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Optical fiber technology



Completepowertransmissionfunction

U.S.Lariancompanysuccessfullycompletedthepowertransmissionfunctionusingopticalfiber,openingupanewwayinthepowerfield.Theyusesemiconductorlaserdiodesatthetransmittingendtoconvertelectricalenergyintolaserlightfortransmissioninopticalfibers,andusesolarcellsasthereceivingenddevice.Thisdeviceuses300-micron-thickgalliumarsenideasaninsulatingsubstrate,coveredwitha20-micron-thicksolarcell.Itisdividedinto6independentareas,whichareconnectedinseriesbygold-platedairbridges.Whenthelaserlighttransmittedbytheopticalfiberhitsthesolarcell,thelightenergyimmediatelybecomeselectricalenergy.Thevoltagegeneratedbyeachareaisexactly1volt,andsixareasinserieshave6volts,whichisenoughforthecontrolcircuitofmostsensors.

Wideapplication

Ifthepowerofthelaserdiodeiscontinuouslyincreased,coupledwithacompletepowertransmissionsystem,opticalfibertransmissioncanbewidelyusedinmilitary,industrial,commercialandotheraspects.TheFrenchBogenlaboratory,whichspecializesincomputers,electronicequipment,signalprocessingandimagetechnology,useslightsolitonsandshortpulsestoachievedistortion-freetransmissioninopticalfibers.Thistechnologycansolvetheproblemsofdispersionandnon-lineareffects,andthereisnoneedtoinstallmultipleregenerationdevicesalongtheopticalcable.Itonlyneedstosetupanamplifierevery100kilometersorsowhenworking.Thesolitarywaveletscantraverseeachotherwithoutinterferingwitheachother.Itissaidthatthisnewtechnologyisusedforsubmarinesubmarinewithintherangeof6450-12900kilometers,whichcansolvetheproblemofcommunicationdifficulties.AnirregularcarriersignalopticalfibercommunicationtechnologydevelopedbyAmericancommunicationsecurityexpertsisspecificallydesignedtodealwithtoday'sincreasinglyrampantandsophisticatedeavesdroppers.Thistechnologyfirstconvertsusefulinformationsuchasvoiceintodigitalpulsesignals,andthenencodesthesedigitalpulsesignalsandmodulatesthemontorandommicrowavecarriersthatchangeirregularly.Whensending,thelasertransmittingdevicetransmitstheirregularcarriersignalcarryinginformationtothereceiverthroughtheopticalfibercommunicationsystem.Thereceiver'slaserreceiverusesspecialtechnologytosynchronizeanddynamicallycoordinatewiththesendinglaserdevice,andfinallycompletesthetaskofdemodulatingtheusefulsignalfromtheirregularcarrier.Usingthistechnology,theeavesdropperswillnolongerbeuseful,theywillonlyhearchaoticnoises.TheAustralianPaulineCompanyhasrecentlydevelopedanopticalfiberscalethatcanweightruckswithoneopticalfiberandonelaser.Thiskindoffiberweigherusesakindoffiberwithveryspecialresistancecharacteristics.Whenitisunderpressureortension,thefiberwillbeslightlydeformed,causingthecharacteristicsofthelasertochange.Atthistime,thedetectorwillimmediatelylearnthischangeandconvertitintoanelectricalsignalchange.Thisisreflectedonthedisplaypaneloftheinstrument.Sincetheopticalfiberismadeofglass,ithasmoistureresistanceandradiationresistance.Moreimportantly,itiseasytoinstallandmaintain,andissuitableforinstallationonmainroadsinurbanareas,aroundfactories,airportsandrunways,warehouses,andports.Carryout24hoursofcontinuouswork.Therefore,inadditiontoweighing,itcanalsoplayamonitoringrole,andtheaccuracyisfargreaterthanthatofexistingelectronicdevices.

Plasticopticalfiber

AccordingtoarecentreportintheUS,aplasticopticalfiberdevelopedbyBostonOpticalFiberCorporation,Massachusetts,hasatransmissionspeed30timesfasterthanthecurrentstandardcopperwire.Moreover,itislighterinweight,moreflexibleandlowerincostthanglassfiber.Thiskindofopticalfiberusestherefractionoflightorthejumpingmodeoflightinthefibertoreachahighertransmissionspeed,andcantransmitdataataspeedof3megabitspersecondwithin100meters.Atpresent,370,000kilometersofsubmarineopticalcableshavebeenlaidallovertheworld.Thislengthcanalmostcircletheearth10times.Moreover,becauselasersareusedatbothends,thereisnolongeraneedforrepeaterstoamplifysignalsduringtransmission.Thiswillgreatlyreducethecostandcallcosts.Accordingtoreports,theworld'slargestcapacitysubmarineopticalcableconnectingEuropeandtheUnitedStatesisabouttoopen.Thissubmarinecommunicationfiberopticcablethatconnectstheworldisbeinglaid.Thisisthemostmagnificentprojectinthefieldofcommunicationinthe20thcenturyandissupportedby30internationaltelecommunicationsorganizationsaroundtheworld.ItcrossestheAtlanticOcean,crossestheMediterraneanSea,passesthroughtheRedSeaandtheIndianOcean,andcrossestheStraitofMalaccaintothePacificOcean.Thetotallengthisnearly320,000kilometers,connecting175countriesandregions,anditcanmake2.4millionphonecallsortransmithundredsofthousandsofcompressedpicturesatthesametime.Theentireprojectcost14billionU.S.dollarsandisexpectedtobecompletedin2003.

Principleofcomposition

Opticalfibertechnologygenerallyconsistsofthreeparts:theopticalsignaltransmittingend,theopticalfiberusedtotransmittheopticalsignal,andtheopticalsignalreceivingend.

Thefunctionoftheopticalsignaltransmittingendistoconverttheelectricsignaltobetransmittedintoanopticalsignalthroughanelectro-opticalconversiondevice.Currently,thetransmittingendelectro-opticalconversiondevicegenerallyusesalight-emittingdiodeorasemiconductorlasertube.Theoutputlightpowerofthelight-emittingdiodeisrelativelylow,thesignalmodulationrateisrelativelylow,butthepriceischeap,andtheoutputlightpowerandthedrivingcurrentarebasicallylinearwithinacertainrange,whichismoresuitableforshort-distance,low-speed,andanalogsignaltransmission;Theoutputpowerofthediodeislarge,thesignalmodulationrateishigh,butthepriceisrelativelyhigh,anditissuitableforlong-distance,high-speed,digitalsignaltransmission.Thefunctionoftheopticalfiberistotransmittheopticalsignalatthetransmittingendtothereceivingendoftheopticalsignalwithaslittleattenuationanddistortionaspossible.Atpresent,theopticalfiberisgenerallyusedinthenear-infraredbandof0.84µm,1.31µm,1.55µmwithgoodtransmittanceofmultimodeorsingleModesilicafiber.Thefunctionoftheopticalsignalreceivingterminalistorestoretheopticalsignaltothecorrespondingelectricalsignalthroughthephotoelectricconversiondevice.Thephotoelectricconversiondevicegenerallyusesasemiconductorphotodiodeoranavalanchephotodiode.Thelightemittingwavelengthofthelightsourceconstitutingtheopticalfibertransmissionsystemmustmatchthewavelengthbandofthelowlosswindowofthetransmissionfiberandthepeakresponsebandofthephotoelectricdetectiondevice.Thetransmittingendelectro-opticalconversiondeviceusesahigh-brightnessnear-infraredsemiconductorlight-emittingdiodewithacentralemissionwavelengthof0.84µm,thetransmissionfiberusesamultimodequartzfiber,andthereceivingendphotoelectricconversiondeviceusesasiliconphotodiodewithapeakresponsewavelengthof0.8µmto0.9µm.Afurtherintroductiontoeachpartisgivenbelow.

Opticalsignaltransmitter

Thedrivingandmodulationcircuitofthelight-emittingdiodeusedinthesystemisshowninFigure2.Thesignalmodulationadoptsthemethodoflightintensitymodulation,andthelightintensityadjustmentpotentiometerisusedtosendAdjustthestaticdrivingcurrentflowingthroughtheLED,therebychangingtheemittedlightpowerofthelight-emittingdiodeaccordingly.Thesetstaticdrivingcurrentadjustmentrangeis0-20mA,correspondingtothepanellighttransmissionintensitydrivingdisplayvalue0-2000units,whenthedrivingcurrentWhenitissmaller,theemittedlightpowerofthelight-emittingdiodeandthedrivingcurrentarebasicallylinear.Theaudiosignalisisolatedbythecapacitor,resistornetworkandtheopampandcoupledtothenegativeinputofanotheropamp,overlappingwiththestaticdrivingcurrentofthelight-emittingdiodeThelight-emittingdiodeisaddedtosendanopticalsignalthatchangeswiththeaudiosignal,andtheopticalsignaliscoupledtothetransmissionfiberthroughanopticalfibercoupler.Thelowendofthetransmittablesignalfrequencycanbedeterminedbythecapacitorandresistornetwork,andthelowfrequencyresponseofthesystemisnotgreaterthan20Hz

Theopticalsignalreceivingend

istheworkingprinciplediagramoftheopticalsignalreceivingend,thetransmissionfiberTheopticalsignalfromthetransmittingendcouplestheopticalsignaltothephotoelectricconversiondevicephotodiodethroughtheopticalfibercoupler.Thephotodiodeconvertstheopticalsignalintoacurrentsignalproportionaltoit.Thephotodiodeshouldbereversebiasedwhenitisused.Current-to-voltageconversionconvertsthephotocurrentsignalintoavoltagesignalproportionaltoit.Theaudiosignalcontainedinthevoltagesignaliscoupledtotheaudiopoweramplifierviaacapacitorandresistancetodrivethespeakertosound.Thefrequencyresponseofthephotodiodeisgenerallyhigh,andthehighfrequencyresponseofthesystemmainlydependsontheresponsefrequencyoftheoperationalamplifier.

Transmissionfiber

Currently,theopticalfiberusedforopticalcommunicationgenerallyusessilicafiber,whichiscoveredwithalayerofrefractiveindexn1insidethecorewithalargerrefractiveindexn1.Inthecladdinglayer,thelightistotallyreflectedattheinterfacebetweenthecoreandthecladdinglayerandisrestrictedtopropagateinthecore.AsshowninFigure5,theopticalfiberisactuallyadielectricwaveguide.Transmissionalongthefiber,thecorediameterofthefiberisgenerallyfromafewmicronstohundredsofmicrons.Accordingtothetransmissionmode,itcanbedividedintomulti-modefiberandsingle-modefiber.Accordingtotherefractiveindexdistributionmodeofthefiber,itcanbedividedintorefractiveindexsteptypeandrefractiveindex.Gradedopticalfiber.Therefractiveindexsteptypefibercontainstwocircularlysymmetricalcoaxialmedia,bothofwhichareuniformintexture,buthavedifferentrefractiveindexes.Therefractiveindexoftheouterlayerislowerthanthatoftheinnerlayer.

Gradientindexfiberisakindoffiberwhoserefractiveindexisgradedalongthecrosssectionofthefiber.Thepurposeofchangingtherefractiveindexistomakethegroupvelocityofvariousmodessimilar,therebyreducingmodaldispersionandincreasingcommunicationbandwidth.Multimoderefractiveindexstep-typefibersproduceinter-modedispersionduetothedifferentgroupvelocitiesofeachmodetransmission,andthetransmissionbandwidthislimited.Multimoderefractiveindexgradedfiberincreasesthebandwidthofsignaltransmissionduetoitsspecialrefractiveindexdistribution,whichmakesthegroupspeedoftransmissionofeachmodethesame.Single-modefiberisafiberthatonlytransmitsasingleopticalmode.Atpresent,single-modeopticalfibersaremostlyusedinlong-distanceopticalcommunications.

Themaintechnicalindicatorsofsilicafiberincludeattenuationcharacteristics,numericalapertureanddispersion.Numericalaperture:Numericalaperturedescribesthecharacteristicsofopticalfiberwhencoupledwithlightsources,detectorsandotheropticaldevices.Itssizereflectstheabilityoftheopticalfibertocollectlight.AsshowninFigure5,thelightincidentontheendfaceoftheopticalfiberwithinthesolidangle2θmaxistotallyreflectedattheinternalinterfaceoftheopticalfiberandistransmitted,andthelightincidentontheendfaceoftheopticalfiberoutsidethe2θmaxrangeisTheinternalinterfaceofthefiberdoesnotproducetotalreflectionbutistransmittedtothecladdingandisimmediatelyattenuated.Thenumericalapertureofthefiberisdefinedas:NA=Sinθmax,anditsvalueisgenerallybetween0.1and0.6,correspondingtoθmaxIntherangeof90to330,multimodefibershavelargernumericalapertures,andsingle-modefibershaverelativelysmallnumericalapertures.Therefore,single-modefibersgenerallyrequireLDsemiconductorlasersastheirlightsources.

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