Fast Reconfigurable SOA-Based Wavelength Conversion of Advanced Modulation Format Data

15 Pages·2017·2.7 MB·English

applied
sciences
Article
Fast Reconfigurable SOA-Based Wavelength
†
Conversion of Advanced Modulation Format Data
YiLin1,*,AravindP.Anthur1,SeanP.ÓDúill1 ID,FanLiu2,YonglinYu2andLiamP.Barry1
1 SchoolofElectronicEngineering,DublinCityUniversity,Dublin9,Ireland;[email protected](A.P.A.);
[email protected](S.P.ÓD.);[email protected](L.P.B.)
2 WuhanNationalLaboratoryforOptoelectronics,HuazhongUniversityofScienceandTechnology,
Wuhan430073,China;fl[email protected](F.L.);[email protected](Y.Y.)
* Correspondence:[email protected];Tel.:[phone]
† ThispaperisanextendedversionofpaperpublishedintheTheOpticalFiberCommunicationConference
andExhibition,OFC2017heldinLosAngeles,19–23March2017.
Received:31August2017;Accepted:3October2017;Published:10October2017
Abstract: We theoretically analyze the phase noise transfer issue between the pump and the
wavelength-converted idler for a nondegenerate four-wave mixing (FWM) scheme, as well as
studythevectortheoryinnonlinearsemiconductoropticalamplifiers(SOAs), inordertodesign
a polarization-insensitive wavelength conversion system employing dual co-polarized pumps.
A tunable sampled grating distributed Bragg reflector (SG-DBR) pump laser has been utilized
to enable fast wavelength conversion in the sub-microsecond timescale. By using the detailed
characterizationoftheSGDBRlaser,wediscussthephasenoiseperformanceoftheSGDBRlaser.
Finally, we present a reconfigurable SOA-based all-optical wavelength converter using the fast
switchingSGDBRtunablelaserasoneofthepumpsourcesandexperimentallystudythewavelength
conversion of the single polarization quadrature phase shift keying (QPSK) and polarization
multiplexed(Pol-Mux)QPSKsignalsat12.5-Gbaud. Awidetuningrange(>10nm)andlessthan
50nsand160nsreconfigurationtimehavebeenachievedforthewavelengthconversionsystemfor
QPSKandPM-QPSKsignals,respectively. Theperformanceundertheswitchingenvironmentafter
therequiredreconfigurationtimeisthesameasthestaticcasewhenthewavelengthsarefixed.
Keywords: all-opticalnetworks;wavelengthconversiondevices;tunablelasers
1. Introduction
The massive growth in the demand for bandwidth for multimedia services and interactive
networksisshapinganewerafortoday’scommunicationnetworks. Futurenetworksneedtooffer
bandwidth-hungryapplicationsliketelemedicine,IP-TV,virtualrealitygaming,video-on-demand,
andhigh-speedinternetaccess,combinedwithguaranteedQualityofService[1]. Inopticalnetworks
thatemploywavelengthdivisionmultiplexed(WDM),theuseofopticalswitchingtechnologiesona
burstorpacketlevel,combinedwithadvancedmodulationformats,wouldachievegreaterspectral
efficiencyandutilizetheexistingbandwidthmoreefficiently. All-opticalwavelengthconverterswhich
typicallycompriseoftunablepumplasers,nonlinearmedia,andthetunableopticalfilter,areexpected
tobeoneofthekeycomponentsinthesebroadbandnetworks. Theycanbeusedtointerfacedifferent
networks and potentially increase the capacity of a communication system [2,3]. The wavelength
converterscanbealsousedatthenetworknodestoavoidcontentionandtodynamicallyallocate
wavelengthstoensureoptimumuseoffiberbandwidth[4].
Recently,significantworkhasbeenundertakenonwavelengthconversionbasedonfour-wave
mixing(FWM)byusingdifferentnonlineardevicessuchasnonlinearsemiconductoropticalamplifiers
(SOAs)[5],highlynonlinearfiber(HNLF)[6–8],periodicallypoledLithiumNiobate(PPLN)[9],and
Appl.Sci.2017,7,1033;doi:10.3390/app7101033 www.mdpi.com/journal/applsci
Appl.Sci.2017,7,1033 2of15
nonlinear waveguides [10–12]. Nonlinear SOAs give the best conversion efficiency among these
nonlinear devices because they are active devices [13]. To perform the wavelength conversion of
datasignalswheretheinformationisencodedontothephaseandamplitudeoftheopticalcarrier,
e.g., quadrature phase shift keying (QPSK) and 16-ary state quadrature amplitude modulation
(16-QAM),thewavelengthconversionprocessmustpreservetheamplitudeandphaseinformation.
Therefore, FWM is required as the wavelength conversion process because FWM preserves the
amplitudeandphaseinformationoftheinputsignal. Inaddition,FWMistransparenttothedifferent
modulationformatswheretheinformationcanbeencodedintheamplitude,phase,andpolarization
oftheopticalcarrier[14],andisalsotransparenttothesignalbaudrate(thewavelengthconversion
oftheon-offkeying(OOK)signalat100Gbaudhavingbeendemonstratedin[15]). Recently,alotof
workhasbeendoneonthewavelengthconversionofdatasignalsemployingdifferentmodulated
formatsincluding(Differential)QPSK,8-PhaseShiftKeying(PSK),16-QAM,and64-QAMbasedon
FWMinSOAsusingsinglepumpanddualpumpwavelengthconversionsystems[16–27].
In-additiontophaseandamplitude,opticalcommunicationnetworksareemployingpolarization
multiplexed(Pol-Mux)modulationformatstoachievetwicethespectralefficiencybyencodingdata
ontwoorthogonalpolarizationsofthelight. Whendataispresentintwoorthogonalpolarizations,
itisveryimportanttoensurethatFWM-basedwavelengthconvertersaretransparenttoPol-Mux
modulationformats. VectortheoryofFWMinnonlinearmediaiswellunderstoodandutilizedfor
achievingpolarization-insensitivewavelengthconversion[28]. Vectortheorywasstudiedandverified
innonlinearSOAsandthiswasutilizedtodesignapolarization-insensitivewavelengthconversion
schemeforPol-Muxmodulationformatsutilizingdualco-polarizedpumpsinourearlierwork[29].
Utilizingtheunderstandingfromthesephasenoiseandpolarizationstudies,wedesignedthemost
efficientandimpairment-freewavelengthconverterbasedonFWM.Weusethisdesigntostudythe
dynamiccharacteristicsoftheFWM-basedwavelengthconverterandpresenttheresults,whenthe
dataiscommunicatedinbursts/packets.
OneimportantissuethatneedstobeconsideredwhenusingFWMforall-opticalwavelength
conversionofdatasignals,wheretheinformationisencodedontothephaseoftheopticalcarrier,isthe
phasenoisetransferissuebetweenthepumpandtheconvertedidler. In[30],asimplerelationship
betweenthelinewidthofthesignal,pump,andconvertedidlerintheFWMprocesswasfoundby
theoreticalanalysisandverifiedexperimentally. ForthecaseofthesinglepumpFWMschemeshown
inFigure1a,thelinewidthoftheconvertedidleristhelinewidthofthesignalplusfourtimespump
linewidth,andforthecaseofthedualpumpFWMschemeshowninFigure1b,thelinewidthofthe
convertedidlerisequaltothesummedlinewidthsofthesignalandthedualpumps. Thephasenoise
transferproblemwillcauseasignificanteffectfortheFWM-basedwavelengthconversionofdatawith
phaseencoding,asitbecomesmoredifficulttorecoverthedatawiththeinduceduncertaintyinthe
phaseofthewavelengthconvertedidler. Thus,thelinewidthisanimportantparameterforthepump
lasertobeemployedintheFWM-basedwavelengthconversionsystem.
An example of a typical all-optical packet-switched network which employs wavelength
conversionisillustratedinFigure1c. IPpacketsenterthenetworkthroughtheedgerouterwhere
theyareretransmittedonanewwavelengthtoavoidcontention[31]. Byemployingtheall-optical
wavelengthconverterinthisnetwork,itcanenablerapidroutingofthesamewavelengthchannels
fromanydirectiontoanydirection,andeasilyavoidthecontentioninwhichdifferentpacketswith
thesamewavelengthsaretryingtoleavetheedgerouter. Thewavelengthconverterconsistsoffast
tuning tunable lasers as the pumps, an SOA, and a tunable optical filter. The wavelength of the
wavelength-convertedsignalthroughFWMcanbepreciselyalteredbytuningthewavelengthofthe
pumpsources,andafterthenonlinearwavelengthconversionprocessintheSOA,theconvertedsignal
isthenfilteredoutbyusingatunableopticalfilterandsenttothenextnetworknode. Theswitching
speed of this SOA-based wavelength converter mainly depends on the tunable pump lasers and
thetunableopticalfilter[32],butinthisworkwefocusontheeffectofthetunablelaser. Thelaser
wavelengthtuningspeedcanbeaslowasseveralnsforvarioustunablelaserdesignssuchasthe
Appl. Sci. 2017, 7, 1033 3 of 15
Appl.Sci.2017,7,1033 3of15
the sampled grating distributed Bragg reflector (SG-DBR) laser [33], the digital super-mode
distributed Bragg grating (DS-DBR) laser [34], and the modulated grating Y-branch (MGY) laser [35].
sampledgratingdistributedBraggreflector(SG-DBR)laser[33],thedigitalsuper-modedistributed
These lasers can also achieve a wide tuning range (>45 nm), a high side mode suppression ratio
Bragggrating(DS-DBR)laser[34],andthemodulatedgratingY-branch(MGY)laser[35]. Theselasers
(SMSR > 35 dB), and large output power (>10 dBm). All the components in the wavelength converter
canalsoachieveawidetuningrange(>45nm),ahighsidemodesuppressionratio(SMSR>35dB),
(lasers and SOA) can be integrated and these results motivate the construction of a compact, optically-
andlargeoutputpower(>10dBm). Allthecomponentsinthewavelengthconverter(lasersandSOA)
integrated, and rapidly reconfigurable all-optical wavelength converter.
canbeintegratedandtheseresultsmotivatetheconstructionofacompact,optically-integrated,and
Most of the previous research on wavelength conversion is undertaken by using static, fixed-
rapidlyreconfigurableall-opticalwavelengthconverter.
wavelength lasers. However, wavelength converters with rapid, dynamic wavelength
Most of the previous research on wavelength conversion is undertaken by using static,
re-configurability would be required to bypass the power-hungry electronic switches with high
fixed-wavelength lasers. However, wavelength converters with rapid, dynamic wavelength
latency in the switching nodes of future transparent optical networks. The main focus of this paper
re-configurabilitywouldberequiredtobypassthepower-hungryelectronicswitcheswithhighlatency
is thus to develop a wavelength converter comprised of an SOA as the nonlinear element and a fast-
intheswitchingnodesoffuturetransparentopticalnetworks. Themainfocusofthispaperisthusto
switching SG-DBR tunable laser as one of the pump sources. In Section 2, we initially discuss the
developawavelengthconvertercomprisedofanSOAasthenonlinearelementandafast-switching
theory of the phase noise transfer issue in FWM and the dual co-polarized pumping scheme. In
SG-DBRtunablelaserasoneofthepumpsources. InSection2,weinitiallydiscussthetheoryofthe
Section 3, using the detailed characterization of the SGDBR laser, we discuss the phase noise of the
phasenoisetransferissueinFWMandthedualco-polarizedpumpingscheme. InSection3,usingthe
SGDBR. In Section 4, we provide a complete set of experimental results for single polarization QPSK
detailedcharacterizationoftheSGDBRlaser,wediscussthephasenoiseoftheSGDBR.InSection4,
[36] and experimentally demonstrate rapid wavelength conversion of a PM-QPSK signal with the
weprovideacompletesetofexperimentalresultsforsinglepolarizationQPSK[36]andexperimentally
switching time of tens of nanoseconds using a fast-switching, tunable laser as one of the pumps in a
demonstraterapidwavelengthconversionofaPM-QPSKsignalwiththeswitchingtimeoftensof
dual wavelength pumping scheme. After the reconfiguration time, the performance under a
nanosecondsusingafast-switching,tunablelaserasoneofthepumpsinadualwavelengthpumping
switching scenario is the same as the case when the wavelengths are held static. The experimental
scheme. Afterthereconfigurationtime,theperformanceunderaswitchingscenarioisthesameasthe
results indicate that the incoming signal can be precisely and quickly converted to the required
casewhenthewavelengthsareheldstatic. Theexperimentalresultsindicatethattheincomingsignal
wavelength channel on a nanosecond timescale.
canbepreciselyandquicklyconvertedtotherequiredwavelengthchannelonananosecondtimescale.

Source
node
Input signal Core
router
SOA TOF Edge
Tunable laser
Control signals router
Tunable optical filter
(a) Edge
Core
Input signal router
router
Control signals Tunable laser SOA TOF
Tunable laser Tunable optical filter Optical core network Destination
Control signals
(b) (c) node
Figure 1. (a) A single pump semiconductor optical amplifier (SOA)-based wavelength converter
Figure 1. (a) A single pump semiconductor optical amplifier (SOA)-based wavelength converter
employing one fast tunable pump laser. (b) A dual-pump SOA-based wavelength converter
employingonefasttunablepumplaser.(b)Adual-pumpSOA-basedwavelengthconverteremploying
employing fast tunable pump lasers where the output wavelengths can be chosen by appropriately
fasttunablepumplaserswheretheoutputwavelengthscanbechosenbyappropriatelyselectingthe
selecting the wavelength of the tunable lasers. The wavelengths of the tunable lasers are adjusted by
wavelengthofthetunablelasers. Thewavelengthsofthetunablelasersareadjustedbythecontrol
the control signals to the SG-DBR devices. (c) Schematic of an all-optical packet-switched network.
signalstotheSG-DBRdevices.(c)Schematicofanall-opticalpacket-switchednetwork.
2. Theory
2. Theory
22.1.1. .PPhhaases eNNooisies eaanndd FFWWMM
Consider two pumps with electric field intensities given by (cid:1831)(cid:4652)(cid:1318)→ and (cid:1831)(cid:4652)(cid:1318)→ mixing with a signal
Considertwopumpswithelectricfieldintensitiesgivenby E(cid:3043)(cid:2870) and E(cid:3043)(cid:2869) mixingwithasignal
having an electrical field intensity of →(cid:1831)(cid:4652)(cid:1318)(cid:3046), generating an idler throughp 2FWM wpi1th a field intensity of
havinganelectricalfieldintensityof E ,generatinganidlerthroughFWMwithafieldintensityofE.
(cid:1831). Let the frequencies of the mixing psumps and the signal be represented as (cid:2033) , (cid:2033) , and (cid:2033) , i
(cid:3036) (cid:3043)(cid:2869) (cid:3043)(cid:2870) (cid:3046)
Letthefrequenciesofthemixingpumpsandthesignalberepresentedasω ,ω ,andω ,respectively.
respectively. Let the phase of the mixing pumps and the signal be represepn1tedp 2as (cid:2038) , s(cid:2038) , and (cid:2038)
(cid:3043)(cid:2869) (cid:3043)(cid:2870) (cid:3046)
Letthephaseofthemixingpumpsandthesignalberepresentedasφ ,φ ,andφ respectively. The
respectively. The frequency and the phase of the idler are related to thpe1 mipx2ing freqsuencies by,
frequencyandthephaseoftheidlerarerelatedtothemixingfrequenciesby,
(cid:2033) = (cid:2033) − (cid:2033) + (cid:2033) (1)
(cid:3036) (cid:3043)(cid:2870) (cid:3043)(cid:2869) (cid:3046)
ω = ω − ω + ω (1)
i p2 p1 s
(cid:2038) = (cid:2038) − (cid:2038) + (cid:2038) (2)
(cid:3036) (cid:3043)(cid:2870) (cid:3043)(cid:2869) (cid:3046)
φ = φ − φ + φ (2)
i p2 p1 s
Appl.Sci.2017,7,1033 4of15
Unlessbothpumpshavecorrelatedphasenoise,thephasenoiseoftheidlerisrelatedtothephase
noise of the mixing frequencies by the following relationship when the phase noise of the mixing
frequenciesisuncorrelated[16,30],
∆σ2 = ∆σ2 +∆σ2 +∆σ2 (3)
i p1 p2 s
where∆σ2representsthevarianceofthephasenoise. Forwhitefrequencyphasenoise,thelinewidth
(representedby∆ω)andthephaseerrorvariancearelinearlyrelated[37]. Therefore,thelinewidthof
theidlerduetowhitephasenoiseisrelatedtothelinewidthofthemixingpumpsandthesignalby,
∆ω = ∆ω + ∆ω + ∆ω (4)
i p1 p2 s
ThusitcanbeobservedfromEquation(4)thatthephasenoiseoftheidlerwillbethesumofthe
phasenoiseofthemixingfrequencies.
2.2. FWMwithDualCo-PolarizedPumps
WenextlookatthevectortheoryofFWMinnonlinearSOA.Thestatesofpolarizationofthe
two co-polarized pumps and signal are shown in Figure 2a. Pump1 and pump2 are co-polarized.
Forsimplicity,weconsideralinearlypolarizedinputsignalatanarbitraryangleθwithrespecttothe
pumps.TheoutputspectrumofthisFWMschemeisgiveninFigure2b.Theconvertedidlersgenerated
bythebeatingbetweenthedualco-polarizedpumpshaveopticalfrequenciesω −ω +ω and
p1 p2 s
ω −ω +ω . WeshowthattheFWMwavelengthconversionschemepreservesthepolarization
p2 p1 s
properties of the signal within the idler at frequency ω −ω +ω . The electric field of the two
p2 p1 s
→ → →
co-polarizedpumps(E and E )andsignal(E )aregivenby,
p1 p2 s
→ (cid:0) (cid:0) (cid:1)(cid:1)→
E = A exp j ω t+φ x (5)
p1 p1 p1 p1
→ (cid:0) (cid:0) (cid:1)(cid:1)→
E = A exp j ω t+φ x (6)
p2 p2 p2 p2
→ (cid:104) → →(cid:105)
E = A exp(j(ω t+φ )) cos(θ)x +sin(θ)y (7)
s s s s
whereA ,A ,andA aretheamplitudesofpump1,pump2,andthesignal,respectively. Theidler
p1 p2 s
atafrequencyofω = ω − ω +ω isgeneratedbytheresultantoftwobeatingprocessesgiven
i p2 p1 s
by[38–40],
→ (cid:18)→ →∗ (cid:19)→ (cid:18)→ →∗ (cid:19)→
(cid:0) (cid:1) (cid:0) (cid:1)
E =r ω − ω E ·E E +r ω −ω E ·E E , (8)
i p2 p1 p2 p1 s s p1 s p1 p2
(cid:0) (cid:1) (cid:0) (cid:1)
wherer ω − ω istheefficiencyofthebeatingprocessbetweenthetwopumpsand r ω − ω
p1 p2 s p1
istheefficiencyofthebeatingbetweenthesignalandthepump1[41].
In the case of co-polarized pumps scheme, we tune the wavelengths of the two pumps such
thattheinputsignalisfarenoughfromthepumpsinordertoavoidtheirinteraction,whichmeans
(cid:12) (cid:12) (cid:12) (cid:12)
(cid:12)ωp2−ωp1(cid:12) (cid:28) (cid:12)ωs−ωp1(cid:12). Therefore,theelectricalfieldoftheidlercanbegivenby,
→ (cid:18)→ →∗ (cid:19)→
(cid:0) (cid:1)
E =r ω −ω E ·E E (9)
i p2 p1 p2 p1 s
SubstitutingEquation(5)toEquation(7)inEquation(9)gives,
→ →
E = r(cid:0)ω −ω (cid:1)A∗ A E exp(cid:2)j(cid:0)ω −ω +ω (cid:1)t+(cid:0)φ −φ +φ (cid:1)(cid:3) (10)
i p2 p1 p1 p2 s p2 p1 s p2 p1 s
ItcanbeobservedfromEquation(10)thatthegeneratedidlerhasthesamepolarizationasthat
oftheinputsignal,andtheoutputpowerisindependentofthestateofthepolarizationoftheinput
Appl.Sci.2017,7,1033 5of15
signal. AssumingtheSOAispolarizationindependent,thedualco-polarizedpumpingschemecanbe
usedtoconvertasignalwitharbitrarypolarizationwithaconstantconversionefficiencyandsingle
nonlinearSOA.
Appl. Sci. 2017, 7, 1033 5 of 15
Y
Pump1 Pump2
Signal
Δω
Signal
Idler
Idler
Δω Δω
θ
X ω
Pump1 ωp1 ωp 2 ωp1-ωp2+ωs ωs ωp2-ωp1+ωs
Pump2
(a) (b)
Figure 2. Four-wave mixing (FWM) scheme with dual co-polarized pumps. (a) The X and Y axes
Figure2. Four-wavemixing(FWM)schemewithdualco-polarizedpumps. (a)TheXandYaxes
denote orthogonal states of polarization. In the dual pump FWM scheme, the polarization state of the
denoteorthogonalstatesofpolarization.InthedualpumpFWMscheme,thepolarizationstateofthe
wavelength-converted idler is that of the input signal provided both pumps have the same
wavelength-convertedidleristhatoftheinputsignalprovidedbothpumpshavethesamepolarization.
polarization. (b) Spectral locations of the pumps, signal, and idlers.
(b)Spectrallocationsofthepumps,signal,andidlers.
3. Linewidth Characterization of SGDBR Pump Lasers for Wavelength Conversion
3. LinewidthCharacterizationofSGDBRPumpLasersforWavelengthConversion
Among the tunable lasers, the SGDBR laser has been proven to be a suitable fast tunable laser
Acamndoindagteth [4e2t]u, annadb tlheel satsreurcstu,trhe eofS tGhiDs tByRpel oafs elarsehra iss sbheoewnnp irno Fviegnurteo 3ba.e Ita iss ucoimtapbrliesefdas otf tounnea gbalien laser
candisdeacttieon[4, 2o]n,ea pnhdatshe esescttriuocnt,u arnedo tfwtoh igsratytipneg osefcltaiosenrs.i Dssuhe otow tnhein peFriigoudr ien 3wah.iIcthi sthceo mtwpor gisreadtinogfso anree gain
sampled differently, each grating has different period of reflection maxima. Therefore, the Vernier
section,onephasesection,andtwogratingsections. Duetotheperiodinwhichthetwogratingsare
effect is enabled and enhances the overall tuning range considerably. The function of the phase
sampleddifferently,eachgratinghasdifferentperiodofreflectionmaxima. Therefore,theVernier
section is to allow the cavity modes to be shifted independently of the grating’s reflection peak,
effectisenabledandenhancestheoveralltuningrangeconsiderably. Thefunctionofthephasesection
achieving continuous tuning. As the data is modulated on the phase of the laser in the transmission
istoallowthecavitymodestobeshiftedindependentlyofthegrating’sreflectionpeak,achieving
systems employing advanced modulation formats, the phase noise of the lasers becomes an
contiinmupoourstatnut nfainctgo.r Ains dtheteerdmaitnainigs mthoe dtrualnastmedissoionnt hpeerpfohramsaenocef t[h43e].l aFsoerr thine tShGeDtBraRn lsamseirs, sbiootnh sthyes tems
emplogayiinn gseactdiovna nancedd thme opdasusliavtei otunnfionrgm seacttsi,otnhse cpanh acosentnriobiustee otof tthhee olavseerrasll bpehcaosme neosiasen oifm thpeo lratsaenrt [4fa4c].t orin
deterImn ionridnegr ttoh echtraarancstmeriizses itohne ppehrafsoer nmoaisne coef [t4h3e] .SGFoDrBtRh elaSseGr,D thBeR frlaeqseure,nbcoyt mhothdeulgataiionn s(eFcMti)o nnoaisned the
passivspeetcutrnuinmg, swehcitciohn hsacsa bneecno nptrroibvuedte toto bteh ea ovvereyr asluliptahbales emneoaissueroemftehnet loafs ethre[ 4p4h]a.sIen nooridsee rotfo lacshearrsa tcot erize
be employed in coherent systems [45], can be obtained by the technique using a coherent receiver
thephasenoiseoftheSGDBRlaser,thefrequencymodulation(FM)noisespectrum,whichhasbeen
with a narrow-linewidth LO [37]. As shown in Figure 3b, white noise from the gain section and low
provedtobeaverysuitablemeasurementofthephasenoiseoflaserstobeemployedincoherent
frequency carrier noise from the passive tuning sections are observed. Due to the advance in digital
systems[45], canbe obtainedbythe techniqueusing acoherent receiverwith anarrow-linewidth
signal processing (DSP), the low frequency excess phase noise can be compensated. A 2nd-order PLL
LO[37]. AsshowninFigure3b,whitenoisefromthegainsectionandlowfrequencycarriernoise
scheme has been used in the carrier phase estimation to track the excess phase noise [46], which
fromthepassivetuningsectionsareobserved. Duetotheadvanceindigitalsignalprocessing(DSP),
presents the ability to employ fast tunable laser for higher order modulation formats.
thelowfrequencyexcessphasenoisecanbecompensated. A2nd-orderPLLschemehasbeenusedin
thecarrierphaseestimationtotracktheexcessphasenoise[46],whichpresentstheabilitytoemploy
fasttunablela(sae)rforhigherordermodulationformats. (b)
Apart from that, the phase noise of the SGDBR laser isCstarrorinegr lnyodiseependent on the injection
currentsoneacFhrosnetctionG,aainndthPehalsineewBidatchkvariesbetweenseveral100’skHztoseveralMHzwith
differentinjectedcurrents. TheoutputwavelengthandphasenoiseoftheSGDBRlaserasafunctionof
theinjectedcurrentsinthefrontsectionaremeasuredandshowninFigure4,with90mAcurrenton
thegainsectionand0mAcurrentonthephaseandbacksection. Thewhitenoiseandcarriernoise
arerepresentedbythehighfrequencylinewidthandlowfrequeWnchyitlein neowisiedth,whicharecalculated
fromtheFM-noisespectrumbythesimpleequation: ∆ν=π···S(f)[47]. Thehighfrequencylinewidth
isrelatedtotheFMnoiselevelinthefrequencyrangebeyond500MHz,whereasthelowfrequency
linewidthcontributedbythepassivetuningsectionsisextractedbythemeanvalueoftheFM-noise

inthefrequencyrangeunder50MHz. ItcanbeobservedfromFigure4,bychangingthecurrents
Figure 3. (a) Structure of the sampled, grating-distributed Bragg reflector (SGDBR) laser; (b)
intoagratingsection,thattheentirereflectioncombofthegratingsectionshiftsinwavelengthand
frequency modulation (FM) noise spectrum of the SGDBR laser.
thelaserwavelengthjumpsnotonlytoanadjacentlongitudinalmode,butcanalsojumpbyseveral
Appl. Sci. 2017, 7, 1033 5 of 15
Y
Pump1 Pump2
Signal
Δω
Signal
Idler
Idler
Δω Δω
θ
X ω
Pump1 ωp1 ωp 2 ωp1-ωp2+ωs ωs ωp2-ωp1+ωs
Pump2
(a) (b)
Figure 2. Four-wave mixing (FWM) scheme with dual co-polarized pumps. (a) The X and Y axes
denote orthogonal states of polarization. In the dual pump FWM scheme, the polarization state of the
wavelength-converted idler is that of the input signal provided both pumps have the same
polarization. (b) Spectral locations of the pumps, signal, and idlers.
3. Linewidth Characterization of SGDBR Pump Lasers for Wavelength Conversion
Among the tunable lasers, the SGDBR laser has been proven to be a suitable fast tunable laser
candidate [42], and the structure of this type of laser is shown in Figure 3a. It is comprised of one gain
section, one phase section, and two grating sections. Due to the period in which the two gratings are
sampled differently, each grating has different period of reflection maxima. Therefore, the Vernier
Appl.Sci.2017,7,1033 6of15
effect is enabled and enhances the overall tuning range considerably. The function of the phase
Appl. Sci. 2017, 7, 1033 6 of 15
section is to allow the cavity modes to be shifted independently of the grating’s reflection peak,
achieving continuous tuning. As the data is modulated on the phase of the laser in the transmission
nmtoAapnaortt hferromsu pthear-tm, tohdee pahtaaswe anvoeisleen ogft hthwe hSeGreDtBhRe rleaflseerc tiios nstproeankgslyo fdtehpeeVnedrennite ro-tnu ntheed ignrjaetcitniogns
systems employing advanced modulation formats, the phase noise of the lasers becomes an
hcuavrreenret-sa oling neeadch. Bsyecitniocrne, aasnindg ththee lcinuerwreindttsho vnatrhiees gbreattwinegense scetivoenr,atlh 1e0l0o’sw kfHrezq utoe nsecvyelrinale wMiHdtzh wanitdh
important factor in determining the transmission performance [43]. For the SGDBR laser, both the
hdiigffherfernetq iunejencctyedli cnuerwreidntths. Tarhee foouutnpdutt woapvreelseenngtthan anodp ppohsaistee ntroeinsed .of Tthhee SloGwDBfrRe qlauseenr cays ali nfuenwcitdiothn
gain section and the passive tuning sections can contribute to the overall phase noise of the laser [44].
uofs uthaell yinijneccrteedas ceusrurnentitlst hine twhea vfreolennt gstehctjiuomn paroec mcuerass,uwrhedil eanthde sthreonwdnf oinr tFhigeuhrieg h4,f wreiqthu e9n0c mylAin ceuwrirdetnht
In order to characterize the phase noise of the SGDBR laser, the frequency modulation (FM) noise
iosnd tehcer sgepaaesicnetrd su.emcAt, iwosnhm iaconhsd ht a0lso m wbeAef nrce upqrruroeevnnetdc oy tnop tbhheae as p evhenaroysei ss uaenitcdaab nblea bcmeke csaeosucmtriepomenne. nTstah oteef dwthhbe iypteht hanseoei DsneoS iaPsn,e dtoh fc ealarosrepireesrr tanoto iiosne
paroein rtespborefe sethemneptSleoGdy DebdyB iRtnh lceao shheiergrehwn fittr hseyqlsuetseesmnhcs yi[g 4lh5in]f,e rcewaqniud betehn coaybntndaio nlioesdwe a bfryree tqphuere etfneeccrhyrne lidiqnuteoew upisedirntfhgo, r awm chodihcaehtra eantrtre ar nceacselmcivuieslras itoend
efrxopmer tihmweiet Fhn Mtas .n-naAorrcioscweo r-sldpinienecwgtriutdomtht hLbeOys et[h3l7ein] .s eiAwms ipsdhlteoh wemqnu eianat siFouignrue: mrΔe νe3 nb=t, sπw rh·e iSste(uf )lnt o[s4i,s7te]w .f rToohmwe athhvieeg glhea nfinrge tsqheucsteioonfnc tyah nleidnS leGowwDi dBtRh
lias sreerlaatfrereedqc tuhoeo ntshceyen cFtaoMrrsi wenro inticsoheis lebe efvrteowlm ei neth ntehf peo arfsrtsehiqveuel eatuntencriynw gr aasnvecgetelieo bnnegsy tahorenc doob n5sv0eer0vr MseidoH. nDzeu, xewp thoe ertirhmeea easnd ttvh.aeTn hlcoeew i1n 5 fdr4ei8gq.i6uta8el nncmy
wlinaevweliedsnitgghnt ahclo opnfrtortchibeesusStinGedgD (bDByRS Pt,h)w,e t hhpieac lhsoswiisv ferre etpquruneeisnnecgny t seeexdcctebisoysn ptshh ieass beel xantcorkiascept coeadinn btbyAe ctsohhmeo pmweennasiannt evFdai.g lAuu e2r enodf4 -,tohirsed ceFhrM oPsL-enLno ibsye
scheme has been used in the carrier phase estimation to track the excess phase noise [46], which
ainp pthlyei nfrgeq9u0emncAy cruarnrgeen tuonndethr e50g aMinHsze.c Ittio cnanw bheil eobthseerovtehde rfrpoamss Fivigeuturen 4in, gbyse cchtiaonngsinagre ttheer mcuinrraetnedts.
presents the ability to employ fast tunable laser for higher order modulation formats.
Tinhteo 1a5 5g3r.a7t0inngm sewctaivoenl,e tnhgatth t(hpeo einnttiBre) irsecfhleocsteionnb cyoimncbr eoafs tinhge tghreatciunrgr esnecttoionnt hsehiffrtosn itns wecativoenletnog3t.h4 manAd.
the laser wavelength jumps not only to an adjacent longitudinal mode, but can also jump by several
nm to another s(uap)er-mode at a wavelength where the reflection peaks of the Vernier-tuned gratings
(b)
have re-aligned. By increasing the currents on the grating section, the low frequency linewidth and
Carrier noise
high frequency Flirnoenwt idthG aairne foPuhnasde to Bparceksent an opposite trend. The low frequency linewidth
usually increases until the wavelength jump occurs, while the trend for the high frequency linewidth
is decreased. As most low frequency phase noise can be compensated by the DSP, the operation
points of the SGDBR laser with less high frequency noise are preferred to perform data transmission
experiments. According to these linewidth measurements results, two wavelengths of the SGDBR
White noise
laser are chosen to switch between for the later wavelength conversion experiment. The 1548.68 nm
wavelength of the SGDBR, which is represented by the black point A shown in Figure 4, is chosen by
applying 90 mA current on the gain section while the other passive tuning sections are terminated.

The 1553.70 nm wavelength (point B) is chosen by increasing the current on the front section to 3.4
mA. FigureF3i.gu(ar)e S3tr. u(cat)u rSetroufcttuhree saomf tphlee ds,agmrpatleindg, -gdriasttirnibgu-dtiesdtriBbruatgedg rBerflaegcgt orre(fSleGctDorB R(S)GlaDsBeRr;) (bla)sferre; q(ube) ncy
frequency modulation (FM) noise spectrum of the SGDBR laser.
modulation(FM)noisespectrumoftheSGDBRlaser.
B
A

FFiigguurree 44.. MMeeaassuurreedd hhiigghh ffrreeqquueennccyy lliinneewwiiddtthh ((ggrreeeenn ttrriiaanngglleess)),, llooww ffrreeqquueennccyy lliinneewwiiddtthh ((rreedd ddoottss)),,
aanndd oouuttppuutt wwaavveelleennggtthh ((bblluuee ssqquuaarreess)) wwiitthh ddiiffffeerreenntt ccuurrrreennttss oonn tthhee ffrroonntt sseeccttiioonn ooff tthhee SSGGDDBBRR llaasseerr,,
wwiitthh 9900 mmAA ccuurrrreenntt oonn tthhee ggaaiinn sseeccttiioonn,, 00 mmAA ccuurrrreenntt oonn tthheep phhaasseea annddb baacckks seeccttiioonn..
44.. EExxppeerriimmeennttaall SSeettuupp ffoorr RReeccoonnfifigguurraabbllee WWaavveelleennggtthhC Coonnvveerrtteerr
FFiigguurree 55 ddeeppiiccttss tthhee sscchheemmaattiicc ddiiaaggrraamm ooff tthhee eexxppeerriimmeennttaall sseettuupp ooff tthhee SSOOAA--bbaasseedd wwaavveelleennggtthh
ccoonnvveerrssiioonn ooff QQPPSSKK aanndd PPMM--QQPPSSKK ((bblluuee ddaasshheedd)) ssiiggnnaallss uussiinngg aa SSGGDDBBRR llaasseerr aass oonnee ooff tthhee ppuummpp
ssoouurrcceess.. AA nnaarrrrooww lliinneewwiiddtthh eexxtteerrnnaall ccaavviittyy llaasseerr ((EECCLL)) ttuunneedd ttoo 11554422..55 nnmm ffoorr aallll eexxppeerriimmeennttss iiss
uusseedd aass tthhee ssiiggnnaall ssoouurrccee aanndd wwaass mmoodduullaatteedd wwiitthh QQPPSSKK ddaattaa aatt 1122..55--GGbbaauudd uussiinngg aann ooppttiiccaall IIQQ
mmoodduullaattoorr.. TThhee ooppttiiccaall mmoodduullaattoorr iiss ddrriivveenn bbyy eelleeccttrriiccaall ssiiggnnaallss ggeenneerraatteedd bbyy tthhee aarrbbiittrraarryy wwaavveeffoorrmm
generator (AWG) with two uncorrelated pseudo-random bit sequences (PRBS) of 27-1 bits periodicity.
The AWG operated at 25 GSa/s, which gives 2 samples per symbol for the 12.5-Gbaud QPSK signal.
Appl.Sci.2017,7,1033 7of15
generator(AWG)withtwouncorrelatedpseudo-randombitsequences(PRBS)of27-1bitsperiodicity.
Appl. Sci. 2017, 7, 1033 7 of 15
TheAWGoperatedat25GSa/s,whichgives2samplespersymbolforthe12.5-GbaudQPSKsignal.
ThetwoQPSKsignalsareamplifiedbyusingtheradiofrequency(RF)amplifiers. Toimplementthe
The two QPSK signals are amplified by using the radio frequency (RF) amplifiers. To implement the
PM-QPSKsignalscheme,thePM-QPSKsignalisgeneratedbyusingaPol-Muxemulator,consisting
PM-QPSK signal scheme, the PM-QPSK signal is generated by using a Pol-Mux emulator, consisting
ofapolarizationbeamsplitter(PBS)atitsinput,apassivestagewithadelayof4.88ns(61symbols),
of a polarization beam splitter (PBS) at its input, a passive stage with a delay of 4.88 ns (61 symbols),
twopolarizationcontrollers,andapolarizationbeamcombiner(PBC)tocombinethepolarization
two polarization controllers, and a polarization beam combiner (PBC) to combine the polarization
tributaries. Theoutputofthetwopumpsarepassedthroughpolarizationcontrollers,combined,and
tributaries. The output of the two pumps are passed through polarization controllers, combined, and
polarization-aligned using a 3 dB coupler and a PBS. For the wavelength conversion of the QPSK
polarization-aligned using a 3 dB coupler and a PBS. For the wavelength conversion of the QPSK
signaldata,thegeneratedopticalQPSKsignalisthencoupledwithtwopumps(oneSGDBRlaserand
signal data, the generated optical QPSK signal is then coupled with two pumps (one SGDBR laser
onenarrowlinewidthECL)andsentintotheSOA-basedwavelengthconverter. ForbothQPSKand
and one narrow linewidth ECL) and sent into the SOA-based wavelength converter. For both QPSK
PM-QPSKschemes,thepowerofthesignalandthepumpsattheinputoftheSOAaresetat−10dBm
and PM-QPSK schemes, the power of the signal and the pumps at the input of the SOA are set at
and0dBm,respectively,foroptimumconversionefficiency[48]. Thesignalwavelengthundergoes
−10 dBm and 0 dBm, respectively, for optimum conversion efficiency [48]. The signal wavelength
wavelengthconversionthroughnon-degenerateFWMintheSOAoperatingat500mAbiascurrent.
undergoes wavelength conversion through non-degenerate FWM in the SOA operating at 500 mA
TheSOAdeviceusedintheexperimentisanonlinearSOAwithaveryfastgainrecoverytimeless
bias current. The SOA device used in the experiment is a nonlinear SOA with a very fast gain recovery
than25ps. ItoperatesoverC-bandwithatypicalsmallgainof25dB,asaturationpowerof+13dBm,
time less than 25 ps. It operates over C-band with a typical small gain of 25 dB, a saturation power of
andlessthan0.5dBpolarization-dependentsaturatedgain.
+13 dBm, and less than 0.5 dB polarization-dependent saturated gain.

QPSK Transmitter
AWG
Pol-MUX emulator
RF RF
I Q PBS PC PBC Converted Idler
ECL (Signal) Coherent receiver
PC QPSK-Mod Wavelength converter + analysis
τ
DelayPC LO Coh.Rx
3 dB SOA OBPF PC +
ECL (Pump 1) VOA ISO DSP offline
PC 3 dB PC
3 dB OSA
Switching OBPF
PBS VOA OSNR measurement
ASE VOA
Back Phase Gain Front
SGDBR （Pump2） PC Dual pumps Noise loading
Figure 5. Schematic diagram of the reconfigurable SOA-based wavelength conversion of the
Figure5.SchematicdiagramofthereconfigurableSOA-basedwavelengthconversionofthequadrature
quadrature phase shift keying (QPSK) and polarization multiplexed (Pol-Mux)-QPSK (blue dashed)
phase shift keying (QPSK) and polarization multiplexed (Pol-Mux)-QPSK (blue dashed) signals
signals employing a sampled grating distributed Bragg reflector (SGDBR) pump laser. PC:
employing a sampled grating distributed Bragg reflector (SGDBR) pump laser. PC: polarization
polarization controller, PBS: polarization beam splitter, PBC: polarization beam combiner, OBPF:
controller,PBS:polarizationbeamsplitter,PBC:polarizationbeamcombiner,OBPF:opticalband-pass
optical band-pass filter, ISO: isolator, ASE: noise source, VOA: variable optical attenuator, OSA:
filter,ISO:isolator,ASE:noisesource,VOA:variableopticalattenuator,OSA:opticalspectrumanalyzer,
optical spectrum analyzer, LO: local oscillator, Coh.Rx: coherent receiver.
LO:localoscillator,Coh.Rx:coherentreceiver.
After the FWM process in the SOA, the converted idler is then filtered out by using a tunable
AftertheFWMprocessintheSOA,theconvertedidleristhenfilteredoutbyusingatunable
optical bandpass filter (OBPF). For system performance evaluation purposes, the optical signal to
optical bandpass filter (OBPF). For system performance evaluation purposes, the optical signal to
noise ratio (OSNR) of the idler is changed by adding amplified spontaneous emission (ASE) from an
noiseratio(OSNR)oftheidlerischangedbyaddingamplifiedspontaneousemission(ASE)from
Erbium-doped fiber amplifier (EDFA) that is passed through a 2 nm bandwidth tunable optical
anErbium-dopedfiberamplifier(EDFA)thatispassedthrougha2nmbandwidthtunableoptical
bandpass filter. The filtered idler is then passed through a 3 dB splitter with one arm sent to the
bandpassfilter. Thefilteredidleristhenpassedthrougha3dBsplitterwithonearmsenttotheoptical
optical spectrum analyzer (OSA)for the OSNR measurement, and the other arm is passed into the
spectrumanalyzer(OSA)fortheOSNRmeasurement,andtheotherarmispassedintothepolarization
polarization diversity coherent optical receiver and captured by a real-time oscilloscope sampling at
diversitycoherentopticalreceiverandcapturedbyareal-timeoscilloscopesamplingat50GSa/sfor
50 GSa/s for offline DSP processing. The received idler power at the input of the coherent receiver is
offlineDSPprocessing. Thereceivedidlerpowerattheinputofthecoherentreceiverismaintainedat
maintained at −19 dBm. The data captured from the real-time oscilloscope is first resampled to 2
−19dBm. Thedatacapturedfromthereal-timeoscilloscopeisfirstresampledto2samplespersymbol
samples per symbol using a priori knowledge of the clock frequency. Then the constant modulus
usingaprioriknowledgeoftheclockfrequency. Thentheconstantmodulusalgorithm(CMA)[49–51]
algorithm (CMA) [49–51] is utilized to enable polarization de-multiplexing for PM-QPSK signal. An
isutilizedtoenablepolarizationde-multiplexingforPM-QPSKsignal. AnMthpowerfrequencyoffset
Mth power frequency offset compensation method [52–54] is employed to compensate the frequency
compensationmethod[52–54]isemployedtocompensatethefrequencyoffsetbetweentheconverted
offset between the converted idler signal and the local oscillator in the coherent receiver, with m = 4
idlersignalandthelocaloscillatorinthecoherentreceiver,withm=4beingthenumberofdistinct
being the number of distinct phases in the QPSK symbol set. In order to make this frequency offset
compensation method work correctly, it is essential to make sure that the absolute frequency offset
is less than Rs/(2 M), where Rs is the symbol rate. A second order Phase-Locked Loop (PLL) is
employed for the phase noise estimation [46], and the synchronization is achieved by adding training
symbols at the beginning of the data [55] in order to carry out the BER calculation.
Appl.Sci.2017,7,1033 8of15
phasesintheQPSKsymbolset. Inordertomakethisfrequencyoffsetcompensationmethodwork
correctly,itisessentialtomakesurethattheabsolutefrequencyoffsetislessthanRs/(2M),where
Rs is the symbol rate. A second order Phase-Locked Loop (PLL) is employed for the phase noise
estimation[46],andthesynchronizationisachievedbyaddingtrainingsymbolsatthebeginningof
Appl. Sci. 2017, 7, 1033 8 of 15
thedata[55]inordertocarryouttheBERcalculation.
55.. RReessuullttss aanndd DDiissccuussssiioonn
The wavelength of the fast-tuning SGDBR pump laser is switched between two operating modes
Thewavelengthofthefast-tuningSGDBRpumplaserisswitchedbetweentwooperatingmodes
by applying a switching signal to the wavelength tuning sections. In order to benchmark the system
byapplyingaswitchingsignaltothewavelengthtuningsections. Inordertobenchmarkthesystem
performance, we first present the BER performances when pump2 is tuned and fixed to the two
performance, we first present the BER performances when pump2 is tuned and fixed to the two
wavelengths that pump2 will later be dynamically switched between. The wavelength and power of
wavelengthsthatpump2willlaterbedynamicallyswitchedbetween. Thewavelengthandpower
the signal and pump1 (ECL) are kept constant throughout. The input and output spectra of the SOA
ofthesignalandpump1(ECL)arekeptconstantthroughout. Theinputandoutputspectraofthe
when the SGDBR laser is set at 1548.68 nm and 1553.70 nm are shown in Figure 6a,b, respectively,
SOAwhentheSGDBRlaserissetat1548.68nmand1553.70nmareshowninFigure6a,b,respectively,
and it can be observed that the indicated idler of interest has changed wavelength position from Ch1
anditcanbeobservedthattheindicatedidlerofinteresthaschangedwavelengthpositionfromCh1
(1541.395 nm) to Ch2 (1538.684 nm).
(1541.395nm)toCh2(1538.684nm).
(a)
0 Output Pump1
Input
Pump2
-20 Signal
Idler
(Ch 1)
-40
-60
-80
1539 1541 1543 1545 1547 1549 1551 1553
Wavelength (nm)

Figure 6. Input and output spectra of SOA showing the spectral locations of the signal of the external
Figure6.InputandoutputspectraofSOAshowingthespectrallocationsofthesignaloftheexternal
cavity laser (ECL), pump1 (ECL), pump2 (SGDBR), and converted idlers. (a) SOA input and output
cavitylaser(ECL),pump1(ECL),pump2(SGDBR),andconvertedidlers.(a)SOAinputandoutput
spectra when SGDBR is set at 1548.68 nm. (b) SOA input and output spectra when SGDBR is set at
spectrawhenSGDBRissetat1548.68nm. (b)SOAinputandoutputspectrawhenSGDBRissetat
1553.70 nm. The detected idlers are indicated.
1553.70nm.Thedetectedidlersareindicated.
BER performance as a function of OSNR at the receiver for the original signal; the signal after
BERperformanceasafunctionofOSNRatthereceiverfortheoriginalsignal;thesignalafter
SOA and converted signals (idlers) for the QPSK and PM-QPSK signals at 12.5 GBaud are displayed
SOAandconvertedsignals(idlers)fortheQPSKandPM-QPSKsignalsat12.5GBaudaredisplayedin
in Figure 7a,b, respectively. It can be observed that the penalty between the original signal, the signal
Figure7a,b,respectively. Itcanbeobservedthatthepenaltybetweentheoriginalsignal,thesignal
after SOA, and wavelength converted idler is under 0.5 dB for both cases, indicating the quality of
after SOA, and wavelength converted idler is under 0.5 dB for both cases, indicating the quality
the wavelength conversion scheme and potential usefulness. The constellation diagrams of the
wavelength converted idler for a received OSNR of 12 dB and 14.5 dB are also given in Figure 7a,b,
respectively. The phase noise of the FWM components is studied by using a coherent phase noise
measurement technique [37,46]. The ECL’s employed for the signal and pump1 have a linewidth of
around 40 kHz and the linewidth of the 1548.68 nm and 1553.70 nm wavelengths of the SGDBR laser
(employed as the second pump for FWM scheme) are 260 kHz and 230 kHz, respectively. The
linewidth (from the high frequency phase noise region) of the idlers is measured to be around 370
kHz and 300 kHz, as expected when the SGDBR is set to the two operating modes, since the idler
Appl.Sci.2017,7,1033 9of15
ofthewavelengthconversionschemeandpotentialusefulness. Theconstellationdiagramsofthe
wavelengthconvertedidlerforareceivedOSNRof12dBand14.5dBarealsogiveninFigure7a,b,
respectively. ThephasenoiseoftheFWMcomponentsisstudiedbyusingacoherentphasenoise
measurement technique [37,46]. The ECL’s employed for the signal and pump1 have a linewidth
ofaround40kHzandthelinewidthofthe1548.68nmand1553.70nmwavelengthsoftheSGDBR
laser(employedasthesecondpumpforFWMscheme)are260kHzand230kHz,respectively. The
linewidth(fromthehighfrequencyphasenoiseregion)oftheidlersismeasuredtobearound370kHz
Appl. Sci. 2017, 7, 1033 9 of 15
and300kHz,asexpectedwhentheSGDBRissettothetwooperatingmodes,sincetheidlerlinewidth
ilsinthewesidutmh iosf ththee sluinme wofi dththe slionfetwhiedpthusm opf sthaen pdusmigpnsa al,nadn sdiginnatlh, iasncda sien tthheisl cinaesew tihdeth linofewthiedtShG oDf BthRe
pSuGmDpBlRa speurmdopm lainseart edso.minates.

FFiigguurree7 7..( a(a))B Bitiet rerorrrorra trea(tBe E(RB)EvRe)r svuesrsoupst iocapltsiciganl asligtonanlo itsoe nraotiisoe (OraStNioR ()OcSuNrvRe)s fcourrtvhees infopru tthoer igininpault
soigringainl,atlh seigsniganl,a tlhaef tseirgnSaOl Aaf,taenr dSOthAe, caonndv tehrtee cdoindvleerrtsefdo rid1l2e.r5s GfoBra 1u2d.5Q GPBSaKudsi gQnPaSlK. ( sbi)gBnaElR. (vbe) rBsuEsR
OvSeNrsRusc uOrSvNesRf ocrutrhveesin fpour ttohrei gininpaults iogrnigailn,tahl essigignnaal,l tahftee rsiSgOnAal, aafntdert hSOecAo,n avnedrt etdhei dcloenrsvfeorrte1d2 .i5dGleBrsa ufodr
P1M2.5-Q GPBSaKudsi gPnMal-.QPSK signal.
To calculate the limitations of the wavelength conversion scheme, we study the OSNR of the
To calculate the limitations of the wavelength conversion scheme, we study the OSNR of the
idler as a function of the detuning between the two pumps. The different wavelength converted idlers
idler as a function of the detuning between the two pumps. The different wavelength converted
are filtered out by using an optical tunable band-pass filter. The OSNR measurement is then
idlersarefilteredoutbyusinganopticaltunableband-passfilter. TheOSNRmeasurementisthen
undertaken by using the OSA. In order to estimate the signal power and the noise floor correctly, the
undertakenbyusingtheOSA.Inordertoestimatethesignalpowerandthenoisefloorcorrectly,the
solution is to take two consecutive sweeps of the OSA with different resolution bandwidth (RBW)
solutionistotaketwoconsecutivesweepsoftheOSAwithdifferentresolutionbandwidth(RBW)
settings [56]. For the first sweep, OSA 0.2 nm RBW is used for the measurement of the signal power,
settings[56]. Forthefirstsweep,OSA0.2nmRBWisusedforthemeasurementofthesignalpower,
and the second sweep measures the noise power by using 0.1 nm RBW setting. The output OSNR of
the idler wavelength for the case with signal fixed at 1542.5 nm, pump1 fixed at 1549.8 nm, and the
pump2 (SGDBR) tuned from 1548.7 nm to 1564.7 nm is displayed in Figure 8, which shows a tuning
range of around 14 nm can be achieved with more than 9 dB OSNR, which is enough for the
wavelength conversion of the QPSK data at 12.5 Gbaud to get a BER value below the 7% FEC limit
(3.8 × 10−3). Around 11 nm tuning range can be achieved for the wavelength conversion of PM-QPSK
data to get a BER value below the 7% FEC limit. In order to investigate the time-resolved BER [57,58]
performance of the fast-reconfigurable wavelength converter, we apply a square wave current with
500 kHz repetition rate to the front section of the SGDBR to switch the wavelength converted idler
between Ch1 (1541.395 nm) and Ch2 (1538.684 nm), with the other currents applied to the SGDBR
laser held constant and the received OSNR set at 12 dB for the QPSK signal and 14.5 dB for the
Appl.Sci.2017,7,1033 10of15
andthesecondsweepmeasuresthenoisepowerbyusing0.1nmRBWsetting. TheoutputOSNR
oftheidlerwavelengthforthecasewithsignalfixedat1542.5nm,pump1fixedat1549.8nm,and
the pump2 (SGDBR) tuned from 1548.7 nm to 1564.7 nm is displayed in Figure 8, which shows a
tuningrangeofaround14nmcanbeachievedwithmorethan9dBOSNR,whichisenoughforthe
wavelengthconversionoftheQPSKdataat12.5GbaudtogetaBERvaluebelowthe7%FEClimit
(3.8×10−3). Around11nmtuningrangecanbeachievedforthewavelengthconversionofPM-QPSK
datatogetaBERvaluebelowthe7%FEClimit. Inordertoinvestigatethetime-resolvedBER[57,58]
performanceofthefast-reconfigurablewavelengthconverter,weapplyasquarewavecurrentwith
500kHzrepetitionratetothefrontsectionoftheSGDBRtoswitchthewavelengthconvertedidler
Appl. Sci. 2017, 7, 1033 10 of 15
betweenCh1(1541.395nm)andCh2(1538.684nm),withtheothercurrentsappliedtotheSGDBRlaser
hPeMld-QcoPnSsKta snigtnaanld. Tthhee rteimceeiv-reedsoOlvSeNdR BsEeRt amte1a2sduBrefmorentht eisQ cPhSaKrascitgenriazleadn fdor1 4C.h51d bByf ofirxtihneg PthMe- cQePnSteKr
sfrigenquale.nTchye otfim thee-r OesBoPlvFe dtoB CEhR1m aenads uardejmusetnintgis tchhea LraOct etori zthede wfoarvCehle1nbgythfi xoifn Cghth1.e Tchenet eprrofrceeqsus eins ctyheonf
trheepeOaBtePdF ftoor CChh12a. ndadjustingtheLOtothewavelengthofCh1. TheprocessisthenrepeatedforCh2.
40
35
30
25
20
15
10
5
1526 1528 1530 1532 1534 1536 1538 1540 1542
Wavelength of the converted ider
Figure 8. OSNR as a function of the wavelength of the converted idler.
Figure8.OSNRasafunctionofthewavelengthoftheconvertedidler.
We now present results of wavelength conversion when pump2 is dynamically tuned between
Wenowpresentresultsofwavelengthconversionwhenpump2isdynamicallytunedbetween
1547.68 nm and 1553.7 nm. In order to accurately estimate the time-resolved BER performance after
1547.68nmand1553.7nm. Inordertoaccuratelyestimatethetime-resolvedBERperformanceafter
a switching event, a number of switching events are captured via multiple acquisitions using the real-
a switching event, a number of switching events are captured via multiple acquisitions using the
time scope. Each data point in the time-resolved BER curves corresponds to the probability of
real-timescope. Eachdatapointinthetime-resolvedBERcurvescorrespondstotheprobabilityof
receiving an error in a 10 ns period. This means the BER is averaged over a block length of 125
receivinganerrorina10nsperiod. ThismeanstheBERisaveragedoverablocklengthof125symbols
symbols (10 ns), and data captured from 200 switching events is used for the calculation, with a total
(10 ns), and data captured from 200 switching events is used for the calculation, with a total of
of 5 × 104 bits and 105 bits used for calculating each BER point in the time-resolved BER curves for
5×104bits and 105 bits used for calculating each BER point in the time-resolved BER curves for
QPSK and PM-QPSK signal at 12.5 GBaud, respectively. It can be seen from Figure 9a,b that the
QPSK and PM-QPSK signal at 12.5 GBaud, respectively. It can be seen from Figure 9a,b that the
reconfiguration time (time to have a BER better than the 7% FEC limit) after a switch is approximately
reconfigurationtime(timetohaveaBERbetterthanthe7%FEClimit)afteraswitchisapproximately
50 ns and 160 ns for QPSK and PM-QPSK signal, respectively. The red curves in Figure 9a,b show the
50nsand160nsforQPSKandPM-QPSKsignal,respectively. TheredcurvesinFigure9a,bshowthe
temporal frequency offset between the idler and the local oscillator in the coherent receiver after a
temporalfrequencyoffsetbetweentheidlerandthelocaloscillatorinthecoherentreceiveraftera
switching event. It takes around 70 ns for the frequency of the wavelength converted idler for QPSK
switchingevent. Ittakesaround70nsforthefrequencyofthewavelengthconvertedidlerforQPSK
signal to fully stabilize after a switch, and 100 ns for PM-QPSK signal.
signaltofullystabilizeafteraswitch,and100nsforPM-QPSKsignal.
Figure10showstheBERmeasurementasafunctionofOSNRinaswitchingenvironmentwith
differentwaitingtimeaftertheidlerisswitchedtoCh1andCh2whenusingQPSKandPM-QPSK
decoding. Theblueandredcurvesshowtheresultsforthedatawithawaitingtimeof50nsafter
switchingwhenusingQPSKdecodingforCh1andCh2,respectively. Thebrownandgreencurvesare
fora160nswaitingtimewhenusingPM-QPSKdecodingforCh1andCh2,respectively. Thepink
andblackcurvesshowthebadperformancewitha50nswaitingtimeusingPM-QPSKdecodingfor
Ch1andCh2,respectively. TherequiredwaitingtimewhenusingPM-QPSKdecodingislongerthan
QPSKdecodingmainlyduetothelongerconvergencetimeassociatedwiththeCMAmethodusedfor
de-multiplexingthedual-polarizationpackets.