Optical Amplifiers VenkatVenkataramanan
Ins,tuteforOp,calSciences
UniversityofToronto
Plan
Introduc,on
Rare‐earthdopedFiberAmplifiers
RamanAmplifiers
SemiconductorOp,calAmplifiers
GeneralConsidera,ons
Summary
Amazing Bandwidth of Optical Fibers
Fiber‐gradesilicaistransparentfrom1µ ‒ 1.6µ. Thetotalbandwidthitcancarryis
Δλ ≈ c/λ1 ‒ c/λ2=3.0×1014–1.1×1014=11THz
Bandwithofatelephonesignalgoesfrom100Hz–3KHz.ByNyquistCriterion,oneconversa,onrequires2×3KHz=6KHz
So,thenumberofconversa,onsthatcanbesimultaneouslycarriedoverafiberisapproximately,
Nf=1.1×1014/3×103=36billion
So,inprincipleasinglefiberissufficienttocarryten,mesalltheconversionsgeneratedworldwide.
As the signal becomes weak
Amplifier:increasesthestrengthoftheop,calsignal.
Itisananalogdevice,sowhatyouputiswhatyouget;withsomenoise,ofcourse
Repeater:Convertsweakop,calsignalintoelectronicform,useselectronicsignaltodriveatransmi\erthatrecreatesthesignal
Areceiver–transmi\erplacedbacktoback
Regenerator:cleansupdigitalsignalbyremovingnoiseanddistor,onandregenera,ngafreshsignal
Theyhavediscrimina,oncircuitsthatexaminethe,me‐varyingsignal,iden,fiessignalandnoise;cleanthesignal
Why do we need Optical Amplifiers?
Typicalfiberlossaround1.5µmis~0.2dB/km.
A`ertraveling~100km,signalsarea\enuatedby~20dB,
Theyneedtobeamplifiedorsignal‐to‐noisera,o(SNR)ofdetectedsignalsistoolowandbiterrorrate(BER)becomestoohigh(typicallywantBER<10‐9)
Canbedonebydetec,ngtheweakenedsignals,thenmodula,nganewlaserwithmodula,onreadoffthedetectedsignals
This{Op,caltoElectricaltoOp,cal}conversionsrequirescostlyhigh‐speedelectronics(>10GHz)
Bestwaytoamplifyisop,cally,andbestop,calmethodisfiberamplifier(lowestloss,mostefficient,moststable)
Optical Amplifiers : Functions
Transmitter
Receiver
Power Booster
In-line Amplifier
Detector Pre-amplifier
100 km
Principle of Optical Amplification
SpontaneousEmissionversusS,mulatedEmission
Ingeneral,lightisabsorbedasitpropagates
Ifthepopula,onatthehigherenergystateishigherthanalowerstate,lightgetsamplifiedasittravelsthroughthemedium
Amplifiedlightbearsthecharacteris,csofthesignalphoton
E1
E3
E2
E1
E3
E2
Optical Amplifiers :: Characteristics
Anop,calamplifierischaracterizedby:
Gain:ra,oofoutputpowertoinputpower(indB)
Gainefficiency:gainasafunc,onofinputpower(dB/mW)
Gainbandwidth:rangeofwavelengthsoverwhichtheamplifieriseffec,ve
Gainsatura,on:maximumoutputpower,beyondwhichnoamplifica,onisreached
Noise:undesiredsignalduetophysicalprocessinginamplifier
Optical Amplifiers :: Types
Rare‐earthdopedFiberAmplifiers ErbiumDoped(EDFA)–1,500–1,600nmband
PraseodymiumDoped(PDFA)–1,300nmband
Raman(andBrillouin)Amplifiers
SemiconductorOp,calAmplifiers(SOAs)–400–2,000nmband
+ Erbium Doped Fiber Amplifiers
Erbium Doped Fiber Amplifier
• Commerciallyavailablesincetheearly1990’s
• Worksbestintherange1530to1565nm• Gainupto30dB(1000photonsoutperphotonin!
Inside an EDFA
Fibre input/output
Erbium doped fibre
loop
Pump laser WDM Fibre coupler
Erbium doped fiber :: Profile
Rare-earth doped fiber characteristics
Fiberisdrawnbysametechniqueascommunica,onfibers fabricateapreform(cm‐sizesilicatubecoatedwithcorematerial) heatandcollapsetube pullpreformintoafiber
Rare‐earthconcentra,on[RE2O3]istypicallylow afewhundredpartspermillion(ppm)insilicatehosts afewthousandppminfluorideandphosphatehosts
Networkmodifiers(Al,P,La)areaddedtothecoreglasstoincreasesolubilityofRE2O3insilica
Indexmodifiers(Ge,Al)areaddedtothecoretoraiseitsrefrac,veindex
Atusualconcentra,ons(<100ppm),RE2O3hasli\leeffectontheindexprofile
Completeinversioncanbeachievedwith980‐nmpumpingbutnotwith1480‐nmpumping.
Thespontaneouslife,meofthemetastableenergylevel(4I13/2)isabout10ms,whichismuchslowerthanthesignalbitratesofprac,calinterest.
Ass,mulatedemissiondominatesoverspontaneous,amplifica,onisefficient
Erbium doped fiber :: Amplification Process
Erbium‐dopedfiberisusuallypumpedbysemiconductorlasersat980nmor1480nm.
Athree‐levelmodelcanbeusedfor980‐nmpumps,whileatwo‐levelmodelusuallysufficesfor1480‐nmpumps.
At1480nm,silicafibershavelowloss;hencepumpcancopropagatewiththesignal
Pumpmayevenbeplacedremotely
Erbium doped fiber :: Operation
Higherthepopula,oninversionlowertheamplifiernoise
980nmpumpispreferredforlownoiseamplifica,on
Howevermorepowerful1480nmsourcesareavailable
1480
980 820
540
670
Ground state
Metastable state
EDFA :: Operating Wavelengths
SofarwehavefocusedonEDFAsopera,ngintheC‐band(1530‐1565nm).
Erbium‐dopedfiber,however,hasarela,velylongtailtothegainshapeextendingwellbeyondthisrangetoabout1605nm.
Thishass,mulatedthedevelopmentofsystemsintheso‐calledL‐bandfrom1565to1625nm.
Gainspectrumoferbiumismuchfla\erintrinsicallyintheL‐bandthanintheC‐band.
Thismakesiteasiertodesigngain‐fla\eningfiltersfortheL‐band.
PumppowersrequiredforL‐bandEDFAsaremuchhigherthantheirC‐bandcounterparts.
Gain Flatness
Popula,onlevelsatdifferentbandsvary
Andhencethegainvaria,on
SeriouslyaffectsWDMsystems
Toovercomethis Usefilterinsideamplifier
Userfluorideglassfiber
Gain performance of various glass fibers
Gain Flattening : A main issue
ThegainspectrumofanEDFA(oranyotherfiberamplifier)isnotuniform
A`ertravelingthroughafewamplifiers(e.g.,500km),differencebetweenpowerofthesignalsΔPexceeds5–10dBandthebiterrorrateistoostronglydegraded
Lower‐powerchannelsbecomeunusable
Filtersarethenintroducedtoselec,velyaddlossestohighgainwavelengths
Gain Flattening
Gain Dependence on fiber length
Gainisalwaysnega,veattheendofalongenoughfiber
Lengththatmaximizesgainincreaseswiththepumppower
Spectral gain dependence on pump power
Asthepumppowerincreasesgainshi`stowards1530nmfromLband
Atarightpumppowergainisflatoverawidewavelengthrange(GainFla\eningTechnique)
Noise : Amplified Spontaneous Emission
Thelightthatstartss,mulatedemissionoriginatesspontaneously
Whenexcitedstatereleasesitsenergywithouts,mula,on,spontaneousemissionoccurs
Asthistravelsthroughtheamplifyingmedium,thisspontaneousemissionalsogetsamplified
ThiscreatesabackgroundnoisecalledAmplifiedSpontaneousEmission
Thisbroadbandnoiseisverysimilartosta,cinAMRadio
Random spontaneous emission (SE)
Amplification along fiber
EDFA : Parameters
Widebandwidth‐40nm(5000GHz)
Highamplifica,on‐30do40dB
Highoutputpower‐do+20dBm(100mW)
Lownoise‐4dB(NoisefactorF)
Pumpwavelength‐980or1480nm
Nodispersioncompensa,on
EDFA : Schematic
Schema,cdiagramofatwo‐stageerbium‐dopedfiberamplifierforlownoisefigureandhighoutputpoweropera,on
EDFA : Advantages and Disadvantages
EDFAshavehighpumppoweru,liza,on(>50%)
Directlyandsimultaneouslyamplifyawidewavelengthband(>80nm)inthe1550nmregion,witharela,velyflatgain.
Flatnesscanbeimprovedbygain‐fla\eningop,calfilters
Gaininexcessof50dB
LownoisefigureSuitableforlonghaulapplica,ons
EDFAsarenotsmall
Cannotbeintegratedwithothersemiconductordevices
+ Raman Amplifiers
Raman Amplifier
Topologicallysimplertodesign–nospecialdopingisrequired
Usesintrinsicop,calnonlinearityoffiber
Amplifica,ontakesplacethroughoutthelengthoftransmissionfiber
HencealsoknownasDistributedAmplifier
Distributed Amplification
Ramanpumpingisusuallydonebackwards
Gainishigheratthereceiverendofthefiberthanatthetransmi\erend
Thisisveryusefulincompensa,ngforthelosses
Transmitter Optical
Receiver EDFA
Raman Pump Laser
Long Fibre Span
Raman Amplifier
UsesintrinsicpropertyofSilicafiberMediumoftransmissionitselfcombatssignalloss
ThephysicsbehindiscalledS,mulatedRamanSca\ering
Highenergypumpwaveisco‐launchedintothefiber
Ramangaindependsonthepumppowerandfrequencyoffsetbetweenpumpandsignal
Pumpphotongivesupenergytocreateasignalphoton
Residualvibra,onalenergyisabsorbedasphonons
AmplifiedSignal
SignalPump
Phonon Relaxation
Typical configuration
Transmitted
Received
Advantages of backward pumping
BackwardpumpinginRamanamplifierhasseveraladvantages
Ramanisanalmostinstantaneousprocess
PumpnoisestronglyaffectsWDMsignalstobeamplifiedifforwardpumpingisapplied
Ifpumphasaslightfluctua,onin,me,individualbitsareamplifieddifferently
Thisleadstoamplitudefluctua,onsorji\er
Inbackwardpumping,powerfluctua,onsareaveragedout
Raman Gain in Fiber
Dependsmainlyontheop,calfrequencies;butalsoonthepumpfrequencyandpolariza,on
ThereisamaximumRamangainforafrequencyoffsetof13.2 THz.Forexample,apumpwaveat1064 nmleadstothelargestRamangainatasignalwavelengthof1116 nm.
ThepeaksintheRamanspectrumcorrespondtocertainvibra,onmodesofthesilicastructure.
Theusablegainbandwidthisabout48nm
Advantages and Disadvantages
Advantages Variable wavelength amplification possible
Compatible with installed SM fibre
Can be used to "extend" EDFAs
Can result in a lower average power over a span, good for lower crosstalk
Very broadband operation may be possible
Disadvantages High pump power requirements, high pump power lasers have
only recently arrived
Sophisticated gain control needed
Noise is also an issue
EDFA – Raman Amplifiers : Comparison
Characteristic EDFA Raman Amplification band Dopant dependant Depends on pump
wavelengths
Bandwidth 20 nm, More for multiple dopants 48 nm, more for multiple pumps
Gain 20 dB or more; depends on ion concentration, fiber length and pump config
4-11 dB, proportional to pump intensity and eff. fiber length
Saturation Power Depends on gain and material constants Equals about power of pump
Wavelength 980 nm or 1480 nm for EDFA 100 nm lower than signal at peak gain
Combined EDFA and Raman Amplifications
With only an EDFA at the transmit end the optical power level decreases over the fiber length
With an EDFA and Raman the minimum optical power level occurs toward the middle, not the end, of the fiber.
Distance
Opt
ical
Pow
er
EDFA +
Raman
EDFA only
+ Semiconductor Optical Amplifiers
Semiconductor Optical Amplifier
Anelectricalcurrentispassedthroughthedevicethatexciteselectronsintheac,veregion.
Whenphotons(light)travelthroughtheac,veregionitcancausetheseelectronstolosesomeoftheirextraenergyintheformofmorephotonsthatmatchthewavelengthoftheini,alones.
Therefore,anop,calsignalpassingthroughtheac,veregionisamplifiedandissaidtohaveexperienced"gain."
• Bothedges(or"facets")oftheSOAaredesignedtohaveverylowreflec,vitysothattherearenounwantedreflec,onsofthesignalwithinthesemiconductoritself.
• Thisisthemaindifferencefromregularlasersthathavereflec,vefacetsinordertobuilduptheintensityoflightwithinthesemiconductormaterial.
SOA :: Amplification Process
SemiconductorshaveValenceandConduc,onbands
Atthermalequilibriumvalencebandhashigherpopula,on
Underpopula,oninversioncondi,onconduc,onbandwillhavehigherpopula,on
Popula,oninversionisachievedbyforwardbiasingthep‐njunc,on
E1
E2
ValenceBand
ConductionBand
signal
hole electron
SOA :: Design
SOA vs Semiconductor Laser
Bothareverysimilarinprincipleandconstruc,on
Essen,allyFabry‐Perotcavi,es,withamplifica,onachievedbyexternalpumping
ThekeyinSOAispreven,ngself‐oscilla,onsgenera,nglaseroutput
Thisisaccomplishedbyblockingcavityreflec,onsusingbothanan,reflec,on(AR)coa,ngandthetechniqueofanglecleavingthechipfacets.
SOAsareelectricallypumpedbyinjectedcurrent
SOA :: Characteristics
Compact>onlyasmallsemiconductorchipwithelectricalandfiberconnec,ons.
Theoutputpowersaresignificantlysmaller.
Thegainbandwidthissmaller,butdevicesopera,ngindifferentwavelengthregionscanbemade.
Theupper‐statelife,meandthusthestoredenergyaremuchsmaller,sothatthegainreactstochangesinpumppowerorsignalpowerwithinnanoseconds(insteadofmilliseconds).
Changesingainalsocausephasechangesleadingtolinewidthenhancementfactor.
SOAsexhibitmuchstrongernonlineardistor,ons{self‐phasemodula,onandfour‐wavemixing}.
Thenoisefigureistypicallyhigher.
Theamplifica,onisnormallypolariza,on‐sensi,ve.
SOA :: Gain vs Signal Power
InSOAsthegaindynamicsaredeterminedbythecarrierrecombina,onlife,me(fewhundredpicoseconds).
Theamplifiergainwillreactrela,velyquicklytochangesintheinputsignalpower.
Thisdynamicgaincancausesignaldistor,on,whichbecomesmoresevereasthemodulatedsignalbandwidthincreases.
Theseeffectsareevenmoreimportantinmul,channelsystemswherethedynamicgainleadstointerchannelcrosstalk.
ThisisincontrasttoEDFAs,whichhaverecombina,onlife,mesoftheorderofmillisecondsleadingtonegligiblesignaldistor,on.
SOAsalsoexhibitnonlinearbehaviour=>problemssuchasfrequencychirpingandgenera,onofintermodula,onproducts.
Nonlineari,escanalsobeofuseinSOAsasfunc,onaldevicessuchaswavelengthconverters.
SOA :: Applications
Powerbooster–Immediatelya`erLaserDiode
In‐lineamplifier
Detectorpreamplifier
Op,calswitchingelement
Wavelengthconverter
Optical Amplifiers : Comparison
Property EDFA Raman SOA Gain (dB) > 40 > 25 >30
Wavelength (nm) 1530-1560 1280-1650 1280-1650
Bandwidth (3dB) 30-60 Pump dependent 60
Max. Saturation (dBm) 22 0.75 × pump 18
Polarization Sensitivity No No Yes
Noise Figure (dB) 5 5 8
Pump Power 25 dBm >30 dBm < 400 mA
Time Constant 10-2 s 10-15 s 2 x 10-9
Size Rack mounted Bulk module Compact
Switchable No No Yes
Cost Factor Medium High Low
Considerations Powerbooster:Placedimmediatelya`ertransmi\er.Helpincreasethepowerof
thesignal,noisemaynotbethemajorissue SOA
In‐lineamplifier:Compensateforthesignala\enua,onasitpropagates.Neededinlong‐haulnetworks.Noiseplaysaconsiderableroleasthesignalweakens Combina,onofEDFA,FiltersandRamanAmplifiers
Preamplifier:Aweakop,calsignalisusuallyamplifiedbeforeitentersthereceiver.Noiseisacrucialfactor
Transmitter
Receiver
Power Booster
In-line Amplifier
Detector Pre-amplifier
100 km
Other doped fiber amplifiers
Band Name Meaning Wavelength (nm) Technology
O Original 1260-1360 Praseodymium
E Extended 1360-1460 -
S Short 1460-1530 Thulium fiber
C Conventional 1530-1565 Erbium fiber
L Long 1565-1625 Erbium fiber
U Ultra-long 1625-1675 -
References
Erbium‐DopedFiberAmplifiersbyPhilippeC.Becker,N.AndersOlsson,andJayR.Simpson
Rare‐Earth‐DopedFiberLasersandAmplifiers,RevisedandExpandedbyMichelJ.F.Digonnet
RamanAmplifica,oninFiberOp,calCommunica,onSystems(byCliffordHeadleyandGovindAgrawal
RamanAmplifiersforTelecommunica,ons1:PhysicalPrinciplesbyMohammadN.Islam
Ramanamplifica,ondesigninWDMsystems,WebProforumTutorials,h\p://www.iec.org
andvariousinternetresources
Questions?!
Thankyou