DemonstrationofSilicon-On-InsulatorCoherentreceiverforRadio-Over-Fiberapplications

JoseD.SARMIENTO-MERENGUEL1*,JochemVERBIST3,JingZHANG2,4,BartMOENECLAEY3,JohanBauwelinck3,XinYin3,RobertHALIR1,AlejandroORTEGA-

MOÑUX1,ÍñigoMOLINA-FERNÁNDEZ1,GuntherROELKENS2,4.1UniversidaddeMálaga,Dept.deIngenieríadeComunicaciones,ETSITelecomunicación,Campus

deTeatinoss/n,Málaga,29071,Spain2PhotonicsResearchGroup,Dep.INTEC,UGent-imec,9052Ghent,Belgium

3IDLab,Dep.INTEC,UGent-imec,9052Ghent,Belgium4CenterforNano-andBiophotonics,GhentUniversity,9000Ghent,Belgium

*jdsm@ic.uma.es

Radio-Over-Fiber transmission has emerged during the last years as a promisingtechnologyallowingtheseamlessintegrationofopticalandwirelesstechnologies.Oneof the most relevant applications of RoF systems is found in mobile networks,specifically in the transmissionofRFsignals fromthebasestation(BS) to thecentraloffice (CO), as shown in figure 1 (a). Most of the current approaches are based onintensity-modulation/direct-detection (IM-DD) systems [1]. On the other hand,schemes based on complex modulation formats, i.e. phase modulation and coherentdetection(PM-CD),havealsobeenproposed,overcomingthelimiteddynamicrangeoftheIM-DDsystems,andofferingthecoherentdetectionadvantages,suchasmitigationof fiber transmission impairments through signal processing, coherent gain, etc. [2].Moreover,incoherentreceptionschemes,byproperlytuningthelocaloscillator,down-conversion of the RF signal can also be performed, thus relaxing the electronics.Furthermore,takingadvantageofthecoherentgain,theRFamplifierusuallylocatedatthe base station can be theoretically omitted, reducing the operation costs. In thiscontribution, a Silicon-On-Insulator (SOI) coherent receiver with low powerconsumption[3]isdemonstrated,showninFigure1(b),forthereceptionofRoFsignalsintheuplink,fromtheBStotheCO,forthefirsttime.Inthisproofofconcept,coherentgainandimprovederrormagnitude(EVM)comparedtoIM-DDisdemonstrated.

Fig.1.a)ApplicationofRoFtransmissioninthemobileaccessnetwork;b)Microscopepictureofthesiliconphotoniccoherentreceiverwithintegratedtransimpedanceamplifier;c)Setup1,usedforcoherentreceptionofRoFsignals;d)Setup2,usedfor

direct-detectionreceptionofRoFsignals.

Figure 1 (c) and (d) show the two different setups employed for the demonstration.Setup 1, shown in Fig. 1 (c), has been used for the demonstration of RoF coherentreception.Inthissetuplightfromalasersourceissplit, ina1/99powerdivider.Thesmallerfractionisusedforthelocaloscillatorsignal,andtherestissenttoadualdriveMach-Zehndermodulator (DDMZM).ThemodulatedRF signal is generatedat3.5GHzusingavectorsignalgenerator.ThisRFsignalisdividedintwobranches,introducinga180ºphaseshiftinoneofthem.

Thebiaspointof theDDMZMisset to𝑉" so that theRFsignal linearlymodulates theopticalfield.Duetotheinsertionlossesofthegratingcouplersinthecoherentreceiver(𝐼𝐿 ≈ 15𝑑𝐵)ithasbeennecessarytoemployanEDFAamplifierineachoftheopticalcoherentreceiverinputs.Setup2,showninFig.1(d),isusedtoevaluatetheoperationofan IM-DDscheme, inorder tocompare theperformancesofbothscenarios. In thiscasetheMZMisbiasedinitsquadraturepoint,pursuingintensitymodulation.

Fig.2.a)EVMasafunctionoftheon-chipLOpower(PLO).Representativereceived

constellationsareshown(PRFissetto0dBm);b)EVMforcoherentvs.directdetection.AtlowPRFcoherentdetection,(on-chipPLOof10dBm),exhibitsbetterperformance.

In a first experiment, the performance improvement due to the coherent gain isdemonstratedinsetup1,usinga160Mbps16QAMasbasebandsignal.Theerrorvectormagnitude (EVM) of the received constellation was measured as a function of thepowerofthelocaloscillator.Figure2a)showstheresultofthisexperiment,wheretheon-chippowerofthelocaloscillatorissweptfrom-6to10dBm.Itcanbeobservedthatas the LO power increases, the EVM becomes smaller, thus demonstrating theimprovement in the performance. The second experiment consists in demonstratingthat in similar conditions, for low RF signal power (as it happens in real scenarios),coherent reception presents better performance than direct detection. In this case, a160MbpsQPSKbasebandsignalisemployedinbothsetups.Inordertofairlycomparebothschemes,measurementsaredoneforthesameopticalsignalpowerattheinputofthereceivers.Figure2b)showsthemeasuredEVMinbothcases,asafunctionoftheRF signal power. It can be seen how for lower RF signal power the direct detectionexhibits higher EVM than the coherent detection, demonstrating that coherentreception canbe abetter alternative. In summary, the resultspresented in thisworkdemonstrate the operation of a SOI integrated coherent receiver for RoF signalreception.Coherentgain isdemonstrated,which can imply the substitutionof theRFamplifier located in the base stations. Furthermore, the operation of the coherentreceiver has been compared with a direct detection scheme, showing betterperformanceforlowpowerRFsignals.

WeacknowledgefundingfromtheMinisteriodeEconomíayCompetitividad,ProgramaEstatalde Investigación,Desarrolloe InnovaciónOrientadaa losRetosde laSociedad(cofinanciadoFEDER),ProyectoTEC2013-46917-C2-1-R,ProyectoTEC2016-80718-R,and the Universidad de Málaga. The contribution of the IDLab members was partlysupportedbytheiMindsIoTresearchprogramandtheH2020projectFlex5Gware.

References

[1] D.Novaketal.,Radio-Over-FiberTechnologies forEmergingWirelessSystems, IEEEJ.Quant.Electron.,vol.52,no.1,pp.1-11,2016

[2] T.R.Clarketal.,Coherentopticalphase-modulationlink,IEEEPhoton.Technol.Lett.,vol.19,no.16,pp.1206-1208,2007.

[3] J.Zhangetal.,CompactLow-Power-Consumption28-GbaudQPSK/16-QAMIntegratedSiliconPhotonic/ElectronicCoherentReceiver.IEEEPhoton.J.,vol.8,no.1,pp.1-10,2016.

-568

101214161820

PLO (dBm)

EV

M(%

)

a)

-1-3-7 1 3 5 7 9 11 -30 -25 -20 -15 -10 -5 0PRF (dBm)

0

10

20

30

40

50

EV

M(%

)

Coh. DetectionDir. Detection

b)

0.5

1