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Photonic Integrated Circuits for Tunable Delay Lines

A delay line is a device that allows controlling thepropagation delay of signals in a continuous mode,ensuring a true-time-delay operation on largeoperativebandwidths.Inducea temporaldelayona signal it is not aneasytask.Apartdigitalelectronicsthatcantakeadvantageof shift registers for a discrete arbitrary delay, inanalog electronics, microwave and photonics acontinuously variable delay without moving parts isdifficult to achieve. However, integrated photoniccircuits offer a number of solutions to achieve true-time-delay in very compact, flexible and potentiallylowcostdevices.Atypicalapplicationofadelay line isshowninFig.1where an absolute delay on an optical signal or arelativedelaybetweentwosignalsforsynchronizationpurposes is achieved. The delay Δt should becontinuously controllable, induce negligibleattenuation andhave sufficient bandwidth to let thepulsesgothroughundistorted.Therearealargenumberofapplicationswheredelaylinesplayakeyrole:opticalBeamFormingNetworksfor antenna arrays, imaging processing units, radio-on-fiber links and devices, interferometry, sensingunits, etc. In case the signal is electrical, it could beconvenienttomodulatethelightgeneratedbyalaserwith the RF signal, delay in the optical domain andconvertbacktoelectricalbymeansofaphotodiode.

Figure1-Typicaluseofadelayline:synchronizationoftwosignalsUsing a proper technology (Indium Phosphide), thelaser,modulatorandphotodiodecanbeintegratedonthesamephotonicintegratedcircuit(PIC),providinga

very compact, electrically terminated, robust device.This is a classical expedient that goes under thegenericnameofmicrowavephotonics.

Photonic Delay Lines Fig. 2 shows threeexamplesofphotonicsdelay linesintegrated on chip. The coil a) induces a fixed delayanditistheintegratedversionoftheclassicalcoilsofoptical fibers or coaxial cables used in many bulkapplications. This is the basic element to realize, incombinationwithintegrated2x2switches,delaylinesprovidingadiscretenumbersof fixeddelays. Ithasaverysmall footprint, low losses,hugebandwidthanddoesnotrequireanyexternalcontrol.Fig.2showsinb)acoupledRingResonatorsbaseddelay lineand inc)aMach-Zehnderbasedsinglestagedelayline.Bothsolutions provide a continuously tunable delay andcan be electrically controllable thanks to properthermalcontrollersplacedabovethewaveguides.

Figure 2 – a) Fixed delay line; b) Coupled Ring Resonator basedtunable delay line; c) Mach-Zehnder based single stage tunabledelayline. Devices inducing a variable time delay up to fewnanoseconds have been realized and successfullytested.Thesedevicesrequireasuitableelectronicandsoftware to achieve the control and stabilizationagainst thermal and other fluctuations and beadaptivetothevariationsoftheinputsignals.

Wavelengthcontrol

Delaycontrol Delaycontrol

Pics4all │application note

Potentials and Limits The ring-based delay line is a popular solutionbecause of the very small footprint. Tunability isachievedbychangingtheresonantfrequencyandthecombinationof various rings provides shapingof thespectral region in which the delay is achieved. Thebandwidth, instead, is usually narrow and theachievedbandwidth-delayproductisrathersmall.TheMach-Zehnderbaseddelaylinehasamuchlargerbandwidth-delay product and induces a very smalldistortion on the delayed signals. Several stages canbe cascaded to achieve a longer delay, stillmaintainingalargebandwidthasshowninFig.3foradouble stage device. Only one electrical control isrequired to change the coupling coefficient K of thedirectionalcouplers.Further,beinga2x2circuit,itcanprovideadifferentialdelaybetweentwoinputsignals.Thephotonicintegrateddelaylineisperfectlysuitablefor the combination with other functionality, activedevicesaslasers,modulatorsandphotodetectorsandpackaged in a single case with electrical or opticalinputandoutput.Thissolution,withoutmovingpartsis perfectly suitable for applications where speed,numberofdelays,reliabilityandcostsarekeyfactors.For any solution shown in Fig. 2, the inducedattenuationisproportionaltothetotaltimedelayandhence the choice of the most suitable technology(Silicon Photonics, Indium Phosphide or SiliconNitride)isofkeyimportance.The Mach-Zehnder based delay line is patented1 byPolitecnico diMilano and it is available for exclusiveandnon-exclusivelicensing.

Figure 3 - Characteristics of a two-stages Mach-Zehnder based tunable delay line.

1 Patent IT- 102017000053579. 2 http://pics4all.jeppix.eu/

Discuss your application with us Eachspecificapplicationrequiresacarefulevaluationof the requestedrequirements in termsofmaximumdelay, delay resolution, attenuation, bandwidth,speed and control technique. A preliminary analysispermitstoidentifythemostsuitabletechnology.PleasecontactthePICs4Allconsortium2toknowmoreabout the use of PIC technology. The PICs4All isfundedundertheHorizon2020frameworkandbringstogether expertise to support end-users with PICtechnology.Wehelpyouconnectingtotheecosystem ofdesigners,foundries,packagingandtestservices.

Contact information Andrea Melloni, Professor Politecnico di Milano, Italy Website: http://photonics.deib.polimi.it

http://www.polifab.polimi.it Email: andrea.melloni@polimi.it

2 http://pics4all.jeppix.eu/

Photonic Integrated Circuits (PICs) Also known as optical chips, PICs can contain tens to hundreds of optical components. While electronic ICs consist of transistors, capacitors and resistors, a PIC consists of, for example, lasers, modulators, photodetectors and filters, all integrated on a single substrate. These PICs are nowadays extensively used commercially, mainly in datacom and telecom, becoming popular also in sensing. PIC technology has now become accessible to users without a cleanroom, through so-called multi-project wafer runs and open foundries. Indium phosphide based technology is commercially available through SMART Photonics and Heinrich Hertz Institute. Access is coordinated by the JePPIX platform: http://www.jeppix.eu/.