0733-8724 (c) 2015 IEEE. Personal use is permitted, but
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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:
[email protected])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 [email protected], 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.
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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
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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
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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.
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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.
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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
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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.