Ultra-high-density spatial division multiplexing with a few-mode multicore fibre.pdf (PDFy mirror)

0.45 MB·English

ARTICLES
PUBLISHEDONLINE:26OCTOBER2014|DOI:10.1038/NPHOTON.2014.243
Ultra-high-density spatial division multiplexing
fi
with a few-mode multicore bre
R. G. H. van Uden1*, R. Amezcua Correa2*, E. Antonio Lopez2, F. M. Huijskens1, C. Xia2, G. Li2,3,
A. Schülzgen2, H. de Waardt1, A. M. J. Koonen1 and C. M. Okonkwo1*
Single-mode fibres with low loss and a large transmission bandwidth are a key enabler for long-haul high-speed optical
communication and form the backbone of our information-driven society. However, we are on the verge of reaching the
fundamental limit of single-mode fibre transmission capacity. Therefore, a new means to increase the transmission
capacity of optical fibre is essential to avoid a capacity crunch. Here, by employing few-mode multicore fibre, compact
three-dimensional waveguide multiplexers and energy-efficient frequency-domain multiple-input multiple-output
equalization,wedemonstratetheviabilityofspatialmultiplexingtoreachadatarateof5.1Tbits−1carrier−1(net4Tbits−1
carrier−1) on a single wavelength over a single fibre. Furthermore, by combining this approach with wavelength division
multiplexing with 50 wavelength carriers on a dense 50GHz grid, a gross transmission throughput of 255Tbits−1 (net
200Tbits−1)overa1kmfibrelinkisachieved.
W
ith the persistent exponential growth in Internet-driven hole-assisted few-mode multi-core fibre (FM-MCF)20,21, with
traffic, the backbone of our information-driven society, seven few-mode cores, each allowing the LP and two degenerate
01
based on single-mode fibre (SMF) transmission, is LP modes to co-propagate in the X- and Y-polarization. A
11
rapidly approaching its fundamental capacity limits1. In the past, custom-designed butt-coupled integrated three-dimensional (3D)
capacityincreasesinSMFtransmissionsystemshavebeenachieved waveguide is key to multiplexing all 21 spatial LP modes per linear
byexploitingvariousdimensions,includingpolarizationandwave- polarizationbeingsimultaneouslytransmitted.Thefibredesignmini-
lengthdivisionmultiplexing,intandemwithadvancedmodulation mizes intercore crosstalk and reduces the required MIMO equalizer
formats and coherent transmission techniques2. However, the complexity from 42×42 to 7×(6×6), hence reducing energy con-
impending capacity crunch implies that carriers are lighting up sumption.Inaddition,anenergy-efficientMIMOfrequency-domain
dark fibres at an exponentially increasing rate to support societal equalizer (FDE) is used for every core. A single-carrier spectral
capacity demands3. To alleviate the corresponding costs and efficiency of 102bits−1Hz−1 is achieved by encoding 24.3GBaud
increased energy requirements associated with the linear capacity 32 quadrature amplitude modulation (QAM), allowing for next-
scaling from using additional SMFs, spatial division multiplexing generation 5.103Tbits−1carrier−1 gross (4Tbits−1carrier−1 net)
(SDM)withinasinglefibrecanprovideasolution4,5.Byintroducing data rate spatial super channels. Combining the spatial dimension
an additional orthogonal multiplexing dimension, the capacity, with 50 wavelength channels on a 50GHz International
spectral and energy efficiency, and therefore the cost per bit, can Telecommunication Union (ITU) grid, a gross total capacity of
be decreased, which is vital for sustaining the business model of 255Tbits−1 (200Tbits−1 net) is demonstrated, further indicating
majornetworkstakeholders.TofulfiltheSDMpromise,anewpara- the viability of combining few-mode and multicore transmission
digm is envisaged that allows up to two orders of magnitude techniquesinasinglefibretoachieveultra-highcapacity.
capacity increasewithrespect to SMFs6. SDM is achieved through
multiple-input multiple-output (MIMO) transmission, employing Hole-assisted few-mode multicore fibre
the spatial modes of a multimode fibre (MMF)7, or multiple In this work, the key requirement for an ultra-high-density SDM
single-mode cores, as channels8–13. Recently, a distinct type of system is a multicore fibre with high mode density and ultra-low
MMF,thefew-modefibre(FMF),hasbeendevelopedtoco-propa- crosstalk, provided by the few-mode multicore approach. In a
gate three or six linear polarized (LP) modes14–17. Driven by conventional multicore fibre, crosstalk constitutes only intercore
rapidenhancementsinhigh-speedelectronics,digitalsignalproces- crosstalk. However, for FM-MCFs, both intracore (between LP
sing (DSP) MIMO techniques can faithfully recover mixed modes) and intercore crosstalk are to be considered20. In relation
transmission channels18, allowing spectral efficiency increases as to intracore crosstalk, it is inevitable that strong modal inter-
spatial channels occupy the same wavelength. State-of-the-art actions occur due to perturbations, splice points and multi-mode
single-carrier FMF transmission experiments have demonstrated components, along a realistic transmission link22–25. Accordingly,
capacity increases in a single fibre by exploiting six spatial when designing and fabricating FM-MCFs, suppressing intercore
modes, achieving 32bits−1Hz−1 spectral efficiency17. By using crosstalkiscritical.Onemethodforminimizingintercorecrosstalk
multicore transmission, a spectral efficiency of 109bits−1Hz−1 has istofabricateafibrewithalargecore-to-corepitch,butatthedetri-
been demonstrated using 12 single-mode cores19. In this work, ment of information density. In contrast to trench-assisted struc-
we demonstrate ultra-high-capacity transmission over a 1km tures, the hole-assisted structure adopted for such a fibre has the
1COBRAResearchInstitute,DepartmentofElectricalEngineering,EindhovenUniversityofTechnology,DenDolech2,POBox513,5600MB,Eindhoven,
TheNetherlands,2CREOL,TheCollegeofOpticsandPhotonics,UniversityofCentralFlorida,POBox162700,Orlando,Florida32816-2700,USA,3College
ofPrecisionInstrumentandOpto-ElectronicEngineering,TianjinUniversity,Tianjin300072,China.*e-mail:[email protected];[email protected];
[email protected]
NATUREPHOTONICS|ADVANCEONLINEPUBLICATION|www.nature.com/naturephotonics 1

©2014 Macmillan Publishers Limited. All rights reserved.
ARTICLES
NATURE PHOTONICS
DOI:10.1038/NPHOTON.2014.243
a b 0
Core 1 Core 2 Core 3 Core 4 Core 5 Core 6 Core 7
−10
LP arrival time
11
B)
d −20
ht ( LP arrival time
g 01
wei −30
er
0 µm ualiz −40
4 q
E
−50
0 5
Time (ns)
Figure1|Few-modemulticorefibrecharacteristics.a,FM-MCFcross-section,whichisbutt-coupledtothe3Dwaveguide.b,MeasuredMIMOequalizer
responseofallsevencoresforthetransmittedLP X-polarizationtothereceivedLP X-polarizationat1,555.75nm,allowingdifferentialmodedelay
01 01
estimationwithanaccuracyof∼20ps,indicatingasimilarperformanceofallcores.AsallLPmodesareequallyexcited,therespectiveMIMOequalizer
matrixelementsaresimilar.
a ChUT laser
Local oscillator
10 km
Odd/even channel
Laser 1
Loading channels Las...er 50 Multiplexer EDFA 24.3 Gbaudtransmitter EDFAPol4a4ri znastiond100 GHzecointerleaverrre2OEl9avd3teid onnns EDFA 15785551 nnnEsssDFAs CoUT Core switch 32=Dm1 uwlativpelegx1u ekidrme FM-MCF3dDem wualvtiepgleu2xi=1deer CoreswitchCoUT TDM-SDMreceiver
1:18
multiplexer Loading cores
b 0 c d
R
−10 ela 1 2
Odd −−3200tive trans 3 4 5 Core 1 Core 2 Core 3
Even −−1-−02−30400mitted power (d 6 7 40 µm Core 4Core 7 Core 5 Core 4 Core 6
B
)
0
1,542.14
1,561.81
Wavelength (nm)
Figure2|FM-MCFWDM/SDMexperimentaltransmissionset-up.a,TheloadingchannelsandoneCoUTaresimultaneouslymodulatedbya24.3GBaud
16or32QAMconstellationsequence.Consecutively,polarization,carriers,coresandmodesaredecorrelated.The3Dmultiplexerguidesthetransmission
channelsintoandoutoftheFM-MCFviabutt-coupling,wheretheCoUTisvariedthroughallcoresconsecutively.b,Decorrelatedwavelengthspectrumafter
beinginterleavedbyawavelength-selectiveswitch.c,Saturatedcameraimagetakenatthereceiverside,withallcoreslitsimultaneously.d,Independent
coresareexcited,indicatinglowcrosstalkpercore.Rightbottom:selectivelaunchingoftheLP andLP modesinthecentrecore(core4),respectively,
01 11
wherethemodalenergyisconfinedtothecentreofthehexagonalcorestructure.
benefit of improved mode-confinement and intercore crosstalk apart, creating an air-hole to pitch (air-hole diameter d; air-hole
reduction,whichisbestminimizedbytuningtheair-holediameter pitch Λ) ratio of 0.62, corresponding to an intercore crosstalk of
andair-holepitcharoundeachcore.Accordingly,a1kmstep-index under −80dBkm−1, which provides the potential to scale to
seven-coreFM-MCF withan outer diameterof 192µm (±0.5µm) longer transmission distances10,11,20. During measurements, the
and a coated fibre diameter of 37µm was successfully fabricated FM-MCF was wound on a conventional fibre drum with a
(Fig. 1a). For improved placement within future ducts for actual mandrel radius of 15.9cm. The LP mode average measured
01
deployment,thefabricatedfibrecladdingdiametercanbereduced attenuation for all cores is 0.3dB. The measured effective areas
to 150µm without losing its transmission attributes. Individual (A ) for the LP and LP modes are 112 and 166µm2, respect-
eff 01 11
coreswithadiameterof13.1µmandacorerefractiveindexdiffer- ively. Depicted in Fig. 1b are the impulse response measurements
enceof0.36%werehexagonallyarrangedatacore-to-coredistance for all cores, which indicate a differential mode group delay of
of40µm.Air-holeswithdiametersof8.2µmwereplaced13.3µm 4.6psm−1 at 1,550nm. The calculated chromatic dispersions at
2 NATUREPHOTONICS|ADVANCEONLINEPUBLICATION|www.nature.com/naturephotonics

©2014 Macmillan Publishers Limited. All rights reserved.
ARTICLES
NATURE PHOTONICS
DOI:10.1038/NPHOTON.2014.243
a To manage the complexity of the measurement process, the
21 dual polarization SMF inputs transmitter ChUT laser was split into two equal tributaries, with
thesecondtributaryactingasaremotelocaloscillator(LO)forall
700 measurements26,27. As the transmitter and receiver shared the
same laser source, a 10km SMF was inserted into the LO path to
minimize the impact of laser phase coherence. All carriers were
modulated by a lithium niobate (LiNbO ) IQ-modulator, which
3
was driven by two digital-to-analog converters (DACs), represent-
1 FM-MCF output ing the in-phase and quadrature components, to generate a
24.3GBaud 16 or 32QAM signal. Higher-order modulation
formatsareparticularlyinterestingforfibreperformanceinvestigation
b because of their susceptibility to impairments in the transmission
channel.Beforetransmission,themodulatedcarriersweredecorre-
1 2
latedconsecutivelyforpolarization,carriers,modesandcores.The
relative powers of the decorrelated carriers are shown in Fig. 2b,
showingarelativelyequalpowerdistributionovertheentiretrans-
3 4 5
mittedwavelengthband.Inaddition,duetotheexcellentautocor-
relation properties of the transmitted sequences from the DACs,
6 7 0 µm all transmitted channels guided into the mode-multiplexer can be
4 consideredtobeindependentanddistinct.Severalmode-multiplex-
ing techniques have been developed that selectively excite a set of
modes in few-mode fibres, such as phase plates28–30, spot launch-
Figure3|Three-dimensionalwaveguidecharacteristics.a,Schematicof ers31–34andadiabaticallytaperedphotoniclanterns35–37.Incontrast
the3Dwaveguide,wheresetsofthreetransparentwaveguidesareplacedin to the bulky optics associated with phase plates, the latter two
atriangulararrangementtoaddressrespectivefew-modecores.b,3D techniques potentially have the compactness necessary for inte-
waveguideFM-MCFfacetmicroscopeimage. gration into a future transponder. Hence, a customized and
compact3Dwaveguidemultiplexerwasdesignedtosimultaneously
1,550nmfortheLP andLP modesare23and28psnm−1km−1, spot-launch all spatial channels into the FM-MCF. Accordingly,
01 11
respectively,whilethedispersionslopesforLP andLP are0.06 the waveguides in the mode multiplexers were formed in a
01 11
and 0.07psnm−2km−1, respectively. The calculated cutoff wave- 5.3mm×10mmborosilicateglasssubstratebydirectlaserwriting
length is ∼1,300nm for the LP mode and ∼2,100nm for the using focused ultrafast femtosecond laser pulses. Borosilicate glass
21
LP mode, ensuring effective two-mode guidance for C- and supports an extensive wavelength band, covering all key telecom
11
L-bandWDMtransmission. bands ranging from visible light up to 2.2µm. The inscription
techniqueproducescontrollablesubsurfacerefractiveindexmodifi-
Experimentalimplementation cationandallowstherequired3Dpatternoftransparentwaveguides
At the transmitter side (Fig. 2a), 50 conventional low-linewidth to be carefully controlled to ±50nm. As shown in Fig. 3a, the 21
(<100kHz) external cavity lasers (ECLs) on a dense 50GHz ITU SMFinputs(witha127µmpitchV-groove)wereattachedtowave-
frequency grid spanning the telecom C-band wavelength region guides(assignedinsevensetsofthreewaveguides)andinscribedin
from 1,542.14 to 1,561.81nm were used as loading channels. a hexagonal arrangement with a diameter of 80µm to match the
DuringtheFM-MCFperformanceverification,allfew-modecores core arrangement and structure of the FM-MCF. The individual
wereinvestigatedasthecoreundertest(CoUT)wasvariedconsecu- square waveguides have a cross-sectional effective area of 36µm2
tively. By replacing each loading wavelength channel within the (Fig.3b),andeachsetofthreewaveguideswasplacedinatriangular
channel under test (ChUT) laser, quantitative performance arrangement38. This arrangement minimizes insertion losses,
measurementswereperformedviabit-error-rate(BER)estimation. while equally exciting the LP and LP degenerate modes in
01 11
CoUT inputsa AOMAO 2M 1 ED2V.F4OA5A km2 × 1 EDFA CCrreeoocchhee2ee1iirrvveeeennrrtt 4500ss c cGGooSSppT eesrsi−−g11ger−1 Up sampling to 56 GS sgenerDigital re-alignmentator Front-end compensation CD compensation 6×6 MIMO Carrier phase estimation −Log (system BER)b10 42531 131662 QQQAAAMMM 6BBTT×BBTh6eory 16 QATheory 32 QA 270%% H SDD FFEECC--lliimmiitt
M M
EDFA 32 QAM 6 × 6
or 6
ocal oscillat EDFA 1:3 AOM 3 2.4V5O kAm 2 × 1 10 15 20OSNR (d2B5/0.1 nm)30 35 40
L
AOM 4
Figure4|TDM-SDMreceivercharacterization.a,TDM-SDMreceiver,whereonedualpolarizationmodeisreceivedbycoherentreceiver1andtwomodes
areseriallyreceivedbycoherentreceiver2.TheAOMsactassignalgates,withaclosingtimecorrespondingtothepropagationtimeofthedelayfibre,
enabledbythetriggergenerator.Thevariableopticalattenuator(VOA)ensuresequalpowerreceptionofbothmodes.Inthedigitaldomain,theserialized
signalsareparallelizedtoconstructthreereceiveddual-polarizationmodes.b,Performanceofsingle-channeltransmissionreceivedbytheTDM-SDM
receiverwithrespecttoback-to-back(BTB)performance,indicatinganerrorfloorof2×10−4for16QAMand1×10−3for32QAM.
NATUREPHOTONICS|ADVANCEONLINEPUBLICATION|www.nature.com/naturephotonics 3

©2014 Macmillan Publishers Limited. All rights reserved.
ARTICLES
NATURE PHOTONICS
DOI:10.1038/NPHOTON.2014.243
a 2 b 1
Core 1 Core 2 Core 3 Core 4 Core 5 Core 6 Core 7 Core 1 Core 2 Core 3 Core 4 Core 5 Core 6 Core 7
7% HD FEC-limit
m BER) m BER) 20% SD FEC-limit
(syste−Log10 3 −Log (syste10 2
4 3
1,542.14 1,552.18 1,561.81 1,542.14 1,552.18 1,561.81
Wavelength (nm) Wavelength (nm)
c 10 d 10
Core 1 Core 2 Core 3 Core 4 Core 5 Core 6 Core 7 Core 1 Core 2 Core 3 Core 4 Core 5 Core 6 Core 7
B) 8 B) 8
d d
nt loss ( 6 nt loss ( 6
e e
d d
n n
pe 4 pe 4
e e
d d
e- e-
d d
Mo 2 Mo 2
0 0
1,542.14 1,552.18 1,561.81 1,542.14 1,552.18 1,561.81
Wavelength (nm) Wavelength (nm)
Figure5|Transmissionresults.a,16QAMtransmissionperformance.Allchannelsperformwellbelowthe7%harddecisionFEClimitrequiredtoensure
error-freetransmissionfora81.6bits−1Hz−1grossspectralefficiency.b,32QAMtransmissionperformanceiswellbelowthe20%softdecisionFEClimit,
providinga102bits−1Hz−1grossspectralefficiency.c,16QAMtransmissionmeasuredMDLfiguresforalltransmittedchannels,averagingat3.9dB.
d,32QAMmeasuredMDLfigures,averagingat4.4dB.
each core to minimize mode-dependent loss (MDL)34. The MDL receiver receives the first mode and a second coherent receiver
was approximated at 1.5–2dB and the insertion loss on average receives the remaining two modes, demonstrating the potential
was 1.1dB across all 21 waveguides (excluding fibre) at 1,550nm. scaling of received modes in both time and number of dual-
Thewaveguidesweredesignedtominimizepolarization-dependent polarization coherent receivers. A key requirement of this receiver
loss (measured to be <0.2dB), which was incorporated into the subsystem is to have switches with short rise/fall times (<1ns).
MDL approximation. The compact nature of the 3D waveguide Micro-electro-mechanical systems (MEMS) and piezoelectric
allows a highly stable butt-coupled interface to the FM-MCF switches were initially considered, but these cannot support the
end-facets, without requiring additional bulky imaging optics. A required high-speed switching, hence leaving onlyacoustic optical
3Dwaveguidewasusedasboththespatialmultiplexeranddemul- modulator (AOM) and semiconductor optical amplifier (SOA)
tiplexer in this experimental set-up. The total end-to-end loss switches as potential solutions. Unfortunately, a major drawback
measuredaftertransmissionwas12dB(includingmultiplexerand of SOA switches is the addition of parasitic amplitude stimulated
demultiplexer 3D waveguide), which is in line with single-core emission (ASE) noise to the received signals. In this work, low-
few-moderesults34.Crosstalkwasmeasuredbyindividuallyexciting insertion-loss AOM switches were therefore chosen that comply
thedifferentcores.Themeasuredcrosstalkvaluewasbelow−70dB with the rise/fall time of <1ns, providing an excellent choice for
km−1, which was limited by the dynamic range of the Newport acting as high-speed signal gates with a close time of 11µs. This
2832-C power monitor used for the measurement. Figure 2c pre- equates to the propagation time provided by the inserted single-
sents a 1,550nm near-field image captured at the demultiplexer mode delay fibre. The open and close times of the switches were
side, where the camera input power was saturated per core. controlled by a trigger generator and, by combining the gated
Figure 2d demonstrates the saturated power per core for each signals,opticalalignmentoftwodual-polarizationmodesforserial-
individually excited core, showing the low crosstalk provided by izedreceptionbyasinglecoherentreceiverwasachieved.InMIMO
theair-hole-assisteddesign.Forthenear-fieldimage,theobserved coherenttransmission, received modeshavetobeat withthe same
crosstalk was also less than −70dBkm−1, limited by the dynamic LO. Therefore, the signal TDM-SDM scheme was replicated in
range of the near-infrared camera used. The right bottom figure theLOpath.Afteranalog-to-digitalconversionbyreal-timeoscillo-
in Fig. 2d (core 4) shows the individual excitation of the two LP scopes, the modes were parallelized for offline processing.
modesofthecentrecore. Successively, in the digital domain, front-end impairments, chro-
To enable mode measurement after FM-MCF propagation, we maticdispersion,MIMOequalizationandcarrierphaseestimation
firstproposedanddevisedanelegantfew-modemeasurementtech- wereperformed39–41.Keytounravellingthemixedmodesisalow-
nique.TheessenceofthetechniqueisshowninFig.4a,wherethe complexity,andhenceanenergy-efficient,frequency-domainMIMO
novel time-domain multiplexed (TDM) SDM receiver with two equalizer with adaptive step size for high tracking capabilities18,42.
dual-polarizationcoherentreceiversisusedforsimultaneousrecep- The equalizer uses the 50% overlap–save scheme, combined with
tion of the three transmitted modes per core. The first coherent a radix-4 fast-Fourier transform (FFT) to minimize the required
4 NATUREPHOTONICS|ADVANCEONLINEPUBLICATION|www.nature.com/naturephotonics

©2014 Macmillan Publishers Limited. All rights reserved.
ARTICLES
NATURE PHOTONICS
DOI:10.1038/NPHOTON.2014.243
complex computations43, resulting in 5.76 and 4.61 complex transmission capacityof 255Tbits−1 (200Tbits−1 net), consisting
multiplicationsperbitfor16and32QAMtransmission,respectively, of 50 spatial super channels transmitting 5.103Tbits−1carrier−1
furtherhighlightingthelowenergyconsumptionachievedforDSP. (4Tbits−1carrier−1 net) on a dense 50GHz wavelength ITU grid
In addition, this scheme allows residual receiver impairments to with a spectral efficiency of 102bitss−1Hz−1 for fully mixed
be reduced after front-end impairment compensation has been MIMO transmission per core. While readily enabling beyond
performed,resultinginoptimumBERperformance. next-generation capacity per wavelength, the demonstrated trans-
mission system also has the potential to combine 21 legacy SMFs
Results operatingonanITUstandardized50GHzspacedwavelengthgrid
To demonstrate the FM-MCF transmission capabilities, we first into a single fibre. In view of the emerging amplifier multimode
characterized the transmitter and receiver performance by noise and multicore technologies, this work shows that a new class of
loading at the transmitter side for optical signal-to-noise ratio fibres combining few modes and multiple cores will pave the way
(OSNR)measurementsasdepictedinFig.4b.Atthe40dBOSNR towardsfuturehigh-densitySDMtransmissionsystems.
level,withrespecttoback-to-backmeasurements,the6×6single-
channel MIMO measurements for 16 and 32QAM indicate a Received3March2014;accepted12September2014;
modesterrorfloorincreaseof2×10−4and8×10−4forsinglewave- publishedonline26October2014
length transmission, respectively. This BER increase is mainly
attributed to residual channel interference after the frequency- References
domainMIMOequalizerunravelsthechannels. 1. Essiambre,R.J.&Tkach,R.W.Capacitytrendsandlimitsofoptical
With every wavelength channel in all cores enabled simul- communicationnetworks.Proc.IEEE100,1035–1055(2012).
taneously,Fig.5a,bdemonstratesthesuccessfultransmissionof16 2. Winzer,P.J.Beyond100Gethernet.IEEECommun.Mag.48,26–30(2010).
3. Lingle,R.Capacityconstraints,carriereconomics,andthelimitsoffiberand
and 32QAM, respectively, where the forward error correcting
cabledesign.OpticalFiberCommunicationConference(OFC),
(FEC) overhead is assumed to ensure error-free transmission. The paperQM2F1(2013).
average values of BER after WDM transmission (Fig. 5a,b) are 4. Ellis,A.D.,Zhao,J.&Cotter,D.Approachingthenon-linearShannonlimit.
approximately 1×10−3 and 5×10−3, corresponding to respective IEEEJ.LightwaveTechnol.28,423–433(2010).
BER penalties of 8×10−4 and 4×10−3 for 16 and 32QAM trans- 5. Winzer,P.J.Energy-efficientopticaltransportcapacityscalingthroughspatial
multiplexing.IEEEPhoton.Tech.Lett.23,851–853(2011).
missionincomparisonwithsingle-channelperformancesat40dB
6. Richardson,D.J.,Fini,J.M.&Nelson,L.E.Space-divisionmultiplexingin
OSNR. The insets in Fig. 5a,b represent the best- and worst-per- opticalfibres.NaturePhoton.7,354–362(2013).
forming channel constellations of the respective constellation 7. Raddatz,L.,White,I.H.,Cunningham,D.G.&Nowell,M.C.Anexperimental
types, and the gross aggregate transmission capacity of 255.15 andtheoreticalstudyoftheoffsetlaunchtechniquefortheenhancementof
Tbits−1 signifies the potential high-density space-division-multi- thebandwidthofmultimodefiberlinks.IEEEJ.LightwaveTechnol.
16,324–331(1998).
plexing capabilities of the hole-assisted FM-MCF. This successful
8. Sakaguchi,J.etal.305Tb/sspacedivisionmultiplexedtransmissionusing
use of the butt-coupled 3D waveguide multiplexer and demulti- homogeneous19-corefiber.IEEEJ.LightwaveTechnol.31,554–562(2013).
plexer demonstrates the high integration capabilities obtained by 9. Mizuno,T.etal.12-core×3-modedensespacedivisionmultiplexed
combining multicore and multimode SDM techniques. However, transmissionover40kmemployingmulti-carriersignalswithparallelMIMO
equalization.OpticalFiberCommunicationConference(OFC),paper
inherent to space-division-multiplexed transmission is a received
Th5B.2(2014).
power difference between transmitted channels, denoted by the
10.Igarashi,K.etal.1.03-Exabit/s·kmsuper-Nyquist-WDMtransmissionover
MDL. Obtained from mutual information estimation, MDL is a 7,326-kmseven-corefiber.39thEuropeanConferenceandExhibitionof
figure of merit for establishing the theoretical transmission OpticalCommunication,paperPD3.E.3(2013).
capacity44, which is computed through singular value decompo- 11.Zhu,B.etal.Space-,wavelength-,polarization-divisionmultiplexed
transmissionof56-Tb/sovera76.8-kmseven-corefiber.OpticalFiber
sition of the transmission channel matrix, obtained by least-
CommunicationConference(OFC),paperPDPB.7(2011).
squareschannelestimation45,46.TheMDLfiguresofallwavelength
12.Watanabe,T.&Kokubun,Y.Ultra-largenumberoftransmissionchannelsin
carriersarepresentedinFig.5c,d,withaverageestimatedMDLsof spacedivisionmultiplexingusingfew-modemulti-corefiberwith
3.9dBand4.4dBfor16and32QAM,respectively.ThesmallMDL optimizedair-hole-assisteddouble-claddingstructure.Opt.Express22,
differencesareattributedtoslightmisalignmentbetweenthecores 8309–8319(2014).
and (de)multiplexer, as well as wavelength-dependent mode field 13.mTaaknaargae,dHt.reatnasml.i1s.s0i1o-nPwb/isth(1921.S4D-bM/s//2H2z2aWggDreMga/t4e5s6pGecbt/rsa)lcerffioscsietanlcky-.
excitation during the experiment. The observed MDL differences 38thEuropeanConferenceandExhibitionofOpticalCommunication,
are similar to those reported in a previous work for single-core paperTh.3.C.1(2012).
few-mode transmission34, further demonstrating that multicore 14.Sillard,P.etal.Low-DMGD6-LP-ModeFiber.OpticalFiberCommunication
and multimode SDM can be combined to enable ultra-high- Conference(OFC),paperM3F.2(2014).
15.Mori,T.,Sakamoto,T.,Wada,M.,Yamamoto,T.&Yamamoto,F.Six-LP-mode
densitytransmissioncapacity.Currentactivitiesfocusonincreasing transmissionfiberwithDMDoflessthan70ps/kmoverC+Lband.Optical
the number of modes per core14,15 and increasing the number of FiberCommunicationConference(OFC),paperM3F.3(2014).
cores12 to further increasing the spectral efficiency in a single 16.Grüner-Nielsen,L.etal.FewmodetransmissionfiberwithlowDGD,
fibre. By combining both technologies, as demonstrated in this lowmodecoupling,andlowloss.IEEEJ.LightwaveTechnol.30,
3693–3698(2012).
work, a platform for two-dimensional scaling is provided. The
system’slinearandnonlineartransmissionproperties47–50,emerging 17.fReywf,-mR.oedteafil.b3er2.-bOipt/tsi/cHalzFsipbeerctCraolmefmficuineinccaytioWnDCMonfterraennscmei(sOsiFoCn)o,vpearp1e7r7-km
optical components andDSP complexitywilldecide the optimum PDP5A.1(2013).
combinationofthetwodimensions6. 18.VanUden,R.G.H.,Okonkwo,C.M.,Sleiffer,V.A.J.M.,deWaardt,H.&
Koonen,A.M.J.MIMOequalizationwithadaptivestepsizeforfew-mode
Conclusion fibertransmission.Opt.Express22,119–126(2014).
19.Qian,D.etal.1.05Pb/sTransmissionwith109b/s/Hzspectralefficiencyusing
We have demonstrated a robust 50-channel WDM transmission
hybridsingle-andfew-modecores.96thAnnualMeeting,FrontiersinOptics
of 21 spatial LP modes in both polarizations, enabled by a hole- (FiO),paperFW6C.3(2012).
assisted FM-MCF. The signal is coupled into and out of the 20.Xia,C.etal.Hole-assistedfew-modemulticorefiberforhigh-densityspace-
FM-MCFusingcustom-designedlow-loss(<1.5dB)3Dwaveguide divisionmultiplexing.IEEEPhoton.Technol.Lett.24,1914–1917(2012).
21.Saitoh,K.,Matsui,T.,Sakamoto,T.,Koshiba,M.&Tomita,S.Multicore
(de)multiplexers, demonstrating the high potential for integration
hole-assistedfibersforhighcoredensityspacedivisionmultiplexing.Proceedings
into emerging corenetwork transponders. Furthermore, low-com-
of15thOptoElectronicsandCommunicationsConference(OECC),
putational-complexityMIMODSPwasusedtoenableanaggregate 164–165(2010).
NATUREPHOTONICS|ADVANCEONLINEPUBLICATION|www.nature.com/naturephotonics 5

©2014 Macmillan Publishers Limited. All rights reserved.
ARTICLES
NATURE PHOTONICS
DOI:10.1038/NPHOTON.2014.243
22.Jung,Y.etal.MultimodeEDFAperformanceinmode-divisionmultiplexed 41.VanUden,R.G.H.,Okonkwo,C.M.,Chen,H.,deWaardt,H.&Koonen,A.M.
transmissionsystems.OpticalFiberCommunicationConference(OFC),paper J.28Gbaud32QAMFMFtransmissionwithlowcomplexityphaseestimators
JW2A.24(2013). andsingleDPLL.IEEEPhoton.Technol.Lett.26,765–768(2014).
23.Kang,Q.etal.Designoffour-modeerbiumdopedfiberamplifierwithlow 42.Randel,S.,Winzer,P.J.,Montoliu,M.&Ryf,R.Complexityanalysisofadaptive
differentialmodalgainformodaldivisionmultiplexedtransmissions.Optical frequency-domainequalizationforMIMO-SDMtransmission.39thEuropean
FiberCommunicationConference(OFC),paperOTu3G.3(2013). ConferenceandExhibitionofOpticalCommunication,paperTh.2.C.4(2013).
24.Fontaine,N.K.,Ryf,R.,Guan,B.,Neilson,D.T.Wavelengthblockerforfew- 43.Leibrich,J.&Rosenkranz,W.Frequencydomainequalizationwithminimum
modefiberspacedivisionmultiplexedsystems.OpticalFiberCommunication complexityincoherentopticaltransmissionsystems.OpticalFiber
Conference(OFC),paperOTh1B.1(2013). CommunicationConference(OFC),paperOWV1(2010).
25.Ryf,R.etal.Wavelength-selectiveswitchforfew-modefibertransmission. 44.Winzer,P.J.&Foschini,G.J.MIMOcapacitiesandoutageprobabilities
EuropeanConferenceonOpticalCommunications,paperPD1C.4(2013). inspatiallymultiplexedopticaltransportsystems.Opt.Express19,
26.Sleiffer,V.A.J.M.etal.Mode-division-multiplexed3×112-Gb/sDP-QPSK 16680–16696(2011).
transmissionover80kmfew-modefiberwithinlineMM-EDFAandBlindDSP. 45.VanUden,R.G.H.,Okonkwo,C.M.,Sleiffer,V.A.J.M.,deWaardt,H.&
38thEuropeanConferenceandExhibitionofOpticalCommunication,paper Koonen,A.M.J.PerformancecomparisonofCSIestimationtechniquesfor
Tu.1.C.2(2012). FMFtransmissionsystems.IEEEPhotonicsSocietySummerTopicalMeeting
Series,paperWC4.2(2013).
27.Randel,S.etal.6×56-Gb/smode-divisionmultiplexedtransmissionover33-
kmfew-modefiberenabledby6×6MIMOequalization.Opt.Express19, 46.Benvenuto,N.&Cherubini,G.AlgorithmsforCommunicationsSystemsand
16697–16707(2011). theirApplications(Wiley,2002).
28.Ryf,R.etal.Modedivisionmultiplexingover96kmoffew-modefiber 47.Essiambre,R.-J.etal.Inter-modalnonlinearinteractionsbetweenwellseparated
usingcoherent6×6MIMOprocessing.IEEEJ.LightwaveTechnol.30, channelsinspatially-multiplexedfibertransmission.38thEuropeanConference
512–531(2012). andExhibitionofOpticalCommunication,paperTu.1.C.4(2012).
29.Itrpa,nEs.meitssailo.n14o6vλer×160××1590--Gkbmausdpawnasvoeflefnegwt-hm-oandedfimboedrew-ditihviasiognainm-euqltuipalleizxeedd 48.Sgaoinng,spKe.cYtr.a&inKaimfe,wY-.mHo.dMeefiabseurr.eOmpetnictaolfFiinbterramCoomdamluanndicaintitoenrmCoodnafelrBenriclelouin
(OFC),paperW3D.6(2014).
few-modeEDFA.OpticalFiberCommunicationConference(OFC),paper
49.Xiao,Y.,Mumtaz,S.,Essiambre,R.-J.&Agrawal,G.P.Effectofrandomlinear
PDP5A.2(2013). modecouplingonintermodalfour-wavemixinginfew-modefibers.Optical
30.Sleiffer,V.A.J.M.etal.73.7Tb/s(96×3×256-Gb/s)mode-division-
FiberCommunicationConference(OFC),paperM3F.5(2014).
multiplexedDP-16QAMtransmissionwithinlineMM-EDFA.Opt.Express
50.Hayashi,T.,Nakanishi,T.,Sasaki,T.,Saitoh,K.&Koshiba,M.Dependenceof
20,B428–B438(2012). crosstalkincreaseduetotightbendoncorelayoutofmulti-corefiber.Optical
31.Chen,H.etal.Employingprism-basedthree-spotmodecouplersforhigh FiberCommunicationConference(OFC),paperW4D.4(2014).
capacityMDM/WDMtransmission.IEEEPhoton.Technol.Lett.25,
2474–2477(2013). Acknowledgements
32.Chen,H.etal.Employinganintegratedmodemultiplexeronsilicon-on-
TheauthorsacknowledgepartialfundingfromtheEuropeanUnionFramework7
insulatorforfew-modefibertransmission.EuropeanConferenceonOptical
MODEGAPproject(grantagreementno.258033).Thisresearchwasalsopartially
Communications,paperTu.1.B.4(2013). supportedbytheNationalBasicResearchProgrammeofChina(973;project
33.Chen,H.etal.3MDM×8WDM×320Gb/sDP32QAMtransmission #2014CB340100).C.M.O.acknowledgesfundingfromtheSouthKoreanITR&D
overa120kmfew-modefiberspanemploying3-spotmodecouplers.18th programmeofMKE/KIAT(2010-TD-200408-001).E.A.L.acknowledgestheConsejo
OptoElectronicsandCommunicationsConference(OECC),paper NacionaldeCienciayTecnología(CONACyT).TheauthorsthankA.AmezcuaCorreaand
PD3-6-1(2013). P.SillardofPrysmianGroupandN.PsailaofOptoscribefordiscussions.
34.Ryf,R.,Fontaine,N.K.&Essiambre,R.-J.Spot-basedmodecouplersfor
mode-multiplexedtransmissioninfew-modefiber.IEEEPhoton.Technol.Lett. Authorcontributions
24,1973–1976(2012). R.G.H.v.U.andC.M.O.developedtheconceptandconductedthetransmission
35.Yerolatsitis,S.&Birks,T.A.Three-modemultiplexerinphotoniccrystalfibre. experiments.R.A.C.,A.S.andG.L.conceivedtheFM-MCFconcept.R.A.C.,E.A.L.andA.S.
EuropeanConferenceonOpticalCommunications,paperMO.4.A.4(2013). designedandfabricatedthehole-assistedFM-MCF.C.X.modelledthefibre.R.G.H.v.U.,C.
36.Leon-Saval,S.G.,Argyros,A.&Bland-Hawthorn,J.Photoniclanterns:astudy M.O.andF.M.H.designedandcharacterizedthe3D(de)multiplexer.R.G.H.v.U.developed
oflightpropagationinmultimodetosinglemodeconverters.Opt.Express18, theDSPalgorithms.R.G.H.v.U.andC.M.O.designedandverifiedtheTDM-SDMreceiver
8430–8439(2010). concept.C.M.O.,H.d.W.andA.M.J.K.providedoverallleadershipacrossallaspectsofthe
37.Leon-Saval,S.G.etal.Mode-selectivephotoniclanternsforspace-division work.C.M.O.,R.G.H.v.U.andR.A.C.wrotethemanuscript.
multiplexing.Opt.Express22,1034–1044(2014).
38.Fontaine,N.K.,Ryf,R.,Bland-Hawthorn,J.&Leon-Saval,S.G.Geometric Additionalinformation
requirementsforphotoniclanternsinspacedivisionmultiplexing. Supplementaryinformationisavailableintheonlineversionofthepaper.Reprintsand
Opt.Express20,27123–27132(2012). permissionsinformationisavailableonlineatwww.nature.com/reprints.Correspondenceand
39.Savory,S.J.Digitalcoherentopticalreceivers:algorithmsandsubsystems.IEEE requestsformaterialsshouldbeaddressedtoR.G.H.v.U.,R.A.C.andC.M.O.
J.Sel.Top.QuantumElectron.16,1164–1179(2010).
40.Kuschnerov,M.etal.DSPforcoherentsingle-carrierreceivers.IEEEJ. Competingfinancialinterests
LightwaveTechnol.27,3614–3622(2009). Theauthorsdeclarenocompetingfinancialinterests.
6 NATUREPHOTONICS|ADVANCEONLINEPUBLICATION|www.nature.com/naturephotonics

©2014 Macmillan Publishers Limited. All rights reserved.