexing (DWDM)

Transmitting signals at multiple wavelengths through the same fiber with close spacing. Channel spacing usually is 200GHz or less in frequency units, corresponding to 1.6nm in wavelength units at 1550nm. The transmission of many of closely spaced wavelengths in the 1550 nm region over a single optical fiber. Wavelength spacings are usually 100 GHz or 200 GHz which corresponds to 0.8 nm or 1.6 nm. DWDM bands include the C-Band, the S-Band, and the L-Band.

Destructive Interference

Any interference that decreases the desired signal. For example, two light waves that are equal in amplitude and frequency, and out of phase by 180°, will negate one another.


A device that generates an electrical signal when illuminated by light. The most common fiber-optic detectors are photodiodes.

Diameter-mismatch Loss

The loss of power at a joint that occurs when the transmitting fiber has a diameter greater than the diameter of the receiving fiber. The loss occurs when coupling light from a source to fiber, from fiber to fiber, or from fiber to detector.

Dielectric Filter

An optical fiber that selectively transmits one wavelength and reflects others based on interference effects inside the structure. Also called interference filter.

Diffraction Grating

An array of fine, parallel, equally spaced reflecting or transmitting lines that mutually enhance the effects of diffraction to concentrate the diffracted light in a few directions determined by the spacing of the lines and by the wavelength of the light.


An electronic device that lets current flow in only one direction. Semiconductor diodes used in fiber optics contain a junction between regions of different doping. They include light emitters (LEDs and laser diodes) and detectors (photodiodes).

Diode Laser

A semiconductor diode that generates laser light. A current flowing through the diode causes electrons and holes to recombine at the junction layer between p- and n-doped regions, producing excited states that can release energy in the form of light.


Abbreviation for dual in-line package. An electronic package with a rectangular housing and a row of pins along each of two opposite sides.


A device that combines two or more types of signals into a single output. Usually incorporates a multiplexer at the transmit end and a demultiplexer at the receiver end.

Directional Coupler

A coupler in which light is transmitted differently when it goes in different directions.


The stretching of light pulses as they travel in an optical fiber, which increases their duration.

The temporal spreading of a light signal in an optical waveguide caused by light signals traveling at different speeds through a fiber either due to modal or chromatic effects.

Dispersion Compensation

Offsetting the dispersion of one fiber by using different fibers or other components that have dispersion of the opposite sign. Usually done for chromatic dispersion; compensation for polarization-mode dispersion is in development.

Dispersion-compensating Fiber (DCF)

A fiber that has the opposite dispersion of the fiber being used in a transmission system. It is used to nullify the dispersion caused by that fiber.

Dispersion-compensating Module (DCM)

This module has the opposite dispersion of the fiber being used in a transmission system. It is used to nullify the dispersion caused by that fiber. It can be either a spool of a special fiber or a grating based module.

Dispersion-Shifted Fiber (DSF)

Optical fiber with nominal wavelength of zero chromatic dispersion shifted away from 1310nm. Often used for zero dispersion-shifted fiber, which has zero chromatic dispersion at 1550nm and is not used in DWDM system.

Dispersion Management

A technique used in a fiber optic system design to cope with the dispersion introduced by the optical fiber. A dispersion slope compensator (illustrated) is one dispersion management technique.

Dispersion Penalty

The result of dispersion in which pulses and edges smear making it difficult for the receiver to distinguish between ones and zeros. This results in a loss of receiver sensitivity compared to a short fiber and measured in dB. The equations for calculating dispersion penalty are as follows:

Dispersion Slope

The change in dispersion with wavelength

Distributed Bragg Reflection

Reflection of light caused by periodic changes in refractive index in a stack of layers of different composition 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 a 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 that exhibits wide transmission bandwidth and low bending loss to 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.


A cable that delivers service to an individual customer.


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 Cable

A two-fiber cable suitable for duplex 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 a LC but with a shutter over the end of the fiber.

Edge-Emitting Diode (ELED)

An LED that emits light from its edge, producing more directional output than LEDs that emit from their top surface.

Edge-Emitting Laser

A semiconductor laser that emits light in the plane of its junction from the edge of the chip.

Effective Area

The area of a single-mode fiber that carries the light.


Describes the fact that the core or cladding may be elliptical rather than circular.

Electro-Absorption Modulator

A semiconductor diode reverse-modulated so it modulates light passing through it.

Electromagnetic Radiation

Waves made up of oscillating electrical and magnetic fields perpendicular to one another and traveling at the speed of light. Can also be viewed as photons or quanta of energy. Electromagnetic radiation includes radio waves, microwaves, infrared, visible light, ultraviolet radiation, X rays, and gamma rays.


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.


A fiber-optic bundle used for imaging and viewing inside the human body.

End Face

Term 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 backreflection. Typical polish types are PC, UPC, and APC.

End finish

The quality of the end surface of a fiber prepared for splicing or terminated in a connector. For an optical fiber, the optical quality of the surface at the end of the fiber.


Abbreviation for electrical-to-optical converter. A device that converts electrical signals to optical signals, such as a laser diode.

Equilibrium Mode Distribution (EMD)

The steady modal state of a multimode fiber in which the relative power distribution among modes is independent of fiber length.

Erbium-Doped Fiber Amplifier (EDFA)

Optical fiber doped with the rare earth element erbium, which can amplify light at 1530 to

1610nm when pumped by an external light source.


A local-area network standard. The original Ethernet transmits 10 Mbit/s. Other version are Fast Ethernet at 100 Mbit/s, Gigabit Ethernet at 1Gbit/s, and 10 Gigabit Ethernet. A standard protocol (IEEE 802.3) for a 10-Mb/s baseband local area network (LAN) bus using carrier sense multiple access with collision detection (CSMA/CD) as the access method. Ethernet is a standard for using various transmission media, such as coaxial cables, unshielded twisted pairs, and optical fibers.

Evanescent Wave

Guided light waves that extend beyond the boundary of a fiber core into the cladding. Evanescent waves can transfer energy between waveguides. Light guided in the inner part of an optical fiber’s cladding rather than in the core, i.e. the portion of the light wave in the core that penetrates into the cladding.

Excess Loss

Loss of a passive coupler above that inherent in dividing light among the output ports. In a fiber optic coupler, the optical loss from that portion of light that does not emerge from the nominal operation ports of the device.

External Modulation

Modulation of output of a light source by an external device.

Extrinsic Loss

Splice losses arising from the splicing process itself.

Extinction Ratio

The ratio of the low, or OFF optical power level (PL) to the high, or ON optical power level (PH).

Extinction Ratio

The ratio of the power of a plane-polarized beam that is transmitted through a polarizer placed in its path with its polarizing axis parallel to the beam’s plane, as compared with the transmitted power when the polarizer’s axis is perpendicular to the beam’s plane.


Fabry Perot Laser

A laser oscillator in which two mirrors are separated by an amplifying medium with an inverted population, making a Fabry-Perot cavity. Standard diode lasers are Fabry-Perot lasers.

Fall Time

Also called turn-off time. The time required for the trailing edge of a pulse to fall from 90% to 10% of its amplitude; the time required for a component to produce such a result. Typically measured between the 90% and 10% points or alternately the 80% and 20% points.

Fast Axis

In a birefringent material, the index of refraction varies with the direction of vibration of a lightwave. That direction having a low refractive index is the fast axis; at right angles to it is the slow axis, with a high index of refraction.

Faraday Effect

A phenomenon that causes some materials to rotate the polarization of light in the presence of a magnetic field parallel to the direction of propagation. Also called magneto-optic effect.


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 singlemode 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.

Frequency-division Multiplexing (FDM)

A method of deriving two or more simultaneous, continuous channels from a transmission medium by assigning separate portions of the available frequency spectrum to each of the individual channels.


A tube within a connector with a central hole that contains and aligns a fiber.

Fiber Amplifier

An optical fiber doped to amplify light from an external source. The most important type is the erbium-doped fiber amplifier.

Fiber Bandwidth

The lowest frequency at which the magnitude of the fiber transfer function decreases to a specified fraction of the zero frequency value. Often, the specified value is one-half the optical power at zero frequency.

Fiber Bragg Grating (FBG)

An optical fiber in which the core refractive index varies periodically, causing Bragg scatting at wavelengths selected by the period and refractive index. A fiber Bragg grating reflects the selected wavelength and transmits others.

Fiber Fuse

A mechanism whereby the core of a single-mode fiber can be destroyed at high optical power


Fiber Grating

An optical fiber in which the refractive index of the core varies periodically along its length, scattering light in a way similar to a diffraction grating, and transmitting or reflecting certain wavelengths selectively.

Fiber Optic Attenuator

A component installed in a fiber optic transmission system that reduces the power in the optical signal. It is often used to limit the optical power received by the photodetector to within the limits of the optical receiver.

Fiber-Optic Gyroscope

A coil of optical fiber that can detect rotation about its axis.

Fiber Optic Cable

A cable containing one or more optical fibers.

Fiber Optic Communication System

The transfer of modulated or unmodulated optical energy through optical fiber media which terminates in the same or different media.

Fiber Optic Link

A transmitter, receiver, and cable assembly that can transmit information between two points.

Fiber Optic Modems

Fiber optic modems are used in fiber optic networks for sending and receiving data.

Fiber Optic Ribbon

A coherent optical fiber bundle in which the configuration is flat rather than round, giving an output in a line.

Fiber Optic Sensor

Any device in which variations in the transmitted power or the rate of transmission of light in optical fiber are the means of measurement or control. Fibers can be used to measure temperature, pressure, strain, voltage, current, liquid level, rotation and particle velocity

Fiber Optic Span

An optical fiber/cable terminated at both ends which may include devices that add, subtract, or attenuate optical signals.

Fiber Optic Subsystem

A functional entity with defined bounds and interfaces which is part of a system. It contains solid state and/or other components and is specified as a subsystem for the purpose of trade and commerce.


An optical instrument consisting of an objective lens, a coherent (usually flexible) fiber bundle and an eyepiece to examine the output of the fiber bundle.

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 which transmits only part of the incident energy and may thereby change the spectral distribution of energy.

Filled Cable

A cable construction in which the cable core is filled with a gel material that will prevent moisture from entering or passing through the cable.


Fiber in the loop. Fiber-in-the-loop (FITL): Fiber optic service to a node that is located in a neighborhood.

Flooding Compound

A substance surrounding the buffer tubes of a fiber-optic cable, to prevent water intrusion into the interstices in the event of a breach of the jacket.

Fluoride Glasses

Materials that have the amorphous structure of glass but are made of fluoride compounds (e.g., zirconium fluoride ) rather than oxide compounds (e.g., silica). Suitable for very long wavelength transmission. This material tends to be destroyed by water, limiting its use.

FM (Frequency Modulation)

A method of transmission in which the carrier frequency varies in accordance with the signal.

FOTP (Fiber Optic Test Procedure)

Standards developed and published by the Electronic Industries Association (EIA) under the EIA-RS-455 series of standards.

FWM – Four Wave Mixing (FWM)

A nonlinearity common in DWDM systems where multiple wavelengths mix together to form new wavelengths, called interfering products. Interfering products that fall on the original signal wavelength become mixed with the signal, mudding the signal, and causing attenuation. Interfering products on either side of the original wavelength can be filtered out. FWM is most prevalent near the zero-dispersion wavelength and at close wavelength spacings.


Abbreviation for Fabry-Perot. Generally refers to any device, such as a type of laser diode, that uses mirrors in an internal cavity to produce multiple reflections.

Free-Space Optics

Also called free-space photonics. The transmission of modulated visible or infrared (IR) beams through the atmosphere via lasers, LEDs, or IR-emitting diodes (IREDs) to obtain broadband communications.


The number of times an electromagnetic wave oscillates in a second, or the number of wave peaks that pass a point in second; measured in hertz.

FDM – Frequency-Division Multiplexing (FDM)

Combining analog signals by assigning each a different carrier frequency and merging them in a single signal with a broad range of frequencies

Frequency-shift Keying (FSK)

Frequency modulation in which the modulating signal shifts the output frequency between predetermined values. Also called frequency-shift modulation, frequency-shift signaling.

Fresnel Reflection Loss

Reflection losses at the ends of fibers caused by differences in the refractive index between glass and air. The maximum reflection caused by a perpendicular air-glass interface is about 4% or about -14 dB.


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 which can mean fiber to the: premises, curb, home, business, or desk, for example.

Full Duplex

In data transmission, transmitters and receivers that 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

A splice made by melting the tips of two fibers together so they form a solid junction.

Fusion Splicer

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

Furcation Tubing

A protective tubing that protects exposed fiber. Commonly used in terminating multi-fiber cable or “fan-out” situations. Also referred to as buffer tubing.


Abbreviation for fiber under test. Refers to the fiber being measured by some type of test equipment.


Abbreviation for full width half maximum. Used to describe the width of a spectral emission at the 50% amplitude points. Also known as FWHP (full width half power).


Gallium Aluminum Arsenide (GaAlAs)

A semiconductor compound used in LEDs, diode lasers, and certain detectors.

Gallium Arsenide (GaAs)

A semiconductor compound used in LEDs, diode lasers, detectors and electronic components.

Gap Loss

Loss resulting from the end separation of two axially aligned fibers.

Gap Loss Attenuator

An optical attenuator that exploits the principle of gap loss to reduce the optical power level when inserted in-line in the fiber path. e.g., to prevent saturation of the receiver.

Gaussian Beam

A beam pattern used to approximate the distribution of energy in a fiber core. It can also be used to describe emission patterns from surface-emitting LEDs. Most people would recognize it as the bell curve (illustrated).


An abbreviation for Gigabit Ethernet. Gigabit networking, or commonly called 10-Gigabit Ethernet (10GBASE-T), is a communications technology that offers data speeds up to 10 billion bits per second.


Gigabits (billion bits ) per second


Abbreviation for germanium. Generally used in detectors. Good for most fiber optic wavelengths (e.g., 800-1600 nm). Performance is inferior to InGaAs


A substance, resembling petroleum jelly in viscosity, that surrounds a fiber, or multiple fibers, enclosed in a loose buffer tube.


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

Graded-Index Fiber

A fiber in which the refractive index changes gradually with distance from the fiber axis, rather than abruptly at the core-cladding interface.

Graded-Index Fiber Lens

A short segment of a graded-index fiber that focuses light passing through it.


Abbreviation for gradient index. Generally refers to the SELFOC lens often used in fiber optics.

Group Delay

The rate of change of the total phase shift with respect to angular frequency, d /d , through a device or transmission medium, where is the total phase shift, and is the angular frequency equal to 2f , where f is the frequency.

Group Delay Time

The difference in travel time through a fiber for light of different wavelengths.

Group Index

Also called group refractive index. In fiber optics, for a given mode propagating in a medium of refractive index (n), the group index (N), is the velocity of light in a vacuum (c), divided by the group velocity of the mode.

Group Velocity

  1. The velocity of propagation of an envelope produced when an electromagnetic wave is modulated by, or mixed with, other waves of different frequencies.
  2. For a particular mode, the reciprocal of the rate of change of the phase constant with respect to angular frequency.
  3. The velocity of the modulated optical power.



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

Hard-Clad Silica Fiber

A fiber with a hard plastic cladding surrounding a step-index silica core. (Other plastic-clad silica fibers have a soft plastic cladding.)

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.

High Loss Fiber

Optical fiber in which the attenuation exceeds the normally acceptable level for long-haul or data communications use.

Hockey Puck

A polishing fixture used to facilitate the manual finishing of the endfaces of certain types of optical fiber connectors.

Hybrid Cable

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

Hydrogen Losses

Increases in fiber connector attenuation that occur when hydrogen diffuses into the glass matrix and absorbs some light.


Index-Matching Gel (Index-Matching Fluid)

A gel or fluid with refractive index close to glass that reduces refractive-index discontinuities that can cause reflective losses.

Index Matching Material

A substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of an optical fiber, and is used to reduce Fresnel reflection at the fiber endface.

Index of Refraction

The speed of light in a vacuum divided by the speed of light in a material, abbreviated n, which measures how materials refract light.

Index Profile

The refractive index of a fiber as a function of cross section.

Indium Gallium Arsenide (InGaAs)

A semiconductor material used in lasers, LEDs, and detectors.

Indium Gallium Arsenide Phosphide (InGaAsP)

A semiconductor material used in lasers, LEDs, and detectors.

Infrared (IR)

Light with wavelengths longer than 700nm and shorter than about 1mm, invisible to the human eye, which we can feel as heat. Glass optical fibers transmit infrared signals at 700 to about 1650nm in the infrared.

Infrared Emitting Diodes

LEDs that emit infrared energy (830 nm or longer).

Infrared Fiber

Colloquially, optical fibers with best transmission at wavelengths of 2µm or longer, made of materials other than silica glass.

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.

Integrated Optics

Optical devices that perform two or more functions and are integrated on a single substrate; analogous to integrated electronic circuits.


Power per unit solid angle.


Indium Phosphide. A semiconductor material used to make optical amplifiers and HBTs.

Insertion Loss

The loss of power that results from inserting a component, such as a connector, coupler (illustrated), or splice, into a previously continuous path.

Inside Plant

Telecommunications facilities placed inside a building.

Integrated Detector/Preamplifier (IDP)

A detector package containing a PIN photodiode and transimpedance amplifier.


The square of the electric field strength of an electromagnetic wave. Intensity is proportional to irradiance and may get used in place of the term “irradiance” when only relative values are important.

Intensity Modulation (IM)

In optical communications, a form of modulation in which the optical power output of a source varies in accordance with some characteristic of the modulating signal.

Interchannel Isolation

The ability to prevent undesired optical energy from appearing in one signal path as a result of coupling from another signal path. Also called crosstalk.


For light, the way that waves add together, depending on their phase. Constructive interference occurs when the waves are in phase and their amplitudes add. Destructive interference occurs when the waves are 180 degree out of phase and their amplitudes cancel.

Interference Filter

An optical filter that selectively transmits one wavelength and reflects others based on interference effects inside the structure. Also called dielectric filter.


An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces; it can measure a length in terms of the length of a wave of light by using interference phenomena based on the wave characteristics of light. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers.


An optical device that separates a series of optical channels so alternating wavelengths emerge out its two ports. The best known type is a Mach-Zehnder interferometer.

Intermodulation (Mixing)

A fiber nonlinearity mechanism caused by the power dependant refractive index of glass. Causes signals to beat together and generate interfering components at different frequencies. Very similar to four wave mixing.

International Telecommunications Union (ITU)

A civil international organization, headquartered in Geneva, Switzerland, established to promote standardized telecommunications on a worldwide basis. The ITU-R and the ITU-T are committees under the ITU, which is recognized by the United Nations as the specialized agency for telecommunications.

Intrinsic Losses

Loss due to inherent traits within the fiber; for example, absorption, scattering, and splice loss.


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 economical activities.


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



The outer, protective covering of the cable. Also called the cable sheath.

Jumper Cable

A short single fiber cable with connectors on both ends used for interconnecting other cables or testing.



A very strong, very light, synthetic compound developed by DuPont which is used to strengthen optical cables.


One thousand cycles per second.


Abbreviation for kilometer. 1 km = 3,280 feet or 0.62 miles.


Lambertian Emitter

An emitter that radiates according to Lambert’s cosine law, which states that the radiance of certain idealized surfaces depends on the viewing angle of the surface. The radiant intensity of such a surface is maximum normal to the surface and decreases in proportion to the cosine of the angle from the normal. Given by:

Large-Core Fiber

Usually, a fiber with a core of 200µm or more.

Large Effective Area Fiber (LEAF)

An optical fiber, developed by Corning, designed to have a large area in the core, which carries the light.


From L ight Amplification by Stimulated Emission of R adiation, one of the wide range of devices that generates light by that principle. Laser light is directional, covers a narrow range of wavelengths, and is more coherent than ordinary light. Semiconductor diode lasers are the usual light sources in fiber optic systems.

Laser Diode (LD)

A semiconductor that emits coherent light when forward biased.

Lateral Displacement Loss (Lateral Offset Loss)

The loss of power that results from lateral displacement of optimum alignment between two fibers or between a fiber and an active device.

Launch Fiber (Launch cable)

An optical fiber used to couple and condition light from an optical source into an optical fiber. Often the launch fiber is used to create an equilibrium mode distribution in multimode fiber. Also called launching fiber.

LC Connector

LC stands for Lucent Connector. The LC is a small form-factor fiber optic connector.


Wavelengths of about 1570 to 1625nm where some erbium-doped fiber amplifiers operate.

Separate from C-Band.

Leaky Mode

In an optical fiber, a mode having a field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance.

L-I Curve

The plot of optical output (L) as a function of current (I) which characterizes an electrical-to-optical converter. A typical L-I curve is shown at right.

Light ARMOR Cable

A fiber optic cable assembly with ruggedized plastic jacketing providing fiber protection for semi-harsh environment, commercial, or industrial applications.

Light Emitting Diode (LED)

A semiconductor diode that emits incoherent light at the junction between p- and n-doped materials.


An optical fiber or fiber bundle.


An adjective, a synonym for optical, often (but not always) meaning fiber-optic. The path of a point on a wavefront. The direction of the lightwave is generally normal (perpendicular) to the wavefront.


The basic measurement of how well analog-to-digital and digital-to-analog conversions are performed. To test for linearity, a mathematically perfect diagonal line is converted and then compared to a copy of itself. The difference between the two lines is calculated to show linearity of the system and is given as a percentage or range of least significant bits.


The rang of wavelengths in an optical signal, sometimes called spectral width.


A defect in the cleaved end face of an optical fiber, in the form of a sharp protrusion at the edge of the fiber.

LOMMF Laser Optimized Multimode Fiber

LOMMF is the highest capacity medium for 10-gig optical transmission. LOMMF was developed for use with VCSEL lasers. With laser optimized multimode fiber no special terminations or connectors are necessary.

Long Wavelength

A commonly used term for light in the 1300 and 1550 nm ranges.

Longitudinal Modes

Oscillation modes of a laser along the length of its cavity. Each longitudinal mode contains only a narrow range of wavelengths, so a laser emitting a single longitudinal mode has a narrow bandwidth. Distinct from transverse modes.

Loose Tube

A protective tube loosely surrounding a cabled fiber, often filled with gel. A type of fiber optic cable construction where the fiber is contained within a loose tube in the cable jacket.

Loose Tube vs Tight Buffered

Fiber optic cables are constructed in two ways: loose tube and tight buffered. Both contain a type of strengthening member, such as aramid yarn, stainless steel wire strands, or gel-filled sleeves. Each, however, is designed for very different environments.


Attenuation of optical signal, normally measured in decibels. The amount of a signal’s power, expressed in dB, that is lost in connectors, splices, or fiber defects.

Loss Budget

An accounting of overall attenuation in a system.


Primarily used for indoor applications, Low Smoke Zero Halogen (LSZH) cable is designed to reduce toxic emissions in event of fire.


Mach-Zehnder Interferometer

An optical device that separates a series of optical channels so alternating wavelengths emerge out its two ports, sometimes called an interleaver.


In a fiber, all macroscopic deviations of the fiber’s axis from a straight line, that will cause light to leak out of the fiber, causing signal attenuation.

Mandrel Wrapping

In multimode fiber optics, a technique used to modify the modal distribution of a propagating optical signal.


Allowance for attenuation in addition to that explicitly accounted for in a system design.

Mass Splicing

Splicing of many fibers in a cable.

Material Dispersion

Pulse dispersion caused by variation of a material’s refractive index with wavelength.

Mechanical Splice

A splice in which fibers are joined mechanically (e.g., glued or crimped in place) but not fused together. An optical fiber splice accomplished by fixtures or materials, rather than by thermal fusion. The capillary splice, illustrated, is one example of a mechanical splice.

Mean Launched Power

The average power for a continuous valid symbol sequence coupled into a fiber.

MEMS (Micro-electro-mechanical systems)

Tiny moving mirrors fabricated from semiconductor materials.


Tiny bends in a fiber that allow light to leak out and increase loss. Mechanical stress on a fiber that introduces local discontinuities, which results in light leaking from the core to the cladding by a process called mode coupling.


One millionth of a meter or 10-6 meters. Abbreviated µm.

Microscope Fiber Optic Inspection

A microscope used to inspect the end surface of a connector for flaws or contamination or a fiber for cleave quality.


One millionth of a second or 10-6 seconds. Abbreviated µs.


One millionth of a Watt or 10-6 Watts. Abbreviated µW.


Abbreviation for military specification. Performance specifications issued by the Department of Defense that must be met in order to pass a MIL-STD.


Abbreviation for military standard. Standards issued by the Department of Defense.

Minimum Bend Radius

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

Misalignment Loss

The loss of power resulting from angular misalignment, lateral displacement, and fiber end separation.

Modal Dispersion

Dispersion arising from differences in the times that different modes take to travel through multimode fiber.

Modal Noise

Noise that occurs whenever the optical power propagates through mode-selective devices. It is usually only a factor with laser light sources.


An electromagnetic field distribution that satisfied theoretical requirements for propagation in a waveguide or oscillation in a cavity (e.g., a laser). Light has modes in a fiber or laser. A single electromagnetic wave traveling in a fiber.

Mode Coupling

The transfer of energy between modes. In a fiber, mode coupling occurs until equilibrium mode distribution (EMD) is reached.

Mode Evolution

The dynamic process a multilongitudinal mode laser undergoes whereby the changing distribution of power among the modes creates a continuously changing envelope of the laser’s spectrum.

Mode-Field Diameter (MFD)

The diameter of the one mode of light propagating in a single mode fiber, slightly larger than core diameter.

Mode Filter

A device that removes higher-order modes to simulate equilibrium mode distribution. A mode filter is most easily constructed.

Mode Scrambler

A device that mixes modes to uniform power distribution.

Mode Stripper

A device that removes high-order modes in a multimode fiber to give standard measurement

conditions. A device that removes cladding modes.


The process by which the characteristic of one wave (the carrier) modifies another wave (the signal). Examples include amplitude modulation (AM), frequency modulation (FM), and pulse-coded modulation (PCM).

Modulation Index

In an intensity-based system, the modulation index is a measure of how much the modulation signal affects the light output.


A device that imposes a signal on a carrier.

MT Connector

Multi-fiber connector housing up to 24 fibers in a single ferrule.

MT RJ Connector

MT-RJ stands for Mechanical Transfer Registered Jack. MT-RJ is a fiber-optic cable connector that is very popular for small form factor devices due to its small size. Housing two fibers and mating together with locating pins on the plug, the MT-RJ comes from the MT connector, which can contain up to 12 fibers.

MU Connector

MU is a small form factor SC. It has the same push/pull style, but can fit 2 channels in the same footprint of a single SC. MU was developed by NTT.

Multilongitudinal Mode (MLM) Laser

An injection laser diode which has a number of longitudinal modes.

Multimode (Multi Mode)

Transmits or emits multiple modes of light. An optical waveguide with a much larger core (50µm +) than the singlemode waveguide core (2µm to 9µm) and which permits approximately 1,000 modes to propagate through the core compared to only one mode through a singlemode fiber.

Multimode Dispersion

Dispersion resulting from the different transit lengths of different propagating modes in a multimode optical fiber. Also called modal dispersion.

Multimode Fiber – MM

An optical fiber that has a core large enough to propagate more than one mode of light The typical diameter is 62.5 micrometers or 50 micrometers.


A device that combines two or more signals into a single output.


The process by which two or more signals are transmitted over a single communications channel. Examples include time-division multiplexing (TDM) and wavelength-division multiplexing (WDM).


Abbreviation for Mach-Zehnder, a structure used in fiber Bragg gratings and interferometers.

Named for the two men who developed the underlying principles of the structure.


NA Mismatch Loss

The loss of power at a joint that occurs when the transmitting half has a numerical aperture greater than the NA of the receiving half. The loss occurs when coupling light from a source to fiber, from fiber to fiber, or from fiber to detector.

NDSF – Non Dispersion-Shifted Fiber

The most popular type of single-mode fiber deployed. It is designed to have a zero-dispersion wavelength near 1310 nm.

NEXT , RN – Near-end Crosstalk (NEXT, RN)

The optical power reflected from one or more input ports, back to another input port. Also known as isolation directivity.


The part of the infrared near the visible spectrum, typically from 700 to 1500 or 2000nm; it is not rigidly defined.

Near Field Scanning

A technique for measuring the refractive-index profile of an optical fiber by using an extended source to illuminate an endface and measuring the point-by-point radiance at the exit face.


A system of cables or other connections that links many terminals or devices, all of which can communicate with each other through the system.

Neutral Density Filter

Also known as a gray filter. A light filter that decreases the intensity of the light without altering the relative spectral distribution of the energy.


  1. A terminal of any branch in network topology or an interconnection common to two or more branches in a network.
  2. One of the switches forming the network backbone in a switch network.
  3. A point in a standing or stationary wave at which the amplitude is a minimum.


  1. An undesired disturbance within the frequency band of interest; the summation of unwanted or disturbing energy introduced into a communications system from man-made and natural sources.

  1. A disturbance that affects a signal and that may distort the information carried by the signal.
  2. Random variations of one or more characteristics of any entity such as voltage, current, or data.


The deviation from linearity in an electronic circuit, an electro- optic device or a fiber that generates undesired components in a signal. Examples of fiber nonlinearities include SBS, SRS, FWM, SPM, XPM, and Intermodulation.

Normal (angle)

Perpendicular to a surface.


Abbreviation for nonreturn to zero. A common means of encoding data that has two states termed “zero” and “one” and no neutral or rest position.

Numerical Aperture (NA)

The sine of half the angle over which a fiber can accept light. Strictly speaking, this is multiplied by the refractive index of the medium containing the light, but for air the index is almost equal to

  1. The light-gathering ability of a fiber; the maximum angle to the fiber axis at which light will be accepted and propagated through the fiber. NA = sin a, where a is the acceptance angle. NA also describes the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector.

NZ-DSF – Nonzero Dispersion-Shifted Fiber (NZ-DSF)

Single mode fiber with the wavelength of zero chromatic dispersion shifted to just outside of the erbium-fiber amplifier region. Some types have zero dispersion near 1500nm, others near 1625nm. Types with zero dispersion at 1580nm are not usable in the L-band of erbium-doped fiber amplifiers.


OADM – Optical Add/Drop Multiplexer

A device which adds or drops individual wavelengths from a DWDM system.

OCH – Optical Channel

An optical wavelength band for WDM optical communications.


Abbreviation for optical distribution network. Term for optical networks being developed for interactive video, audio, and data distribution.


Abbreviation for optical-to-electrical converter. A device used to convert optical signals to electrical signals. Also known as OEC.


Abbreviation for opto-electronic integrated circuit. An integrated circuit that includes both optical and electrical elements.


Abbreviation for original equipment manufacturer. The manufacturer of any device that is designed and built to be distributed under the label of another company.


Optical Fiber Nonconductive General Purpose. Type OFNG cable must be resistant to the spread of fire and suitable for general-purpose use, with the exception of risers and plenums.


Optical Fiber Nonconductive Plenum. Cable installed in ducts, plenums, and other spaces used for environmental air must be listed as having adequate fire-resistant and low-smoke producing characteristics.


Optical Fiber Nonconductive Riser. Optical fiber cable used in vertical shafts, or in runs between floors, must have fire-resistant characteristics capable of preventing the spread of fire from floor-to-floor.


Abbreviation for optical line termination. Optical network elements that terminate a line signal.


Abbreviation for optical loss test set. A source and optical power meter combined used to measure optical loss.


Abbreviation for optical multiplex section. A section of a DWDM system that incorporates an optical add/drop multiplexer.


Abbreviation for optical network interface. A device used in an optical distribution network to connect two parts of that network.


Abbreviation for optical network termination. Optical network element that terminates a line signal in installations where the fiber extends into the customer premises.


Abbreviation for optical network unit. A network element that is part of a fiber-in-the-loop


Open Systems Interconnection (OSI)

Pertaining to the logical structure for communications networks standardized by the International Organization for Standardization (ISO).

Optical Amplifier

A device that amplifies an input optical signal without converting it into electrical form. The best developed are optical fibers doped with the rare-earth element erbium.

Optical Bandpass

The range of optical wavelengths which can be transmitted through a component.

Optical Channel

An optical signal transmitted at one wavelength. WDM systems transmit multiple channels at separate wavelengths.

Optical Channel Spacing

The wavelength separation between adjacent WDM channels.

Optical Channel Width

The optical wavelength range of a channel.

Optical Continuous Wave Reflectometer (OCWR)

An instrument used to characterize a fiber optic link wherein an unmodulated signal is transmitted through the link, and the resulting light scattered and reflected back to the input is measured. Useful in estimating component reflectance and link optical return loss.

Optical Circulator

A device that transmits light only in one direction through a series of ports, so light can go from port 1 to port 2 and port 2 to port 3, but not from port 2 to port 1.

Optical Directional Coupler (ODC)

A component used to combine and separate optical power.

Optical Fall Time

The time interval for the falling edge of an optical pulse to transition from 90% to 10% of the pulse amplitude. Alternatively, values of 80% and 20% may be used.

Optical Fiber

A glass or plastic fiber that has the ability to guide light along its axis. The three parts of an optical fiber are the core, the cladding, and the coating or buffer.

Optical Isolator

A component used to block out reflected and unwanted light. Also called an isolator.

Optical Link Loss Budget

The range of optical loss over which a fiber optic link will operate and meet all specifications.

The loss is relative to the transmitter output power and affects the required receiver input power.

Optical Networking

Processing and switching signals in optical form as well as transmitting them optically.

Optical Node

The point where signals are transferred from optical fibers to other transmission media, typically twisted-pair wires or coaxial cable.

Optical Path Power Penalty

The additional loss budget required to account for degradations due to reflections, and the combined effects of dispersion resulting from intersymbol interference, mode-partition noise, and laser chirp.

Optical Performance Monitor

A device installed in a WDM system to monitor signals at the transmitted wavelengths.

Optical Power Meter

An instrument that measures the amount of optical power present at the end of a fiber or cable.

Optical Pump Laser

A shorter wavelength laser used to pump a length of fiber with energy to provide amplification at one or more longer wavelengths. See also EDFA.

Optical Return Loss (ORL)

The ratio (expressed in dB) of optical power reflected by a component or an assembly to the optical power incident on a component port when that component or assembly is introduced into a link or system.

Optical Rise Time

The time interval for the rising edge of an optical pulse to transition from 10% to 90% of the pulse amplitude. Alternatively, values of 20% and 80% may be used.

Optical Spectrum Analyzer (OSA)

An instrument that scans the spectrum to record power as a function of wavelength.

Optical Signal-to-Noise-Ratio (OSNR)

The optical equivalent of SNR.

Optical Time-Domain Reflectometer (OTDR)

An instrument that measures transmission characteristics by sending a short pulse of light down a fiber and observing back-scattered light.

Optical Waveguide

Technically, any structure that can guide light. Sometimes used as a synonym for optical fiber, it can also apply to planar light waveguides.

Outside Plant (OSP)

In telephony, all cables, conduits, ducts, poles, towers, repeaters, repeater huts, and other equipment located between a demarcation point in a switching facility and a demarcation point in another switching facility or customer premises.

Overfilled Launch

A condition for launching light into the fiber where the incoming light has a spot size and NA larger than accepted by the fiber, filling all modes in the fiber.


Abbreviation for optical cross-connect. See cross-connect.



Panda is a common style of Polarization Maintaining fiber, using round and symetrical stress rods on either side of the core to induce polarization.

Parabolic Profile

In an optical fiber, a power-law index profile with the profile parameter, g, equal to. Synonym:

quadratic profile.


The region of usable frequency in electronics or wavelength in optics.

Passive Branching Device

A device which divides an optical input into two or more optical outputs.

Passive Component

A component that doesn’t require outside power.

Passive Device

Any device that does not require a source of energy for its operation. Examples include electrical resistors or capacitors, diodes, optical fiber (photo), cable, wires, glass, lenses, and filters.

Passive Optical Network(PON)

A fiber-optic distribution network with no active components between the switching point and the customer.

PC (Fiber Connector Polish)

Abbreviation for physical contact. Refers to an optical connector that allows the fiber ends to physically touch. Used to minimize backreflection and insertion loss.

PCS Fiber – Plastic Clad Silica

Also called hard clad silica (HCS). A step-index fiber with a glass core and plastic or polymer cladding instead of glass.

Peak Power

Highest instantaneous power level in a pulse.

Peak Wavelength

In optical emitters, the spectral line having the greatest output power. Also called peak emission wavelength.


The position of a wave in its oscillation cycle.

Phase Constant

The imaginary part of the axial propagation constant for a particular mode, usually expressed in radians per unit length. See also attenuation.

Phase-shift Keying (PSK)

  1. In digital transmission, angle modulation in which the phase of the carrier discretely varies in relation, either to a reference phase or to the phase of the immediately preceding signal element, in accordance with data being transmitted.

  2. In a communications system, the representation of characters, such as bits or quaternary digits, by a shift in the phase of an electromagnetic carrier wave with respect to a reference, by an amount corresponding to the symbol being encoded. Also called biphase modulation, phase-shift signaling.


An optoelectronic transducer such as a PIN photodiode or avalanche photodiode. In the case of the PIN diode, it is so named because it is constructed from materials layered by their positive, intrinsic, and negative electron regions.

Photodiode – PD

A diode that can produce an electrical signal proportional to light falling upon it.


A term coined for devices that work using photons or light, analogous to “electronic” for devices working with electrons.


Providing an electric current under the influence of light or similar radiation.


Quanta of electromagnetic radiation. Light can be viewed as either a wave or a series of photons.


A short optical fiber permanently attached to a source, detector, or other fiber optic device at one end and an optical connector at the other.

pin Photodiode

A semiconductor detector with an intrinsic (i) region separating the p- and n-doped regions. It has fast linear response an is used in fiber-optic receivers.

Planar Waveguide

A flat waveguide formed on the surface of a flat material. The zone of high refractive index is rectangular in cross-section and guides light in the same way as the core of an optical fiber.

Plastic-Clad Silica (PCS) Fiber

A step-index multimode fiber in which a silica core is surrounded by a lower-index plastic cladding.

Plastic Optical Fiber (POF)

An optical fiber made entirely of plastic compounds. Optical fibers in which both the core and cladding are made of plastic material. Typically their transmission is much poorer than that of glass fibers, and their lowest losses are in the visible region.


Abbreviation for planar lightwave circuit. A device which incorporates a planar waveguide.


An air-handling space such as that found above drop ceiling tiles or in raised floors. Also, a fire code rating for indoor cable.

Plenum Cable

Cable made of fire-retardent material that meets electrical code requirements (UL 910) for low smoke generation and installation in air spaces.

Point-to-Point Transmission

Carrying a signal between two points, without branching to other points.

Polyethylene (PE)

A type of plastic material used for outside plant cable jackets.

Polyvinyl-chloride (PVC)

A type of plastic material used for cable jacketing. Typically used in flame-retardant cables.

PVDF (Kynar®)

Abbreviation used to denote polyvinyldifluoride. A type of material used for cable jacketing.


Alignment of the electric and magnetic fields that make up an electromagnetic wave; normally refers to the electric field. If all light waves have the same alignment, the light is polarized.

Polarization Maintaining Fiber (PM Fiber)

Fibers that maintains the polarization of light that enters it. Examples are PANDA, Bow-Tie, and Elliptical

Polarization Dependent Loss(PDL)

In passive optical components, loss that varies as the polarization state of the propagating wave changes. Expressed as the difference between the maximum and minimum loss in decibels.

Polarization Mode Dispersion (PMD)

Dispersion arising from random fluctuations in how fibers transmit light in vertical and horizontal polarizations.


The optical process, following grinding, that puts a highly finished, smooth and apparently amorphous surface on a lens or a mirror.

Polishing and Abrasive Material

Any of the numerous powders used for grinding and polishing glass, crystal or metal, the chief material being emery and carborundum for grinding, and rouge or the oxides of tin, cerium or other metals for polishing.

Polishing Jig

In fiber optics, a device used to polish a biconic plug to a specified length and surface finish.

Also called a polishing disc.


Abbreviation for passive optical network. A broadband fiber optic access network that uses a means of sharing fiber to the home without running individual fiber optic lines from an exchange point, telco CO, or a CATV headend and the subscriber’s home.


Abbreviation for peak-to-peak. The algebraic difference between extreme values of a varying quantity.


A cylindrical rod of specially prepared and purified glass from which an optical fiber is drawn.

Precision Sleeve Splicing

Optical fiber splicing that uses a capillary tube, of suitable material, to align the mating fibers.

Profile Dispersion

Dispersion attributed to the variation of refractive index contrast with wavelength.


Abbreviation for picosecond. One trillionth of a second or 10-12 seconds.

Pulling Eye

A pulling eye is a device fastened to a fiber cable to which a hook may be attached in order to pull the cable through a duct or small space.


A current or voltage which changes abruptly from one value to another and back to the original value in a finite length of time. Used to describe one particular variation in a series of wave motions. The parts of the pulse include the rise time, fall time, and pulse width, pulse amplitude. The period of a pulse refers to the amount of time between pulses.

Pulse Dispersion

The Spreading out of pulses as they travel along an optical fiber.

Pump Laser

The semiconductor laser that provides the light that excites atoms in a fiber amplifier, putting them in the right state to amplify light. A power source for signal amplification, typically a 980 nm or 1480 nm laser, used in EDFA applications.


Abbreviation for picowatt. One trillionth of a Watt or 10-12 Watts.


Quadrature Phase-shift Keying (QPSK)

Phase-shift keying uses four different phase angles out of phase by 90°. Also called quadriphase

or quaternary phase-shift keying.

Quantum Efficiency

The fraction of photons that strike a detector that produces electron-hole paris in the output current.


Radiation-hardened Fiber

An optical fiber made with core and cladding materials that are designed to recover their intrinsic value of attenuation coefficient, within an acceptable time period, after exposure to a radiation pulse.

Raman Amplifier

A fiber that transfers energy from a strong pump beam to amplify a weaker signal at a longer wavelength, using stimulated Raman scattering.

Rare Earth Doped Fiber

An optical fiber in which ions of a rare-earth element, such as neodymium, erbium or holmium, have been incorporated into the glass core matrix, yielding high absorption with low loss in the

visible and near-infrared spectral regions.

Rayleigh Scattering

The scattering of light that results from small inhomogeneities of material density or composition.


Lines that represent the path taken by light.


A device that detects an optical signal and converts it into an electrical form usable by other devices.

Receive Cable

A known good fiber optic jumper cable attached to a power meter used as a reference cable for loss testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.

Receiver Sensitivity

The minimum acceptable value of received power needed to achieve an acceptable BER or performance. It takes into account power penalties caused by use of a transmitter with worst-case values of extinction ratio, jitter, pulse rise times and fall times, optical return loss, receiver connector degradations, and measurement tolerances. The receiver sensitivity does not include power penalties associated with dispersion, or backreflections from the optical path; these effects are specified separately in the allocation of maximum optical path penalty. Sensitivity usually takes into account worst-case operating and end-of-life (EOL) conditions.


The bending of light as it passes between materials of different refractive index.

Refractive Index

The speed of light in a vacuum divided by the speed of light in a material, abbreviated n, which measures how materials refract light.

Refractive-Index Gradient

The change of refractive index with distance from the axis of an optical fiber. Also called refractive index profile.


A receiver-transmitter pair that detects and amplifies a weak signal for retransmission through another length of optical fiber.


The ratio of detector output to input, usually measured in units of amperes per watt (or microampers per microwatt).

Residual Loss

The loss of the attenuator at the minimum setting of the attenuator.

Return Loss

See optical return loss.

Return Path

A communications connection that carries signals from the subscriber back to the operator. The return path allows for interactive television and on-demand services, such as pay-per-view, video on demand, and interactive games.


Abbreviation for radio frequency. Any frequency within the electromagnetic spectrum normally associated with radio wave propagation.

RF Carrier

An AM technique wherein a carrier, with a frequency much higher than the encoded information, varies according to the amplitude of the information being encoded.

Ribbon Cables

Cables in which many parallel fibers are embedded in a plastic material, forming a flat ribbon-like structure.


Abbreviation for relative intensity noise. Often used to quantify the noise characteristics of a laser.


A cable that forms a closed loop connecting two or more points, so all points remain connected if the cable breaks at one point.

Ring Network

A network topology in which terminals are connected in a point-to-point serial fashion in an unbroken circular configuration.

Rip Cord

Of an optical cable, a parallel cord of strong yarn that is situated under the jacket(s) of the cable for the purpose of facilitating jacket removal preparatory to splicing or breaking out.


A pathway for indoor cables that pass between floors, normally a vertical shaft or space. Also a fire-code rating for indoor cable.

Rise Time

The time it takes output to rise from low levels to peak value. Typically measured as the time to rise from 10% to 90% of maximum output.


A device that directs data packets to their destinations using information in their headers to pick the best path. Distinct from wavelength router.


Abbreviation for return to zero. A common means of encoding data that has two information states called “zero” and “one” in which the signal returns to a rest state during a portion of the bit period.



A proposed designation for wavelengths of 1460 to 1530nm, where optical amplifiers based on thulium-doped fibers are in development.

SC Connector

Abbreviation for subscription channel connector. A push-pull type of optical connector that features high packing density, low loss, low backreflection, and low cost.


Loss of light that is scattered off atoms in different directions, so it escapes from the fiber core. A major component of fiber attenuation.


A defect on a polished optical surface whose length is many times its width. Block reek is a chainlike scratch formed in polishing. A runner cut is a curved scratch caused by grinding. A sleek is a hairline scratch. A crush or rub is a surface scratch or scratches usually caused by mishandling.

Self-phase modulation (SPM)

A fiber nonlinearity caused by the nonlinear index of refraction of glass. The index of refraction varies with optical power level causing a frequency chirp which interacts with the fiber’s dispersion to broaden the pulse.

Semiconductor Optical Amplifier (SOA)

A laser diode without end mirrors coupled to the fibers on both ends. Light coming in either fiber is amplified by a single pass through the laser diode. An alternative to EDFAs.

SFP Loopback

An small form factor test fixture used loop an electrical signal from the Tx side of a port to the Rx side of a port, prior to population with an optical transceiver.

Selfoc™ Lens

A trade name used by Nippon Sheet Glass Company (NSG) for a graded-index fiber lens; a segment of graded-index fibers made to serve as lens.

Semiconductor Laser

A laser in which injection of current into a semiconductor diode produces light by recombination of holes and electrons at the junction between p- and n-doped regions.


An outer protective layer of a fiber optic cable. Also called the cable jacket.

Short Wavelength

A commonly used term for light in the 665, 790, and 850 nm ranges.


Abbreviation for silicon. Generally used in detectors. Good for short wavelengths only (e.g., < 1000 nm).


Silicon dioxide (SiO2).

Signal-to-Noise Ratio (SNR)

The ratio of signal to noise, measured in decibels; an indication of analog signal quality.

Simple Network Management Protocol (SNMP)

The Internet standard protocol for network management software. It monitors devices on the network, and gathers device performance data for management information data bases (MIB).

Silica Glass

Glass made mostly of silicon dioxide, SiO2, used in conventional optical fibers.


Single element (e.g., a simplex connector is a single-fiber connector).

Simplex Cable

A term sometimes used for a single-fiber cable.

Single-Frequency Laser

A laser that emits a range of wavelengths small enough to be considered a single frequency.

Single-longitudinal Mode Laser (SLM)

An injection laser diode which has a single dominant longitudinal mode. A single-mode laser with a side mode suppression ratio (SMSR)< 25 dB.

Single Mode

Containing only one mode. When dealing with lasers, beware of ambiguities because of the difference between transverse and longitudinal modes. A laser operating in a single transverse mode typically does not operate in a single longitudinal mode. A type of low-loss optical waveguide with a very small core (2-9 microns). It requires a laser source for input signals

because of the very small entrance aperture. The smallest of the core radius approaches the wavelength of the source. Consequently, only a singlemode is propagated.

Single Mode Fiber (SMF)

A small-core optical fiber through which only one mode will propagate. The typical diameter is 8-9 microns for 1310/1550nm wavelengths

Single Polarization Fiber

Optical fibers capable of carrying light in only one polarization.

Slab Dielectric Waveguide

An electromagnetic waveguide (a) that consists solely of dielectric materials, (b) in which the dielectric propagation medium has a rectangular cross section, (c) that has a width, thickness, and refractive indices that determine the operating wavelength and the modes the guide will support beyond the equilibrium length, (d) that may be cladded, protected, distributed, and electronically controllable, and (e) that may be used in various applications, such as in integrated optical circuits (IOCs) in which their shape is geometrically more convenient than the optical fibers that are circular in cross section, that are used in fiber optic cables for long-distance transmission.


The name of the mixture of liquid and grinding or polishing compounds used in processing optical materials.


A threaded type of optical connector. One of the earliest optical connectors to be widely used.

Offers poor repeatability and performance.

SM Zipcord Fiber

Zipcord (or zip-cord) is a two fiber cable essentially with two single-fiber cables conjoined by their jackets. The jacket strip can be easily separated from one another for the installation of optical connectors. Zip cord cables may include both loose-buffer and tight-buffer designs.


An optical pulse that naturally retains its original shape as it travels along an optical fiber.


In fiber optics, a transmitting LED or laser diode, or an instrument that injects test signals into fibers.

Spectral Efficiency

The number of data bits per second that can be transmitted in a one Hertz bandwidth range.

Spectral Width

A measure of the extent of a spectrum. For a source, the width of wavelengths contained in the output at one half of the wavelength of peak power. Typical spectral widths are 50 to 160 nm for

an LED and less than 5 nm for a laser diode.

Spectral Width, Full Width, Half Maximum (FWHM)

The absolute difference between the wavelengths at which the spectral radiant intensity is 50 percent of the maximum power.


A permanent junction between two fiber ends.

Splice Organizer

In optical communication, a device that facilitates the splicing or breaking out of fiber optic cables.

Splice Tray

A container that prevents spliced fibers from becoming damaged or being misplaced.


see Coupler.

Splitting Ratio

The ratio of power emerging from output ports of a coupler.

ST Connector

Abbreviation for straight tip connector. Popular fiber optic connector originally developed by AT&T.

Standard Single Mode Fiber

Step-index single mode fiber with zero dispersion at 1310nm; the first type used in fiber optic communications, still widely used.

Star Coupler

A coupler with more than three or four ports.

Star Network

A network in which all terminals are connected through a single point, such as a star coupler or concentrator.

Steady State Modal Distribution

Equilibrium modal distribution (EMD) in multimode fiber, achieved some distance from the source, where the relative power in the modes becomes stable with increasing distance.

Strain Relief

A Method of controlling the bend of a fiber as it exits the connector. Available in various sized depending on the cable size. 900um, 1.6mm, 2.0mm, and 3.0mm, or even the Timbercon Armadillo cable (custom integrated boot shell design).

Strength Member

The part of a fiber optic cable composed of aramid yarn, steel strands, or fiberglass filaments that increase the tensile strength of the cable.


An optical fiber, either multimode or singlemode, in which the core refractive index is uniform throughout so that a sharp step in refractive index occurs at the core-to-cladding interface. It usually refers to a multimode fiber. Such fibers have a large numerical aperture, are simple to connect, but have lower bandwidth than other types of optical fibers.

Step-Index Multimode Fiber

A step-index fiber with a core large enough to carry light in multiple modes.

Step-Index Single-Mode Fiber

A step-index fiber with a small core capable of carrying light in only one mode; this type has zero dispersion at 1310nm.

Stimulated Raman Scattering

Interactions between light and atoms in a transparent material that convert energy from one wavelength to another.

Submarine Cable

A cable designed to be laid underwater.

Surface-Emitting Diode (SLED)

An LED that emits light from its flat surface rather than its side. Simple and inexpensive, with emission spread of a wide angle.

Surface-Emitting Laser

A semiconductor laser that emits light from the wafer surface.


A device that directs light along different fiber paths


A data signal that is sent along with a clock signal. A system in which events, such as signals, occur at evenly spaced time durations. Opposite of asynchronous.


Talkset (fiber optic)

A communication device that allows conversation over unused fibers.

Tap Loss

In a fiber optic coupler, the ratio of power at the tap port to the power at the input port.

Tap Port

In a coupler where the splitting ratio between output ports is not equal, the output port containing the lesser power.


Terabits (trillion, or 1012 bits) per second.

T Coupler

A coupler with three ports.

TEC (TE Cooler)

Abbreviation for thermoelectric cooler. A device used to dissipate heat in electronic assemblies.


Preparation of the end of a fiber to allow connection to another fiber or an active device, sometimes also called “connectorization”.

Termination and Splicing

Termination and splicing equipment for fiber optics include tools or kits for cutting, finishing, positioning, aligning and joining fiber optic cables.


Terminating a fiber is accomplished through preparing the fiber for connection to another fiber or device such as a connector. The goal when terminating is to produce a perfect end to the fiber. The end should be cleanly cut, clear and physically connected to the receiving optical device. This can be accomplished through two means; permanently joining the fibers by welding or gluing the ends of the fiber together, or mechanically aligning the fibers and joining them with transparent gel.

Test Cable

A short single fiber jumper cable with connectors on both ends used for testing. This cable must be made of fiber and connectors of a matching type to the cables to be tested.

Test Kit

A kit of fiber optic instruments, typically including a power meter, source and test accessories used for measuring loss and power.

Test Source

A laser diode or LED used to inject an optical signal into fiber for testing loss of the fiber or other components.

Thermo-Optic Switches

Optical switches controlled by temperature-induced changes in refractive index.

Threshold Current

The minimum current needed to sustain laser action in a diode laser.

Throughput Loss

In a fiber optic coupler, the ratio of power at the throughput port to the power at the input port.

Throughput Port

In a coupler where the splitting ratio between output ports is not equal, the output port containing the greater power.

Tight Buffer

A material tightly surrounding a fiber in a cable, holding it rigidly in place.

Tight Buffered Cable

A protective coating extruded tightly over fiber for mechanical and environmental protection. The coating material is either nylon or PVC. This buffering offers excellent physical and flexing properties, but higher micro-bending sensitivity.

Total Internal Reflection

Total reflection of light back into a material when it strikes the interface with a material having a lower refractive index at an angle below a critical value.


A combination of transmitter and receiver providing both output and input interfaces with a device.

Transverse Modes

Modes across the width of a waveguide, fiber or laser. Distinct from longitudinal modes, which are along he length of a laser.


A device that converts energy from one form to another, such as optical energy to electrical energy.


A device that includes a source and driving electronics. It functions as an electrical-to-optical converter.

Traveling Wave

A wave that (a) propagates in a transmission medium, (b) has a velocity determined by the launching conditions and the physical properties of the medium, and (c) may be a longitudinal or transverse wave.


A unit or assembly of units or sections, and associated fittings, made of metal or other noncombustible materials forming a rigid structural system used to support cables. Includes

ladders, troughs, channels, solid bottom trays, and similar structures.


A network architecture in which transmission routes branch out from a central point.


Abbreviation for Underwriter’s Laboratory. An organization that tests product safety for a wide variety of products. UL approved products carry UL symbol.

Ultraviolet (UV)

Electromagnetic waves invisible to the human eye, with wavelengths about 10 to 400nm, shorter than visible light.


Operating in one direction only.

UPC (Ultra Physical Contact)

Specific to singlemode applications, referring to the endface geometry of a connector ferrule as well as performance characteristics (-55dB Return Loss).


VCSEL (Vertical Cavity Surface Emitting Laser)

A semiconductor laser in which light oscillate vertically (perpendicular to the junction plane) and light emerges from the surface of the waver rather than from the edge of the chip.

Visible Light

Electromagnetic radiation visible to the human eye at wavelengths of 400 to 700nm.

Visual Fault Locator

A device that couples visible light into the fiber to allow visual tracing and testing of continuity.

Some are bright enough to allow finding breaks in fiber through the cable jacket.

VOA (Variable Optical Attenuator)

An attenuator in which the attenuation can be varied.



Wide area network. A wide area network (WAN) is a geographically dispersed telecommunications network. The term distinguishes a broader telecommunication structure from a local area network (LAN). A wide area network may be privately owned or rented, but the term usually connotes the inclusion of public (shared user) networks. An intermediate form of network in terms of geography is a metropolitan area network (MAN).


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

Waveguide Array

An array of curved planar waveguides that separates many optical channels at once. Also called Array Waveguide (AWG).

Waveguide Coupler

A coupler in which light gets transferred between planar waveguides.

Waveguide Dispersion

The part of chromatic dispersion arising from the different speeds light travels in the core and cladding of a single mode fiber (i.e., from the fiber’s waveguide structure).


The distance an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Wavelengths of light are measured in nanometers (10-9 m) or micrometers (10-6m).

Wavelength Division Multiplexing(WDM)

Multiplexing of signals by transmitting them at different wavelengths through the same fiber.

Wavelength Adapter

A device which receives one wavelength and outputs a second wavelength, usually to take a standard signal and convert it to an ITU wavelength.

Wavelength Isolation

A WDM’s isolation of a light signal in the desired optical channel from the unwanted optical channels. Also called far-end crosstalk.

Wavelength Router

An optical device that directs input signals according to their wavelength.

Wavelength Routing Switch (WRS)

A switch, used in optical networks, that routes wavelengths as required to specific terminals in the network.

Wavelength Selective Coupler

A device which couples the pump laser wavelength to the optical fiber while filtering out all other unwanted wavelengths. Used in erbium-doped fiber amplifiers.


Possessing large bandwidth.


A wavelength region where fibers have low attenuation, used for transmitting signals.


Y Coupler

A variation on the tee coupler in which input light is split between two channels (typically planar waveguide) that branch out like a Y from the input.


Zero Dispersion-Shifted Fiber

Fiber with zero chromatic dispersion shifted to 1550nm, used before the advent of DWDM. Zero-Dispersion Wavelength

Wavelength at which net chromatic dispersion of an optical fiber is nominally zero. Arises where waveguide dispersion cancels out material dispersion.

Zipcord (Zip Cord)

A two-fiber cable consisting of two single fiber cables having conjoined jackets. A zipcord cable can be easily divided by slitting and pulling the conjoined jackets apart.

Zip Cord Fiber

Two-fiber cable with two single fiber cables having conjoined jackets. The zipcord cable can be easily divided by slitting and pulling the conjoined jackets apart. Zip cord cables include both loose-buffer and tight-buffer designs.


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