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Fiber Optic Glossary

That 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.

A material such as silicon carbide, aluminum oxide, silica, cerium oxide, emery or rouge that is used to figure, shape, or finish optical elements. Abrasives differ from polishing materials mainly in particle size.

A/B Switch

A device that accepts inputs (optical or electrical) from a primary path and a secondary path to provide automatic or manual switching in the event that the primary path signal is broken or otherwise disrupted. In optical A/B switches, optical signal power thresholds dictate whether the primary path is functioning and signals a switch to the secondary path until optical power is restored to the primary path.

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.

Add/Drop Multiplexing
A multiplexing function offered in connection with SONET that allows lower level signals to be added or dropped from a high-speed optical carrier in a wire center. The connection to the add/drop multiplexer is via a channel to a central office port at a specific digital speed (DS3, DS1, etc.)

Abbreviation for add-drop multiplexer. A device which adds or drops signals from a communications network.

Abbreviation for asynchronous digital subscriber line. See DSL.

Aramid Yarn
Yellow fibers that provide cable tensile strength, support, and additional protection for the optical fiber bundle. Kevlar® is a particular brand of aramid yarn.

Acceptance Angle
The angle over which the core of an optical fiber accepts incoming light; usually measured from the fiber axis. Related to numerical aperture (NA).

Access Network
Part of the telecommunication network that connects to individual and corporate users.

An adapter is a mechanical device designed to align fiber-optic connectors. It contains the split sleeve, also known as the interconnect sleeve, that holds the two ferrules together. Adapters can help mate or connect a variety of fiber optic cables together.

Adapter Sleeve
A mechanical fixture within an adapter body that aligns and holds two terminated fiber connectors. Adapter sleeve material is typically phosphor bronze, ceramic or polymer.

Add-drop multiplexer
A device that drops and/or add one or more optical channels to a signal.

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

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

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

Angle of Incidence
The angle between an incident ray and the normal to a reflecting or refracting surface.

Aerial Plant
Cable that is suspended in the air on telephone or electric utility poles.

Abbreviation for automatic gain control. A process or means by which gain is automatically adjusted in a specified manner as a function of input level or another specified parameter.

Abbreviation for amplitude modulation. A transmission technique in which the amplitude of the carrier varies in accordance with the signal.

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).

Angular Misalignment
Loss at a connector due to fiber end face angles being misaligned.

Abbreviation for American National Standards Institute.  An organization that administers and coordinates the U.S. voluntary standardization and conformity assessment system.

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.

APD (Avalanche Photodiode)
A photodiode that exhibits internal amplification of photocurrent through avalanche multiplication of carriers in the junction region.

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

Armadillo Loopback
A ruggedized fiber optic test adapter designed to loop a signal from the Tx side of a port to the Rx side, simulating a complete connection.

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

ASE (Amplified Spontaneous Emission)
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).

Abbreviation for application-specific integrated circuit. A custom-designed integrated circuit.

Abbreviation for American Society for Testing and Materials. An organization that provides a forum for the development and publication of voluntary consensus standards for materials, products, systems, and services that serve as a basis for manufacturing, procurement, and regulatory activities.

Data that is transmitted without an associated clock signal. The time spacing between data characters or blocks may be of arbitrary duration. Opposite of synchronous.

ATM (Asynchronous Transfer Mode)
A digital transmission switching format, with cells containing 5 bytes of header information followed by 48 data types. A transmission standard widely used by the telecom industry. A digital transmission switching format with cells containing 5 bytes of header information followed by 48 data bytes. Part of the B-ISDN standard.

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.

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

1) In electrical systems, a usually passive network for reducing the amplitude of a signal without appreciably distorting the waveform. 2) In optical systems, a passive device for reducing the amplitude of a signal without appreciably distorting the waveform.

Attenuation-Limited Operation
The condition in a fiber optic link when operation is limited by the power of the received signal (rather than by bandwidth or distortion).

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

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

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

Axial Propagation Constant
For an optical fiber, the propagation constant evaluated along the axis of a fiber in the direction of transmission.

The center of an optical fiber.


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A transmission network that carries high speed telecommunications between locations. This is normally the main portion of a telecommunication network, with branches going to individual buildings. In a local area network, this is usually the link between routers, switches, and bridges.

Backbone Cabling
The inter-building and intra-building cable connections between entrance facilities, equipment rooms and telecommunications closets. Backbone cabling consists of the transmission media, main and intermediate cross-connects and terminations at these locations.

Backbone System
A transmission network that carries high-speed telecommunications between regions (e.g., a nationwide long-distance telephone system). Sometimes used to describe the part of a local area network that carries signals between branching points.

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.

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

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

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

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

The number of signal-level transitions per second in a digital data. For some common coding schemes, this equals bits per second, but this is not true for more complex coding. Bits per second is less ambiguous. A unit of signaling speed equal to the number of signal symbols per second, which may or may not be equal to the data rate in bits per second.

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

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

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

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

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

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

The smallest unit of information upon which digital communications are based; also an electrical or optical pulse that carries this information.

Bit Depth
The number of levels that a pixel might have, such as 256 with an 8-bit depth or 1,024 with a 10-bit depth.

Bit Period (T)
The amount of time required to transmit a logical one or a logical zero.

Bit Error Rate (BER)
The fraction of bits transmitted incorrectly. The fraction of bits transmitted that are received incorrectly.

Popular coax bayonet style connector, Often used for baseband video.

An abbreviation for broadband on passive optical network.

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

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

An essential part of many fiber-optic cable designs, consisting of a layer of woven yarn. Note: In the case of single-fiber loose-buffered or two-fiber "zip-cord" loose-buffered fiber-optic cables, the braid is situated between the buffer tube and jacket. In the case of cables having multiple buffer tubes, the braid is usually situated between the inner jacket and outer jacket.

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.

Covering a wide range of frequencies or having a high data rate. The broadband label is sometimes used for a network that carries many different services or for video transmission.

Broadcast Transmission
Sending the same signal to many different places, like a television broadcasting station. Broadcast transmission can be over optical fibers if the same signal is delivered to many subscribers.

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.

Bus Network
A network topology in which all terminals are attached to a transmission medium serving as a bus. Also called a daisy-chain configuration.

Butt Splice
A joining of two fibers without optical connectors arranged end-to-end by means of a coupling. Fusion splicing is an example.

Eight bits of digital data. (Sometimes parity and check bits are included, so one "byte" may include 10 bits, but only 8 of them are data.)


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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.

Cable Television
Communications system that distributes broadcast and non-broadcast signals as well as a multiplicity of satellite signals, original programming and other signals by means of a coaxial cable and/or optical fiber.

In technology, the wave that is modulated with a signal carrying information. In business, a company that provides telecommunication services.

Carrier-to-Noise Ratio (CNR)
The ratio, in decibels, of the level of the carrier to that of the noise in a receiver's IF bandwidth before any nonlinear process such as amplitude limiting and detection takes place.

Category 5e
Category 5 (CAT5) cable is a popular twisted pair copper cable. It is used for Ethernet cable applications. Category 5e (CAT5e) can support short-run Gigabit Ethernet (1000 Mbps) networking, unlike CAT5 which supports Fast Ethernet (100 Mbps).

An acronym for cable television, derived from Community Antenna TeleVision.

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.

Abbreviation for Consultative Committee on Radio. Replaced by ITU-R.

Abbreviation for Consultative Committee on Telephony and Telegraphy. Replaced by ITU-T.

Abbreviation for code-division multiple access. A coding scheme in which multiple channels are independently coded for transmission over a single wideband channel using an individual modulation scheme for each channel.

A fixed-length data packet transmitted in certain digital systems such as ATM.

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 Coding
Data encoding and error correction techniques used to protect the integrity of data. Typically used in channels with high bit error rates such as terrestrial and satellite broadcast and videotape recording.

Channel Markers
A/B Markers easily identify the TX and RX connector on each end of a fiber optic cable assembly. These channel markers assure that the proper connections are made between the transmitting and receiving ports of a transceiver.

Channel Spacing
The amount of bandwidth allocated per channel.

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

Chirped Pulse
A pulse in which the wavelength changes during the duration of the pulse.

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.

Originally a physical connection that transmits electricity or signals. Now also a communication channel that guarantees a fixed transmission capacity.

Circuit Switching
making temporary physical or virtual connections between two points, which guarantees a fixed transmission capacity.

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.

CLEC (Competitive Local Exchange Carrier)
A company that offers local telephone service in competition against dominant phone companies.

Abbreviation for cable modem termination system.

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.

Coaxial cable - cable with a central metallic conductor surrounded by an insulator that is covered by a metallic sheath that runs the leg nth of the cable. 1) A cable consisting of a center conductor surrounded by an insulating material and a concentric outer conductor and optional protective covering. 2) A cable consisting of multiple tubes under a single protective sheath. This type of cable is typically used for CATV, wideband, video, or RF applications.

A device, also called an encoder that converts data by the use of a code, frequently one consisting of binary numbers, in such a manner that reconversion to the original form is possible.

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.

Reducing the number of bits needed to encode a digital signal, typically by eliminating long strings of identical bits or bits that do not change in successive sampling intervals (e.g., video frames).

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.

Industry slang for metal wire, either twisted-pair or coaxial cable.

Copper vs Fiber
In general, fiber has many advantages over copper. In copper networks, loss increases with signal frequency- High data rates increase power loss and therefore decrease transmission distances. In fiber optic networks, loss does not change with signal frequency.

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.

Crimp Sleeve
A crimped metal cylinder that holds the connector to the cable through the cable's strength member.

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.

Cross-gain Modulation (XGM)
A technique used in wavelength converters where gain saturation effects in an active optical device, such as a semiconductor optical amplifier (SOA), allow the conversion of the optical wavelength. Better at shorter wavelengths (e.g. 780 nm or 850 nm).

Cross-phase Modulation (XPM)
A fiber nonlinearity caused by the nonlinear index of refraction of glass. The index of refraction varies with optical power level which causes different optical signals to interact.

Crosstalk (XT)
1) Undesired coupling from one circuit, part of a circuit, or channel to another. 2) Any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates and undesired effect in another circuit or channel.

Abbreviation for carrier sense multiple access with collision detection. A network control protocol in which (a) a carrier sensing is used and (b) while a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, waits for a jam signal, and then waits for a random time interval before trying to send that frame again.

Customer Premises Equipment (CPE)
Terminal, associated equipment, and inside wiring located at a subscriber's premises and connected with a carrier's communication channel(s) at the demarcation point (demarc), a point established in a building or complex to separate customer equipment from telephone company equipment.

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.


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A format for component digital video tape recording working to the ITU-R 601, 4:2:2 standard using 8-bit sampling.

The VTR standard for digital composite (coded) NTSC or PAL signals that uses data conforming to SMPTE 244M.

A composite digital video recording format that uses data conforming to SMPTE 244M.

An uncompressed tape format for component digital video which has provisions for HDTV recording by use of 4:1 compression.

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 Dependent Jitter
Also called data dependent distortion. Jitter related to the transmitted symbol sequence. DDJ is caused by the limited bandwidth characteristics, non-ideal individual pulse responses, and imperfections in the optical channel components.

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.

Abbreviation for decibel relative to a carrier level.

Decibels relative to 1mW.

Decibels relative to 1 µW.

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.

Abbreviation for data circuit-terminating equipment. 1) In a data station, the equipment that performs functions such as signal conversion and coding, at the network end of the line between the data terminal equipment (DTE) and the line, and may be a separate or an integral part of the DTE or of intermediate equipment. 2) The interfacing equipment that may be required to couple the data terminal equipment (DTE) into a transmission circuit or channel and from a transmission circuit of a channel into the DTE.

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.

Feedback arising from reflection distributed through a structure.

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.

Digital Subscriber Line (DSL)
A service that transmits digital signals to homes at speeds of hundreds of kilobits to tens of metabits per second over twisted-pair wires at higher frequencies than voice telephone signals. There are several variations.

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.

Thick liquid or paste used to prepare a surface or a varnish-like substance used for waterproofing or strengthening a material.

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.

Abbreviation for data signaling rate. The aggregate rate at which data pass a point in the transmission path of a data transmission system expressed in bits per second (bps or b/s).

A cable that delivers service to an individual customer.

A transmission rate in the North American digital telephone hierarchy. Also called T-carrier.

Abbreviation for data terminal equipment.
1) An end instrument that converts user information into signals for transmission or reconverts the received signals into user information.
2) The functional unit of a data station that serves as a data source or a data sink and provides for the data communication control function to be performed in accordance with link protocol.

Abbreviation for data terminal ready. In a communications network, a signal from a remote transmitter that the transmitter is clear to receive data.

Abbreviation for digital television. Any technology, using any of several digital encoding schemes, used in connection with the transmission and reception of television signals. Depending on the transmission medium, DTV often uses some type of digital compression to reduce the required digital data rate. Except for artifacts of the compression, DTV is more immune (than analog television) to degradation in transmission, resulting in a higher quality of both audio and video, to the limits of signal reception.

Dual Attachment Concentrator
A concentrator that offers two attachments to the FDDI network which are capable of accommodating a dual (counter-rotating) ring.

Dual Ring (FDDI Dual Ring)
A pair of counter-rotating logical rings.

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).



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