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In an optical fiber, loss of optical power resulting from conversion of that power into heat. Intrinsic causes of absorption in a fiber involve tails of the ultraviolet and infrared absorption bands. Extrinsic components causing loss include (a) impurities, (e.g., the OH ion and transition metals ions) and (b) defects (e.g., results of thermal history and exposure to nuclear radiation). See also: Attenuation.

Acceptance Angle
The half-angle of the cone within which all incident light is totally internally reflected by the fiber core. For graded-index fibers, acceptance angle is a function of position on the entrance face of the core.

In fiber optics, a device for coupling two connectors.

Adjustable Attenuater
An attenuater in which the level of attenuation is varied with an internal adjustment. Also known as Variable Attenuater.

A positive number that indicates the core refractive index shape and ranges from one to infinity.

The distance between high or low points of a waveform or signal. Also referred to as the wave "height".

Amplitude Modulation (AM)
A transmission technique in which the amplitude of the carrier is varied in accordance with the signal.

Any physical property indexed, controlled, or represented by another physical property capable of representing it accurately. Usually refers to a system that codes data by measuring voltages, rather than discrete signals (digitally).

Analog Signal
An electrical signal that varies continuously over an infinite range of voltage or current values, as opposed to a digital signal, which varies discreetly between two values, usually one and zero. It is easiest to think of analog signals as sine waves of various sizes. Compare with Digital Signal.

Analog-to-Digital Converter
A device that converts an analog signal, that is, a signal in the form of a continuously variable voltage or current, to a digital signal, in the form of bits.

Angle of Incidence
The angle between an incident ray and the normal to a reflecting or refracting surface. See also: Critical Angle; Total Internal Refection.

Angle of Refraction
The angle formed between a refracted ray and the normal to the surface. This angle lies in a common plane with the angle of incidence.

Unit of length equal to 10 meters.

Angular Alignment
The alignment of two optical fibers with respect to the angle formed by their axes.

Angular Misalignment Loss
The optical power loss caused by angular deviation from the optimum alignment of source to optical fiber, fiber-to-fiber, or fiber-to-detector. See also: Extrinsic Joint Loss; Intrinsic Joint Loss; Lateral Offset Loss.

Abbreviation for American National Standards Institute. A voluntary organisation that helps set standards and also the U.S. in the International Standards Organisation (ISO).

The manner in which hardware or software is structured. Architecture typically describes how the system or program is constructed, how its components fit together; also refers to the protocols and interfaces modules or components of the system. Network architecture defines the functions and description of data formats and procedures used for communication between nodes or workstations.

ASCII (American Standard Code for Information Interchange)
A system used to represent letters, numbers, symbols and punctuation as bytes of binary signals (ones and zeros).

A mode of data transmission wherein the occurrence of each character is not related to a fixed time frame of reference. Compare with synchronous transmission.

A method of transmitting data. A low-cost alternative to synchronous communications. One or more bits are added to the beginning and the end of each data character in asynchronous communications. This allows the receiver of the signal to recognise the characters being sent. Asynchronous is the simplest form of communications since it does not require the sender and receiver to each have a clock to time each other to "stay in synch". However, the addition of these extra bits (called "framing" bits) means that more bits have to be transmitted in asynchronous communications to get the same message across than in other methods, like synchronous communications.

Asynchronous Transfer Mode (ATM)
The technology selected by the Consultative Committee on International Telegraphy and Telephony (CCITT) to deliver broadband-ISDN services for the world-wide telecommunications network. It is fast, packet-sized technology based on a fixed packet (or cell) size.

Asynchronous Transmission
A mode of data transmission whereby each bit of information is generated separately with some stop/start code to indicate the interval between bits.

In an optical fiber, the diminution of average optical power. In optical fibers, attenuation results from absorption, scattering, and other radiation losses. Attenuation is generally expressed in dB without a negative sign. Calculations and equations involving loss show and use the negative sign. Attenuation is often used as a synonym for attenuation coefficient, expressed in dB/Km.

Attenuation Coefficient
A factor expressing optical power loss per unit of length, expressed in dB/Km.

Attenuation-Limited Operation
The condition prevailing when the received signal amplitude (rather than distortion) limits performance. See also: Bandwidth-Limited Operation; Distortion-Limited Operation.

An electronic transducer, either fixed or adjustable, that reduces the amplitude of a wave with causing significant distortion.

Automatic Optical Inspection
A system for automatic industrial process control or measurement, consisting of an optical module for image acquisition, a segmentation processor to isolate the image from its background, and an image analysis processor.

Avalanche Photodiode (APD)
A photodiode that shows gain in its output power that it receives through avalanche multiplication of photo current. Note: As the reverse-bias voltage approaches the breakdown voltage, hole-electron pairs created by absorbed photons acquire sufficient energy to create additional hole-electron pairs when they collide with ions; thus, a multiplication (signal gain) is achieved. See also: Photon; PIN Photodiode.

Average Wavelength
The average of the two wavelengths for which the peak optical power has dropped to half.

Axial (Lateral) Misalignment Loss
The optical power loss caused by two fiber axes that are parallel but not on the same central axis.

Axial Ray
A light ray that travels along the optical fiber's axis. See also: Skew Ray.

That portion of scattered light which returns in a direction generally reverse to the direction of propagation.

The range of frequencies handled by a device or system.

Bandwidth-Limited Operation
Usually refers to the maximum effective transfer rate data; the condition prevailing when the system bandwidth, rather than the amplitude or power of the signal, limits performance.. The condition is reached when material and modal dispersion distort the shape of the waveform beyond specified limits.

A method of communication in which a signal is transmitted at its original frequency without being impressed on a carrier.

A unit of signalling 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.

Baud Rate
Measurement of data transmission speed, expressed in bits per second or bps.

Bayonet Coupling
A quick coupling device for plug and receptacle connection, designed to lock a connector into an adapter or receptacle. An example is an ST(r) connector.

Beam Diameter
The distance between two diametrically opposed points at which the irradiance is a specified fraction of the beam's peak irradiance; most commonly applied to beams that are circular or nearly circular in cross section. Synonym: Beamwidth.

Beam Divergence
The increase in beam diameter with increase of distance from the source.

A device for dividing an optical beam into two or more separate beams; often a partially reflecting mirror.

Bend Loss
A form of increased attenuation in a fiber that results from bending a fiber around a restrictive curvature (a macrobend) or from minute distortions in the fiber (microbends).

Bend Radius
Radius of curvature that a fiber can bend without breaking.

BER Floor
A limiting of the bit-error ratio (BER) in a digital fiber optic system as a function of received power due to the presence of signal degradation mechanisms or noise.

Bit Error Rate (BER)
In digital applications it is the ratio of bits received in error to bits sent. BERs of 10 (one error bit to a billion sent) are common.

Bit Rate
The number of bits of data transmitted over a phone line per second. You can usually figure how many characters per second you will be transmitting, in synchronous communications, if you divide the bit rate by ten. The total bits transmitted will depend on re-transmissions, which depends on the noise of the line, etc.

Buffer Coating
A protective layer, such as an acrylic polymer, applied over the fiber cladding for protective purposes.

Buffer Tube
A hard plastic tube, having an inside diameter several times that of a fiber, that holds one or more fibers.

Bypass Switch
An FDDI station option that ensures ring integrity. The bypass switch has a default state when power is lost. When a power failure occurs, optical connections are made within the DAS that reconnect the incoming primary ring fiber with the primary ring fiber. Similar connections are made on the secondary ring. This allows network operation to continue without the need for reconfiguration, which can still be employed for emergency situation.

Cable Bend Radius
Cable bend radius during installation infers that the cable is experiencing a tensile load. Free bend infers a lower allowable bend radius since it is at a condition of no load/

Cable Jacket
The outermost protective covering applied over the internal cable elements (i.e. core, cladding, buffer and strength member).

An access-driven form of replication. Data is replicated and stored locally as it is referenced; only an initial reference to a piece of data requires access to the master copy. Updates merely invalidate the cached items.

Carrier Frequency
The electromagnetic wave frequency selected to transmit information. Optical carrier frequency is from the infrared, visible or ultraviolet spectrum areas.

Abbreviation for Comite Consulatif International de Telegraphie et Telephonie. An international communications standards. Based in Geneva, Switzerland. Concerned with devising and proposing recommendations for international telecommunications.

Centre Wavelength (Laser)
The nominal value central operating wavelength defined by a peak mode measurement where the effective optical power resides

Centre Wavelength (LED)
The average of the two wavelengths measured at the half amplitude points of the power spectrum.

Chromatic Dispersion
The speed of an optical pulse travelling in a fiber changes as its wavelength changes. Since any practical light source has a certain spectral width, this effect results in pulse broadening called chromatic dispersion. Chromatic dispersion can be measured by the measurement of travel time at different wavelengths.

A low-refractive-index, glass or plastic that surround the core of a fiber. Optical cladding promotes total internal reflection for the propagation of light in a fiber.

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

Cladding Mode Stripper
A device that encourages the conversion of cladding modes to radiation modes; as a result of its use, cladding rays are stripped from the fiber. A cladding mode stripper often uses a material having a refractive index equal to or greater than that of the waveguide cladding to induce this conversion.

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.

A material put on a fiber during the drawing process to protect it from the environment.

The process by which a divergent or convergent beam of radiation is converted into a beam with the minimum divergence possible the system (ideally a parallel bundle or rays). See also: Beam Divergence.

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

A passive device in which optical power from several input fibers is collected at a common point. See also: Coupler.

In a wire or cable, the measurement of the location or the centre of the conductor with respect to the geometric centre of the circular insulation.

Connector Return Loss
The amount of power reflected from the connector to connector interface. Return loss values are expressed as dB.

Connector Variation
The maximum value in dB of the difference in insertion loss between mating optical connectors. Also known as Optical Connector Variation.

The light conducting portion of a fiber, defined by its high refractive index. The core is normally in the centre of a fiber, bounded by concentric cladding of lower refractive index.

Core Eccentricity
A measure of the displacement of the centre of the core relative to the cladding centre.

Core Ellipticity (non-circularity)
A measure of the departure of the core from roundness.

A device whose purpose is to distribute optical power among two or more ports, or to concentrate optical power from two or more fibers into a single port. Couplers may be active or passive.

Coupling Efficiency
The efficiency of optical power transfer between two components.

Coupling Loss
The power loss suffered when coupling light from one optical device to another.

Crimp Sleeve
A hollow, metal cylinder that when crimped, holds the connector to the cable via the strength member of the cable.

Critical Angle
The smallest angle at which a meridional ray may be totally reflected within a fiber at the core-cladding interface. When light propagates in a homogeneous medium of relatively high refractive index (n) onto an interface with a homogenous material of lower index (n).

Cut-off Wavelength
For a single mode fiber, the wavelength above which the fiber exhibits single mode operation.

Abbreviation for continuous wave. Usually used to refer to the constant optical output from an optical source when it is biased but not modulated with a signal.

Data Compression
Storing of digital data with techniques that consume less memory space than basic methods.

Data Frame
Consists of the Destination Address, Source Address, Length Field, LLC Data, Pad and Check Frame Sequence.

Data Rate
The maximum number of bits of information which can be transmitted per second, as in a data transmission link; typically expressed as megabits per second (Mb/s).


Decibel referenced to a milliwatt.

An abbreviation for decibel millivolt. The level at any point in a system expressed in dB's above or below a 1 millivolts (dBmV). Zero dBmV is equal to 1 millivolt across 75 ohms. The level at any point in a system expressed in dBs above or below a 1millivolt/75 ohm standard is said to be the level in decibelmillivolts or dBmV.

Decibel referenced to a microwatt.

Decibel (dB)
The standard unit used to express gain or loss of optical power. A standard logarithmic unit for the ratio of two powers, voltages or currents. In fiber optics, the ratio is power.

Greatest relative difference in refractive index of core and cladding.

A transducer that provides an electrical output signal in response to an incident optical signal. The current is dependent on the amount of light received and the type of device.

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

Dichroic Filter
An optical filter that transmits light selectively according to wavelength.

As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefronts as well as with each other to form various diffraction patterns.

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.

The cause of bandwidth limitations in a fiber. Dispersion causes a broadening of input pulses along the length of the fiber. Two major types are: mode dispersion caused by differential optical path lengths in a multimode fiber and material dispersion caused by a differential delay of various wavelengths of light in a waveguide material.

Distortion-Limited Operation
Generally synonymous with bandwidth-limited operation.

Distributed Feedback Laser (DFB)
An injection laser diode which has a Bragg reflection grating in the active region, in order to suppress multiple longitudinal modes and enhance a single-longitudinal mode.

A material, usually germania or boron oxide, added to silica to change its index of refraction.

Characteristic of data transmission. Either full or half duplex. Full permits simultaneous, two-way communication. Half means only one side can talk at once.

Duplex Cable
A two-fiber cable suitable for duplex transmission.

Duplex (Two Position) Connectors (FDDI)
Used as the physical connections between stations on the FDDI ring to connect fiber optic cables. The connectors are polarised to prevent the transmitting and receiving fibers from becoming inadvertently interchanged.

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

Duty Cycle
In a digital transmission, the ratio of high levels to low levels.

Dynamic Range
In a transmission system, the ratio of the overload level to the noise level of the system, usually expressed in decibels. The ratio of the specified maximum level of a parameter (e.g., power, voltage, frequency, or floating point number representation) to its minimum detectable or positive value, usually expressed in decibels.

Eccentricity (Connector)
Displacement of the fiber hole from the true centreline of the ferrule's outside diameter.

Abbreviation for edge-emitting LED.

End Separation Loss
The optical power loss caused by distance between the end of a fiber and a source, detector, or another fiber.

Abbreviation for electrical-to-optical converter.

Equilibrium Mode Distribution (EMD)
The condition in a multimode optical fiber in which the relative power distribution among the propagating modes is independent of length. Synonym: Steady-State Condition. See also: Equilibrium Length; Mode; Mode Coupling.

Extrinsic Fiber Loss
Optical fiber loss caused by imperfect alignment of fibers in a connector or splice. Contributors include angular misalignment, axial misalignment, end separation, and end finish.

Extrinsic Loss
In a fiber interconnection, that portion of loss that is not intrinsic to the fiber but is related to imperfect joining, which may be caused by the connector or splice. Contributors include angular misalignment, axial misalignment, end separation, and end finish. Generally synonymous with insertion loss.

FDM - Frequency Division Multiplex
See Frequency Division Multiplexing. Method by which the available transmission frequency range is divided into narrower bands, each used for a separate channel. As utilised buy broadband technology, the frequency spectrum is divided up among discrete channels, to allow one user or a set of users access to single channels.

A component of a connector that holds fiber in place and aids in its alignment, usually cylindrical in shape with a hole through the centre.

Fiber Bandwidth
The 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 Buffer
Material used to protect an optical fiber or cable from physical damage, providing mechanical isolation or protection. Fabrication techniques include both tight jacket or loose tube buffering as well as multiple buffer layers.

Fiber Bundle
An assembly of unbuffered optical fibers. Usually used as a single transmission channel as opposed to multifiber cables, which contain optically and mechanically isolated fibers, each of which provides a separate channel.

Fiber Cleaving
The controlled fracture of an optical fiber along a crystalline plane resulting in a smooth surface.

Fiber Core
The inner portion of the fiber that transmits light. The index of refraction is higher than that of the cladding that surrounds it.

Fiber Optics
Light transmission through optical fibers for communication or signalling.

Fiber Optic Attenuator
A component that is installed in a fiber optic transmission system to reduce 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 Test Procedure (FOTP)
Standard test procedures developed and published by the Electronic Industries Association (EIA) under EIA-RS-455 series of standards.

A bundle of systematically arranged fibers that transmits a full color image which remains undisturbed when the bundle is bent. With an objective lens on one end of the bundle, and a magnifying lens at the other, the system becomes a flexible fiberscope used to view objects that could not be seen by direct viewing.

Flux Budget
Optical power attenuation permitted between any two transceivers. This attenuation allowance is for optical connector losses, optical cable attenuation and the optical cable attenuation and the optical power division in a Codestar passive fiber optic coupler. The sum of these attenuations, losses and divisions must not exceed the flux budget.

Free-Space Optical Interconnect
A type of internal photonic connection in an integrated circuit in which a holographic grating is used to focus light at points on a silicon chip, maximising the speed of signal propagation.

Frequency-Division Multiplexing (FDM)
A system that allows the transmission of more than one signal over a common path, by assigning each signal a different frequency band.

Frequency Modulation (FM)
A method of transmission in which the carrier frequency varies in accordance with the signal.

Frequency Response
The change of gain with frequency.

Fresnel Reflection
Reflection of a portion of the light incident on a planar interface between two homogenous media having different refractive indices. Fresnel reflection occurs at the air-glass interfaces at entrance and exit ends of an optical fiber. Resultant transmission losses (on the order of 4% per interface) can be virtually eliminated by use of anti-reflection coating or index matching material.

Abbreviation for Fiber-to-the-Home.

Fundamental Mode
The lowest order mode of a waveguide.

Fusion Splice
A splice accomplished by the application of localised heat sufficient to fuse or melt the ends of two lengths of optical fiber, forming a continuous single fiber.

An exchange of predetermined signal for purposes of control when a connection is established between two data sets....A preliminary procedure, usually part of communications protocol, to establish a connection.

A term used to identify any of the four elements: chlorine, fluoride, bromide and iodine, grouped together because their chemical properties are similar.

Hard-Sheath Cable
Cable or wire contained within a continuous inner of outer metallic sheath.

Harmonic Distortion
Interference resulting from some type of harmonic signal, measured in decibels (dB).

Harmonic Frequency
A frequency that appears as the result of a strong signal at a lower frequency.

High definition television.

Head End
The main or top transmitting device in a broadband network where all transmission are distributed.

That part of a message, at the beginning, which contains destination, address, source address, message numbering, and possible other information. It helps direct the message along its journey.

Hybrid Optical Integrated Circuit
Device in which the various circuit elements are fabricated in different substrate materials and then appropriately joined together so that the various substrate materials can be chosen to optimise the performance of each type of device in the optical integrated circuit.

Integrated detector/preamplifier.

Abbreviation for the Institute of Electrical and Electronic Engineers. A publishing and standards-making body responsible for many standards used in LANs.

Incoherent Fiber Bundle
A bundle of filaments of optical glass or other transparent materials that transmit light only, not optical images. The arrangement of the individual fibers in the bundle is not sufficiently regular to transmit optical images.

Index Matching Material
A material, often a liquid or a cement, whose refractive index is nearly equal to the core index, used to reduce Fresnel reflections from an optical fiber's end face.

Index of Refraction
The ratio of the velocity of light in a vacuum to the velocity of light in a given medium.

Index Profile
In a graded-index optical fiber, the refractive index as a function of radius.

Injection Laser Diode (ILD)
A solid state semiconductor device consisting of at least one p-n junction capable of emitting coherent, stimulated radiation under specified conditions.

Insertion Loss
Total optical power loss caused by insertion of an optical component such as a connector, splice, or coupler into a previously continuous path.

Interchannel Isolation
The ability to prevent undesired optical energy from appearing in one signal path as a result of coupling from another signal path; cross talk.

Connectors from a manufacturer may be intermixed and mated with another manufacturer's connectors.

Intrinsic Joint Loss
Loss caused by fiber-parameter (e.g., core dimensions, profile parameter) mismatches when two non identical fibers are joined.

(Integrated Services Digital Network). A standard protocol (format) for digital telecommunications transmissions. ISDN is an international standard for transmitting digital information (text, sound, voice, data, video, etc.)

Data transmission where timing is derived from the signal carrying the data. Not timing or clock lead is provided at the customer interface.

Statistical variance in switching delay. Cell size determines worst-case switching delay. Small ATM cell size means low jitter and gives benefits of cut-through switching without the implementation complexity. Deviations from the ideal timing of an event which occur at high frequencies. Low frequency deviations are tracked by the clock recovery and do not directly affect the timing allocations within a bit cell. Jitter is not tracked by the clock recovery and directly affects the timing allocations in a bit cell.

Jitter, Data Dependent (DDJ)
Jitter that is 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.

Jitter, Deterministic (DJ)
Timing distortions caused by normal circuit effects in the transmission system. Deterministic jitter is often subdivided into duty cycle distortion (DCD) caused by propagation differences between the two transitions of a signal and data dependent jitter (DDJ) caused by the interaction of the limited bandwidth of the transmission system components and the symbol sequence.

Jitter, Duty Cycle Distortion (DCD)
Distortion usually caused by propagation delay differences between low-to-high and high-to-low transitions. DCD is manifested as a pulse width distortion of the nominal baud time.

Jitter, Random (RJ)
Jitter due to thermal noise which may be modelled as a Gaussian process. The peak-to-peak value of random jitter is of a probabilistic nature, and thus any specific value yields an associated bit error rate.

Tensile strength in thousands of pounds per square inch.

A uniform radiance distribution.

Lambertian Radiator
An optical source which has radiance uniform in all directions, proportional to the cosine of the angle from the normal.

Local Area Network

A device that produces monochromatic, coherent light through stimulated emission. Most lasers used in fiber optic communications are solid-state semiconductor devices....An acronym for Light Amplification by Stimulated Emission of Radiation.... A device which transmits an extremely narrow and coherent beam of electromagnetic energy in the visible light spectrum. See also: Injection Laser Diodes, Stimulated Emission.

Laser Chirp
A phenomenon in lasers where the wavelength of the emitted light changes during modulation.

Laser Modulation
Turning the laser on the off.

Lasing Threshold
The lowest excitation level at which a laser's output is dominated by stimulated emission rather that spontaneous emission. See also: Laser, Spontaneous Emission, Stimulated Emission.

The time interval between when a network station seeks access to a transmission channel and when access is granted or received. Same as waiting time. In a bridge of router, it is the amount of time elapsed between receiving and retransmitting the LAN packet. The length of time the packet is struck in a bridge or router.

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

Lateral Offset Loss
An optical power loss caused by transverse or lateral deviation from optimum alignment of source to optical fiber, fiber-to-fiber, or fiber-to-detector.

Launch Angle
The angle between the light ray and optical axis of an optical fiber or fiber bundle.

Launch Fiber
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 modal distribution in multimode fiber. Also referred to as Launching Fiber.

In a strict sense, the visible spectrum nominally covering the wavelength range of 400 nm to 750 nm. In the laser and optical communications field, the much broader portion of the electromagnetic spectrum that can be handled by the basic optical techniques used for the visible spectrum extending from the near-ultraviolet region of approximately 0.3 micrometers through the visible region and into the mid-infrared region to 30 micrometers.

Light Diffusion
The scattering of light by reflection or transmission. Diffuse reflection results when light strikes an irregular surface such as frosted window or the surface of a frosted or coated light bulb.

Light Emitting Diode (LED)
A semiconductor device which emits incoherent light from a p-n junction (when biased with an electrical current). Light may exit from the junction strip edge or from its surface (depending on the device's structure).

Lightguide Cable
An optical fiber, multiple fiber, or fiber bundle which includes a cable jacket and strength members, fabricated to meet optical, mechanical and environmental specifications.

Electromagnetic waves in the region of optical frequencies. The term "light" was originally restricted to radiation visible to the human eye, with wavelengths between 400 and 700 nm. However, it has become customary to refer to radiation in the speed regions adjacent to visible light as "light" to emphasise the physical and technical characteristics they have in common with visible light.

Lightwave Data Communication
Made possible by fiber optic technology. Based on the fact that pulses of light transmitted over fiber will enable media networks to communicate over greater distances at higher rates of speed, with complete immunity to electrical interference of any type, offer greater reliability at lower cost than can electrical, copper-based networks, with either coaxial cable or twisted pair wiring. The conversion of electrical signals to lightwave signals is accomplished via transceivers containing special compound semiconductors made of gallium arsenide and indium phosphide. These two compounds have enabled the implementation of lightwave communication at extremely high data rates. Ground loops, electromagnetic interference and a host of related problems that are associated with transmission over copper wire or cable are eliminated. Fiber optic networks are the most cost-effective, thoroughly reliable, easily-maintained networks in the world.

Local Area Network (LAN)
A geographically limited network interconnecting electronic equipment.

Longitudinal Mode
An optical waveguide mode with boundary conditions determined along the length of the optical cavity.

Type of diagnostic test in which the transmitted signal is returned to the sending device after passing through a data communications link or network. A test typically runs on a four-wire circuit. The two transmit leads are joined to the two receive leads. A signal is then applied around the loop. This allows a technician or built-in diagnostic circuit to compare the returned signal with the transmitted signal and get some sense of what's wrong. Loopbacks are often done by excluding one piece of equipment after another.

Loose-Tube Buffering
A cable construction in which the optical fiber is placed in a plastic tube having an inner diameter much larger than the fiber itself. The loose tube isolates the fiber from the exterior mechanical forces acting on the cushion. The space between the tube and the fiber is often filled with a gel to cushion the fiber.

See Absorption, Angular Misalignment Loss, Attenuation, Backscattering, End Separation Loss, Extrinsic Joint Loss, Insertion Loss, Intrinsic Joint Loss, Lateral Offset Loss, Material Dispersion, Microbending, Reflection, Transmission Loss.

In an optical fiber, all macroscopic deviations of the fiber's axis from a straight line; distinguished from microbending.

Metropolitan Area Network. A network linking LANs and other networks at many sites with a city area.

Material Dispersion
Light impulse broadening caused by various wavelengths of light travelling at different velocities through a fiber. Material dispersion increased with the increasing spectral width of the source.

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

Mechanical Splicing
Joining two fibers together by mechanical means to enable a continuous signal. Elastomeric splicing is one example of mechanical splicing.

Microbend Loss
A form of increase attenuation caused by: a) having the fiber curved around a restrictive radius or curvature, or b) microbends caused by minute distortions in the fiber imposed by externally induced perturbations. Excessive bend loss may result from poor drawing or cable manufacturing techniques. In an optical fiber, loss caused by sharp curvatures involving local axial displacements of a few micrometers and spatial wavelengths of a few millimetres. Such bends may result from fiber coating.

In an optical fiber, loss caused by shape curvature involving axial displacements of a few micrometers and spatial wavelengths of a few millimetres. Such bends may result from fiber coating, cabling, packaging, installation, etc. Note: Microbending can cause significant radiative losses from mode coupling.

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

Modal Dispersion
Term for multimode dispersion, which is pulse broadening due to optical power running via different waveguide modes

In any cavity or transmission line, one of those electromagnetic field distributions that satisfies Maxwell's equations and boundary conditions. The field pattern of a mode depends on the wavelength, refractive index and cavity or waveguide geometry.

Mode Coupling
In an optical fiber, the exchange of power among modes. The exchange of power may reach statistical equilibrium after propagation over finite distance that is designated the equilibrium length.

Mode Evolution
The dynamic process a multilongitudinal 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 optical energy in a singlemode fiber. Because the MFD is greater than the core diameter. MFD replaces core diameter as a practical parameter.

Mode Filter
A device to remove high order modes to stimulate equilibrium mode distribution in a short length of optical fiber.

Mode Hopping
Dynamic changing of discrete modes in a source spectrum.

Mode Scrambler
A device for inducing mode coupling in an optical fiber.

Mode Partitioning
Change in the dominant mode as a function of time.

the process by which the characteristic of one wave (the carrier) is modified by another wave (the signal). Examples include amplitude modulation (AM), frequency modulation (FM), and pulse-coded modulation (PCM). Electrical switch to control the light source.

Monochromatic Light
Light consisting of a single wavelength or a very narrow band of wavelengths.

Mean Time Between Failure. Used by manufacturers to measure reliability of equipment. Almost always measured in hours. Keep in mind that an MTBF figure for a LAN card or a computer is always longer than what you will find in your own experience, because testing methods by manufacturers in laboratories do not reproduce office environments very well.

Multilongitudinal Mode Laser (MLM)
An injection laser diode which has a number of longitudinal modes.

Multimode Distortion
In a multimode optical fiber, the pulse distortion resulting from differential mode propagation rates.

Multimode Fiber
A fiber that supports propagation of more than one mode.

Multiple Reflection Noise (MRN)
The fiber optic receiver noise resulting from the interference of delayed signals from two or more reflection points in a fiber optic span.

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

Refers to analogue or voice-band data rates or transmission.

Near-end Crosstalk
The optical power reflected from one or more input ports, back to another input port.

Network Management
The collection of processes, tools and methods necessary to design, install, operate and maintain a network.

Nodal Points
Of all the rays passing through a lens from off-axis objects point to its corresponding image point, there is always one ray whose direction in the image space is equal to that in the object space. The nodal points are the two points at which these two external rays appear to intersect the axis.

Noise Equivalent Power (NEP)
The radiant power that produces a signal-to-noise ratio of unity at the output of a given optical detector.

Numerical Aperture (NA)
The "lightgathering ability" of a fiber, defining the maximum angle to the fiber axis at which light will be accepted and propagated through the fiber. NA = sin0, where 0 is the acceptance angle. NA also is used to describe the angular spread of light from a central axis, as in exiting a fiber, emitting from a source, or entering a detector. For multimode fiber, the numerical aperture usually ranges from .2 to approximately .5. Singlemode fiber generally has a numerical aperture on the order of .1.

Abbreviation for optical-to-electrical converter.

Optical Axis
The line passing through both centres of curvatures of the optical surfaces of a lens; the optical centreline for all the centres of a lens system.

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

Optical Continuous Wave Reflectometer (OCWR)
An instrument used to characterise 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 Directional Coupler (ODC)
A component used to combine and separate optical power.

Optical Isolator
A component used to block out reflected and other unwanted light.

Optical Launch Point
Focal point coming out of the laser. From this point you launch into fiber.

Optical Path Power Penalty
The additional lost budget required to account for degradation due to reflections and the combined effects of dispersion resulting from intersymbol interference, mode-partition noise and laser chirp.

Optical Power (LED)
Radiant power, expressed in watts.

Optical Power Budget
See Flux Budget

Optical Reference Plane
The plane that defines the optical boundary between the plug and the receptacle.

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

Optical Time Domain Relectometer (OTDR)
A method of characterising a fiber wherein an optical pulse is transmitted through the fiber and the resulting backscatter and reflections are measured as a function of time. Useful in estimating attenuation coefficient as a function of distance and identifying defects and other localised losses.

Optical Waveguide
Dielectric waveguide with a core consisting of optically transparent material of low attenuation (usually silica glass) and with cladding consisting of optically transparent material of lower refractive index than that of the core. It is used for the transmission of signals with lightwaves and is frequently referred to as a fiber. In addition, there are some optical components, such as laser diodes, which are referred to as optical waveguides.

Pertaining to a device that responds to optical power, emits or modifies optical radiation, or utilises radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer.

A cathode-ray tube with attendant amplifiers and control circuits for measuring and studying the waveforms of small currents and voltages. A cathode ray tube (CRT) oscilloscope is particularly convenient for studying repetitive phenomena, but a tube with a long-delay phosphor can be used to analyse a single electrical pulse. An oscilloscope equipped with a camera becomes an oscillograph.

A group of binary digits, including data and call control signals, switched as a composite whole. The data, all control signals and possible error control information, are arranged in a specific format.

Packet Switching
A data communications technique in which data is transmitted by means of addressed packets and a transmission channel is occupied for the duration of transmission of the packet only. The channel is then available for use by packets being transferred between different data terminal equipment. A data transmission method, using packets, whereby a channel is occupied only for the duration of transmission of the packet. The packet switch sends the different packets from different data conversations along the best route available in any particular order. At the other end, the packets are reassembled to form the original message which is the sent to the receiving computer. Because packets need not be sent in a particular order, and because they can go any route as long as they reach their destination, packet switching networks can choose the most efficient rout and send the most efficient number of packets down the route before switching to another route to send more packets. The other advantage of packet switching is the unified format that every message is moulded into.

Paraxial Rays
Rays which are nearly parallel with the optical axis.

Parity Bit
An additional bit added to a group of bits. That additional parity bit means that adding up all the bits in every byte will produce an odd or even number, depending on whether you choose odd or even parity. Parity bits are added for error detection.

Phase Modulation
Modulation is the process of using a medium to carry information. A torch beam could be "modulated" by turning it on and off, thus sending digital information. An electrical sine wave travelling down a twisted wire pair can also be modulated to carry information, a sine wave that is defined by its frequency, amplitude and phase. These are the only three parameters of a sine wave that can thus be changed to carry information. Phase modulation is typical used in higher speed modems.

The conductivity increase exhibited by some non-metallic materials resulting from the free carriers generated when photon energy is absorbed in electronic transmissions.

The current that flows through a photosensitive device as the result of exposure to radian power.

An optoelectronic transducer, such as a pin photodiode or avalanche photodiode.

A two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. Photodiodes are used for the detection of optical power and the conversion of optical power to electrical power.

The quantum of electromagnetic energy, generally regarded as a discrete particle having zero mass, no electrical charge, and an indefinitely long lifetime.

Photovoltaic Effect
Production of a voltage difference across a p-n junction resulting from the absorption of photon energy. The voltage difference is caused by internal drift of holes and electrons.

Physical Layer
Layer one of the OSI reference modes; encodes, modulates and transmits data across physical links (i.e., the transmission medium, such as coaxial cable) on the network; also defines the network's physical signalling characteristics.

A short length of optical fiber, permanently fixed to a component, used to couple power between the component and a transmission fiber. A fiber optic connector terminated to one end of an optical fiber cable.

A device used to convert optical signals to electrical signals in a receiver. For relatively fast speeds and moderate sensitivity in the 0.75um to 1.1um area wavelength, the silicon photodiode is most commonly used. Avalanche photodiodes (APD) combine the detection of optical signals with internal amplification of photocurrent. The internal gain is realised through avalanche multiplication of carriers in the junction region. The advantage in using an APD is its higher signal-to-noise ration, especially at high bit rates.

Plastic Clad Silica (PCS) Fiber
A fiber with a glass core and a plastic cladding.

Point-to-point transmission - Transmission of data between only two stations or nodes i.e., one sender and one receiver. Point-to-Point link - A circuit which connects two (and only two) nodes without passing through an intermediate node.

The direction of the electric field in the lightwave.

Polarisation Mode Dispersion
Pulse spreading in a single mode fiber which happens because of the different group velocities for each of the two perpendicular polarisation's of light travelling in the fiber.

A glass structure from which an optical fiber waveguide may be drawn.

Fusing with a low current to clean the fiber end. Precedes fusion splicing.

Primary Coating
The plastic coating applied directly to the cladding surface of the fiber during manufacture to preserve the integrity of the surface.

Pulse-Coded Modulation (PCM)
A technique in which an analogue signal, such as a voice, is converted into a digital signal by sampling the signal's amplitude and expressing the different amplitudes as a binary number. The sampling rate must be twice the highest frequency in the signal.

Pulse Dispersion
The widening of an optical pulse as it travels the length of a fiber. The property limits the useful bandwidth of the fiber and is usually expressed in terms of nanoseconds of widening per kilometre. The principal mechanisms are material dispersion and multimode distortion effect.

Pulse Spreading
The dispersion of incoming optical signals along the length of an optical fiber.

Radiant Power
The time rate of flow of radiant energy, expressed in watts.

The emission and/or propagation of energy through space or through a medium in the form of either waves or corpuscular emission.

Radius Ferrule (PC Finish)
The ferrule of the connector has a dome tip which allows the fiber face to make physical contact with the fiber face of another PC connector. PC finish reduces back reflection

A geometric representation of a light path through an optical medium; a line normal to the wavefront indicating the direction or radiant energy flow.

Rayleigh Scattering
Scattering by refractive index fluctuations that are small with respect to wavelength. The scattered field is inversely proportional to the power of the wavelength.

Receiver Sensitivity
The minimum acceptable value of average received power at point R to achieve a 10 bit error ratio. It takes into account power penalties caused by use of a transmitter with worst-case values of extinction ratio, jitter, pulse rise and fall times, optical return loss at point S, receiver connector degradations, and measurement tolerances. The receiver sensitivity does not include power penalties associated with dispersion, jitter, or reflections from the optical path; these effects are specified separately in the allocation of maximum optical path penalty. Sensitivity takes into account worst-case operating and end-of-life (EOF) conditions.

The part of a message or system that can be thrown away without losing the essential information of service. The part of a system that duplicates the essential tasks to take over should the original fail. Redundancy is built into many systems - or you can build redundancy in, at your option - to insure your system will always work....That portion of the total information contained in a message which can be eliminated without the loss of essential information, such as characters used only for checking. Also used to describe a communications facility in which there is a spare "back up" device for each important component of the system.

Reference Surface
The surface of an optical fiber which is used to contact transverse alignment elements of a connector or another component.

The ratio of reflected power to incident power. Note: In optics, frequently expressed as optical density or as a percent; in communication applications, generally expressed in dB.

The abrupt change in direction of a light beam at an interface between two dissimilar media so that the light beam returns into the medium from which it originated.

An instrument for measuring the reflectance of a surface.

The bending of a beam of light at an interface between two dissimilar media or in a medium whose refractive index is a continuous function of position (graded-index medium).

Refractive Index (of a medium)
The ratio of the velocity of light in a vacuum to its velocity in the medium. Synonym: Index of Refraction.

Refractive Index Profile
The description of refractive index as a function of radius in a fiber.

Residual Loss
The loss of the attenuater at the minimum setting of the attenuater.

The ratio of an optical detector's electrical output to it's optical input. The precise definition depends on the detector type; general expressed in amperes per watt or volts per watt of incident optical power.

The change in direction of light rays or photons after striking a small particle or discontinuity. It may also be regarded as the diffusion of a light beam caused by the inhomogeneity of the transmitting medium.

Acronym for Small Computer System Interface.

Secondary Mode Group
A second complete set of spectral lines separate from the main group of spectral lines in an optical spectrum.

For a fiber optic receiver, the minimum optical power required to achieve a specified level of performance, such as bit error ratio.

Shot Noise
Noise caused by random current fluctuations arising from the discrete nature of electrons.

Side Mode
Any observable mode of an optical device's spectrum which is not the dominant mode.

Signal-to-Noise Ratio (SNR, S/N)
The ratio of the received optical power, with fill signal averaging, divided by the noise floor for the detector; the ratio of signal level to noise level, related to bit error rate performance.

Transmission in only one direction. Generally a communications system or device capable of transmission in one direction only. See Duplex.

Singlemode Fiber
An optical fiber that supports only one mode of light propagation above the cut-off wavelength.

Abbreviation for Surface-emitting LED.

Snell's Law
The law of refraction: when light is incident on two homogenous isotropic media with a common boundary.

Synchronous Optical Network

The light emitter, either an LED or laser diode, in a fiber optic link; a device that when properly driven will produce information carrying optical signals.

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 peak power. Typical spectral widths are 20 to 60 nanometer for an LED and 2 to 5 nanometer for a laser diode.

Spectral Window
A wavelength region of relatively high transmittance, surrounded by regions of low transmittance.

Speed of Light
186,000 miles per second.

A permanent junction between optical fibers. May be thermally fused or mechanically applied.

A passive device which divides optical power among several output fibers from a common point.

Spontaneous Emission
Radiation emitted when the internal energy of a quantum mechanical system drops from an excited level to a lower level without regard to the simultaneous presence of similar radiation. Examples of spontaneous emission include radiation from an LED and radiation from an injection laser below the lasing threshold.

ST(r) Connector
A type of connector used on fiber optic cable utilising a spring loaded twist and lock coupling similar to the BNC connectors used with coaxial cabling.

Stabilised Light Source
An LED or laser diode that emits light with a controlled and constant spectral width, central wavelength, and peak power with respect to time and temperature.

Step-Index Fiber
An optical fiber, either multimode or single mode, 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.

Stimulated Emission
Radiation emitted when the internal energy of a quantum mechanical system drops from an excited level to a lower level when induced by the presence of radiant energy at the same frequency. An example is the radiation from an injection laser diode above the lasing threshold.

Stop Bit
In asynchronous transmission, the last transmitted element in each character. This bit completes the character and alerts the receiver to get ready to accept another character. See also: Asynchronous.

Strain Relief
A technique involving methods of termination or installation which reduces the transmission of mechanical stresses to the fiber in connector/cable terminations.

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

Synchronous Optical Network (SONET)
A standard for fiber optic telecommunications interfaces.

Synchronous Transmission
Transmission in which there is a constant time between successive bits, characters, or events. The timing is achieved by sharing of clocking....A transmission method in which the synchronising of characters is controlled by timing signals generated at the sending and receiving stations (as opposed to start/stop communications). Both stations operate continuously at the same frequency and are maintained in a desired phase relationship. Any of several data codes may be used for the transmission, so long as the code utilised the required line control character. (Also call "bi-sync" or "binary synchronous".)

Transparent synchronous transmitter-receiver interface.

Transmission Control Protocol/Internet Protocol

Tee Coupler
A three-port optical coupler.

Thermal Noise
Noise resulting from thermally induced random fluctuation in current in the receiver's load resistance.

Time Division Multiplexer
A device which permits the simultaneous transmission of many independent channels into a single high-speed data stream by dividing the signal into successive alternate bits.

Time Division Multiplex (TDM)
The process or device by which more than one signal can be sent over a single channel by using different time intervals for the different signals. This may be done by varying the pulse duration, pulse amplitude and pulse position.

The manner in which the nodes or stations of a network are connected.

Total Internal Reflection
The total reflection that occurs when light strikes an interface at angles of incidence (with respect to the normal) greater than the critical angle.

A module composed of a receiver and transmitter.

Transmission Loss
Total loss encountered in transmission through a system.

UTP to OF converter
Unshielded twisted pair to fiber optic converter.

WAN (Wide Area Network)
A network of connected computers that covers a great geographical area.

A conducting or dielectric structure able to support and propagate one or more electromagnetic field patterns (modes).

The physical distance covered by one cycle of wave.

Wide Area Network
A data communications network designed to served an area of hundreds or thousands of miles. Public and private packet switching networks and the nation-wide telephone network are good examples of wide area networks.

Zirconium Arc
A small bulb containing a conducting gas, an arc being formed between a metal ring and a tiny zirconium electrode near the centre of the ring. The heated zirconium emits light, and constitutes a convenient laboratory point source.