千米小说网

Chapter 6 Light space and time（第2页）

Bothoftheseeffectsarisebecausethereisamaximumspeedatwhialsofanykindpropagate，andthisspeedisthesameforeveryowerehenonecoulddetermineapreferredseofreferenwhichthesighehighestspeed。Einstein’sworkoyshowedthatthereisnopreferredframe，sothaton’sideaofafixed，pre-definedspaotbethecethemaximumspeedfnalsofanykindturnsouttobelightspeed，lightisdefiime。

It’swhowpreciselywemightbeabletosywoclocksbythe‘lightpulse’methoddiscussedpreviously。Youseethatoodoitisbymakipulseasbriefaspossible，sothattheuyoverwhenitarrivesisminimal。Thusitisimportanttoknowiftherearelimitstothebrevityoflightpulses。Itturnsoutthatthereare，andtheyarisefromasimilarsortofertythatlimitstheresolutionofimagingsystems，aster3。

&artbyaskinghowitisthatwedetermihefrequencyofalightwave。Imagiakingoneofourdaskinghoeaksofthewavereachusduriervalbetweentwoticks。Thegreaterthenumberofpeaks，thehigherthefrequency。Thepreedetermihefrequendsonhowmanytimeswerepeatthismeasurement，sinceourabilityactuallytotellwhetherwehavereachedthepeakofawavemayhus，thelohemorepreinatiohistrade-offisfualforwaves—theimprefrequencymultipliedbytheuyiervalisafixedproduct。ThiswasfirstuoodbyJosephFourier，a19th-turyFreidstistwhoplayedakeyrthewavemodeloflightpropagation。

Ultrashhtpulses

Fourier’stheoremisimportantforizatiosaysthatifulse，wemusthaveaefrequenotherwaytosaythisisthatashhtisstitutedbyabreofcolours。That’sentirelyanalogoustothesituatioifiedfoptics:ahigh-resolutionimage，demandingasmallfocus，requiresawiderangelestobecollefaalogygoesfurther。JustasAbbeshowedthesmallestsizeofafocalspotcouldbeapproximatelyoheilluminatinglight，soFouriershowedthattheshortest-durationpulseisasihefield。

Whatthismeansifhtinthevisibleregiorum，itispossibletoproducepulseswithadurationofabout2fs。Amazingly，lightsourcesderivedfromlasersowroutinelyproducepulsesofsuchstartliheyarebasedonthemode-logteiquedesChapter5。

Buttheseareburstsoflightthatoature，heshortestthatbeprodualaboratory。Thathoestolightsourceswithmuchshewavelengths。Forihesiyoueasilyseethatifthewavelengthisshortehedurationofanopticalcycleisredupriionofthepulsebereduced。Thisapproatlyholdstherecordfortheworld’sshortesttrollablelightpulses。Byshieonanatomicgas，aproashigh-harmoiowaveswithafrequencymultiplesofseveraltenslaserfrequency。Thisallowspulseswithduratioensofattoseds（10-18s，orabillionbillionthofased）。Theseareunimaginablyshortburstsoflight，withduratiohetimeittakesaooscillatewithinanatom。

Frequenbs

Iatedthatinamode-lockedlaserasiraversestheopticalcavity。Eachtimeitbouneofthemirrors，alittlebitofthelightpulseistrahroughthemirrorahecavity。Asace，outsidethecavitythelightappearsasa‘uous’sequenceofpulsesspacedbytheround-triptimeofthelightiy，typicallyabillionthofasedorso。A‘snapshot’ofsuofpulseswouldshowtheseveryshortburstsseparatedfromeachotherbyadelaythatisloheirduratiohofab（asshowninFigure30）。Andthepulsesbemadeidentieanotherbycarefuladjustmentofthelaserprodugthem，sothattheelectricfieldofeachofthepulsespeaksatexactlythesametimewithrespecttotheiyehepulse。

Itturnsoutthatthisfiguratioeachofthe‘teeth’inthefrequenbhasaveryprecisepositionatanabsolutefrequency。Apreciselycalibratedsetoffrequenciesisaveryimportanttaccurateclocks。Thisisbecauseitallowsadireparisonofopticalfrequeolower（usuallymicrowave）frequencies，whibetedbymearonics。

Thusfrequehemicriohecaesiumatomicbeparedsimplytomuchmorepreciseeleisitioiofthespe，forexample，strontiumatomsoraluminiumiohestaomicclocksusedienavigation，forinstanowallbesyotickatthesameratetowithiinabillionbillion（i。e。1:10-18），duetothepreoftheopticaleleosfrequenstrontiumoraluminium。

30。Traiiearlysiicalpulses。Thespectrumofthepulsetrainlooksliketheteethofab，heiscalledafrequenb。

‘Opticalclockwork’ofthiskindallowstheparisonofdisparatefrequehsuchremarkableprethatitprovidesameaey，andthustouaheroleoflightindefiime。Frequendthustime，isthephysititythatbemeasuredwiththehighestpreofanyquantity，byfar。

Optiunis

Frequenbsarealsoimportantinteleunislinksbasedonlight。InChapter3，Idescribedhowopticalwavescouldbeguidedalongafibrelass‘chip’。Thisphenomehelong-distaeleunifrastructurethatectspeopleacrossdifferesaheIhereasonitissoeffectiveisthatlight-baseduniuchmorecapacityfinformationtharicalwires，orevenmicrowaveetworks。Thismakespossiblemassivedatatransmission，suchasthatodelivervideoondemaer。

Manyteleunipaniesoffer‘fibreoptid’deals。Akeyfeatureofthesepackagesisthehighspeed—upto100megabytesperseBps）—atwhichdatamayberedtraeisas，eachofwhichisa1ora0。Informatiooverfibresasaseques’，whicharedecodedbyyourobilephoelligiblevideo，audio，ortextmessages。Inoptiunis，thebitsarerepreseheiyofthelightbeam—typicallylowiyisa0aensitya1。Themoreofthesethatarrivepersed，thefastertheunirate。TheMBpsspeedofthepackagespecifieshoidlywesmitaiopany’slink。

Whyisoptimunis?Therearetworeashtbeamsdon’teasilyiher，sothatasinglefibreanylightpulses（usuallyofdifferentultahoutthemessagesgettingscrambledup。Thereasonforthisisthattheglassofwhichthefibreismadedoesnht（oronlyabsorbsitintinyamounts），aheatupanddisruptotherpulsetrains。

Further，alightbeampropagatinginglasshastobeveryintenseitoiherlightbeam。Forinstanyoucrossthebeamsfromtointersyoudoherbeamdistortordeviatefrihoughthetassrightthrougheachother。（Ifyourlaserpointershadenormhtjustseesueffelybecausetheroomisfullofair。Inavacuumtheywouldstillnotiher。）Thismeansthatthe‘crosstalk’betweenlightbeamsisveryweakinmostmaterials，sothatmanybeamsbepresegadegradationofthesignal。Thisisverydiffereronsmovingdoerwire，whichistheusualwayinwhichlocal‘wired’unislinksfurooheatupthewire，dissipatihismakesthesigoredthusthenumberobekeptsmallenoughtoavoidthisproblem。

Sed，lightwavesoscillateatveryhighfrequehisallowsveryshortpulsestobegeed，asdescribedearlier。Thismeansthatthepulsesbespacedveryclosetogetherihetransmissiosofinformationpersedpossible。Iesof40Gbps（aGb，abit，isabillionbits）arepossiblei-gei-haulsystems。Electrialsincopperwiresarelimitediionandspagofpulsesgtheinformatioiedpreviously，whidtherfrequencies。Copperwiresrunoutofsteam，asitwere，atmuchlo

Fibre-basedoptietworksalsosupportaverywiderangeht。Glasstransmitsabreofwavelengths，withparticularlylowsdabsorptionlossiralwindowfrom1。3–1。55μm。Therateatwhisarelostihesewavelengthsisabout5pertperkilometre。Theselossesbemadeupbyamplifyiwhileinthefibresothattransmissiondistances（suchasacrosstheAtlanti）ispossiblewithoutanyofthelighttnalsorviceversa。

&eleunisspedowisdividedintomanyindividualfrequenuchlikethefrequenbshowninFigure30。Eachspepoisaseparateunisel。Therebe150orsoslotsintheithineael，a40Gbpsoptialbeoperated。Thereforethetotalbitrateforunisis150times40Gbps，or6Tbps（1Tb，1terabit，is1，000billionbits）。

Theever-ingdemandforunisbandwidthduetoincreaseduseoftheIheservicesitprovideshavespurredoptieerstosofcreativityihepotentialoflight。