FIBER OPTICAL GLOSSARY

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    OPTICAL FIBER

    GLOSSARY.

    SHOKRI ELGEBALI12/21/2010

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    fiber opticsglossary-The following terms were

    defined with the assistance of

    Panduit Corporation, a leadingmanufacturer of wiring andnetwork cabling products(www.panduit.com) and JeffHecht, noted optical

    networking consultant andauthor.

    Adapter- A mechanical device designed

    to align fiber-optic connectors.It contains the split sleeve(interconnect sleeve) thatholds the two ferrules together.

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    Adapter Sleeve- A mechanical fixture withinthe adapter body that alignsand holds two terminated fiberconnectors. Adapter sleevematerial is typically phosphor

    bronze, ceramic or polymer.

    Absorption- The absorbing of light energywithin an optical fiber due to

    natural impurities in the glass.Absorption and scattering arethe main cause of attenuation(signal loss) in an optical fiber.

    -

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    -Acceptance Angle- The angle at which the core of

    the fiber will take in light.

    - Numerical Aperture.

    -The light-gathering ability of anoptical fiber, as determined bythe square root of thedifference of the squares of therefractive indexes of the core(n 1 ) and the cladding (n 2 ),and as expressed in theequation: NA = (n 1 2 - n 2 2 )1 / 2 Fiber optic transmissionsystems (FOTS) are based onthe principle of total internal

    reflection, meaning that alllight injected into the fiber isretained in the fiber.Theobjective is to retain all

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    components of the opticalsignal in the core. However, a

    light source naturally injectssome light rays into the core atangles less than the criticalangle, which is perpendicularto the plane of thecore/cladding interface. Atsuch severe angles, theincident light rays penetratethe interface and enter thecladding, where they may belost. The numerical aperture

    essentially is an indication ofhow well an optical fiberaccepts and propagates light.As illustrated in Figure N-2,optical fiber with a small NA

    (top) requires more directional,i.e., collimated, light, whereasfiber with a large NA (bottom)does not.The higher NA allows

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    the fiber to accept more lightand propagate more modes.The

    NA is mathematically equal tothe sine of the angle ofacceptance. Note: The NA isimportant in multimode fiber(MMF). It is not, however, acritical measurement in single-mode fiber (SMF), as the smallcore supports only a singlemode of propagation and,therefore, the light is neitherreflected nor refracted.The

    light-accepting ability can alsobe defined in terms of the coneof acceptance, which is themaximum angle at which thefiber will accept incident light,

    represented in threedimensional view. angle ofacceptance.

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    Collimation.-The process by which a beam of

    radiant electromagnetic energyis lined up to minimizedivergence or convergence.Ideally, a collimated beam is abundle of parallel rays perfectly

    lined up along an optical line-of-sight (LOS) between atransmitter and receiver,perhaps through, and in perfectparallel with, a waveguide. In a

    fiber optic transmission system(FOTS), a perfectly collimatedoptical beam would beperfectly lined up with the fibercore.

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    Cone of Acceptance.

    -The maximum angle,represented in three-dimensional view as a cone, atwhich an optical fiber willaccept incident light. Withinthat cone, as defined by thoseangles, a light source can injectan optical signal into the fibercore and the signal will remainin the core, reflecting off of theinterface between the core and

    cladding, as illustrated inFigure C-5. At a more severeangle, i.e., outside the cone,the signal will penetrate theinterface and enter, and

    perhaps be lost in, thecladding. The angle ofacceptance and, therefore, thecone of acceptance are

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    determined by the difference inindex of refraction (IOR)

    between the core and cladding.

    Critical Angle.-Light striking the interface

    between two substances caneither reflect off of thesubstance it encounters orenter it, with the differencedepending on the nature of thesubstances and the angle at

    which the incident light raystrikes the interface. A glassoptical fiber (GOF) comprisesan inner core of glass of a givenrefractive index, or index of

    refraction (IOR), surroundedby one or more layers ofcladding of lower refractiveindex.The critical angle is

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    measured from the normal,which is at 90 degrees from

    (i.e., perpendicular to) thesurface of the core/claddinginterface, or boundary. If, asillustrated in Figure C-6, theincident light rays strike theinterface at an angle greaterthan the critical angle, theyreflect off the interface, withthe angle of reflection beingthe same as the angle ofincidence.The light rays glance

    off of the interface, so to speak.If, on the other hand, theincident light rays strike theboundary at an angle less thanthe critical angle, they enter

    the cladding, where they eitherare lost or refracted back intothe core, depending on the type

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    of fiber and the angle ofincidence.

    Total InternalReflection (TIR).

    -The complete reflection of alight ray as it strikes theinterface between the mediumin which it is traveling and a

    medium with a lower refractiveindex, or index of refraction(IOR) at an angle greater thanthe critical angle, which ismeasured from theperpendicular at the point ofreflection. Depending on thespecific nature of the glassoptical fiber (GOF) and its

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    manner of construction, obliquelight rays striking the interface

    between the core and claddingvariously are reflected orrefracted back into the core,which is the primary light-conducting medium. If theoptical fiber is a step-indexfiber, and the angle ofincidence is greater than thecritical angle, the light rays arereflected back into the core. Ifthe angle is less than the

    critical angle, the light rayspenetrate the core/claddinginterface, where they are lost.If the fiber is a graded-indexfiber, the light rays also reflect

    off the core/cladding interfaceif the angle of incidence isgreater than the critical angle.As the angle decreases below

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    the critical angle, the light raysenter the cladding, where they

    gradually gain velocity andbend, or refract, back into thecore. The lesser the angle, thegreater the penetration and thegreater the associatedincreases in velocity andrefraction. If the angle of theincident light ray is too severe,the light ray will penetrate thecore/cladding interface and belost in the cladding of either

    type of cable.Total internalreflection essentially confinesthe optical signal to the coreconducting material, therebymaintaining signal strength

    over a distance.Total internalreflection is the fundamentalprinciple that makes fiber optictransmission possible.

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    Add/Drop MultiplexerA device that includes or removesone or more optical channels to a

    signal passing through it. Seeadd/drop multiplexer.

    Aramid YarnAn ingredient in optical fiber cable

    that provides support, protectionand tensile strength. Also referredto as KEVLAR, which is a brand ofaramid yarn.

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    ATM (asynchronous

    transfer mode)- A network technology thatswitches optical and electronicsignals that are broken into 53-byte cells.

    ATM (AsynchronousTransfer Mode).

    -A fast-packet, connection-oriented, cell-switchingtechnology for broadbandsignals.ATM was an outgrowthof the ITU-T developmentefforts towards broadband

    integrated services digitalnetwork (B-ISDN). Although B-ISDN faltered, ATM became theswitching technology of choice

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