Absorption: That portion of optical attenuation resulting from the conversion of optical power to heat. Caused by impurities such as hydroxyl ions and exposure to nuclear radiation.

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

Angular misalignment: The loss of optical power caused by deviation from optimum alignment of fiber-to-fiber of fiber-to-wave-guide. Loss at a connector due to fiber angles being misaligned.

AR: Anti-reflection (coating).

Attenuation: The decrease in signal strength along a fiber optic wave-guide caused by absorption and scattering. Attenuation is usually expressed in dB/km.

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 photo diode (APD): A photo diode that exhibits internal amplification of photo current through avalanche multiplication of carriers in the junction region.

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Back reflection, back scatter: The return of a portion of scattered light to the input end of the fiber; the scattering of light in the direction opposite to its original propagation.

Bandwidth: The range of frequencies within which a fiber optic wave-guide 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 specific limits.

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

Baud: 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.

Beamsplitter: 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.

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

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

Bit-error rate (BER): The fraction of bits transmitted that are received incorrectly.

Break-out kit: Used to build up the outer diameter of fiber cable for connecting.

Broadband: A method of communication where the signal is transmitted by being impressed on a high-frequency carrier.

Buffer: A protective coating applied directly to the fiber.

Bus network: A network topology in which all terminals are attached to a transmission medium serving as a bus.

Bypass: The ability of a station to isolate itself optically from a network while maintaining the continuity of the cable plant.

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Cable: One or more optical fibers enclosed within protective covering(s) and strength members.

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

CATV: Originally "Community Antenna TeleVision," now "CAble TeleVision."

CCITT: French acronym for International Consultative Committee on Telephony and Telegraphy. Now "ITU."

CCTV: Closed-circuit television.

CD: Compact disk. Often used to describe high-quality audio, CD-quality audio, or short-wavelength lasers.

Center wavelength (laser): The nominal value central operating wavelength. It is the wavelength defined by a peak mode measurement where the effective optical power resides.

Center wavelength (LED): The average of the two wavelengths measured at the half amplitude points of the power spectrum.

Central office: The nerve center of the telephone system.

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

Cladding: Material that surrounds the core of an optical fiber. The lower index of refraction in the cladding causes the transmitted light to travel down the core.

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

Cleave: 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.

Coating: The material surrounding the cladding of a fiber. It is generally a soft plastic material that protects the fiber from damage. Not to be confused with the buffer.

Concatenation: The process of connecting pieces of fiber together. Concentrator: A multiport repeater.

Connector: A mechanical or optical device that provides a demountable connection between two fibers or a fiber and a source or detector.

Connector plug: A device used to terminate an optical fiber conductor cable.

Connector receptacle: The fixed or stationary half of a connector that is mounted on a panel/bulkhead. Receptacles mate with plugs.

Connector variation: The maximum value in dB of the difference in insertion loss between mating optical connectors (e.g., with re-mating, temperature cycling, etc.). Also known as optical connector variation.

Cooled lasers: Lasers mounted with thermoelectric coolers to stabilize the wavelength and energy by keeping the laser operating temperature stable.

Core: The light-conducting central portion of an optical fiber, composed of material with a higher index of refraction than the cladding. The portion of the fiber that transmits light.

Counter-rotating: An arrangement whereby two signal paths, one in each direction, exists in a ring topology.

Coupler: An optical device that combines or splits power from optical fibers.

Coupling ratio/loss: The ratio/loss of optical power from one output port to the total output power, expressed as percent.

Cutback method: A technique of measuring optical fiber attenuation by measuring the optical power at two points at different distances from the test source.

Cutoff wavelength: In single-mode fiber, the wavelength below which the fiber ceases to be single mode.

CW (continuous wave): Usually used to refer to the constant optical output from an optical source when it is biased (i.e., turned on) but not modulated with a signal. Can yield the purest spectrum for DWDM.

CW laser: Continuously ON laser, used as a light source for external modulators or test equipment; not modulated by drive voltage or current.

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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.

Dense wavelength division multiplexing (DWDM): Multiplexing of signals by transmitting them at different wavelengths through the same fiber.

Detector: An opto-electric transducer used in fiber optics to convert optical power to electrical current. In fiber optics, usually a photo diode.

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.

Dichroic filter: An optical filter that transmits light according to wavelength, based upon wavelength interference at a filter surface.

Diffraction grating: An array of fine, parallel, equally spaced reflecting or transmitting lines that mutually enhance the effects of defraction to concentrate the diffracted light in a few directions determined by the spacing of the lines and by the wavelength of the light. Behaves similarly to a prism.

Direct modulation laser: Laser directly modulated by the electrical voltage and current.

Dispersion: The spreading out of light pulses as they travel in an optical fiber; proportional to length.

Dispersion-shifted fiber: Standard single-mode fibers exhibit optimum attenuation performance at 1550 nm and optimum bandwidth at 1300 nm. Dispersion-shifted fibers are made so that both attenuation and bandwidth are optimum at 1550 nm.

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.

Downstream laser: Sends data and information from the "head end" or source out to the distribution node; complement of return path laser.

Dual-window fiber: This term is used in two ways. For multi-mode fibers, the term means that the fiber is optimized for 850 nm and 1300 nm operations. For single-mode fibers, the term means that the fiber is optimized for 1300 nm and 1500 nm operation.

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

Duplex cable: A two-fiber cable suitable for duplex transmission. Usually two-fiber strands surrounded by a common jacket.

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

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EA modulator: Electro-absorptive optical device to electrically attenuate the laser light at microwave rates. Usable at digital data rates to over 10 GHz. Usually made of LiNbO3.

Erbium doped fiber amplifier (EDFA): Common type of optical light amplifier used in 1550 nm fiber optic systems.

ESCON (enterprise systems connection): A duplex optical connection used for computer-to-computer data exchange.

Excess loss: In a fiber optic coupler, the optical loss from that portion of light that does not emerge from the nominal operation ports of the device.

Extrinsic loss: In a fiber optic interconnection, that portion of loss not intrinsic to the fiber but related to imperfect joining of a connector or splice.

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Fabry Perot: A type of semiconductor laser diode that uses the "Fabry Perot" filter principle as the working mechanism. The Fabry Perot filter selects wavelengths using a light interference pattern produced by precisely spaced and parallel surfaces.

Faraday effect: A phenomenon that causes some materials to rotate the polarization of light in the presence of a magnetic field.

FC: A threaded optical connector that originated in Japan. Good for single-mode or multi-mode fiber and applications requiring low back reflection.

Fiber feeder service: Fibers used in "Fiber to the Curb (FTTC)" applications from the splice point to the fiber node.

Fiber optic jumper: Used to interconnect transmission equipment to the fiber optic patch panel.

Fiber test equipment: Used for the testing, maintenance, restoration, and inspection of fiber systems.

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

Fresnel reflection: The reflection that occurs at the planar junction of two materials having different refractive indices; Fresnel reflection is not a function of the angle of incidence.

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

Graded-index fiber: An optical fiber whose core has a non-uniform index of refraction. The core is composed of concentric rings of glass whose refractive indices decrease from the center axis. The purpose is to reduce modal dispersion and thereby increase bandwidth.

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Hard-clad silica fiber: A fiber with a hard plastic cladding surrounding a step-index silica core.

Head end: The central facility where signals are combined and distributed in a cable television system.

High-power laser: May be several vastly different power levels, depending upon applications. For fiber test equipment, high-power pulse lasers typically range from 60 mW to 150 mW; for direct modulation fiber communication lasers, high power is 1 mW to 5 mW; for high-power EA modulated lasers, high power is 20 mW to 30 mW.

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Index of refraction: The ratio of the speed of light in a vacuum to the speed of light in a material.

Index-matching gel: A gel or fluid with a refractive index close to glass that reduces refractive-index discontinuities that can cause reflective loss.

Infrared: Wavelengths longer than 700 nm and shorter than 1 nm. We cannot see infrared radiation but can feel it as heat. Transmission of glass optical fibers is best in the infrared at wavelengths of 1100-1600 nm. (see Light)

Injection laser: Another name for a semiconductor or laser diode.

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

Intrinsic loss: Splice loss arising from differences in the fibers being spliced.

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Junction laser: A semiconductor diode laser.

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Large-core fiber: Usually a fiber with a core of 200 µm or more.

LASER/laser diode: Light Amplification by Stimulated Emission of Radiation Ðone of the wide range of devices that generates light by that principle. Laser light is directional, covers a narrow range of wavelengths, and is more coherent than ordinary light. Semiconductor diode lasers are the standard light sources in fiber optic systems. Lasers emit light by stimulated emission.

Laser diode wavelengths: Usual standard wavelengths for Oki laser diodes are 1550 nm and 1310 nm. For WDM applications, laser diodes may be specified at different sub-wavelengths of the standard. For example 1550 nm may have subsets in 0.8 nm steps, 1310 may be specified in 1 nm steps. 0.8 nm at 1550 and 1 nm at 1310 are approximately 100 GHz spacing, in ordinary radio frequency terms. Additional laser wavelengths sold by Oki are 1625 nm "service channel" and 1480 optical pump for optical amplifiers. A new laser frequency for Oki is the 980 nm pump laser.

Launch cable: Used to connect fiber optic test equipment to the fiber system.

Light: Electromagnetic radiation visible to the human eye at 400 nm to 700 nm. The term is also applied to electromagnetic radiation with properties similar to visible light, including the invisible near-infrared radiation in most fiber optic communication systems.

Light-emitting diode (LED): A semiconductor diode that emits incoherent, but chromatically pure light at the junction between p- and n-doped materials. LEDs emit light by spontaneous emission.

Light guide: An optical fiber, fiber bundle or light conducting material.

Local loop: The part of the telephone network extending from the central (switching) office to the subscriber.

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

Loss: Attenuation of optical signal, normally measured in decibels.

Loss budget: An accounting of overall attenuation in a system.

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

Material dispersion: Pulse dispersion caused by variation of a material's refractive index with wavelength.

Mechanical splice: A splice in which fibers are joined mechanically (e.g., glued, crimped, or otherwise held in place) but not fused together.

Micro bending: Tiny bends in a fiber that allow light to leak out and thus increase loss. Micro bending is worse at longer wavelengths.

Modal dispersion: Dispersion arising from differences in the times that different modes take to travel through multi-mode fibers. Mode: An electromagnetic field distribution that satisfies theoretical requirements for propagation in a wave-guide or oscillation in a cavity (e.g., a laser). Light has modes in a fiber or laser.

Mode field diameter: The diameter of the one mode of light propagating in a single-mode fiber, typically slightly larger than the core diameter.

Mode stripper: A device that removes high-order modes in a multi-mode fiber to give standard measurement conditions. Multi-mode: Transmits or emits multiple modes of light.

Multi-mode fiber (MMF): Fiber where light propagates in many modes. Used for shorter distances; usually a larger diameter fiber of plastic or other low-cost material.

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

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

Noise equivalent power (NEP): The optical input power to a detector needed to generate an electrical signal equal to the inherent electrical noise.

Normal (angle): Perpendicular to the surface.

Numerical aperture (NA): The sine of half the angle over which a fiber can accept light. Strictly speaking, this is multiplied by the refractive index of the medium containing the light, but that equals one (1) for air, the normal medium for which NA is measured.

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OC-x (ex: OC3, OC12, OC48) optical carrier: A carrier rate specified in the SONET standards.

Optical amplifier: A device that amplifies the input optical signal without converting it to electrical form. The best developed are optical fibers doped with the rare earth element erbium.

Optical demultiplexer: A device that separates two or more optical wavelengths from a single input or fiber into multiple fibers or detectors. Similar to a prism or diffraction grading. Optical loss test set: An optical power meter and light source calibrated for use together.

Optical multiplexer: A device that combines two or more optical wavelengths into a single output or fiber. Similar to the reverse of a prism or diffraction grading.

Optical time domain reflectometer (OTDR): An optical test instrument that measures transmission characteristics by sending a short pulse of light down a fiber and observing the reflected, back scattered light.

Optical wave-guide: Technically, any structure that can guide light. Sometimes used as a synonym for optical fiber, it can also apply to planar light wave-guides.

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Packing fraction: The fraction of the surface area of a fiber-optic bundle that is fiber core.

Peak power: Highest instantaneous power level in a pulse.

Photo detector: A light detector.

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

Photonic: A term coined for devices using photons, analogous to "electronic" for devices working with electrons.

Photons: Quantum of electromagnetic radiation. A "particle" of light.

PIN diode: A diode with a large intrinsic (non-doped) region sandwiched between highly conductive p and n doped semiconducting regions. Photons (light energy) absorbed in this region creates electron-hold pairs that are then separated by an electric field, generating an electric current. The PIN diode is a an important detector type, due to its simplicity, stability and bandwidth. May be made from silicon for short/visible wavelenghts, or 3-5 material (InGaAs/InP) for IR wavelengths.

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

Planar wave-guide: A wave-guide fabricated in a flat material such as a thin film.

Plastic-clad silica (PCS) fiber: A step-index multi-mode fiber in which a silica core is surrounded by a lower index plastic cladding. Plenum cable: Cable made of fire retardant material that meets electrical code requirements (UL 910) for low smoke generation and installation in air spaces.

Polarization maintaining fiber (PMF): Fiber where light propagates predominately in one mode and maintains a fixed polarization. Used for maximum distance and bandwidth; usually a rectangular (~5 x ~9 micron) fiber of monolithic quartz.

Point-to-point transmission: Carrying a signal between two end points, without branching to other points.

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

Polarization maintaining fiber: Fiber that maintains the polarization of light that enters it.

Pulse dispersion: The spreading out of pulses as they travel along an optical fiber.

Pulse laser: Laser driven by voltage and current pulse to maximum output, then extinguished by removal of all drive. Used in OTDRs for testing fiber systems. May be cooled for stability.

Pump laser: 1480 nm or 980 nm power laser used to drive optical amplifiers by exciting the rare earth doped fiber. Usually used in pairs for reliability.

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Quantum efficiency: The fraction of photons that strike a detector which produce electron hole pairs in the output current.

Quaternary: A semiconductor compound made of four elements (e.g., InGaAsP).

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Radiometer: An instrument, distinct from a photometer, to measure power (watts) of electromagnetic radiation.

Rays: Straight lines that represent the path taken by light.

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

Recombination: Combination of an electron and a hole in a semiconductor that releases energy, sometimes leading to light emission.

Refraction: The bending of light as it passes between materials having a different refractive index.

Refractive index: Ratio of the speed of light in a vacuum to the speed of light in a material.

Refractive index gradient: The change in refractive index with respect to the distance from the axis of an optical fiber.

Regenerator: A receiver-transmitter pair that detects a weak signal, cleans it up, then sends the regenerated signal through another length of fiber. Usually separate from an optical amplifier.

Repeater: Often a receiver-transmitter pair that detects, cleans up, and amplifies a weak signal for retransmission through another length of optical fiber. Sometimes a repeater contains multiple regenerators, one for each fiber in a cable.

Responsivity: The ratio of a detector's output to input, usually measured in units of amperes per watt (or micro amperes per microwatt).

Ribbon cable: Cable in which many fibers are embedded in a plastic material in parallel, forming a flat ribbon-like structure.

Ring architecture: A network scheme in which a transmission line forms a complete ring. If the ring is broken, signals can still be sent among the terminals.

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

RPL: Return path laser. Sends data "upstream" or back to the "head end."

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Selfoc lens: A trade name used by the Nippon Sheet Glass Co. for a graded-index fiber lens. A segment of graded-index fiber made to serve as a lens.

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

Service channel: Monitor laser path of wavelength outside of data channel. Performance monitored to verify fiber status.

Sheath: An outer protective layer of a fiber optic cable.

Signal-to-noise ratio: The ratio of signal to noise, measured in decibels. An indication of the signal quality in an analog system.

Silica glass: Glass made mostly of silicon dioxide (SiO2) used in conventional optical glass.

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

Single-frequency laser: A laser that emits a range of wavelengths small enough the be considered a single frequency.

Single mode: Containing only one mode. When dealing with lasers, be aware of ambiguities because of the difference between transverse and longitudinal modes. A laser operating in a single transverse mode typically does not operate in a single longitudinal mode.

Single mode fiber (SMF): Fiber where light propagates predominately in one mode. Used for maximum distances; usually a small, precise diameter (8-9 microns) fiber of monolithic quartz.

Single polarization fibers: Optical fibers capable of carrying light in only one polarization.

Smart structures (or smart skins): Materials containing sensors (fiber optic or other types) to measure their properties during fabrication or use.

Soliton: A optical pulse that regenerates to its original shape at certain points as it travels along an optical fiber. Solitons can be combined with optical amplifiers to carry signals very long distances.

SONET (Synchronous Optical NETwork): A standard for fiber optic transmission that is part of B-ISDN.

Splice: A permanent junction between two fiber ends.

Splitting ratio: The ratio of power emerging from two output ports of a coupler.

Star coupler: A coupler with more than three ports.

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

Step-index single-mode fiber: A step-index fiber with a small core capable of carrying light in only one mode.

Submarine cable: A cable designed to be laid underwater.

Subscriber loop: The part of the telephone network from the central office to individual subscribers.

Surface-emitting diode: An LED that emits light from its flat surface rather than its side. Simple and inexpensive, with emission spread over a wide range.

Synchronous digital hierarchy (SDH): The international version of SONET.

Synchronous optical network: See SONET.

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T coupler: A coupler having three ports.

Ternary: A semiconductor compound made of three elements (e.g., GaAlAs).

III-V (3-5) semiconductor: A semiconductor compound made of one or more elements from the IIIA (3A) column of the periodic table (Al, Ga, or In) and one or more of the elements from the VA (5A) column (N, P, As, or Sb). Used in LEDs, diodes lasers and detectors. Silicon and germanium are from the IV column of the periodic table.

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

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

Transverse modes: Modes across the width of a wave-guide (e.g., a fiber or laser). Distinct from longitudinal modes, which are along the length.

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

Trunk line: A transmission line running between telephone switching offices.

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Ultraviolet: Electromagnetic waves invisible to the human eye, with wavelengths about 100 nm - 400 nm. Considered too short to be visible to the human eye. 350 nm - 400 nm is considered soft-UV (see Light).

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Visible light: Electromagnetic radiation visible to the human eye from wavelengths of 400 (blue) - 700 nm (red) (see Light).

Voice circuit: A circuit capable of carrying one telephone conversation or its equivalent. The standard subunit in which telecommunications capacity is counted. The U.S. analog equivalent is 4 kHz. The digital equivalent is 56 kbits/s in North America and 64 kbits/s in Europe.

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Wave guide: A structure that guides electromagnetic waves along its length. An optical fiber is an optical wave-guide.

Wave guide dispersion: The part of chromatic dispersion arising from the different speeds light travels in the core and cladding of a single-mode fiber (i.e., from the fiber's wave-guide structure).

Wavelength: The distance an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Wavelengths of light are measured in nanometers (nm) or micrometers (microns).

Wavelength division multiplexing (WDM): Multiplexing of signals by transmitting them at different wavelengths through the same fiber.

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Y coupler: A variation on the "T" coupler in which input light is split between two channels that branch out like a "Y" from the input.

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Zero dispersion wavelength: A wavelength at which net chromatic dispersion of an optical fiber is nominally zero. This arises when wave-guide dispersion cancels out material dispersion.