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