Raman Amplifiers for Telecommunications 1 : Physical Principles

331 Pages·2003·6.36 MB·English

by  Mohammad N. Islam

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ToshimitsuAsakura
Hokkai-GakuenUniversity BoMonemar
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UniversityofMannheim
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E-mail:[email protected] E-mail:[email protected]
Mohammed N. Islam (Ed.)
Raman Amplifiers
for Telecommunications 1
Physical Principles
Foreword by Robert W. Lucky
With222Figures
MohammedN.Islam
DepartmentofElectricalEngineeringandComputerScience
UniversityofMichiganatAnnArbor
1110EECSBuilding
1301BealAvenue
AnnArbor,MI48109-2122
[email protected]
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XteraCommunications,Inc.
500WestBethanyDrive,Suite100
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USA
[email protected]
LibraryofCongressCataloging-in-PublicationData
Ramanamplifiersfortelecommunications1:physicalprinciples/editor,MohammedN.Islam.
p.cm.–(Springerseriesinopticalsciences;v.90/1)
Includesbibliographicalreferencesandindex.
ISBN0-387-00751-2(hc.:alk.paper)
1.Fiberoptics. 2.Opticalcommunications. 3.Ramaneffect. 4.Opticalamplifiers.
I.Islam,MohammedN. II.Series.
TL5103.592.F52R352003
(cid:1)
621.38275–dc21 2003044945
ISBN0-387-00751-2 ISSN0342-4111 Printedonacid-freepaper.
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To my loving wife,
Nasreen
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Foreword
IremembervividlythefirsttimethatIheardaboutthefiberamplifier.Atthattime,
of course, it was the erbium-doped fiber amplifier, the predecessor of the Raman
amplifierthatisthesubjectofthisbook.
ItwasanearlymorninginaforgottenyearinMurrayHill,NewJerseyatoneof
ourBellLabsmonthlyresearchstaffmeetings.Abouttwentydirectorsandexecutive
directors of research organizations clustered around a long table in the imposing
executiveconferenceroom.ArnoPenzias,thevicepresidentofresearch,presidedat
thefootofthetable.
Everyonewhoparticipatedinthoseresearchstaffmeetingswilllongremember
theircultureandatmosphere.Arnowouldpickanarbitrarystartingpointsomewhere
aroundthetable,andthedesignatedpersonwouldheadtothefrontofthetabletogive
ashorttalkon“somethingnew”inhisorherresearcharea.Thisfirstspeakerwould
invariablyfiddlehelplesslywiththecontrolsembeddedinthepodiumthatcontrolled
the viewgraph projector, but eventually we would hear machinery grinding in the
back room as a large hidden mirror moved into place. We would all wait quietly,
arranging and choosing our own viewgraphs from the piles that lay on the table in
frontofeveryparticipant.
Therulesforthestaffmeetingwerethateachspeakerwasallowedsevenminutes
andthreeviewgraphs.However,inspiteofArno’sbesteffortstoenforcethisregimen,
everyonetooktoolongandusedtoomanyviewgraphs.Variousattemptsatusingloud
timersandotherincentivesallfailed.Noonecouldgivearespectabletalkonaresearch
topicforwhichtheyhadpassionatefeelingsinsevenminutes.
Another rule was that anyone could forfeit his talk by simply saying, “I pass.”
This forfeiture was always accepted without comment, but new directors always
asked their friends about whether this would constitute a black mark against their
performance.Nooneknewforsure,butrumorhaditthatitwasunwisetopassunless
youweretrulydestituteofmaterial.Afterall,theimplicationwouldbethattherewas
nothingnewinyourresearchorganizationforthelastmonth—notagoodindication
ofyourmanagementskills.
Withnoonepassing,andeveryonespeakingtoolong,thesestaffmeetingssome-
timesseemedendless.Computerscientistswouldtalkaboutnewconstructsinsoft-
viii Foreword
ware,systemspeoplewouldtalkaboutnewtechniquesforspeechrecognition,physi-
cistswouldtalkaboutsomenewlaser,chemistswouldshowdiagramsofneworganic
materials,andsoforth.Itdidn’ttakelongforeachtalktoexceedtheunderstanding
ofmostlistenersofwhateverspecialtywasbeingdiscussed.Ialwaysleftwithapro-
foundsenseofthelimitationsofmyownknowledge,butwithanexhilaratinginkling
intotheunfoldingofscience.Itwas,perhaps,thebestoftheoldresearchmodelin
BellLabs,andinretrospectIcansaythatinthisnewcompetitiveworldImissthose
oldscientific-stylemanagementmeetings.
ItwasinsuchameetingthatIfirstheardaboutthefiberamplifier.Idon’tknow
whetherIhadbeenpayingattention,butIwasimmediatelygalvanizedbytheimpli-
cationsofthisnewdiscovery.Onewordcametomeandblazedacrossmymind:that
wordwas“transparency.”
Surprisingly, in my experience I am not always immediately enthusiastic about
anewtechnologyuponinitialexposure.Onemightthinkthatthepotentialsofgreat
breakthroughs are self-evident, but that does not seem to be the case. When I first
heardabouttheinventionofthelaser,Ihadnopremonitionthatlaserswouldbecome
the primary instrument of the world’s telecommunications traffic.When one of the
inventorsofpublickeycryptographytoldmehisideaforhavingtwokeys,Iscoffed
atthenaivetéofhisconcept.Irememberthinkingonfirsthearingaboutwhatisnow
theprincipalalgorithmfordatacompressionthatIthoughtitwasonlyatheoretical
exercise.Somytrackrecordforsuchinsightsisnotaltogethergood.
However,withthefiberamplifierIwenttotheotherextreme.Iforesawadramatic
revolution in communications. I spoke up at the staff meeting that morning to say
that this invention would transform the architecture of communications networks.
Thiswouldleadtotransparentnetworks,Isaid,andthatthiswouldnotnecessarily
begoodforAT&T.Igotcarriedawaywiththisvision,andwentontosaythatpri-
vatenetworkscouldhavetheirownwavelengthstravelingtransparentlythroughthe
network,untouchedbythecommoncarrierinthemiddle.Oneprivatenetworkmight
have“blue”light(figurativelyspeaking,ofcourse,becausewe’renottalkingabout
visible wavelengths) whereas another would have “green.” I foresaw a plug on the
wallthatpassedonlythechosenwavelength,whichwouldbeownedexclusivelyby
thatparticularcustomer’snetwork.AT&Twouldthusbedeprivedoftheopportunity
to process signals for value-added services.AT&T, in fact, wouldn’t have any idea
whatwaspackedintothosewavelengths.
Well,thathasn’texactlyhappened,buttoday’sopticalnetworksaremovingto-
wards increased transparency, and Raman amplifiers will accelerate this trend.The
advantagesoftransparencyarecompelling.Agreatmanyconstituentsignalscanbe
amplifiedcheaplyinonefellswoop.Moreimportantly,thisamplificationisindepen-
dentofthebitrates,protocols,waveforms,multiplexing,oranyotherparticularsof
thetransmissionformat.Thedesignisn’t“lockedin”toanyspecificformat,andas
thesedetailschange,theamplificationremainsaseffectiveasever.Inthecaseofthe
Raman amplifier, the bandwidth is so enormous that adjectives seem inadequate to
describeitspotentialforbulkamplification.
Transparencyinthenetworkissoattractivethatprobablytheonlyreasonitisn’t
done is that it is so difficult to achieve. One reason is, of course, the necessity for
periodicallyunbundlingthesignaltoaddordropsubcomponents.Inthedigitalworld
Foreword ix
thishasusuallymeantacompletedemultiplexingandremultiplexingoftheoverall
signal,anexpensiveoperation.Theopticalworldopensupthepossibilityofselective
transparency for certain wavelengths whereas others are unpacked to do add-drop
multiplexing.
Sonetworktopologysetslimitsontransparency.Buttheotherreasontransparency
ishardtoachieveistheimplicitaccumulationofimpairmentsasasignalincurssucces-
siveamplifications.Itisironicthatthetelephonenetworkwasessentiallytransparent
forthefirsthalf-centuryofitsexistence.Until1960thelong-haultransmissionsys-
tems used analogue amplification to boost levels as the signal traversed the nation.
Theinventionofthetriodevacuumtubeenabledthefirsttranscontinentaltransmis-
sionsystemtobedeployedinthe1920s.Itwasamarvelousfeattobeabletosenda
bandofsignals3000milesacrossthecountry,passingthroughmanyamplifiers,ac-
cumulatingnoiseanddistortionalongtheway,butstillprovidingintelligiblespeech
attheotherend.Someolderreaderswillrememberwhenlongdistancephonecalls
soundedcracklyand“distant.”Now,ofcourse,itisimpossibletotellhowfaraway
aconnectionis.Theyallsoundlocal,becauseofdigitaltransmission.
Digitaltransmissionwasthetriumphofthe1960s.Thoughnowitseemsobvious,
engineersfoundthephilosophyofdigitizationhardtograspforseveraldecadesafter
theinventionofpulsecodemodulationbyReevesin1939.Thereisatrade-offhere:
bandwidthagainstperfectibility.A3kHzvoicesignal,forexample,istransformedby
ananalogue-to-digitalconverterintoa64,000kbpsstreamofbits,greatlyexpanding
the necessary transmission bandwidth. However, this digital signal can be regener-
atedperfectly,removingnoiseanddistortionperiodicallyasnecessary.Amiracleis
achievedasthebitsarriveacrossthecountryinthesamepristineformaswhenthey
left.
So it was that all long distance transmission was converted to digital format.
The introduction of the first lightwave transmission systems hurried this change,
inasmuchaslightwavesystemsweredeemedtobe“intrinsicallydigital”becauseof
theirnonlinearitiesandthelackofamplifiers.Noonecaredmuchatthetime—the
early1980s—buttheentiredesignofthenetworkwaspredicatedonthetransmission
of64kbpsvoicechannels.Themultiplexinghierarchy,theelectronicswitching,the
synchronizationandtiming,andthetransmissionformatassumedthateverythingwas
packaged into neat little voice channels.That, of course, was before the rise of the
Internet.
Nowopticalamplificationhasreversedthistrendofthelasthalf-centurytowards
digitizationbaseduponahierarchyofvoicesignals.Itisn’tjustthatopticalamplifiers
haveanenormousbandwidth.Theydosomethingthoseoldtriodevacuumtubescould
neverdo:theyamplifywithoutsubstantiallyincreasingthenoiseanddistortionofthe
signal.Ramanamplifiersareparticularlygoodinthisway.Moreover,becauseRaman
amplificationisdistributedacrossthewholespanofthefiber,thesignallevelnever
drops as low as it does when discrete amplifiers are employed. In a system using
discrete amplifiers the signal level is at its lowest and most vulnerable right before
thepointofamplification.
Back at that research staff meeting I was concerned about the implications of
transparencytothearchitectureofthenetwork.Atransparentnetworkis,bydefinition,
a “dumb” network. It doesn’t do anything to the signal; it can’t, because it doesn’t