Dictionary of Optical Terms

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    ANSIIIEEEStd 812-1984

    An American National StandardIEEE Standard Definitions of

    Terms Relating to Fiber Optics

    SponsorFiber Optics Committee of theIEEE Communications Society

    Approved June 10,1982IEEE Standards Board

    Approved July 30,1985American National Standards Institute

    @ Copyright 1984by

    The Institute of Electrical and Electronics Engineers, Inc345 East 47th Street, New York,NY 10017, USANo part of this publication ma y be reproduced in anyfm,in an electronic retrieval systemOT othenuise,withuut prior written permission of the publisher.

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    IEEE Standards documents are developed within the Technical Commit-tees of the IEEE Societies and the Standards Coordinating Committees ofthe IEEE Standards Board. Members of the committees serve voluntarilyand without compensation. They are not necessarily members of the In-stitute. The standards developed within IEEE represent a consensus of thebroad expertise on the subject within the Institute as well as those activi-ties outside of IEEE which have expressed an interest in participating inthe development of the standard.

    Use of an IEEE Standard is wholly voluntary. The existence of an IEEEStandard does not imply that there are no other ways to produce, test,measure, purchase, market, or provide other goods and services related tothe scope of the IEEE Standard. Furthermore, the viewpoint expressed atthe time a standard is approved and issued is subject to change broughtabout through developments in the state of the art and comments receivedfrom users of the standard. Every IEEE Standard is subjected to review atleast once every five years for revision or reaffirmation. When a documentis more than five years old, and has not been reaffirmed, it is reasonable toconclude that its contents, although still of some value, do not whollyreflect the present state of the art. Users are cautioned to check to deter-mine that they have the latest edition of any IEEE Standard.

    Comments for revision of IEEE Standards are welcome from any inter-ested party, regardless of membership affiliation with IEEE. Suggestionsfor changes in documents should be in the form of a proposed change oftext, together with appropriate supporting comments.

    Interpretations: Occasionally questions may arise regarding the meaningof portions of standards as they relate to specific applications. When theneed for interpretations is brought to the attention of IEEE, the Institutewill initiate action to prepare appropriate responses. Since IEEE Standardsrepresent a consensus of all concerned interests, it is important to ensurethat any interpretation has also received the concurrence of a balance ofinterests. For this reason IEEE and the members of its technical commit-tees are not able to provide an instant response to interpretation requestsexcept in those cases where the matter has previously received formalconsideration.

    Comments on standards and requests for interpretations should be ad-dressed to: Secretary, IEEE Standards Board345 East 47th StreetNew York, NY 10017


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    Foreword(This Foreword is not a part of IEEE Std 812-1984, IEEE Standard Definitions of Terms Relating to Fiber Optics.)This document is based on the NTIA-SP-794 Optical Waveguide Communications Glossary, issued inSeptember 1979 by the U.S. Department of Commerce; its updated version issued in the latter part of

    1981; and the input from Project 812 Ad Hoc Working Group in April 1981. The contributors to the finaldocument include A. G. Hanson, L. R. Bloom, R. L. Galawa, E. M. Gray, G. W. Day, M. Young, A. H.Cherin, T. L. Gower, C. K. Kao, F. P. Kapron, B. S. Kawasaki, R. L. Lebduska, R. E. Love, P. Reitz,M. Kincaid, J. G. Nault, F. Sladen, J. Masterson, J. Fridman, D. Stone, and G. P. Kurpis.

    The final draft (# 2 ) of this document was accepted by the Fiber Optics Subcommittee of the Trans-mission Systems Committee of the IEEE Communications Society, and submitted to the IEEE StandardsBoard for approval. This project has been coordinated with the following outside organizations: SAE,ASTM, EIA and IEC.Suggestions for improvement of this standard will be welcomed. They should be sent to theSecretaryIEEE Standards BoardInstitute of Electrical and Electronics Engineers345 East 47th StNew York, New York 10017.At the time this standard was approved in committee, the Fiber Optics Subcommittee (presently theOptical Communications Committee of the IEEE Communications Society) had the following mem-bership:

    J. G. Nault, ChairmanB. BaschK. ChangG. DayJ. FridmanD.HannaA. Hudson

    N . KarlovacM. KincaidG . P. KurpisH. M a a sB. McNicholS. Personick

    W. SchumacherJ. SheppardJ. Sipress0. Szentesi0. Wanaseua

    When the IEEE Standards Board approved this standard on June 10, 1982, it had the following mem-bership:Irvin N. Howell, Jr., Chairman Irving Kolodny, Vice Chairman

    Sava I . Sherr, SecretargG . Y. R. Allen Len S. Corey Donald T. Michael*J. J. Archambault Donald C. Fleckenstein A. R. ParsonsJames H. B e d Jay Forster J. P. RiganatiJohn T. Boettger Kurt Greene Robert W. SeelbachJ. V. Bonucchi Joseph L. Koepfinger Jay A. StewartEdward Chelotti John E. May Clifford 0. SwansonEdward J. Cohen Robert E. Weiler

    *Member emeritus

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  • 8/3/2019 Dictionary of Optical Terms


    IEEE Standard Definitions of TermsRelating toabsorption. In an optical waveguide, that portionof attenuation resulting from conversion ofoptical power into heat. Note: Intrinsic compo-nents consist of tails of the ultraviolet andinfrared absorption bands. Extrinsic componentsinclude impurities: for example, the OH- ion ,andtransition metal ions; and defects: for example,results of thermal history and exposure tonuclear radiation. See: attenuation.acceptance angle. Half the vertex angle of thatcone within which optical power may be coupledinto bound modes of an optical waveguide.Notes: (1) Acceptance angle is a function of posi-tion on the entrance face of the core when therefractive index is a function of radius in thecore. In that case, the local acceptance angle isarcsin dn2(r> - nlwheren ( r ) = local refractive indexn2= minimum refractive index of the claddingThe sine of the local acceptance angle is some-times referred to as the local numerical aperture.(2) Power may be coupled into leaky modes atangles exceeding the acceptance angle. See:launch numerical aperture; power-law indexprofile.access coupler. A device placed between twowaveguide ends to allow signals to be withdrawnfrom or entered into one of the waveguides. See:optical waveguide coupler.acousto-optic effect. A periodic variation ofrefractive index caused by an acoustic wave.Note: The acousto-optic effect is used in devicesthat modulate and deflect light. See: modulation.

    Fiber Opticsactive laser medium. The material within alaser, such as crystal, gas, glass, liquid, or semi-conductor, that emits coherent radiation (orexhibits gain) as the result of stimulated elec-tronic or molecular transitions to lower energystates. Syn: laser medium. See: laser; opticalcavity.aligned bundle. A bundle of optical fibers inwhich the relative spatial coordinates of eachfiber are the same at the two ends of the bundle.Note: The term coherent bundle is oftenemployed as a synonym, and should not be con-fused with phase coherence or spatial coherence.Syn: coherent bundle. See: fiber bundle.alpha profile. See: power-law index profile.angle of deviation. In optics, the net angulardeflection experienced by a light ray after one ormore refractions or reflections. Note: The term isgenerally used in reference to prisms, assumingair interfaces. The angle of deviation is then theangle between the incident ray and the emergentray. See: reflection; refraction.angle of incidence. The angle between an inci-dent ray and the normal to a reflecting or refract-ing surface. See: critical angle; total internalreflection.angstrom (A). A unit of optical wavelength(obsolete).1 A = mNote: The angstrom has been used historically inthe field of optics, but it is not an SI (Interna-tional System) unit.angular misalignment loss. The optical power


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    loss caused by angular deviation from theoptimum alignment of source to optical wave-guide, waveguide to waveguide, or waveguide todetector. See: extrinsic joint loss; gap loss;intrinsic joint loss; lateral offset loss.anisotropic. Pertaining to a material whose elec-trical or optical properties are different for differ-ent directions of propagation or differentpolarizations of a traveling wave. See: isotropic.antireflection coating. A thin, dielectric ormetallic film (or several such films) applied to anoptical surface to reduce the reflectance andthereby increase the transmittance. Note: Theideal value of the refractive index of a singlelayered film is the square root of the product ofthe refractive indices on either side of the film,the ideal optical thickness being one quarter of awavelength. See: dichroic filter; Fresnel reflec-tion; reflectance; transmittance.APD. See: avalanche photodiode.attenuation. In an optical waveguide, the dimi-nution of average optical power. Note: In opticalwaveguides, attenuation results from absorption,scattering, and other radiation. Attenuation isgenerally expressed in decibels (dB). However,attenuation is often used as a synonym forattenuation coefficient, expressed as dB/km. Thisassumes the attenuation coefficient is invariantwith length. See: attenuation coefficient; cou-pling loss; differential mode attenuation; equi-librium mode distribution; extrinsic joint loss;leaky modes; macrobend loss; material scat-tering; microbend loss; Rayleigh scattering;spectral window; transmission loss; waveguidescattering.attenuation coefficient. The rate of diminutionof average optical power with respect t o distancealong the waveguide. Defined by the equation

    whereP(z) = power at distance z along the guideP(0) = power at z = 0a = attenuation coefficient in dB/km ifz is inkm. From this equation,

    az = -10 log,, -E;]

    This assumes that a is independent of z; if other-wise, the definition shall be given in terms ofincremental attenuation as:

    or, equivalently,

    See: attenuation; attenuation constant; axialpropagation constant.attenuation constant. For a particular mode,the real part of the axial propagation constant.The attenuation coefficient for the mode poweris twice the attenuation constant. See: attenua-tion coefficient; axial propagation constant;propagation constant.attenuation-limited operation. The conditionprevailing when the received signal amplitude(rather than distortion) limits performance. See:bandwidth-limited operation; distortion-lim-ited operation.avalanche photodiode (APD). A photodiodedesigned to take advantage of avalanche multi-plication of photocurrent. Note: As the reverse-bias voltage approaches the breakdown voltage,hole-electron pairs created by absorbed photonsacquire sufficient energy to create additionalhole-electron pairs when they collide with ions;thus a multiplication (signal gain) is achieved.See: photodiode; PIN photodiode.axial propagation constant. The propagationconstant evaluated along the axis of a waveguide(in the direction of transmission). Note: The realpart of the axial propagation constant is theattenuation constant while the imaginary part isthe phase constant. Syn: axial propagationwave number. See: attenuation; attenuationcoefficient; attenuation constant; propagationconstant.axial propagation wave number. See: axialpropagation constant.axial ray. A light ray that travels along theoptical axis. See: geometric optics; fiber axis;meridional ray; paraxial axis; skew ray.


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    axial slab interferometry. See: slab inter- beamwidth. See: beam diameter.ferometry.backscattering. The scattering of light into adirection generally reverse to the original one.See: Rayleigh scattering; reflectance; reflec-tion. birefringence. See: birefringent medium.

    bidirectional transmission. Signal transmissionin both directions along an optical waveguide orother component.

    bandpass filter. See: optical filter.bandwidth. See: fiber bandwidth.bandwidth-limited operation. The conditionprevailing when the system bandwidth, ratherthan the amplitude (or power) of the signal, lim-its performance. The condition is reached whenthe system distorts the shape of the waveformbeyond specified limits. For linear systems, band-width-limited operation is equivalent to distor-tion-limited operation. See: attenuation-limitedoperation; distortion-limited operation; linearoptical element.barrier layer. In the fabrication of an opticalfiber, a layer that can be used to create a bound-ary against OH- ion diffusion into the core. See:core.baseband response function. See: transferfunction (of a device).beam diameter. The distance between twodiametrically opposed points at which the irra-diance is a specified fraction of the beams peakirradiance; most commonly applied to beams thatare circular or nearly circular in cross section.Syn: beamwidth. See: beam divergence.beam divergence. (1) For beams that are cir-cular or nearly circular in cross section, theangle subtended by the far-field beam diameter.(2) For beams that are not circular or nearly cir-cular in cross section, the far-field angle sub-tended by two diametrically opposed points in aplane perpendicular to the optical axis, at whichpoints the irradiance is a specified fraction of thebeams peak irradiance. Generally, only the max-imum and minimum divergences (correspondingto the major and minor diameters of the far-fieldirradiance) need be specified. See: beam diame-ter; collimation; far-field region.beamsplitter. A device for dividing an opticalbeam into two or more separate beams; often apartially reflecting mirror.

    birefringent medium. A material that exhibitsdifferent indices of refraction for orthogonal lin-ear polarizations of the light. The phase velocityof a wave in a birefringent medium thus dependson the polarization of the wave. Fibers mayexhibit birefringence. See: refractive index (of amedium).blackbody. A totally absorbing body (whichreflects no radiation). Note: In thermal equilib-rium, a blackbody absorbs and radiates at thesame rate; the radiation will just equal absorptionwhen thermal equilibrium is maintained. See:emissivity.bolometer. A device for measuring radiantenergy by measuring the changes in resistance ofa temperature-sensitive device exposed to radia-tion. See: radiant energy; radiometry.Boltzmanns constant. The number k thatrelates the average energy of a molecule to theabsolute temperature of the environment. k isapproximately 1.38 . J /K.bound mode. In an optical waveguide, a modewhose field decays monotonically in the trans-verse direction everywhere external to the coreand which does not lose power to radiation. Spe-cifically a mode for whichn(a)k s p =sn(0)kwherep = imaginary part (phase constant) of the

    axial propagation constantn(a ) = refractive index at r = athe core radius n(0) = refractive index atr = Ok = free-space wavenumber 2 r / AA = wavelength.

    Bound modes correspond to guided rays in theterminology of geometric optics. Note: Except ina monomode fiber, the power in bound modes ispredominantly contained in the core of the fiber.Syn: guided mode; trapped mode. See: cladding


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    mode; guided ray; leaky mode; mode; nor-malized frequency; unbound mode.bound ray. See: guided ray.Brewsters angle. For light incident on a planeboundary between two regions having differentrefractive indices, that angle of incidence atwhich the reflectance is zero for light that has itselectric field vector in the plane defined by thedirection of propagation and the normal to thesurface. For propagation from medium 1 tomedium 2, Brewsters angle isarctan (n,ln,)See: angle of incidence; reflectance; refractiveindex (of a medium).

    brightness. An attribute of visual perception, inaccordance with which a source appears to emitmore or less light; obsolete. Notes: (1) Usageshould be restricted to nonquantitative referenceto physiological sensations and perceptions oflight. (2) Brightness was formerly used as a syn-onym for the photometric term l u m i n an c e and(incorrectly) for the radiometric term r ad i an c e .See: radiance; radiometry.buffer. See: fiber buffer.bundle. See: fiber bundle.cable. See: optical cable.cable assembly. See: multifiber cable; opticalcable assembly.cavity. See: optical cavity.chemical vapor deposition (CVD) technique. Aprocess in which deposits are produced by heter-ogeneous gas-solid and gas-liquid chemical reac-tions at the surface of a substrate. Note: TheCV D method is often used in fabricating opticalwaveguide preforms by causing gaseous mate-rials to react and deposit glass oxides. Typicalstarting chemicals include volatile compounds ofsilicon, germanium, phosphorus, and boron,which form corresponding oxides after heatingwith oxygen or other gases. Depending upon itstype, the preform may be processed further inpreparation for pulling into an optical fiber. See:preform.

    chirping. A rapid change (as opposed to long-term drift) of the emission wavelength of anoptical source. Chirping is often observed inpulsed operation of a source.chromatic dispersion. See: dispersion.cladding. The dielectric material surrounding thecore of an optical waveguide. See: core; nor-malized frequency; optical waveguide; toler-ance field.cladding center. The center of the circle thatcircumscribes the outer surface of the homoge-neous cladding, as defined under tolerance field.See: cladding; tolerance field.cladding diameter. The length of the longestchord that passes through the fiber axis and con-nects two points on the periphery of the homoge-neous cladding. See: cladding; core diameter;tolerance field.cladding mode. A mode that is confined by vir-tue of a lower index medium surrounding thecladding. Cladding modes correspond to claddingrays in the terminology of geometric optics. See:bound mode; cladding ray; leaky mode; mode;unbound mode.cladding mode stripper. A device that encour-ages the conversion of cladding modes to radia-tion modes; as a result, the cladding modes arestripped from the fiber. Often a material having arefractive index equal to or greater than that ofthe waveguide cladding. See: cladding; claddingmode.cladding ray. In an optical waveguide, a ray thatis confined to the core and cladding by virtue ofreflection from the outer surface of the cladding.Cladding rays correspond to cladding modes inthe terminology of mode descriptors. See: clad-ding mode; guided ray; leaky ray.coherence area. The area in a plane perpen-dicular to the direction of propagation overwhich light may be considered highly coherent.Commonly the coherence area is the area overwhich the degree of coherence exceeds 0.88. See:coherent; degree of coherence.coherence length. The propagation distanceover which a light beam may be considered


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    TERMS RELATING TO FIBER OPTICS IEEEStd 812-1984coherent. If the spectral linewidth of the sourceis AA and the central wavelength is A,,, the coher-ence length in a medium of refractive index n isapproximately Azln AX See: degree of coher-ence; spectral width.coherence time. The time over which a propaga-ting light beam may be considered coherent. It isequal to coherence length divided by the phasevelocity of light in a medium; approximatelygiven by A$/cAXwhere

    A = central wavelengthAA = spectral linewidthc = velocity of light in vacuumSee: coherence length; phase velocity.

    coherent. Characterized by a fixed phase rela-tionship between points on an electromagneticwave. Note: A truly monochromatic wave is per-fectly coherent at all points in space. In practice,however, the regioyi of high coherence mayextend only a finite distance. The area on thesurface of a wavefront over which the wave maybe considered coherent is called the coherencearea or coherence patch; if the wave has anappreciable coherence area, it is said to be spa-tially coherent over that area. The distance paral-lel to the wave vector along which the wave maybe considered coherent is called the coherencelength; if the wave has an appreciable coherencelength, it is said to be phase or length coherent.The coherence length divided by the velocity oflight in the medium is known as the coherencetime; hence a phase coherent beam may also becalled time (or temporally) coherent. See: coher-ence area; coherence length; coherence time;degree of coherence; monochromatic.coherent bundle. See: aligned bundle.coherent radiation. See: coherent,,collimation. The process by which a divergentor convergent beam of radiation is convertedinto a beam with the minimum divergence possi-ble for that system (ideally, a parallel bundle ofrays). See: beam divergence.concatenation (of optical waveguides). Thelinking of optical waveguides, end to end.concentricity error. When used in conjunctionwith a tolerance field to specify core/cladding

    geometry, the distance between the center of thetwo concentric circles specifying the claddingdiameter and the center of the two concentriccircles specifying the core diameter. See: clad-ding; cladding diameter; core; core diameter;tolerance field.connector. See: optical waveguide connector.connector insertion loss. See: insertion loss.conservation of radiance. A basic principle stat-ing that no passive optical system can increasethe quantity L n ?whereL = radiance of a beamn = local refractive indexFormerly called conservation of brightness orthe brightness theorem. See: brightness; radi-ance.core. The central region of an optical waveguidethrough which light is transmitted. See: cladding;normalized frequency; optical waveguide.core area. The cross sectional area enclosed bythe curve that connects all points nearest theaxis on the periphery of the core where therefractive index of the core exceeds that of thehomogeneous cladding by k times the differencebetween the maximum refractive index in thecore and the refractive index of the homogene-ous cladding, where k is a specified positive ornegative constant I k I < 1.See: cladding; core;homogeneous cladding; tolerance field.core center. A point on the fiber axis. See: fiberaxis; optical axis.core diameter. The diameter of the circle thatcircumscribes the core area. See: cladding; core;core area; tolerance field.cosine emission law. See: Lamberts cosinelaw.coupled modes. Modes whose energies areshared. See: mode.coupler. See: optical waveguide coupler.coupling. See: mode coupling.


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    IEEEStd 812-1984

    coupling efficiency. The efficiency of opticalpower transfer between two optical components.See: coupling loss.coupling loss. The power loss suffered whencoupling light from one optical device to another.See: angular misalignment loss; extrinsic jointloss; gap loss; insertion loss; intrinsic jointloss; lateral offset loss.critical angle. When light propagates in a homo-geneous medium of relatively high refractiveindex (nhig3onto a planar interface with ahomogeneous material of lower index (nlOw),hecritical angle is defined byarcsin (nlow/nhig,JNote: When the angle of incidence exceeds thecritical angle, the light is totally reflected by theinterface. This is termed total internal reflection.See: acceptance angle; angle of incidence;reflection; refractive index (of a medium); stepindex profile; total internal reflection.curvature loss. See: macrobend loss.cutback technique. A technique for measuringfiber attenuation or distortion by performing twotransmission measurements. One is at the outputend of the full length of the fiber. The other iswithin 1 m-3 m of the input end, access beinghad by Gutting back the test fiber. See: attenua-tion.cutoff wavelength. That wavelength greaterthan which a particular waveguide mode ceasesto be a bound mode. Note: In a single modewaveguide, concern is with the cutoff wavelengthof the second order mode. See: mode.CVD. See: chemical vapor deposition.D* (pronounced D-star). A figure of meritoften used to characterize detector performance,defined as the reciprocal of noise equivalentpower (NEP), normalized to unit area and unitbandwidth.D* = d A m / N E PwhereA = area of the photosensitive region of thedetector

    (Af) = effective noise bandwidth


    S y n : specific detectivity. See: detectivity; noiseequivalent power.dark current. The external current that, underspecified biasing conditions, flows in a photosen-sitive detector when there is no incident radia-tion.degree of coherence. A measure of the coher-ence of a light source; the magnitude of thedegree of coherence is equal to the visibility V ofthe fringes of a two-beam interference experi-mentwhere

    V = m a x - m i nI m a x + I m i n

    Imax intensity at a maximum of the inter-ference patternImin intensity at a minimum

    Note: Light is considered highly coherent whenthe degree of coherence exceeds 0.88, partiallycoherent for values less than 0.88, and incoherentfor very small values. See: coherence area;coherence length; coherent; interference.density. See: optical density.detectivity. The reciprocal of noise equivalentpower (NEP). See: noise equivalent power(NEP).dichroic filter. An optical filter designed totransmit light selectively according to wavelength(most often, a high-pass or low-pass filter). See:optical filter.dichroic mirror. A mirror designed to reflectlight selectively according to wavelength. See:dichroic filter.dielectric filter. See: interference filter.differential mode attenuation. The variation inattenuation among the propagating modes of anoptical fiber.differential mode delay. The variation in propa-gation delay that occurs because of the differentgroup velocities of the modes of an optical fiber.S y n : multimode group delay. See: group veloc-ity; mode; multimode distortion.

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    differential quantum efficiency. In an opticalsource or detector, the slope of the curve relat-ing output quanta to input quanta.diffraction. The deviation of a wavefront fromthe path predicted by geometric optics when awavefront is restricted by an opening or an edgeof an object. Note: Diffraction is usually mostnoticeable for openings of the order of a wave-length. However, diffraction may still be impor-tant for apertures many orders of magnitudelarger than the wavelength. See: far-field diffrac-tion pattern; near-field diffraction pattern.diffraction grating. An array of fine, parallel,equally spaced reflecting or transmitting linesthat mutually enhance the effects of diffractionto concentrate the diffracted light in a few direc-tions determined by the spacing of the lines andthe wavelength of the light. See: diffraction.diffraction limited. A beam of light is diffrac-tion limited if: (1) the far-field beam divergence isequal to that predicted by diffraction theory, or(2) in focusing optics, the impulse response orresolution limit is equal to that predicted bydiffraction theory. See: beam divergence angle;diffraction.diffuse reflection. See: reflection.diode laser. See: injection laser diode (ILD).directional coupler. See: tee coupler.dispersion. A term used to describe the chro-matic or wavelength dependence of a parameteras opposed to the temporal dependence which isreferred to as distortion. The term is used, forexample, to describe the process by which anelectromagnetic signal is distorted because thevarious wavelength components of that signalhave different propagation characteristics. Theterm is also used to describe the relationshipbetween refractive index and wavelength. Note:Signal distortion in an optical waveguide iscaused by several dispersive mechanisms: wave-guide dispersion, material dispersion, and profiledispersion. In addition, the signal suffers degra-dation from multimode dis tor t ion , which is often(erroneously) referred to as multimode disper-sion. Syn: chromatic dispersion (Redundant).See: distortion; intramodal distortion; materialdispersion; material dispersion parameter;multimode distortion; profile dispersion; pro-

    file dispersion parameter; waveguide disper-sion.distortion. A change of signal waveform shape.Note: In a multimode fiber, the signal can sufferdegradation from multimode distortion. In addi-tion, several dispersive mechanisms can causesignal distortion in an optical waveguide: wave-guide dispersion, material dispersion, and profiledispersion. See: dispersion; profile dispersion.distortion-limited operation. The condition pre-vailing when the distortion of the received signal,rather than its amplitude (or power), limits per-formance. The condition is reached when thesystem distorts the shape of the waveformbeyond specified limits. For linear systems,distortion-limited operation is equivalent tobandwidth-limited operation. See: attenuation-limited operation; bandwidth-limited opera-tion; distortion; multimode distortion.divergence. See: beam divergence.double crucible method. A method of fabricat-ing an optical waveguide by melting core andclad glasses in two suitably joined concentriccrucibles and then drawing a fiber from the com-bined melted glass. See: chemical vapor deposi-tion technique.D-star. See: D*.effective mode volume. The square of the prod-uct of the diameter of the near-field pattern andthe sine of the radiation angle of the far-field pat-tern. The diameter of the near-field radiation pat-tern is defined here as the full width at halfmaximum and the radiation angle at half max-imum intensity. Note: Effective mode volume isproportional to the breadth of the relative distri-bution of power amongst modes in a multimodefiber. It is not truly a spatial volume but ratheran optical volume equal to the product of areaand solid angle. See: mode volume; radiationpattern.electroluminescence. Nonthermal conversion ofelectrical energy into light. One example is thephoton emission resulting from electron-holerecombination in a p n junction such as in a lightemitting diode. See: injection laser diode.electro-optic effect. A change in the refractiveindex of a material under the influence of an


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    electric field. Notes: (1) Pockels and Kerr effectsare electro-optic effects that are respectively lin-ear and quadratic in the electric field strength.(2) Electro-optic is often erroneously used as asynonym for optoelectronic. See: optoelectronic.emissivity. The ratio of power radiated by a sub-stance to the power radiated by a blackbody atthe same temperature. Emissivity is a function ofwavelength and temperature. See: blackbody.equilibrium coupling length. See: equilibriumlength.equilibrium length. For a specific excitationcondition, the length of multimode optical wave-guide necessary to attain equilibrium mode distri-bution. Note: The term is sometimes used torefer to the longest such length, as would resultfrom a worst-case, but undefined excitation. Syn:equilibrium coupling length; equilibriummode distribution length. See: equilibriummode distribution; mode coupling.equilibrium mode distribution. The conditionin a multimode optical waveguide in which therelative power distribution among the propagat-ing modes is independent of length. Syn: steady-state condition. See: equilibrium length; mode;mode coupling.equilibrium mode distribution length. See:equilibrium length.equilibrium mode simulator. A device oroptical system used to create an approximationof the equilibrium mode distribution. See: equi-librium mode distribution; mode filter.evanescent field. A time varying electromagneticfield whose amplitude decreases monotonically,but without an accompanying phase shift, in aparticular direction is said to be evanescent inthat direction.excess insertion loss. In an optical waveguidecoupler, the optical loss associated with that por-tion excess insertion loss. In an optical wave-guide coupler, the optical loss associated withthat portion of the light which does not emergefrom the nominally operational ports of thedevice. See: optical waveguide coupler.extrinsic joint loss. That portion of joint lossthat is not intrinsic to the fibers (that is, loss

    caused by imperfect jointing). See: angular mis-alignment loss; gap loss; intrinsic joint loss;lateral offset loss.far-field diffraction pattern. The diffraction pat-tern of a source (such as a light emitting diode[LED], injection laser diode [ILD], or the outputend of an optical waveguide) observed at aninfinite distance from the source. Theoretically, afar-field pattern exists at distances that are largecompared with s 2 / A

    wheres = characteristic dimension of the sourceA = wavelength

    Example: If the source is a uniformly illuminatedcircle, then s is the radius of the circle. Note: Thefar-field diffraction pattern of a source may beobserved at infinity or (except for scale) in thefocal plane of a well-corrected lens. The far-fieldpattern of a diffracting screen illuminated by apoint source may be observed in the image planeof the source. Syn: Fraunhofer diffraction pat-tern. See: diffraction; diffraction limited.far-field pattern. See: far-field radiation pat-tern.far-field radiation pattern. See: radiation pat-tern.far-field region. The region, fa r from a source,where the diffraction pattern is substantially thesame as that at infinity. See: far-field diffractionpattern.FDHM. Abbreviation for ful l duration at halfmaximum. See: full width (duration) half max-imum.ferrule. A mechanical fixture, generally a rigidtube, used to confine the stripped end of a fiberbundle or a fiber. Notes: (1) Typically, individualfibers of a bundle are cemented together within aferrule of a diameter designed to yield a max-imum packing fraction. (2) Nonrigid materialssuch as shrink tubing may also be used for fer-rules for special applications. See: fiber bundle;packing fraction; reference surface.FET photodetector. A photodetector employingphotogeneration of carriers in the channel regionof a field-effect transistor (FET) structure to pro-


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    vide photodetection with current gain. See: pho-tocurrent; photodiode.fiber. See: optical fiber.fiber axis. The line connecting the centers of thecircles that circumscribe the core, as definedunder tolerance field. S y n : optical axis. See:tolerance field.fiber bandwidth. The lowest frequency at whichthe magnitude of the fiber transfer functiondecreases to a specified fraction of the zero fre-quency value. Often, the specified value is one-half the optical power at zero frequency. See:transfer function.fiber buffer. A material that may be used to pro-tect an optical fiber waveguide from physicaldamage, providing mechanical isolation or pro-tection or both. Note: Cable fabrication tech-niques vary, some resulting in firm contactbetween fiber and protective buffering, othersresulting in a loose fit, permitting the fiber toslide in the buffer tube. Multiple buffer layersmay be used for added fiber protection. See: fiberbundle.fiber bundle. An assembly of unbuffered opticalfibers. Usually used as a single transmissionchannel, as opposed to multifiber cables, whichcontain optically and mechanically isolatedfibers, each of which provides a separate chan-nel: Notes: (1) Bundles used only to transmitlight, as in optical communications, are flexibleand are typically unaligned. (2) Bundles used totransmit optical images may be either flexible orrigid, but shall contain aligned fibers. See:aligned bundle; ferrule; fiber optics; multifibercable; optical cable; optical fiber; packingfraction.fiber optics (FO). The branch of optical tech-nology concerned with the transmission of radi-ant power through fibers made of transparentmaterials such as glass, fused silica, or plastic.Notes: (1) Telecommunication applications offiber optics employ flexible fibers. Either a singlediscrete fiber or a nonspatially aligned fiber bun-dle may be used for each information channel.Such fibers are often referred to as opt ical wav e-guides to differentiate from fibers employed innoncommunications applications. ( 2 ) Variousindustrial and medical applications (typicallyhigh-loss) flexible fiber bundles in which individ-

    ual fibers are spatially aligned, permitting opticalrelay of an image. An example is the endoscope.(3) Some specialized industrial applicationsemploy rigid (fused) aligned fiber bundles forimage transfer. An example is the fiber opticsfaceplate used on some high-speed oscilloscopes.flux.* Synonym for radiant power.*Obsolete

    Fraunhofer diffraction pattern. See: far-fielddiffraction pattern.frequency response. See: transfer function (ofa device).Fresnel diffraction pattern. See: near-fielddiffraction pattern.Fresnel reflection. The reflection of a portion ofthe light incident on a planar interface betweentwo homogeneous media having different refrac-tive indices. Notes: (1) Fresnel reflection occursat the air-glass interfaces at entrance and exitends of an optical waveguide. Resultant transmis-sion losses (on the order of 4% per interface) canbe virtually eliminated by use of antireflectioncoatings or index matching materials. (2) Fresnelreflection depends upon the index difference andthe angle of incidence; it is zero at Brewstersangle for one polarization. In optical elements, athin transparent film is sometimes used to givean additional Fresnel reflection that cancels theoriginal one by interference. This is called anantireflection coating. See: antireflection coat-ing; Brewsters angle; index matching mate-rial; reflectance; reflection; refractive index.Fresnel reflection method. The method formeasuring the index profile of an optical fiber bymeasuring the reflectance as a function of posi-tion on the end face. See: Fresnel reflection;index profile; reflectance.full width (duration) half maximum. A meas-ure of the extent of a function. Given by the dif-ference between the two extreme values of theindependent variable at which the dependentvariable is equal to half of its maximum value.The term du r a t i on is preferred when the inde-pendent variable is time. Note: Commonlyapplied to the duration of pulse waveforms, thespectral extent of emission or absorption lines,and the angular or spatial extent of radiation pat-terns.


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    fundamental mode. The lowest order mode of awaveguide. In fibers, the mode designated LP,, orHE,. See: mode.fused quartz. Glass made by melting naturalquartz crystals; not as pure as vitreous silica. See:vitreous silica.fused silica. See: vitreous silica; fused quartz.fusion splice. A splice accomplished by theapplication of localized heat sufficient to fuse ormelt the ends of two lengths of optical fiber,forming a continuous, single fiber.FWHM. See: full width (dura tion) half max-imum.gap loss. That optical power loss caused by aspace between axially aligned fibers. Note: Forwaveguide-to-waveguide coupling, it is commonlycalled "longitudinal offset loss." See: couplingloss.Gaussian beam. A beam of light whose electricfield amplitude distribution is Gaussian. Whensuch a beam is circular in cross section, theamplitude is

    wherer = distance from beam centerw = radius at which the amplitude is l / e of itsvalue on the axisw = the beamwidth

    See: beam diameter.Gaussian pulse. A pulse that has the waveformof a Gaussian distribution. In the time domain,the waveform is

    whereA is a constant and a is the pulse half duration

    at the l / e points. See: full width (dura tion) halfmaximum.geometric optics. The treatment of propagationof light as rays. Note: Rays are bent at the inter-face between two dissimilar media or may becurved in a medium in which refractive index isa function of position. See: axial ray; merid-


    ional ray; optical axis; paraxial ray; physicaloptics; skew ray.graded index optical waveguide. A waveguidehaving a graded index profile in the core. See:graded index profile; step index optical wave-guide.graded index profile. Any refractive index pro-file that varies with radius in the core. Distin-guished from a step index profile. See:dispersion; mode volume; multimode opticalwaveguide; normalized frequency; opticalwaveguide; parabolic profile; profile disper-sion; profile parameter; refractive index; stepindex profile; power-law index profile.group index (denoted N ) . For a given modepropagating in a medium of refractive index n,the velocity of light in vacuum c divided by thegroup velocity of the mode. For a plane wave ofwavelength A, it is related thus to the refractiveindex:N = n - A(dn/dA)See: group velocity; material dispersion pa-rameter.group velocity. ( 1 ) for a particular mode, thereciprocal of the rate of change of the phase con-stant with respect to angular frequency. Note:The group velocity equals the phase velocity ifthe phase constant is a linear function of theangular frequency. (2) Velocity of the signal mod-ulating a propagating electromagnetic wave. See:differential mode delay; group index; phasevelocity.guided mode. See: bound mode.guided ray. In an optical waveguide, a ray that iscompletely confined to the core. Specifically, aray at radial position r having direction so that0 s sin 8 (r) 6 n"(r) - n 2 (U)]*whereO(r) = the angle ray makes with the wave-

    guide axisn(r) = refractive indexn(a) = refractive index at the core radius

    Guided rays correspond to bound (or guided)modes in the terminology of mode descriptors.

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    Syn: bound ray; trapped ray. See: bound mode;leaky ray.HE, mode. Designation for the fundamentalmode of an optical fiber. See: fundamentalmode.heterojunction. A junction between semiconduc-tors that differ in their doping level conduc-tivities, and also in their atomic or alloycompositions. See: homojunction.homogeneous cladding. That part of the clad-ding wherein the refractive index is constantwithin a specified tolerance, as a function ofradius. See: cladding; tolerance field.homojunction. A junction between semiconduc-tors that differ in their doping level conduc-tivities but not in their atomic or alloycompositions. See: heterojunction.hybrid mode. A mode possessing components ofelectric and magnetic field vectors in the direc-tion of propagation. Note: Such modes corre-spond to skew (nonmeridional) rays. See: mode;skew ray; transverse electric mode; transversemagnetic mode.ILD. See: injection laser diode.impulse response. The function h ( t ) describingthe response of an initially relaxed system to animpulse (Dirac-delta) function applied at timet = 0. The root-mean-square (rms) duration, urms,of the impulse response is often used to char-acterize a component or system through a singleparameter rather than a function:

    Note: The impulse response may be obtained bydeconvolving the input waveform from the out-put waveform, or as the inverse Fourier trans-form of the transfer function. See: root-mean-square (rms) pulse duration; transfer func-tion.inclusion. Denoting the presence of extraneousor foreign material.


    incoherent. Characterized by a degree of coher-ence significantly less than 0.88. See: coherent;degree of coherence.index dip. A decrease in the refractive index atthe center of the core, caused by certain fabrica-tion techniques. Sometimes called profile dip.See: refractive index profile.index matching material. A material, often aliquid or cement, whose refractive index is nearlyequal to the core index, used to reduce Fresnelreflections from a fiber end face. See: Fresnelreflection; mechanical splice; refractive index.index of refraction. See: refractive index (of amedium).index profile. In an optical waveguide, therefractive index as a function of radius. See:graded index profile; parabolic profile; power-law index profile; profile dispersion; profiledispersion parameter; profile parameter; stepindex profile.infrared (IR). The region of the electromagneticspectrum between the long-wavelength extremeof the visible spectrum (approximately 0.7 pm)and the shortest microwaves (approximately 1mm).injection fiber. See: launching fiber.injection laser diode (ILD). A laser employing aforward-biased semiconductor junction as theactive medium. S y n : diode laser; semiconductorlaser. See: active laser medium; chirping;laser; superradiance.insertion loss. The total optical power losscaused by the insertion of an optical componentsuch as a connector, splice, or coupler.integrated optical circuit (IOC). An optical cir-cuit, either monolithic or hybrid, composed ofactive and passive components, used for couplingbetween optoelectronic devices and providingsignal processing functions.intensity. The square of the electric field ampli-tude of a light wave. Intensity is proportional toirradiance and may be used in place of the termi r radiance when only relative values are impor-tant. See: irradiance; radiant intensity; radi-ometry.

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    interference. In optics, the interaction of two ormore beams of coherent or partially coherentlight. See: coherent; degree of coherence;diffraction.interference filter. An optical filter consisting ofone or more thin layers of dielectric or metallicmaterial. See: dichroic filter; interference;optical filter.interferometer. An instrument that employs theinterference of light waves for purposes of meas-urement. See: interference.intermodal distortion. See: multimode distor-tion.intramodal distortion. That distortion resultingfrom dispersion of group velocity of a propagat-ing mode. It is the only distortion occurring insingle mode waveguides. See: dispersion; distor-tion.intrinsic joint loss. That loss, intrinsic to thefiber, caused by fiber parameter (for example,core dimensions, profile parameter) mismatcheswhen two nonidentical fibers are joined. See:angular misalignment loss; extrinsic jointloss; gap loss; lateral offset loss.IOC. See: integrated optical circuit.ion exchange technique. A method of fabricat-ing a graded index optical waveguide by an ionexchange process. See: chemical vapor disposi-tion technique; double crucible method;graded index profile.IR. See: infrared.irradiance. Radiant power incident per unit areaupon a surface, expressed in watts per squaremeter. Pow e r de n s i t y is colloquially used as asynonym. See: radiometry.isolator. A device intended to prevent returnreflections along a transmission path. Note: TheFaraday isolator uses the magneto-optic effect.isotropic. Pertaining to a material whose elec-trical or optical properties are independent ofdirection of propagation and of polarization of atraveling wave. See: anisotropic; birefringentmedium.

    Lamberts cosine law. The statement that theradiance of certain idealized surfaces, known asLambertian radiators, Lambertian sources, orLambertian reflectors, is independent of the anglefrom which the surface is viewed. Note: The radi-ant intensity of such a surface is maximum nor-mal to the surface and decreases in proportion tothe cosine of the angle from the normal. Syn:cosine emission law.Lambertian radiator. See: Lamberts cosinelaw.Lambertian reflector. Sw: Lamberts cosinelaw.laser. A device that produces optical radiationusing a population inversion to provide LightAmplification by Stimulated Emission of Radia-tion and (generally) an optical resonant cavity toprovide positive feedback. Laser radiation m aybe highly coherent temporally, or spatially, orboth. See: active laser medium; injection laserdiode; optical cavity.laser diode. See: injection laser diode.laser medium. See: active laser medium.lasing threshold. The lowest excitation level atwhich a lasers output is dominated by stimu-lated emission rather than spontaneous emission.See: laser; spontaneous emission; stimulatedemission.lateral offset loss. A power loss caused bytransverse or lateral deviation from optimumalignment of source to optical waveguide, wave-guide to waveguide, or waveguide to detector.S y n : transverse offset loss.launch angle. The angle between the light inputpropagation vector and the optical axis of anoptical fiber or fiber bundle. See: launch numer-ical aperture.launch numerical aperture (LNA). The numer-ical aperture of an optical system used to couple(launch) power into an optical waveguide. Notes:(1) LNA may differ from the stated NA of a finalfocusing element if, for example, that element isunderfilled or the focus is other than that forwhich the element is specified. (2) LNA is one ofthe parameters that determine the initial distribu-


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    tion of power among the modes of an opticalwaveguide. See: acceptance angle; launchangle.launching fiber. A fiber used in conjunctionwith a source to excite the modes of anotherfiber in a particular fashion. Note: Launchingfibers are most often used in test systems toimprove the precision of measurements. Syn:injection fibers. See: mode; pigtail.leaky mode. In an optical waveguide, a modewhose field decays monotonically for a finite dis-tance in the transverse direction but whichbecomes oscillatory everywhere beyond thatfinite distance. Specifically, a mode for which[ n 2 ( a ) k 2- ( Z / U ) ~ ] * < p < n ( a ) kwherep = imaginary part (phase term) of the axialpropagation constant1 = azimuthal index of the moden ( a ) = refractive index at r = a, he core

    radiusk = free-space wavenumber, 2n / / h , and X =wavelength

    Leaky modes correspond to leaky rays in the ter-minology of geometric optics. Note: Leaky modesexperience attenuation, even if the waveguide isperfect in every respect. Syn: tunnelling mode.See: bound mode; cladding mode; leaky ray;mode; unbound mode.leaky ray. In an optical waveguide, a ray forwhich geometric optics would predict total inter-nal reflection at the core boundary, but whichsuffers loss by virtue of the curved core bound-ary. Specifically, a ray at radial position r havingdirection so thatn2(r)- n 2 ( a ) s sin2 8 (r )and

    [nz(r)- n 2 ( a ) l[ l - (r/a)2cos2 +(r) ]in2 8(r) s

    where8(r) = angle the ray makes with the wave-guide axisn ( r ) = refractive indexa = core radius+(r) = azimuthal angle of the projection ofthe ray on the transverse plane

    Leaky rays correspond to leaky (or tunnelling)modes in the terminology of mode descriptors.Syn: tunnelling ray. See: bound mode; clad-ding ray; guided ray; leaky mode.LED. See: light emitting diode.light. (1) In a strict sense, the region of the elec-tromagnetic spectrum that can be perceived byhuman vision, designated the visible spectrumand nominally covering the wavelength range of0.4 pm to 0.7 pm. (2 ) In the laser and opticalcommunication fields, custom and practice haveextended usage of the term to include the muchbroader portion of the electromagnetic spectrumthat can be handled by the basic optical tech-niques used for the visible spectrum. This regionhas not been clearly defined but, as employed bymost workers in the field, may be considered toextend from the near-ultraviolet region ofapproximately 0.3 pm, through the visible region,and into the mid-infrared region to 30 pm. See:infrared (IR); optical spectrum; ultraviolet(UV).light current. See: photocurrent.light emitting diode (LED). A p n junctionsemiconductor device that emits incoherentoptical radiation when biased in the forwarddirection. See: incoherent.light ray. The path of a point on a wavefront.The direction of a light ray is generally normal tothe wavefront. See: geometric optics.lightguide. See: optical waveguide.line source. (1) In the spectral sense, an opticalsource that emits one or more spectrally narrowlines as opposed to a continuous spectrum. (2) Inthe geometric sense, an optical source whoseactive (emitting) area forms a spatially narrowline. See: monochromatic.line spectrum. An emission or absorption spec-trum consisting of one or more narrow spectrallines, as opposed to a continuous spectrum. See:monochromatic; spectral line; spectral width.linear element. A device for which the outputelectric field is linearly proportional to the inputelectric field and no new wavelengths or modula-tion frequencies are generated. A linear elementcan be described in terms of a transfer functionor an impulse response function.


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    linearly polarized (LP) mode. A mode for mate rial dispersion paramete r (M).which the field components in the direction ofpropagation are small compared to componentsperpendicular to that direction. Note: The LE'description is an approximation which is validfor weakly guiding waveguides, including typicaltelecommunication grade fibers. See: mode;weakly guiding fiber.

    M ( A ) = -1/c (dN/dA) = A/c (d%z/dX2)wheren = refractive index

    N = group indexN = n - A(dn/dA)linewidth. See: spectral width.LNA. See: launch numerical aperture.longitudinal offset loss. See: gap loss.loss. See: absorption; angular misalignmentloss; attenuation; backscattering; differentialmode attenuation; extrinsic joint loss; gaploss; insertion loss; intrinsi c joint loss; lateraloffset loss; macrobend loss; material scatter-ing; microbend loss; nonlinear scattering;Rayleigh sca tteri ng; reflection; transmissionloss; waveguide scattering.LP mode. See: linearly polarized mode.LP,, mode. Designation of the fundamental lin-early polarized (LP) mode. See: fundamentalmode.macrobend loss. In an optical waveguide, thatloss attributable to macrobending. Macrobendingusually causes little or no radiative loss. Syn:curvature loss. See: macrobending; microbendloss.macrobending. In an optical waveguide, all mac-roscopic deviations of the axis from a straightline; distinguished from microbending. See: mac-robend loss; microbend loss; microbending.magneto-optic. Pertaining to a change in a mate-rial's refractive index under the influence of amagnetic field. Magneto-optic materials generally.are used to rotate the plane of polarization.material absorption. See: absorption.material dispersion. That dispersion attributableto the wavelength dependence of the refractiveindex of material used to form the waveguide.Material dispersion is characterized by the mate-rial dispersion parameter M. See: dispersion; dis-tortion; material dispersion parameter; profiledispersion parameter; waveguide dispersion.

    A = wavelengthc = velocity of light in vacuumNotes: (1) For many optical waveguide materials,M is zero at a specific wavelength )b, usuallyfound in the 1.2 p m to 1.5 p m range. The signconvention is such that M is positive for wave-lengths shorter than A, and negative for wave-lengths longer than A,. (2) Pulse broadeningcaused by material dispersion in a unit length ofoptical fiber is given by M times spectral line-width (AA), except at A = &, where terms pro-portional to AA2 are important. [See Note (l).]See: group index; material dispersion.material scattering. In an optical waveguide,that part of the total scattering attributable to theproperties of the materials used for waveguidefabrication. See: Rayleigh scattering; seatter-ing; waveguide scattering.mechanical splice. A fiber splice accomplishedby fixtures or materials, rather than by thermalfusion. Index matching material may be appliedbetween the two fiber ends. See: fusion splice;index matching material; optical waveguidesplice.meridional ray. A ray that passes through theoptical axis of an optical waveguide (in contrastwith a skew ray, which does not). See: axial ray;geometric optics; numerical aperture; opticalaxis; paraxial ray; skew ray.microbend loss. In an optical waveguide, thatloss attributable to microbending. See: micro-bend loss.microbending. In an optical waveguide, sharpcurvatures involving local axial displacements ofa few micrometers and spatial wavelengths of afew millimeters. Such bends may result fromwaveguide coating, cabling, packaging, installa-tion, etc. Note: Microbending can cause signifi-cant radiative losses and mode coupling. See:macrobending.


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    misalignment loss. See: angular misalignmentloss; gap loss; lateral offset loss.modal noise. Noise generated in an optical fibersystem by the combination of mode dependentoptical losses and fluctuation in the distributionof optical energy among the guided modes or inthe relative phases of the guided modes. Sy n:speckle noise. See: mode.mode. In any cavity or transmission line, one ofthose electromagnetic field distributions that sat-isfies Maxwells equations and the boundary con-ditions: The field pattern of a mode depends onthe wavelength, refractive index, and cavity orwaveguide geometry. See: bound mode; claddingmode; differential mode attenuation; differen-tial mode delay; equilibrium mode distribu-tion; equilibrium mode simulator; fundamen-tal mode; hybrid mode; leaky modes; linearlypolarized mode; mode volume; multimode dis-tortion; multimode laser; multimode opticalwaveguide; single mode optical waveguide;transverse electric mode; transverse magneticmode; unbound mode.mode coupling. In an optical waveguide, theexchange of power among modes. The exchangeof power may reach statistical equilibrium afterpropagation over a finite distance that is desig-nated the equilibrium length. See: equilibriumlength; equilibrium mode distribution; mode;mode scrambler.mode dispersion. See: multimode distortion.mode (or modal) distortion. See: multimodedistortion.mode filter. A device used to select, reject, orattenuate a certain mode or modes.mode mixer. See: mode scrambler.mode scrambler. (1) A device for inducing modecoupling in an optical fiber. (2) A device com-posed of one or more optical fibers in whichstrong mode coupling occurs. Note: Frequentlyused to provide a mode distribution that is inde-pendent of source characteristics or that meetsother specifications. Syn: mode mixer. See:mode coupling.mode stripper. See: cladding mode stripper.

    mode volume. The number of bound modes thatan optical waveguide is capable of supporting;for V > 5, approximately given by V2/2 and(V2/2) [ g / ( g + 2)1, respectively, for step indexand power-law profile waveguides

    whereg = profile parameterV = normalized frequency

    See: effective mode volume; mode; normalizedfrequency; power-law index profile; step indexprofile; V number.modulation. A controlled variation with time ofany property of a wave for the purpose of trans-ferring information.monochromatic. Consisting of a single wave-length or color. In practice, radiation is neverperfectly monochromatic but, at best, displays anarrow band of wavelengths. See: coherent; linesource; spectral width.monochromator. An instrument for isolatingnarrow portions of the spectrum.monomode optical waveguide. See: singlemode optical waveguide.multifiber cable. An optical cable that containstwo or more fibers, each of which provides aseparate information channel. See: fiber bundle;optical cable assembly.multifiber joint. An optical splice or connectordesigned to mate two multifiber cables, providingsimultaneous optical alignment of all individualwaveguides. Note: Optical coupling betweenaligned waveguides may be achieved by varioustechniques including proximity butting (with orwithout index matching materials), and the useof lenses.multilayer filter. See: interference filter.multimode distortion. In an optical waveguide,that distortion resulting from differential modedelay. Note: The term mul t imode d i spers ion isoften us&das a synonym; such usage, however, iserroneous since the mechanism is not dispersivein nature. Sy n: intermodal distortion; mode (ormodal) distortion. See: distortion.


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    multimode group delay. See: differential modedelay.multimode laser. A laser that produces emissionin two or more transverse or longitudinal modes.See: laser; mode.multimode optical waveguide. An optical wave-guide that will allow more than one bound modeto propagate. Note: May be either a graded indexor step index waveguide. See: bound mode;mode; mode volume; multimode distortion;normalized frequency; power-law index pro-file; single mode optical waveguide; step indexoptical waveguide.NA. See: numerical aperture.near-field diffraction pattern. The diffractionpattern observed close to a source or aperture,as distinguished from far-field diffraction pattern.Note: The pattern in the output plane of a fiber iscalled the near-field radiation pattern. Syn:Fresnel diffraction pattern. See: diffraction;far-field diffraction pattern; far-field region.near-field pattern. See: near-field radiationpattern; radiation pattern.near-field region. The region close to a source,or aperture. The diffraction pattern in this regiontypically differs significantly from that observedat infinity and varies with distance from thesource. See: far-field diffraction pattern; far-field region.near-field radiation pattern. See: radiation pat-tern.near-field scanning. The technique for measur-ing the index profile of an optical fiber byilluminating the entrance face with an extendedsource and measuring the point-by-point radianceof the exit face. See: refracted ray method.

    noise equivalent power (NEP). At a given mod-ulation frequency, wavelength, and for a giveneffective noise bandwidth, the radiant power thatproduces a signal-to-noise ratio of 1 at the outputof a given detector. Notes: (1) Some manufactur-ers and authors define NEP as the minimumdetectable power per root unit bandwidth; whendefined in this way, NEP has the units ofwatts/(hertz)*. Therefore, the term is a mis-

    nomer, because the units of power are watts.See: D* ; detectivity. (2) Some manufacturersdefine NEP as the radiant power that produces asignal-to-dark-current noise ratio of unity. This ismisleading when dark-current noise does notdominate, as is often true in fiber systems.nonlinear scattering. Direct conversion of aphoton from one wavelength to one or moreother wavelengths. In an optical waveguide, non-linear scattering is usually not important belowthe threshold irradiance for stimulated nonlinearscattering. Note: Examples are Raman and Bril-louin scattering. See: photon.normalized frequency. A dimensionless quantity(denoted by V ) , given by

    wherea = waveguide core radiusA = wavelength in vacuumn, and n2 = the maximum refractive index inthe core and refractive index of the homoge-neous cladding, respectively

    In a fiber having a power-law profile, the approx-imate number of bound modes is (Vz/2) g/ (g +2)], where g is the profile parameter. Syn: Vnumber. See: bound mode; mode volume; para-bolic profile; power-law index profile; singlemode optical waveguide.numerical aperture (NA). (1) The sine of thevertex angle of the largest cone of meridionalrays that can enter or leave an optical system orelement, multiplied by the refractive index of themedium in which the vertex of the cone islocated. Generally measured with respect to anobject or image point and will vary as that pointis moved. (2) For an optical fiber in which therefractive index decreases monotonically from n ,on axis to n2 n the cladding the numerical aper-ture is given byNA = 4-(3) Colloquially, the sine of the radiation oracceptance angle of an optical fiber, multipliedby the refractive index of the material in contactwith the exit or entrance face. This usage isapproximate and imprecise, but is often encoun-


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    tered. See: acceptance angle; launch numericalaperture; meridional ray; radiation angle;radiation pattern.

    optic axis. In an anisotropic medium, a directionof propagation in which orthogonal polarizationshave the same phase velocity. Distinguished fromoptical axis. See: anisotropic.optical axis. In an optical waveguide, syn-onymous with j i b e r axis .optical blank. A casting consisting of an opticalmaterial molded into the desired geometry forgrinding, polishing, or (in the case of opticalwaveguides) drawing to the final optical/mechan-ical specifications. See: preform.optical cable. A fiber, multiple fibers, or fiberbundle in a structure fabricated to meet optical,mechanical, and environmental specifications.Syn: optical fiber cable. See: fiber bundle;optical cable assembly.optical cable assembly. An optical cable that isconnector terminated. Generally, an optical cablethat has been terminated by a manufacturer andis ready for installation. See: fiber bundle;optical cable.optical cavity. A region bounded by two ormore reflecting surfaces, referred to as mirrors,end mirrors, or cavity mirrors, whose elementsare aligned to provide multiple reflections. Theresonator in a laser is an optical cavity. Syn: res-onant cavity; See: active laser medium; laser.optical combiner. A passive device in whichpower from several input fibers is distributedamong a smaller number (one or more) of inputfibers. See: star coupler.optical conductor.* See: optical waveguide.*Deprecatedoptical connector. See: optical waveguide con-nector.optical coupler. See: optical waveguide cou-pler.optical data bus. An optical fiber network, inter-connecting terminals, in which any terminal cancommunicate with any other terminal. See:optical link.

    optical density. A measure of the transmittanceof an optical element expressed by: log,(l/T) or-log,,T, or where T is transmittance. The analo-gous term log,,(l/R) is called reflection density.Note: The higher the optical density the lowerthe transmittance. Optical density times 10 isequal to transmission loss expressed in decibels(dB); for example, an optical density of 0.3 corre-sponds to a transmission loss of 3 dB.See: trans-mission loss; transmittance.optical detector. A transducer that generates anoutput signal when irradiated with optical power.See: optoelectronic.optical fiber. Any filament or fiber, made ofdielectric materials, that guides light, whether ornot it is used to transmit signals. See: fiber bun-dle; fiber optics; optical waveguide.optical fiber cable. See: optical cable.optical fiber waveguide. See: optical wave-guide.optical filter. An element that selectively trans-mits or blocks a range of wavelengths.optical link. Any optical transmission channeldesigned to connect two end terminals or to beconnected in series with other channels. Note:Sometimes terminal hardware (for example,transmittedreceiver modules) is included in thedefinition. See: optical data bus.optical path length. In a medium of constantrefractive index n, the product of the geometricaldistance and the refractive index. If n is a func-tion of position,optical path length = nds,where ds is an element of length along the path.Note: Optical path length is proportional to thephase shift a light wave undergoes along a path.See: optical thickness.optical power. See: radiant power.optical repeater. In an optical waveguide com-munication system, an optoelectronic device ormodule that receives a signal, amplifies it (or, inthe case of a digital signal, reshapes, retimes, orotherwise reconstructs it) and retransmits it. See:modulation.


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    optical spectrum. Generally, the electromagneticspectrum within the wavelength region extendingfrom the vacuum ultraviolet at 40 nm to the farinfrared at 1 mm. See: infrared; light.optical thickness. The physical thickness of anisotropic optical element, times its refractiveindex. See: optical path length.optical time domain reflectometry. A methodfor characterizing a fiber wherein an opticalpulse is transmitted through the fiber and theresulting light scattered and reflected back to theinput is measured as a function of time. Useful inestimating attenuation coefficient as a function ofdistance and identifying defects and otherlocalized losses. See: Rayleigh scattering; scat-tering.optical waveguide. (1) Any structure capable ofguiding optical power. (2) In optical communica-tions, generally a fiber designed to transmitoptical signals. Syn: lightguide; optical conduc-tor (Deprecated); optical fiber waveguide. See:cladding; core; fiber bundle; fiber optics; mul-timode optical waveguide; optical fiber; singlemode waveguide; tapered fiber waveguide,optical waveguide connector. A device whosepurpose is to transfer optical power between twooptical waveguides or bundles, and that isdesigned to be connected and disconnectedrepeatedly. See: multifiber joint; optical wave-guide coupler.optical waveguide coupler. (1) A device whosepurpose is to distribute optical power among twoor more ports. See: star coupler; tee coupler.(2) A device whose purpose is to couple opticalpower between a waveguide and a source ordetector.optical waveguide preform. See: preform.optical waveguide splice. A permanent jointwhose purpose is to couple optical powerbetween two waveguides.optical waveguide termination. A configurationor a device mounted at the end of a fiber orcable which is intended to prevent reflection.See: index matching material.optically active material. A material that canrotate the polarization of light that passes


    through it. Note: An optically active materialexhibits different refractive indices for left andright circular polarizations (circular birefrin-gence). See: birefringent medium.optoelectronic. Pertaining to a device thatresponds to optical power, emits or modifiesoptical radiation, or utilizes optical radiation forits internal operation. Any device that functionsas an electrical-to-optical or optical-to-electricaltransducer. Notes: (1) Photodiodes, light emittingdiodes (LED), injection lasers, and integratedoptical elements are examples of optoelectronicdevices commonly used in optical waveguidecommunications. (2) Electro-optical is oftenerroneously used as a synonym. See: electro-optic effect; optical detector.output angle. See: radiation angle.packing fraction. In a fiber bundle, the ratio ofthe aggregate fiber cross-sectional core area tothe total cross-sectional area (usually within theferrule) including cladding and interstitial areas.See: ferrule; fiber bundle.parabolic profile. A power-law index profilewith the profile parameter g equal to 2. Syn:quadratic profile. See: graded index profile;multimode optical waveguide; power-lawindex profile; profile parameter.paraxial ray. A ray that is close to and nearlyparallel with the optical axis. Note: For purposesof computation, the angle 8 between the ray andthe optical axis is small enough for sin 8 or tan 8to be replaced by 0 (radians). See: light ray.PCS. See: plastic clad silica.peak wavelength. The wavelength at which theradiant intensity of a source is maximum. See:spectral line; spectral width.phase coherence. See: coherent.phase cons tant . The imaginary part of the axialpropagation constant for a particular mode, usu-ally expressed in radians per unit length. See:axial propagation constant.phase velocity. For a particular mode, the ratioof the angular frequency to the phase constant.See: axial propagation constant; coherencetime; group velocity.

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    photoconductivity. The conductivity increaseexhibited by some nonmetallic materials, result-ing from the free carriers generated when photonenergy is absorbed in electronic transitions. Therate at which free carriers are generated, themobility of the carriers, and the length of timethey persist in conducting states (their lifetime)are some of the factors that determine theamount of conductivity change. See: photo-electric effect.photocurrent. The current that flows through aphotosensitive device (such as a photodiode) asthe result of exposure to radiant power. Internalgain, such as that in an avalanche photodiode,may enhance or increase the current flow but isa distinct mechanism. See: dark current; pho-todiode.photodiode. A diode designed to produce pho-tocurrent by absorbing light. Photodiodes areused for the detection of optical power and forthe conversion of optical power to electricalpower. See: avalanche photodiode (APD); pho-tocurrent; PIN photodiode.photoelectric effect. (1) External photoelectriceffect The emission of electrons from the irradi-ated surface of a material. Syn: photoemissiveeffect. (2) Internal photoelectric effect photo-conductivity.photoemissive effect. See: photoelectric effect(external).photon. A quantum of electromagnetic energy.The energy of a photon is hvwhere

    h = Plancks constantv = optical frequency

    See: nonlinear scattering; Plancks constant.photon noise. See: quantum noise.photovoltaic effect. The production of a voltagedifference across a pn junction resulting fromthe absorption of photon energy. The voltage dif-ference is caused by the internal drift of holesand electrons. See: photon.physical optics. The branch of optics that treatslight propagation as a wave phenomenon ratherthan a ray phenomenon, as in geometric optics.

    pigtail. A short length of optical fiber, perma-nently fixed to a component, used to couplepower between it and the transmission fiber. See:launching fiber.PIN diode. A diode with a large intrinsic regionsandwiched between p- and n-doped semicon-ducting regions. Photons absorbed in this regioncreate electron-hole pairs that are then separatedby an electric field, thus generating an electriccurrent in a load circuit.Plancks constant. The number h that relatesthe energy E of a photon with the frequency v ofthe associated wave through the relation E = hu.h = 6.626 J * s. See: photon.plane wave. A wave whose surfaces of constantphase are infinite parallel planes normal to thedirection of propagation.plastic clad silica fiber. An optical waveguidehaving silica core and plastic cladding.power. See: irradiance; radiant intensity; radi-ant power.power density. See: irradiance.power-law index profile. A class of gradedindex profiles characterized by the followingequations:n(r) = n,(l - 2A(r/a)9 r SCLn(r) = n, = nl(l - 2A)+r SCLwhere

    n2, - n:2nA =

    wheren(r) = refractive index as a function of radiusn, = refractive index on axisn, = refractive index of the homogeneouscladdinga = core radiusg = a parameter that defines the shape of theprofile.

    Notes: (1) a is often used in place of g. Hence,this is sometimes called an alpha profile. (2) Forthis class of profiles, multimode distortion issmallest when g takes a particular value depend-


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    ing on the material used. For most materials, thisoptimum value is around 2. When g increaseswithout limit, the profile tends to a step indexprofile. See: graded index profile; mode volume;profile parameter; step index profile.preform. A glass structure from which an opticalfiber waveguide may be drawn. See: chemicalvapor deposition technique; ion exchangetechnique; optical blank.primary coating. The material in intimate con-tact with the cladding surface, applied to pre-serve the integrity of that surface. See: cladding.profile. See: graded index profile; index profile;parabolic profile; power-law index profile; stepindex profile.profile dispersion. (1) In an optical waveguide,that dispersion attributable to the variation ofrefractive index contrast with wavelength, wherecontrast refers to the difference between themaximum refractive index in the core and therefractive index of the homogeneous cladding.Profile dispersion is usually characterized by theprofile dispersion parameter, defined by the fol-lowing entry. ( 2 ) In an optical waveguide, thatdispersion attributable to the variation of refrac-tive index profile with wavelength. The profilevariation has two contributors: (a) variation inrefractive index contrast, and (b) variation inprofile parameter. See: dispersion; distortion;refractive index profile.profile dispersion parameter (P).

    n, A dAN I A dA(A) = - -where

    n,, N , are, respectively, the refractive andgroup indices of the core, and n , d n A s therefractive index of the homogeneous cladding,N , = n, - A (dnJdA), and A is the refractiveindex constant. Sometimes it is defined with thefactor (-2) in the numerator. See: dispersion.profile parameter. The shape-defining parameterg for a power-law index profile. See: power-lawindex profile; refractive index profile.propagation constant. For an electromagneticfield mode varying sinusoidally with time at agiven frequency, the logarithmic rate of change,

    with respect to distance in a given direction, ofthe complex amplitude of any field component.Note: The propagation constant is a complexquantity.pulse broadening. An increase in pulse duration.Note: Pulse broadening may be specified by theimpulse response, the root-mean-square pulsebroadening, or the full-duration-half-maximumpulse broadening. See: impulse response; root-mean-square pulse broadening; full width(duration) half maximum.pulse distortion. See: distortion.pulse duration. The time between a specifiedreference point on the first transition of a pulsewaveform and a similarly specified point on thelast transition. The time between the lo%, 50%,orl / e points is commonly used, as is the root-mean-square (rms) pulse duration. See: root-mean-square pulse duration.pulse length. Often erroneously used as a syn-onym for pulse duration.pulse width. Often erroneously used as a syn-onym for pulse duration.quadratic profile. See: parabolic profile.

    quantum efficiency. In an optical source ordetector, the ratio of output quanta to inputquanta. Input and output quanta need not bothbe photons.quantum noise. Noise attributable to the dis-crete or particle nature of light. Syn: photonnoise.quantum-noise-limited operation. Operationwherein the minimum detectable signal is limitedby quantum noise. See: quantum noise.radiance. Radiant power, in a given direction,per unit solid angle per unit of projected area ofthe source, as viewed from that given direction.Radiance is expressed in watts per steradian persquare meter. See: brightness; conservation ofradiance; radiometry.radiant emittance. Radiant power emitted into afull sphere (4n- sr) by a unit area of a source;expressed in watts per square meter. Syn: radi-ant exitance. See: radiometry.24

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    radiant energy. Energy that is transferred byway of electromagnetic waves, that is, the timeintegral of radiant power; expressed in joules.See: radiometry.radiant exitance. See: radiant emittance.radiant flux.* See: radiant power.*Obsolete

    radiant incidence. See: irradiance.radiant intensity. Radiant power per unit solidangle, expressed in watts per steradian. See:intensity; radiometry.radiant power. The time rate of flow of radiantenergy, expressed in watts. The prefix is oftendropped and the term power is used. Colloquialsynonyms: flux; optical power; power; radiantflux. See: radiometry.radiation angle. Half the vertex angle of thecone of light emitted by a fiber. Note: The cone isusually defined by the angle at which the far-fieldirradiance has decreased to a specified fractionof its maximum value or as the cone withinwhich can be found a specified fraction of thetotal radiated power at any point in the far field.Syn: output angle. See: acceptance angle; far-field region; numerical aperture.rad iat ion mode. In an optical waveguide, amode whose fields are transversely oscillatoryeverywhere external to the waveguide, and whichexists even in the limit of zero wavelength. Spe-cifically, a mode for which

    wherep = imaginary part (phase term) of the axialpropagation constantI = azimuthal index of the moden(a) = refractive index at r = a, the coreradiusk = free-space wavenumber, 2 r r / A , where A isthe wavelength

    Radiation modes correspond to refracted rays inthe terminology of geometric optics. Sgn:unbound mode. See: bound mode; leaky mode;mode; refracted ray.radiation pattern. Relative power distribution as

    a function of position or angle. Notes: (1) Near-field radiation pattern describes the radiant emit-tance (W * m-? as a function of position in theplane of the exit face of an optical fiber. (2) Far-field radiation pattern describes the irradiance asa function of angle in the far-field region of theexit face of an optical fiber. (3) Radiation patternmay be a function of the length of the wave-guide, the manner in which it is excited, and thewavelength. See: far-field region; near-fieldregion.radiometry. The science of radiation measure-ment. The basic quantities of radiometry arelisted in Table 1.ray. See: light ray.Rayleigh scattering. Light scattering by refrac-tive index fluctuations (inhomogeneities in mate-rial density or composition) that are small withrespect to wavelength. The scattered field isinversely proportional to the fourth power of thewavelength. See: material scattering; scatter-ing; waveguide scattering.reference surface. That surface of an opticalfiber which is used to contact the transverse-alignment elements of a component such as aconnector. For various fiber types, the referencemight be the fiber core, cladding, or buffer layersurface. Note: In certain cases the reference sur-face may not be an integral part of the fiber. See:ferrule; optical waveguide connector.reflectance. The ratio of reflected power to inci-dent power. Note: In optics, frequently expressedas optical density or as a percent; in communica-tion applications, generally expressed in decibels(dB). Reflectance may be defined as specular ordiffuse, depending on the nature of the reflectingsurface. Formerly: reflection. See: reflection.reflection. The abrupt change in direction of alight beam at an interface between two dissimilarmedia so that the light beam returns into themedium from which it originated. Reflection froma smooth surface is termed specular, whereasreflection from a rough surface is termed diffuse.See: critical angle; reflectance; reflectivity;total internal reflection.reflectivity. The reflectance of the surface of amaterial so thick that the reflectance does notchange with increasing thickness; the intrinsic


  • 8/3/2019 Dictionary of Optical Terms


    IEEEStd 812-1984

    Table 1Radiometric Terms


    Term Name Symbol Quantity unitRadiant energy Q Energy joule (J)Radiant Power 4 PowerSyn: Optical PowerIrradiance E Power incident per unit area W . m-2(irrespective of angle)

    Irradiance per unit wavelengthinterval at a given wavelengthsphere) per unit area

    W . m-2 . nm-'Spectral irradiance E ,Radiant emittance W Power emitted (into a full W . m-'Syn: Radiant excitanceRadiant intensity I Power per unit solid angle W . sr-lRadiance L Power per unit angle per unit W . S r i . m-'projected area

    Radiance per unit wavelengthintervalata given wavelength (W . sr-') . (m-2 . nm - 1)Spectral radiance LA

    reflectance of the surface, irrespective of otherparameters such as the reflectance of the rearsurface. N o longer in common usage. See:reflectance.refracted near-field scanning method. See:refracted ray method.refracted ray. In an optical waveguide, a raythat is refracted from the core into the cladding.Specifically a ray at radial position r havingdirection so that

    n2(r)- nz((a) s sin2 6 ( r )1 - (r/a)2cos2c$(r)where

    +(r) = azimuthal angle of projection of theray on the transverse planeO(r) = angle the ray makes with the wave-guide axisn ( r ) = the refractive index at the core radiusa = core radius

    Refracted rays correspond to radiation modes inthe terminology of mode descriptors. See: clad-ding ray; guided ray; leaky ray; radiationmode.

    Syn: refracted near-field scanning method. See:refractions; refracted ray.refraction. The bending of a beam of light intransmission through an interface between twodissimilar media or in a medium whose refractiveindex is a continuous function of position(graded index medium). See: angle of deviation;refractive index (of a medium).refractive index (of a medium). Denoted by n,the ratio of the velocity of light in vacuum to thephase velocity in the medium. Syn: index ofrefraction. See: cladding; core; critical angle;dispersion; Fresnel reflection; fused silica;graded index optical waveguide; group index;index matching material; index profile; lin-early polarized mode; material dispersion;mode; normalized frequency; numerical aper-ture; optical path length; power-law indexprofile; profile dispersion; scattering; stepindex optical waveguide; weakly guiding fiber.refractive index contrast. Denoted by A, ameasure of the relative difference in refractiveindex of the core and cladding of a fiber, givenby

    (n? - nz")A =refracted ray method. The technique for meas- 2nuring the index profile of an optical fiber byscanning the entrance face with the vertex of a where n 1 nd n2 are, respectively, the maximumhigh numerical aperture cone and measuring the refractive index in the core and the refractivechange in power of refracted (unguided) rays. index of the homogeneous cladding.


  • 8/3/2019 Dictionary of Optical Terms



    refractive index profile. The description of therefractive index along a fiber diameter. See:graded index profile; parabolic profile; power-law index profile; profile dispersion; profiledispersion parameter; profile parameter; stepindex profile.regenerative repeater. A repeater that isdesigned for digital transmission. Syn: regenera-tor. See: optical repeater.regenerator. See: regenerative repeater.repeater. See: optical repeater.resonant cavity. See: optical cavity.responsivity. The ratio of an optical detector'selectrical output to its optical input, the precisedefinition depending on the detector type; gener-ally expressed in amperes per watt or volts perwatt of incident radiant power. Note: Sensitivityis often used incorrectly a s a synonym.rms pulse duration. See: root-mean-square(rrns) pulse duration.root-mean-square (rms) deviation. A singlequantity characterizing a function given, forf(z),by

    whereMO = S_ "m f (x>dxMI = l / M om xf(z)dz

    Note: The term rms deviation is also used inprobability and statistics, where the normaliza-tion, M O , is unity. Here, the term is used in amore general sense. See: impulse response;root-mean-square (rms) pulse broadening;root-mean-square (rrns) pulse duration; spec-tral width.root-mean-square (rrns) pulse broadening. Thetemporal rms deviation of the impulse responseof a system. See: root-mean-square (rrns) devia-tion; root-mean-square (rrns) pulse duration.root-mean-square (rrns) pulse duration. A spe-cial case of root-mean-square deviation wherethe independent variable is time andf(t) is pulsewaveform. See: root-mean-square (rrns) devia-tion.

    scattering. The change in direction of light raysor photons after striking a small particle or parti-cles. It may also be regarded as the diffusion of alight beam caused by the inhomogeneity of thetransmitting medium. See: leaky modes; mate-rial scattering; mode; nonlinear scattering;Rayleigh scattering; refractive index (of amedium); unbound mode; waveguide scatter-ing.semiconductor laser. See: injection laser diode(ILD).sensitivity. Imprecise synonym for responsivity.In optical system receivers, the minimum powerrequired to achieve a specified quality of per-formance in terms of output signal-to-noise ratioor other measure.shot noise. Noise caused by current fluctuationsdue to the discrete nature of charge carriers andrandom or unpredictable (or both) of chargedparticles from an emitter. Note: There is often a(minor) inconsistency in referring to shot noisein an optical system: many authors refer to shotnoise loosely when speaking of the mean squareshot noise current (amp') rather than noisepower (watts). See: quantum noise.single mode optical waveguide. An opticalwaveguide in which only the lowest order boundmode (which may consist of a pair oforthogonally polarized fields) can propagate atthe wavelength of interest. In step index guides,this occurs when the normalized frequency