0733-8724 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2015.2400399, Journal of Lightwave TechnologyJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, MONTH XXXX 1A Simultaneous Variable Optical Weight and Delayin a Semiconductor Optical Amplier forMicrowave PhotonicsMatthew P. Chang, Member, IEEE, Noelle Wang, Ben Wu, Member, OSA, and Paul R. Prucnal, Fellow, IEEEAbstractInthispaper, wedemonstratehowasinglesemi-conductor optical amplier can serve as a simultaneous variableoptical weight and tunable optical delay for microwave photonics.The device weight, or power transmission, anddelaycanbecontrolledsimultaneouslyandindependentlyfromeachotherby varying the input optical power and the semiconductor biascurrent. The dual functionalityis achievedbycombiningtheeffectsofslowandfastlightwithcross-gainmodulationinthesemiconductor. Weexperimentallydemonstrateatunabledelayrangeof 100psandRFgainrangeof -6dBto+3dBfora600 MHz microwave signal and show how the weight and delayof the device canbe separately tuned. The delay range andbandwidth of the device are limited by the semiconductor carrierlifetime, characteristic of slow and fast light. As a simultaneousoptical weight anddelay, aswell asawavelengthconverter, asemiconductor optical amplier operating in this manner can bea compact and versatile element in microwave photonics, radio-over-ber, and general analog optical signal processing.Index TermsSemiconductor optical amplier (SOA), slowand fast light, optical delay line, microwave photonics, cross-gainmodulation (XGM).I. INTRODUCTIONVARIABLEoptical weightsanddelaysarefundamentalbuildingblocksineveryoptical signal processor. Theydrive myriad applications from microwave photonics to opticalphysical layer security [1], [2] to optical neuromorphic com-puting [3]. Microwave photonics, in particular, rely heavily ontunableopticalweightsanddelaystoimplementpreciseandrecongurable microwave photonic lters [4][7].There are many microwave photonic analog signal process-ing applications where the simultaneous control of weightanddelayisnecessary, suchasradio-frequencyinterferencecancellation[8][10] andoptical phased-arrayantennas[5],[11]. However, most integrated optical delay technologiesinadvertantlyaffectthesignalweight, ortransmission, whileadjusting the delay, making it difcult to simultaneouslycontrol both weight and delay. For example, optical microring[12], [13] and microdisk resonators [14] adjust delays bychangingthe resonator characteristics throughthermal [12]andcarrier-injection[14] effects, or bydetuningtheopticalcarrier from a xed resonance [13]. Regardless of the tuningAll authors are with the Lightwave Communications Research Lab, Depart-ment of Electrical Engineering, PrincetonUniversity, Princeton, NJ08544,USA (email: mpchang@princeton.edu)Copyright c 2015 IEEE. Personal use of this material is permitted.However, permissiontousethis material for anyother purposes must beobtained from the IEEE by sending a request to pubs-permissions@ieee.org.method, the signal transmission changes as the spectral loca-tionofthesignalshiftsrelativetotheresonance, effectivelycouplingweightanddelay. Similarly, forslowandfastlightin a semiconductor [15][21], coherent population oscillations(CPO) are used to create a resonance-based time shift of thesignal, which also affect the transmission of the medium.Efforts havebeenmadetorealizeanamplitude-invariantintegrated optical delay. Microring delay lines using the sep-aratecarrier tuningtechnique[22] havebeendemonstratedwith no amplitude shifts while tuning the delay. However, thetechniquerequiresanadditionalopticalsidebandlterandaseparatecarrierphaseshifter, addingfabricationcomplexity.Absorptiondoublets instimulatedBrillouinscattering[23],[24] and cascaded electroabsorbers and semiconductor opticalampliers(SOAs)[25]havebeenusedtocontrol amplitudeinslowandfast light. Therst schemerequiresbulkyberandisnot integratable, whilethesecondrequirescascadingmultipledevices. IntegratedwaveguideBragggratings[26],[27] havebeenresearchedtoreplicatethesuccess of berBragggratingtunable delays inber optics. However, thegratingfabricationissensitive,andgroupdelayripplesfromfabrication imperfections are the main drawback here [27].In this paper, we expand upon our previous work [28] anddemonstrate a simple technique to obtain not just amplitude-invariant optical delay, but simultaneously controllable weightanddelayinasingleSOA. Thedual tunabilityisachievedby combining slow and fast light with cross-gain modulation(XGM) in the SOA. We show that the weight and delay of theSOAaretunablethroughtwoelectroniccontrol parameters:the SOAbias current and the input optical pump power.TheSOAs physical effects aregovernedbysemiconductorcarrier dynamics so that the device latency is dictated by thesemiconductor carrier lifetime (nanoseconds). As an SOA, thedevicenotonlyactsasatunableweightanddelay, butalsoperformswavelengthconversionandnaturallylendsitselftointegration. Themulti-purposenatureofthisdevice, itslowlatency, and its semiconductor compatibility make it well-adaptedtobeastandaloneltertapinmicrowavephotoniclters. We demonstrate a proof-of-concept lter in this paper.The remaining sections of this paper are organized asfollows. In section II, we describe the physical principlesbehindthedevice. InsectionIII, weshowtheexperimentalsetupandthemethodsusedtocontrol theSOAweight anddelay. InsectionIV,wepresenttheexperimentalresultsandconsider device latency, noise, and nonlinearity. In Section Vwe conclude by summarizing our results.0733-8724 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2015.2400399, Journal of Lightwave TechnologyJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, MONTH XXXX 2II. PHYSICAL PRINCIPLESTheSOAoperatesunder twoprimaryphysical principlestoproduceatunableweight anddelay: slowandfast lightand XGM. Slowand fast light [16], [17], [29] refers tocontrolling the group velocity of light in a propagating mediumbygeneratingamaterialdispersion.Insemiconductors,slowand fast light is created via CPO, where the beating betweentwooptical waves leads tooscillations inthegroundstatepopulation at the beat frequency [29], [30]. In microwavephotonics, thetwoopticalwavesareconvenientlyplayedbythe optical carrier andits RFsidebands. CPOgenerates aresonance centered around the optical carrier frequency with abandwidth limited by the semiconductor carrier lifetime [17].For anSOAoperatedinthegainregime, asit ishere, theresonance is a gain hole, which, through the Kramers-Kronigrelations, results in fast light [31], [32]. From here on, we usethe term time advance rather than time delay to represent thetime shift achieved by the SOA.The fast light effect in the SOA is controlled by the strengthof the population oscillations. This is typically performed byvaryingthepoweroftheinputopticalsignal, whichwecallthe optical pump (Ppump), [17], [29], [30] or the SOA excitedcarrier population, which is controlled by the SOA bias current(ISOA) [17], [21]. However, these twocontrol parameters,even when varied together, couple the optical time advance andoutputpowertogether,renderingthesimultaneouscontroloftime advance and weight difcult. As an example, to increasethe fast light effect one must either increasePpumporISOA,which both increase the output power. One could increase oneparameterwhiledecreasingtheothertomaintainaconstantoutput power, but thiswouldreducethefast light effect. Toindependently control fast light and the output power, a controlparameterisneededthat pushesthefast light effect andtheoutput power in opposite directions.Inour scheme, XGMprovides the means bywhichtheoptical pump parameter can be transformed to push fastlightandoutputpowerinoppositedirections.Aweakprobesignal, detunedfromtheoptical pumpbyseveral timestheinverse carrier lifetime to avoid inducing unwanted CPO withthe pump, is injectedintothe SOAalongwiththe strongmodulatedpumpthat isinducingthefast light. Duetogainsaturation, the higher (lower) the pump power, the lower(higher) the output probe power [33]. Additionally, the pumpsignal is modulated onto the probe wave by XGM. Now, therelationship between fast light and output power for the opticalpumppower parameter has beenreversed: ahigher Ppumpresults in a stronger fast light effect, but a lower output power.In contrast, a higher ISOA results in a stronger fast light effectandahigheroutput power. Thus, theSOAusingXGMcanincreasethefast light effect byincreasingbothPpumpandISOA, yet balance each others gain effects to achieve arbitraryloss or gain within practical limits; the optical weight and timeadvance have been decoupled.We model the combined effects of CPOand XGMbyusingthetheoretical treatment appliedbyMrket. al. [18],[34] for the modulation response of an SOA. The modulatedcomponentsofthepumpandprobewaves, P1andP2,!""" #""" $""" %"""!"&"&!"Microwave Frequency (MHz)XGM Gain (dB) SOA gp = 150 cm-1, P1 = -1 dBm gp = 150 cm-1, P1 = +10 dBm gp = 60 cm-1, P1 = 0 dBm gp = 60 cm-1, P1 = +13 dBm Modulated Pump CW Probe Fig. 1. Gainof the modulatedprobe wave as a functionof microwavefrequency based on Eqn. 2. Four different pump power and SOA bias currentcombinationsareplottedtoshowntheindependentcontrolofoutputpowerandgroupvelocity. Thesolidcurvesrepresent congurationsthat result insimilar dispersions (i.e. similar group velocities), but different output powers.The dashed curve represents a conguration, which results in a similar outputpower to the middle solid curve, but much lower dispersion (i.e. slower light).respectively, can be described by [18]dP1dz= (g a) P1gP1/Ps1 +P1/Psi P1(1)dP2dz= (g a) P2gP2/Ps1 +P1/Psi[P2 + P1exp(ikz)] (2)Intheseequations, aistheinternal loss, Pjistheaveragepowerwherej =1, 2forpumpandprobe, respectively, Psisthesaturationpower, isthemicrowavefrequency, isthesemiconductorcarrierlifetime, andk=k2 k1istheeffective propagation constant of the modulated waves in thereference frame of the probe.g is the saturated gain given byg=gp1 +P1/Ps(3)wheregpisthepeakgaincoefcient. It isassumedthat thepumppower issignicantlylarger thantheprobepower sothat the probe does not contribute to gain saturation. The initialconditionsusedinthesimulationwereP1(L)=P10whereP10is theaverageinput pumppower andLis thecrystallength,P2(0)= 5dBm, P1(L)=mP10wheremisthemodulation index, andP2(0) = 0.Toshowthe possibilityof independentlycontrollingtheoutput probepowerandthetimeadvanceusingPpumpandISOA, the expression for the probe signal is solved numericallyfor four different congurations of Ppump and ISOA. ChangesinPpumpare implemented in the model through the averageand modulated pump powers, P1and P1, respectively,while changes in ISOAare implementedthroughthe peakgaincoefcient, gp. The remainingparameters usedinthesimulationareL=1mm, m=.4, a=15cm-1, Ps=.70733-8724 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2015.2400399, Journal of Lightwave TechnologyJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, MONTH XXXX 3EDFA Isolator Circulator BandpassFilter !2 NetworkAnalyzer VOA PC EAM Pump !1=1551.72nm Probe (P = -5 dBm) !2=1553.33 nm

x(t) !x(t !" )! (Ppump, ISOA)!(Ppump, ISOA)ISOA Ppump Electrical Signal Optical Pump Optical Probe Detector SOADUT Fig. 2. Experimental setup used to measure the weight and time advance ofthe SOA. EAM = electroabsorption modulator, PC = polarization controller,EDFA=erbium-dopedberamplier,VOA=variableopticalattenuator. = signals time of travel and = signal gain.mW,P2=5 dBm, and = 800 ps. The results areplotted in Fig. 1, with the relevant parameters labeled oneachcurve. The solidcurves represent congurations withsimilar resonances, and therefore similar dispersions and groupvelocities, but different output powers, showingthe abilitytocontrol output powerindependentlyoftimeadvance. Thedashed curve represents a conguration with a similar outputpower as the middle solid curve (exactly the same at 1 GHz),but a signicantly weaker resonance, indicating the ability tocontrol time advance independently from output power.III. EXPERIMENTAL SETUPTheSOAusedinthisexperiment isa1mmlongdevicefrom Amphotonix. It employs a buried heterostructure designwith a strained bulk active region to provide low polarizationdependence. The bulk active region is .1 m thick with .1 mthick separate connement heterostructures [35]. The SOA hasanexperimentallymeasuredtransparencycurrent of 15mAandasaturationpower pf -3dBm. Theexperimental setupusedtocharacterizetheoptical weight andtimeadvanceoftheSOAisshowninFig. 2. A1551.72nmpumplaser ismodulatedbyanelectroabsorptionmodulator(EAM), whichreceives a microwave electrical signal from a network analyzer.The network analyzer will ultimately be used to determine thesignal weight andtimeadvanceproducedbytheSOA. Themodulated pump is boosted to 14 dBm power by an erbium-dopedber amplier beforebeingattenuatedtothedesiredpower byavariableoptical attenuator. Theresultingpumppower, Ppump, is the rst control parameter. The pump is thenpassed to the SOA under test by an isolator and circulator.Upon entering the SOA, the pump beats with its RFsidebands, inducing CPO. The CPOstrength depends ontheoptical pumppower, Ppump, andtheSOAexcitedcar-rier densitythroughtheSOAbias current, ISOA, whichisthesecondcontrol parameter. AfterpropagatingthroughtheSOA, the signal has been advanced compared to transmissionthrough an unpumped SOA, and the relative time advance canbemeasuredbydetectingtheoutput signal fromtheSOA.However,thisschemestillresultsinacoupledtimeadvanceand output power, as discussed in the previous section.To separately control the output optical power, the RFsignal is transferred fromthe optical pump to a separate,strongly detuned optical probe (detuning>> CPO resonancebandwidth)viaXGMintheSOAasdepictedinFig.2.Theprobe is a 1553.33 nm laser with signicantly weaker power(P = -5 dBm) than Ppump to avoid perturbing the SOA gain. Itis passed into the SOA by a circulator and counter-propagatesagainst theoptical pump. Thechoiceofcounter-propagationwas made to improve isolation between the optical pump andprobe at the output. Inside the SOA, the RF signal modulatedontheoptical pumpisimprintedontotheoptical probebyXGM, whichalsoinverts the signal. As mentionedintheprevious section, the power of the modulated probe is inverselyrelated to the original pump power because of cross-gainsaturation. Theoptical probeexitstheSOAviaacirculatorandislteredtoremoveunwantedwavelengths. Finally, theoptical probe is detected, and the output RF signal is comparedto the input RF signal by the network analyzer to extract thesignals weight and time advance.IV. EXPERIMENTAL RESULTS AND ANALYSISTheeffectsof PpumpandISOAonthegainandadvanceof the RF signal after propagating through the SOA are inves-tigated, rst asafunctionof PpumpandISOAindividually,andthenasafunctionofboth. Theseresultsshowhowthecombined effects of XGM and CPO can be used to implementan independently tunable optical weight and advance.A. Tunable Time AdvanceFigures3aand3bshowtheexperimentallymeasuredrel-ativetimeadvanceupto4GHzmicrowavefrequencyasafunctionof PpumpandISOA, respectively. Bothplotsshowthe delay-bandwidth roll-off that is characteristic of slow andfast light. In Fig. 3a, ISOAis held constant at 80 mA, whilePpumpis stepped from 0 dBm to 14 dBm in increments of 2dB. Larger pump powers generate a stronger CPO effect, andthusalarger groupvelocityintheSOA, sosignal advanceincreases with Ppump as observed in Fig. 3a. The time advanceforeachvalueof Ppumpismeasuredrelativetothesignalstimeoftravel whenPpump=0dBm, whichcorrespondstothelowest groupvelocity. Thedeviceachievesamaximumadvance of 80 ps at low frequency. The effect has a bandwidthof 1/(2s), where sis thesemiconductor carrier lifetime[29]. From the measurements,sis extrapolated to be around150 ps, which is within the expected range of this SOA.Figure3bshowssimilar measurementsfor timeadvance,this time as a function ofISOA, withPpumpheld constant at5 dBm. ISOA is stepped from 30 mA to 100 mA in incrementsof10mA, andthetimeadvanceismeasuredrelativetothetime of travel atISOA= 25 mA. The higher the bias current,thestronger theCPO, soadvanceincreases withISOA, asobserved in Fig. 3b. The maximumtime advance at lowfrequency is 150 ps and begins to saturate around 100 mA biascurrent due to gain saturation as a function of bias current.To conrm that CPO is the physical effect causing the timeadvance, the RF signal gain was measured using the identicalsystem congurations used to produce the data in Fig. 3. The0733-8724 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2015.2400399, Journal of Lightwave TechnologyJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, MONTH XXXX 41000 2000 3000 400020020406080Frequency (MHz)Relative Advance (ps)1000 2000 3000 400050050100150200Frequency (MHz)Relative Advance (ps)0 1000 2000 3000 4000201510505Frequency (MHz)Normalized Gain (dB)0 1000 2000 3000 4000010203040Frequency (MHz)Normalized Gain (dB)30 mA100 mA2 dBm14 dBmPpump = 5 dBmISOA = 80 mAISOA = 80 mA14 dBm2 dBm30 mA100 mAPpump = 5 dBm(a)(c)(b)(d) (a) (b) Fig. 3. Relative advance up to 4 GHz as a function of (a)Ppump(ISOA= 80 mA) and (b)ISOA(Ppump= 5 dBm). Advance is measured relative to thetime of travel at 0 dBm in (a) and 25 mA in (b).1000 2000 3000 400020020406080Frequency (MHz)Relative Advance (ps)1000 2000 3000 400050050100150200Frequency (MHz)Relative Advance (ps)0 1000 2000 3000 4000201510505Frequency (MHz)Normalized Gain (dB)0 1000 2000 3000 4000010203040Frequency (MHz)Normalized Gain (dB)30 mA100 mA2 dBm14 dBmPpump = 5 dBmISOA = 80 mAISOA = 80 mA14 dBm2 dBm30 mA100 mAPpump = 5 dBm(a)(c)(b)(d)(a) (b) Fig. 4. Normalized gain up to 4 GHz as a function of (a)Ppump(ISOA= 80 mA) and (b)ISOA(Ppump= 5 dBm). Gain is normalized to signal powerat 0 dBm in (a) and 25 mA in (b).resultsareshowninFig. 4. Figure4ashowsthegainofthesystemfordifferent Ppumpvalues,normalizedtothegainatPpump= 0 dBm. A resonance is visible at lower frequencies,characteristicof CPO, withlarger pumppowers generatingdeeper andwider resonances. The resonance bandwidthisidentical to the advance bandwidth in Fig. 3a, indicating thatCPO is the source of the time advance. In addition, note thatas Ppump increases, the signal gain decreases. Figure 4b showsthe gain for different ISOAvalues, normalized to the gain atISOA=25mA. Asimilar trendcanbeseen: larger ISOAvalues lead to larger resonances with a bandwidth identical toFig. 3b. As expected, asISOAincreases, so does the gain.Thereappearstobedifferent resonancebandwidthswhenchanging ISOAcompared to changing Ppump. This is anillusion caused by our choice of different baseline biasing con-ditions for plotting gain in Fig. 4a and b. We have conrmedthisbyplottingthedatainbothFig. 4aand4brelativetoacommon baseline and found similar bandwidths.B. Tunable WeightWhile Fig. 3a and 3b show that adjustingISOA andPpumpcanbeusedtocreateatimeadvance, Fig. 4aand4bshowthat adjusting ISOAand Ppumpalsoinadvertentlychangestheoutput signal power. Thiscouplingwill beusedtoouradvantage.Note that PpumpandISOApush timeadvance inthesamedirectionbut pushgaininoppositedirections. Inotherwords, asPpumpandISOAincrease, sodoesadvance(see Fig. 3a and 3b). On the other hand, asPpumpincreases,gaindecreases; however, as ISOAincreases, gainincreases(see Fig. 4a and 4b).Theinverse relationshipbetweensignal gainand Ppumporiginatesfromcross-gainsaturationintheSOAandisthekey relationship that permits the independent control of weightandtimeadvance, asmentionedinsectionI. Therefore, byadjustingISOAandPpumptogether inabalancedmanner,advance and gain can be adjusted independently.To illustrate this, gain and advance were measured asfunctionsofbothISOAandPpumpat 600MHzmicrowavefrequency, andthe results are shownas gainandadvancecontour plots in Fig. 5a and 5b, respectively. Gain is calculatedby subtracting the loss of the modulator-photodetector-RFamplier systemfromthetotal RFsystemgaintoexamineonlythegaincontributionsfromtheSOAintheXGMcon-guration. The key observation is that constant gain contoursexist within the ISOA and Ppump parameter space. The 3 dBand 6 dB gain contour lines are drawn in Fig. 5a, but a fullrange of -15 dB to +15 dB can be achieved. Furthermore, Fig.5b shows the corresponding advance contour plot as a functionofPpumpandISOA, with advance contours up to 120 ps. Acomparisonofthegainandadvancecontourlinesshowthattheyrunnearlyorthogonal toeachother inthePpumpandISOAparameterspace. ThegaincontourlinesfromFig. 5aare overlayed onto Fig. 5b, and the -6 dB, -3 dB, and +3 dBcontour lines each span across at least 100 ps of advance.We now arrive at the main result of this paper: by adjustingISOAandPpumpacrossdifferent gainandadvancecontourlines in theISOA-Ppump parameter space, the SOA can serveas both a tunable weight and time advance. While the tunableweight, advance, andbandwidthrangesshownherearenotstate-of-the-art, this technique applies to any SOA. Designingan optimal SOA is one direction of future work.0733-8724 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2015.2400399, Journal of Lightwave TechnologyJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, MONTH XXXX 53 2 1 3 2 1 (b) (a) Fig. 5. Contourplotsof(a)gainand(b)advanceasafunctionof ISOAand Ppump at 600 MHz microwave frequency. Gain contours are overlaid onboth plots. Advance contours are shown in (b) only.The experimental results obtained here, particularly in Fig.3, appeartodifferfromtheresultsobtainedin[18]despitethesimilarexperiments. However, wepoint out that theRFfrequencies used here, DC to 4 GHz, are small in comparisonto the former experiment, DC to 40 GHz. At low frequencies,thewavelengthof theRFsignal issignicantlylarger thanthe device length and propagation effects are thus negligible.Indeed, at low frequency the results obtained here more closelyresemble those obtained in the previous work.C. Application in Microwave PhotonicsIn Fig. 6, we show the SOAs ability to act as a simultaneousoptical weight and time advance by measuring the SOA outputusing an optical oscilloscope while biasing the SOA along twodifferent gain contour lines. The input is a 0 dBm, 600 MHzsinewavegeneratedbyanRFsignalgenerator.Thetopandbottom sets of curves, which are articially offset for clarity,correspond to biasing the system along the +3 dB and -3 dBgaincontourlines, respectively. Thebiasingconditionsusedto generate curves 1-3 in both sets of curves are indicated onthe corresponding contour lines in Fig. 5b.Ineachset of curves, theSOAshowsatunableadvancerange of 120 ps, while exhibiting very little amplitude changealong a single gain contour line. On the other hand, the SOAcan act as an optical weight by jumping between gain contours,as shown by the difference between the top and bottom sets of1 0.5 0 0.5 150050100Time (ns)Power (W)120 ps120 ps112332SOA + 3 dB - 3 dB 150 100 50 0 Fig. 6. Outputfromanoscilloscopewhilebiasingthedevicealongthe+3dB(topsetofcurves)and-3dB(bottomsetofcurves)gaincontours. Thebiasingconditionsusedtogeneratecurves1-3for bothsetsof curvesarelabeled on their respective gain contours in Fig. 5b. No normalization of theamplitude was performed.curves. The power of the top curves is twice the power of thebottom curves as opposed to four times (i.e. 6 dB) because theoscilloscopeusedtoproduceFig. 6measuresopticalpower,whereas the gain contours in Fig. 5 correspond to RF power.Some nonlinear distortion leads to a small discrepancy in themeasuredtimeadvanceranges betweenFig. 6andFig5b.Noise and distortion will be discussed in the next section.To demonstrate the application of this device, we use it asthe tuning element in a microwave photonic notch lter, shownin Fig. 7a. The lter is constructed so that as the time advanceof theSOAis increased, shorteningthepathlengthof thebottom tap, the notch shifts to lower frequencies. In addition,bytuningthe device alonga gaincontour line, the notchdepthremainsconstant whileadjustingthenotchfrequency,characteristicofanamplitude-invariant optical delay. Figure7b shows the results of tuning the SOAs advance on the ltersfrequency response. The notch frequency shifts from 579 MHzto 633 MHz while maintaining a nearly constant notch depth of30 dB. The notches in Fig. 7b correspond to the second orderinterferenceminima(thelterhasanotchatDCbecauseofXGMinversion), or, put anotherway, thenotchfrequenciescorrespondtoasinglefreespectralrangeofthelter. Thus,the time difference between the two arms of the lter is givenby the inverse of the notch frequency. Using this information,themaximumtunableadvanceofthesystemalongthisgaincontour line is calculated to be 145 ps.D. Device Latency, Noise, and NonlinearityWe now discuss a few important properties of the device: la-tency, noise, and nonlinearity. The device latency is determinedbythetimerequiredforadjustmentstoPpumpandISOAtoalter the SOAs properties. For Ppump, a change in pump powertakes about 100 ns to propagate to the SOA in our setup. Thiscorresponds to 20 meters of ber and can be shortened simplybyusingshorter ber. Inanintegratedcircuit, achangein0733-8724 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2015.2400399, Journal of Lightwave TechnologyJOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. XX, NO. X, MONTH XXXX 6EAM Weight/ Delay SOA VOA (a) Fig. 7. (a) Simplied schematic of the notch lter used to test the simultaneousweight and advance and (b) frequency response of the tunable lter. The notchfrequency is tuned from 579 MHz to 633 MHz, indicating a 145 ps shift.power could propagate to the SOA much quicker, easily downtoananosecond. For ISOA, achangeinbiascurrent takeseffect on the order of the semiconductor carrier lifetime. FortheSOAusedhere,wemeasuredthisparameterthroughtheCPO resonance bandwidth to be about 150 ps. Therefore, weestimate that the device latency, in an integrated system, is ontheorderofananosecond. Practicallyspeaking, thelatencywill be dictated by the controlling electronics.There are three major sources of noise and distortionin the SOA: amplied spontaneous emission (ASE) noise,nonlinearities, andgroupvelocitydispersion. Thesesourcesare all dependent on the pump power and the SOAbiascurrent, which must be considered. It is well-known that SOAsexhibit relatively large ASEbackgrounds, leading to largenoisegures. Thenoisegureof thisSOAis8dB, whichincreases with decreasing bias current due to the reducedgain. However, we must also account for the effects ofXGM. We experimentally observe lower signal-to-noise ratios(SNR) at lower bias currents, where excessive gain saturationsignicantly reduces the XGM gain, in addition to the risingnoise gure. The extremes of the two parameters also inducenonlinearities. For example, withlowbiascurrent andhighpump power, the pump so strongly depletes the gain thatXGM distorts the transferred signal. On the other hand, withhigh bias currents and low pump powers, if the pump powerdrops signicantly below the SOA saturation power, the pumpbecomes too weak to drive XGM.Group velocity dispersion (GVD) is a consequence ofthe SOAs dispersive mediumandcauses pulse andsignaldistortioninslowandfast light devices[24]. TheeffectsofGVD are manifest in Fig. 3 and 4, which explicitly show thelimitedbandwidthof thetimeadvanceandgainresonance,respectively. The GVD is worse for higher pump powers andbias currents, where the induced material dispersion is larger.Tosummarize, theextremesofSOAbiascurrent andpumppower induce larger nonlinearities and noise; in addition, highbias currents createagreater signal dispersion. Theproperlimits on the bias current and pump power are in the middle ofthe parameter space and must be determined by the applicationtolerance for SNR, dynamic range, and bandwidth.V. CONCLUSIONWe have demonstrated how a single SOA can be used as asimultaneous weight and delay for microwave photonics. TheSOA uses a combination of slow and fast light and XGM togenerate a tunable time advance, while independently control-lingthesignal weight. TheSOAsweight andtimeadvancecanbe adjustedbyusingtwoseparate control parameters:the SOAbias current and the input optical pump power.Byaccessingthefull two-dimensional parameter space, thedevicecanachieve100psof continuouslytunableadvanceand-6to+3dBoftunableRFgainat600MHzmicrowavefrequency. Thedevicelatencyisfundamentallyontheorderof nanoseconds, andis basedonthesemiconductor carrierlifetimeandthetimeittakesforthepumppowerchangetopropagatetotheSOA. AsanSOA, thisdevicelendsitselfto integration, and can potentially serve as a complete, high-speed, and compact tunable lter tap. It is particularly usefulinsystems that requirethesimultaneous recongurationofoptical weight and delay, and wavelength conversion.There are several paths for future work. First, different SOAdeviceparametersandgeometriesshouldbeinvestigatedtoimprove the advance andweight ranges, as well as noise,linearity, and bandwidth. For example, greater semiconductorcarrier lifetimes will increase advance range but will alsoreducebandwidth.Second,arigorousstudyofthedistortionand noise associated with this technique could provide usefulinsight indesigninganoptimal device. Third, it wouldbeuseful tofabricate anintegratedversionof this device forintegrated microwave photonics.ACKNOWLEDGMENTTheauthorsgratefullythankDr. YanhuaDeng, AlexTait,Dr. BhavinJ. Shastri, andJen-TangLu, for their helpandsuggestions on the project.REFERENCES[1] B. Wu, Z. Wang, Y. Tian, M. Fok, B. J. 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Technol., vol. 14, no. 6, pp. 942954, June 1996.[34] J. Mrk, A. Mecozzi, and G. Eisenstein, The modulation response of asemiconductor laser amplier, IEEE J. Sel. Topics Quantum Electron.,vol. 5, no. 3, pp. 851 860, May/June 1999.[35] C. Michie, A. E. Kelly, J. McGeough, I. Armstrong, I. Andonovic, andC. Tombling, Polarization-insensitivesoasusingstrainedbulkactiveregions, J. Lightw. Technol., vol. 24, no. 11, pp. 3920 3927, November2006.MatthewP. Chang (S12) received the Bachelor of Science degree inelectrical engineering from the Pennsylvania State University. He is currentlypursuinghisPh.D. at PrincetonUniversityasamember of theLightwaveCommunications ResearchLabwithProfessor Paul Prucnal. His researchinterestsincludemicrowavephotonics, RFinterferencecancellation, beam-forming, and integrated photonics.Mr. ChangisamemberoftheIEEEPhotonicsSocietyandtheOpticalSociety of America (OSA). He is a Gordon Wu Fellow at Princeton University,andarecipient oftheNational DefenseScienceandEngineeringGraduateFellowship (NDSEG) and the Excellence in Teaching Award at Princeton.Noelle Wang is working towards her Bachelor of Science degree at RutgersUniversity, Rutgers, New Brunswick, NJ.Ben Wu is currently working towards the Ph.D. degree in electrical engineer-ingdepartment at PrincetonUniversity, Princeton, NJ. Hisresearchfocuseson all-optical signal processing, noise analysis and optical steganography.Paul R. Prucnal (S75-M79-SM90-F92) received the A.B. degree fromBowdoinCollege(summacumlaude), withHighest Honors inMathandPhysics. He then received the M.S., M.Phil., and Ph.D. degrees from ColumbiaUniversity. He is currently a Professor of Electrical Engineering at PrincetonUniversity, where he has also served as the Founding Director of the CenterforPhotonicsandOptoelectronicMaterials, andiscurrentlytheDirectorofthe Center for Network Science and Applications. He has held visiting facultypositions at the University of Tokyo and University of Parma.Prof. Prucnal was anArea Editor of the IEEETRANSACTIONSONCOMMUNICATIONSfor optical networks, andwas Technical Chair andGeneral Chair of theIEEETopical MeetingonPhotonicsinSwitchingin1997and1999, respectively. HeisaFellowofIEEEwithreferencetohisworkonoptical networks andphotonicswitching, aFellowof theOSA,andarecipientoftheRudolfKingslakeMedalfromtheSPIE,citedforhisseminal paper onphotonicswitching. In2006, hewas awardedtheGoldMedal from the Faculty of Physics, Mathematics and Optics from ComeniusUniversity in Slovakia, for his contributions to research in photonics. He hasreceivedPrincetonEngineeringCouncil LifetimeAwardfor ExcellenceinTeaching, theUniversityGraduateMentoringAward, andtheWalterCurtisJohnsonPrizeforTeachingExcellenceinElectricalEngineering,aswellasthe Distinguished Teacher Award from Princetons School of Engineering andAppliedScience. Heiseditorofthebook, Optical CodeDivisionMultipleAccess: Fundamentals and Applications.