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Laser Photonics Rev. 7, No. 4, 539553 (2013) / DOI 10.1002/lpor.201200050LASER & PHOTONICSREVIEWS

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Abstract Orthogonal frequency division multiplexing (OFDM)can provide spectrally efficient communication channels be-cause it can utilize carrier orthogonality and various impairmentmitigation methods. An optical OFDM signal can be generatedelectronically to multiplex lower-rate carriers. In recent advance-ments, OFDM signals are also shown to be generated anddemultiplexed by all-optical discrete Fourier transform (DFT),overcoming the speed limit of electronics for >Tbps capacity.High-performance DFT devices, such as arrayed waveguidegrating (AWG) or planar lightwave circuit (PLC), are criticallyrequired to obtain strong orthogonality for scalable all-opticalOFDM (AO-OFDM) system implementations. Advanced tech-niques such as coherent modulation and detection with dig-ital impairment mitigation are also important for long-reachAO-OFDM transmissions. More recently, optical superchannelschemes have been introduced utilizing coherent detection formulti-Tbps AO-OFDM transmissions. This paper reviews thedevice and system aspects for the AO-OFDM technology, in-cluding a generalized theoretical model to provide an indepthunderstanding.

Optical orthogonal frequency division multiplexedtransmission using all-optical discrete Fourier transformJune-Koo K. Rhee1,, Neda Cvijetic2, Naoya Wada3, and Ting Wang2

1. Introduction

The optical-fiber communications research community hasachieved significant technology breakthroughs to reachfiber channel capacities beyond 10 Tbps during the pastdecade, and approached the physical limit of an optical-fiber channel. There are several experimental reports of 10100 Tbps single-mode fiber transmission [18], while100-Gbps single-channel coherent communicationtransceivers have recently become commercially avail-able. The most important technology breakthroughs inthis area include single-carrier digital coherent opticalcommunication [3] with frequency- and time-domainequalizations [9, 10], optical OFDM [1116], andAO-OFDM [2, 68, 1733]. Single-carrier coherent com-munication utilizes M-ary phase and amplitude shift keyingmodulation, such as quadrature phase shift keying (QPSK)and quadrature amplitude modulation (QAM), combinedwith heterodyne detection and digital signal processingadapted from mobile communication technologies, andcan successfully mitigate most fiber-optic transmissionimpairments. Transmission capacity can be extended

1 Dept. of Electrical Engineering, KAIST, 291 Daehangro, Yuseonggu Daejeon 305-701, S. Korea2 NEC Laboratories America, 4 Independence Way Princeton, NJ 08540, USA3 Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi Koganei, Tokyo 184-8795, JapanCorresponding author: e-mail: [email protected].

even further with polarization diversity multiplexing(PDM). Recently, experimental results and field trialsdemonstrating Tbps data transmission over ultralong haulfiber distances have been reported by many researchgroups [1, 7, 8]. Meanwhile, the OFDM communicationtechnique exploited within the framework of mobilecommunications has also been studied for fiber-optictransmission. With electronic OFDM multiplexing anddemultiplexing, optical-fiber systems [13, 14] have shownthe potential for Tbps transmission.

All the aforementioned technologies largely dependon the performance and capacity of electronic process-ing, which is being rapidly developed to provide higher-capacity and lower-cost implementations. However, thecapacity and power-consumption limits of electronic pro-cessors present a bottleneck whenever scalability is con-cerned, if compared with equivalent all-optical meth-ods. Figure 1a shows a typical schematic design of anoptical OFDM system, which consists of an electronicfast Fourier transform (FFT), digitalanalog and analogdigital converters (DAC and ADC, respectively), optical in-phase/quadrature-phase modulators (IQ MOD), and optical

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540 J.-K. K. Rhee et al.: Optical OFDM transmission using all-optical discrete Fourier transform

Figure 1 (online color at:www.lpr-journal.org) Schematicdesigns of optical OFDM (a) andall-optical OFDM (b). I/Q De-mux(Demod) and I/Q Mux(Mod)stand for in-phase/quadrature-phase demultiplexer (demodula-tor) and multiplexer (modulator)in coherent communication,respectively. ADC and DACstand for analog-to-digital anddigital-to-analog converters, re-spectively. OH stands for opticalhybrid. For IM/DD AO-OFDMcommunication, IQ Mod and OHcan be replaced with an onoffmodulator and photodetector,respectively, and DAC/ADCand M-ary Mod/Demod are notrequired.

hybrid IQ detectors [15,16]. In general, the maximum avail-able capacity of an optical OFDM is limited by the speed ofDAC and ADC, as well as the power consumption and foot-prints of silicon chips for FFTs and adaptive IQ detections.Even though lab experiments have demonstrated single-carrier optical OFDM capacity higher than Tbps by offlinedemodulation that is achieved by software processing, thetrue real-time optical OFDM system can achieve only upto 100 Gbps presently [17]. In order to scale the channelcapacity beyond single-carrier application, we can considermulticarrier OFDM where carriers are all-optically multi-plexed to have intercarrier orthogonality. Such all-opticalOFDM can overcome the electronic processing limit ofsingle-carrier systems, such as in wavelength division mul-tiplexed (WDM) transmission that incorporates multipletime division multiplex (TDM) channels in optical fibers.

As shown in Fig. 1b, multicarrier optical data symbolscan be generated individually from optical carrier sources[19]. In an AO-OFDM reference model by Lee et al. [20],FFTs and serial-to-parallel and parallel-to-serial convert-ers are replaced with all-optical DFTs and optical delays,respectively, and hence the AO carrier multiplexing anddemultiplexing are achieved at multiples of the electronicsspeed with nearly no power consumption. In addition, in-stead of using a single fast DAC/ADC, optical IQ MOD,and optical hybrid IQ demodulator, AO-OFDM is imple-mented with many slower devices. Carrier orthogonalitycan be preserved, even with arbitrary modulation formatsas long as the modulation periods are equal and synchro-nized, providing modulation format transparency [7, 21].Each carrier can be multiplexed by PDM to double thetransmission capacity [19].

The AO-OFDM scheme has further advanced to AO-OFDM superchannel schemes, where optical carriers aredemultiplexed orthogonally by coherent multicarrier detec-tion using an electrical FFT, instead of demultiplexing by anall-optical DFT. In a typical embodiment of the superchan-nel scheme, carriers are partitioned to several groups andcoherent multicarrier detection is applied locally to eachgroup. Nonetheless, the AO-OFDM superchannel trans-mission utilizes the same carrier orthogonality propertiesas an optically demultiplexed AO-OFDM. The first AO-OFDM superchannel system was demonstrated by Masudaet al. [1], while the first terabit AO-OFDM superchannel ex-periment using QPSK modulation was achieved by Chan-drasekhar et al. [22]. To mark a new phase of AO-OFDMsuperchannel technology maturity, multi-Tbit/s AO-OFDMsuperchannel field trials using an all-optical OFDM trans-mitter architecture with digital coherent carrier demulti-plexing were shown in [7]. Finally, record-high AO-OFDMspectral efficiency using 16-QAM and beyond 10 000 kmtransmission distance was demonstrated by Huang et al.[23, 24].

This review paper is organized as follows. InSection 2, a simple system-level theoretical model is pro-vided to explain how optical orthogonality can be at-tained by AO-OFDM schemes including superchannel con-cept, followed by a review of AO-OFDM multiplexerand demultiplexer device technologies in Section 3. InSection 4, we introduce system demonstration reviewsof an AO-OFDM direct detection system and an AO-OFDM superchannel system. Section 5 concludes the re-view discussions with anticipated future developments andapplications.

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Laser Photonics Rev. 7, No. 4 (2013) 541

Figure 2 (online color at:www.lpr-journal.org) Schematicillustrations of the principle ofoperation of all-optical OFDMdemultiplexer consisting ofdelay array and DFT circuitry forthe output port n = 2, (a), asan example, and its applicationfor a receiver, (b). In (a), thesinusoidal curves indicate twocarriers that are delayed andphase shifted.

2. Theoretical model for all optical OFDM

2.1. All optical OFDM transmitter and receiverdesigns

When modulated carriers are packed too closely to eachother, so that carrier spectra overlap partially, it is criticalto demultiplex individual carriers while avoiding crosstalk.An OFDM receiver demultiplexes closely packed carriers,eliminating the intercarrier interference (ICI) at certain timepositions. In order to investigate the principle of operation,let us consider a conceptual OFDM demultiplexer model ata receiver, as illustrated in Fig. 2a with 4 carriers (N = 4). Anall-optical OFDM receiver implements two main functions:a delay array for serial-to-parallel conversion and a DFTprocessor [20,25]. Denoted by Tm() = e jm and mn =e j

2N mn , respectively, the delay and DFT transfer functions

of the mth circuit path can be combined to produce thefrequency-domain transfer function for the nth carrier asfollows:

Dn() = 1NN1m=0

Tm()mn

= 1N

N1m=0

e j(m+ 2N mn) n = 0, 1, . . , N 1,(1)

where is the angular frequency, and is the chip pe-riod. The OFDM symbol period is T = N . Accordingly,the carrier spacing in the angular frequency is defined by 2/N . Neglecting the constant time delay factor ofe j(N1), we obtain

Dn() = 1NN1m=0

e jm (n) n = 0, 1, . . , N 1.(2)

In order to gain an insight into the demultiplexing pro-cess, Fig. 2 shows two input carriers 1 = and 2 = 2,modulated by a rectangle function of duration of 2 4(two symbols in the same modulation state) arriving at thedemultiplexer, as an example. From inspection of Fig. 2, itis evident that the delaying and phasing of the replicas ofthe carrier 1 create destructive interference, whereas all

the replicas of 2 sum up constructively. It is also worthobserving that complete orthogonality between the two car-riers holds only for a certain period, where all the replicasfor each carrier interfere destructively. Figure 2b is thecorresponding receiver system model in which N opticaldetectors can be incorporated.

In principle, orthogonality is preserved with any kind ofmodulation format as long as i) each modulation state lastsfor To = N or longer, where the cyclic prefix T = Kis defined by temporal duration in excess of N , ii) therise and fall times of modulation are short enough com-pared with , and iii) the modulation is applied to allcarriers with the same modulator clock phases so thatall the carriers arrive at the receiver at the same time,in order to arrange ICI from all other carriers such thatit is located on the symbol boundaries in the time do-main. The resulting symbol configurations are illustratedin Fig. 3, in which the ICI-free region is indicated by thewhite space in the time domain; Figs. 3a and b depict howICI free periods are formed by all-optic DFT processesdue to destructive interference. This property allows theuse of advanced modulation formats such as M-ary phaseshift key (PSK), QAM, and even another layer of opticalOFDM, as well as simple nonreturn-to-zero onoff keying(NRZ-OOK) for intensity-modulation/direct-detection(IM/DD) systems and differential PSK (DPSK). In thissense, AO-OFDM is modulation-format transparent.

Figure 4 shows a more precise numerical analysisto illustrate the orthogonality behavior in a 4-carrier(N = 4) transmission system with a carrier spacingB /2 of 10 GHz, corresponding to a symbol period ofTo = N = 100 ps. In order to show that ICI occurs onlywhen a carrier is modulated, i.e. only on the boundarybetween symbols, unit step modulation is applied to acarrier at 195.00 THz in the system model shown in Fig. 4a.The corresponding demultiplexed optical waveforms areplotted in Fig. 4b: The target signal waveform at the 195.00-THz carrier frequency is demultiplexed to form a slowlyincreasing optical power waveform, and ICI waveforms forother carrier output ports of other frequencies are shown inthe plots for ICIs. All ICI waveforms last only for a periodof To, and hence one can infer that if carriers are modulatedat every To, an interference-free time position can be foundat every To, as indicated with red arrows in the plots. Thisbehavior in two-carrier experiments was first demonstrated

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Figure 3 (online color at: www.lpr-journal.org) A schematic illustrationof an OFDM symbol structureshown in the time domain; isthe OFDM sampling space, andTo = N and T = k are theminimum symbol and cyclic prefixperiods, respectively, in an N = 4carrier OFDM system. (a) and (b)represent optical carriers producedby short optical pulses; (c) and (d)by a cw source. The color codes in-dicate phase rotation of the carrierwith respect to a reference carrerfrequency, e.g. the first carrier. (b)and (d) compare how cyclic pre-fixes can be applied in pulsed andcw mode OFDM symbol structures.Symbol values are dectected onlyin the ICI-free regions.

by Sanjoh et al. [18] and its 30 100 Gb/s transmissionis achieved by Sano [19]. Because the interference-freeinterval is narrow compared to a symbol period, as shownin Fig. 3, the demultiplexed symbol should be sampled op-tically at each output port of an AO-OFDM demultiplexeror electrically after optical-to-electrical conversion inconjunction with digital signal processing. As depicted inFig. 3d [18], the modulation symbol period T can be madelonger than To, keeping the rise and fall times unchanged,to increase the interference-free range. This is equivalent toadding a cyclic prefix (Fig. 3b) used in conventional OFDMtransmission systems [25, 26]. The cyclic prefix is alsoused to mitigate time-domain impairments in fading radiochannels, for example. The same application can mitigatechromatic dispersion effects of the optical-fiber medium[27, 34].

An OFDM transmitter is required to produce frequency-locked optical carriers that exactly match the optical fre-quencies of the OFDM demultiplexer of the receiver. Thereare several transmitter designs that can generate such fre-quency arrangements. A comb spectrum source, producedby a modelocked laser or by a continuous wave (cw) lasermodulated by an over-driven phase modulator, is the mostwidely adopted method. A comb spectrum is used to gen-erate optical OFDM carriers through aggregation with a

pulse-mode AO-OFDM multiplexer or a cw-mode AO car-rier multiplexer.

The most comprehensive transmitter model that canpresent insights into AO-OFDM communication is apulse-mode transmitter model with a pulsed optical sourceas shown in Fig. 5, which gives an immediate analogy withelectrically generated OFDM symbols [20]. A short pulsetrain with a pulse width narrower or equal to generatedby a modelocked laser diode at a repetition rate B is splitinto N replicas, and each of them is individually modulatedand fed into each input port of the AO-OFDM multiplexer(MUX). An input pulse at an input port i is split into Nreplicas for all different output ports k = 0, 1, . . N1. Pulsesshown in the boxes of the figure indicate the time positionsof optical signal with respect to the OFDM symbolslots. The iDFT block introduces phase shifts betweeninput port i and output port k according to the followingexpression:

ik e j2N ik . (3)

This is the same as the discrete Fourier transform (DFT)used in the electrical OFDM communication. At the outputsof the iDFT block, a set of optical delays are introduced asshown in Fig. 5. This delay array has delays of multiples of


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Figure 4 (online color at:www.lpr-journal.org) A numer-ical example of AO-OFDMdemultiplexer characteristics fora 410 GHz OFDM system, (a)system model where the laseris modulated to a on state att = 100 ps, (b) demultiplexedsignal and ICIs into other carrierports after demultiplex. Redarrows indicate the time-domainsampling positions for datarecovery at the center of eachsymbol period that correspondsto a 10 Gbit/s data rate.

Figure 5 (online color at: www.lpr-journal.org) Conceptualschematic of all-optical ODFM multiplexer of a pulse-mode trans-mitter design. The spectrum illustrations on the bottom shows themodulation broadening.

and N delayed signals are combined by a standard N 1coupler, providing parallel-to-serial conversion. The corre-sponding output of this OFDM symbol includes multiplecarriers with designated frequencies. Applying the same

analysis used to obtain Eq. (2), we find the OFDM multi-plexer function for the ith carrier input as

Mi () = 1NN1k=0

e jk (i). (4)

At the receiver, the same principle of operation is ap-plied, following the system model of Eq. (1). The symbolsare delayed by a matched set of an optical delay array,followed by the DFT, output signals filtering, and photode-tection to recover the input data.

In order to construct the mathematical model for pulse-mode OFDM signal multiplexing and demultiplexing, letus consider single-pulse propagation through an input porti and output port n at an AO-OFDM multiplexer and de-multiplexer, respectively. The frequency-domain amplituderepresentation of a single pulse for the ith carrier can be de-fined as Ai () = ai(), where ai and () denote thedata modulation and the modulation spectrum of the pulse,respectively. Here, the pulse width is assumed to be thesame as . The corresponding OFDM signal is represented




as

Si () = Ai ()Mi () = 1N ai()N1k=0

e jk (i).

(5)

Then an optical pulse sequentially propagating through thesystem from the ith port of an AO-OFDM multiplexer tothe nth port of a demultiplexer is obtained by

Ri,n() = Si ()Dn()

= 1N 2

ai()N1m=0

N1k=0

e jk (i)e jm (n).

(6)

The corresponding time-domain representation of the re-ceived and demultiplexed signal can be obtained by aFourier transform:

ri,n(t) = 12 +

Ri,n()e jt d.

Using the Fourier transform () = + (t )e jt dt ,where (t) is the time-domain representation of (), and + e

j(tt k+m )d = 2(t t k + m ), we ob-tain the optical field of the demultiplexed signal as

ri,n(t) = 1N 2 aiN1m=0

N1k=0

e2 j(kimn)/N(t (k m) ).

(7)

The evaluation of Eq. (7) for i = n is zero at t = sT(= s N ) for integer s as long as (t) = 0 for |t | > /2.For the case of i = n, i.e. for the target carrier being demul-

tiplexed, we find

ri,i (t) = 1N 2 aiN(t) + N1

p=1(N p)((t p )e2 j(pi/N )

+ (t + p )e2 j(pi/N ))).

(8)When N carriers are multiplexed with data modulation ofeach carrier xi (t) =

s ais(tsT ), where ais is the data

sequence of the ith carrier, the output at the demultiplexerport is given by

rn(t) = 1N 2N1i=0

N1m=0

N1k=0

e2 j(kimn)/N xi (t (k m) ).

(9)It is noted that in a pulse-mode AO-OFDM transmitter, theperformance of the optical data modulator can be easedbecause orthogonality depends on the performance of theAO-OFDM multiplexer and the shape of the pulse.

A cw-mode AO-OFDM transmitter generates a multi-plexed OFDM signal either from a comb spectrum sourceor by a set of frequency-controlled cw sources. Figure 6ashows the key principles of AO-OFDM superchannel gener-ation that can be demultiplexed and demodulated by digitalcoherent carrier multiplexing [1, 8, 22, 27], which has beenwidely exploited in high-speed AO-OFDM transmissionexperiments. At the transmitter, a multitone generator canbe used as an optical multicarrier source with phase-lockedoptical tones at integer multiples of the driving source clockfrequency B. Such a multicarrier source can be realizedusing a mode-locked laser synchronized at its fundamen-tal frequency [28], concatenated overdriven optical phasemodulators [6, 35], or a recirculating loop where a newtone is generated at each pass through the loop [23]. Inthe case of multicarrier tone generation via wideband fre-quency modulation, the optical powers of the generated

Figure 6 (online color at: www.lpr-journal.org) Conceptual schematic of cw-mode all-optical ODFM transmitter designs by the use of (a)phase-locked tone generation for superchannel generation, and (b) individual free-running cw carrier sources with precise wavelengthcontrol. The AO carrier MUXs in both designs can be achieved by various design options. Tones and carriers are generated with exactcarrier spacing B. The modulators can use advanced coherent modulation formats at the B baud rate.


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carriers will follow an nth-order Bessel function of the firstkind, such that intercarrier power variation can be large.A flexible band WSS [36] can then be employed with anall-optical carrier demultiplexer to equalize the intercarrierpowers, while an optical carrier demultiplexer, such as anAWG, can be used to separate the orthogonal frequencycomponents.

A comprehensive understanding of orthogonality canbe extended from the fact that orthogonality is indepen-dent of carriercarrier phase correlation, as inferred fromFig. 4. As shown in Fig. 6b, a set of free-running cw laserscan produce AO-OFDM carriers as long as the laser fre-quencies are precisely tuned to the receiver AO-OFDMdemultiplexer frequencies and the modulation clock phasesare aligned to produce orthogonality. In this configuration,however, the instantaneous power waveform of an OFDMsymbol is not deterministic because the carriers are notphase-locked. Subsequently, when coherent detection withelectrical adaptive equalization by digital signal processingis used at a receiver, the impact of ICI may not be elimi-nated as it manifests fast beat noise fluctuation due to fastphase rotations of free-running sources. In addition, fibernonlinearity impairments may not be deterministic, mak-ing nonlinear penalty equalization by receiver digital signalprocessing difficult. Nonetheless, this scheme can also bemodulation-format transparent.

A generalized mathematical model for cw-mode AO-OFDM can be obtained by modification of Eqs. (5) and(6). A modulated carrier i with a modulation spectrumSi() = ai(i), where () is the modulation spec-trum due to optical data modulation, propagating throughthe OFDM system to an AO-OFDM demultiplexer at thereceiver through the nth port is denoted by

Ri,n() = Si ()Dn() = 1N ai ( i)N1m=0

e jm (n).

(10)Using the similar derivation for Eq. (7), we obtain the

corresponding time-domain representation of the opticalfield of the demultiplexed signal as

ri,n (t) = 1N ai ejit

N1m=0

e2 jm(in)/N(t + m ) (11)

The evaluation of Eq. (11) with step-function modulationin the N = 4 example is shown in Fig. 4. For i = n, Eq. (11)is simplified as

ri,i (t) = 1N ai ejit

N1m=0

(t + m ). (12)

The carrier modulation symbol ai can be detected by director coherent detection. For i = n, Eq. (11) is zero at t = sTfor integer s as long as (t) = 0 for |t | > T/2. This isan important modulator requirement for a cw-mode AO-OFDM transmitter.

Figure 7 (online color at: www.lpr-journal.org) AO-OFDM sym-bol propagation in dispersive fiber observed at different outputports of the AO-OFDM demultiplexer with 20 km of SMF28 fiberinserted in the setup shown in Fig. 4a, for comparison with thewaveforms of Fig. 4b. Red arrows indicate a reasonable choiceof the subrate sampling positions for data recovery.

2.2. Subrate OFDM Transmission

In an optical OFDM system, transmission impairments cancause two-fold penalties of intersymbol interference (ISI)and intercarrier interferences (ICI). The ISI penalty in anOFDM system is the same as that of a single-carrier system.On the other hand, the ICI degrades the orthogonal propertyof an OFDM system, and hence can be more critical thanthe ISI penalty. As an example, orthogonality degradationdue to fiber chromatic dispersion (CD) penalty is moresevere if the OFDM symbol bandwidth exceeds 100 GHz.In order to model an example of CD penalty in an AO-OFDM demultiplexing, Eq. (10) can be modified as

Ri,n() = Si ()C()Dn()

= 1N

ai ( i) e j2/2N1m=0

e jm (n).(13)

Here, the fiber CD transfer function C() = e j2/2 is used,where = DL2/2c; D, L , , and c, are the fiber CDcoefficient, fiber length, wavelength, and speed of light,respectively. Using the system model of Fig. 4a with afiber inserted in front of the demultiplexer, the ISI and ICIbehaviors are evaluated by a numerical Fourier transformof Eq. (13). This data shows a 4-carrier example wherethe carrier at 195.00-THz turns on at t = 100 ps, and thecorresponding ISI and ICIs are observed at the receiverAO-OFDM demultiplexer ports as shown in Fig. 7. Fromthe data plots, one can see the ISI and ICI are spread in thetime domain, making the orthogonal property poor. In orderto mitigate such interference penalties, the OFDM carrierbaud rate BC can be lowered below B so that detection points




can be sampled at a lower rate as can be seen in compar-ison of Fig. 4b and Fig. 7. Such subrate OFDM transmis-sion can be achieved by cyclic prefix insertion and subrate(BC

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Figure 9 (online color at: www.lpr-journal.org) AWG configura-tion consisting of delay array and DFT.

AO-OFDM-capable AWG device was first designed to beused not for DFT, but for generation and processing ofoptical codes in packet switching or optical code-divisionmultiple access (OCDMA) systems [37, 38]. A speciallydesigned AWG with a two-slab coupler connected by an ar-ray of waveguides (Fig. 9) can produce and detect OCDMAcodes with Fourier coding, which is equivalent to OFDMsymbols, as discussed in [20].

The first slab waveguide distributes the input fromthe single port to the waveguide array consisting of Nwaveguides with incremental delay differences of toperform serialparallel conversion as illustrated in Fig. 2a.The waveguide ports on the second slab coupler arecarefully placed to produce the Nth-order DFT phaserelations between N input and N output ports, i.e. the DFTof a parallel optical field input signal am (m = 0,1, . . N1)produces a parallel optical field output bn (n = 0,1, . . N1)as depicted in Fig. 10a, according to the definition

Figure 10 (online color at: www.lpr-journal.org) Slab couplerconfiguration (a), and its equivalent two-lens imaging systemmodel (b).

of DFT:

bn =N1m=0

ammn =N1m=0

ame j 2N mn. (14)

It is interesting to observe that the slab coupler of Fig. 10aperforms the spatial DFT equivalently to a two-lens systemseparated by a focal length f, as illustrated in Fig. 10b,where the curvature radius R of the slabs equals thefocal length f. In this design, the light distribution in theoutput plane is a Fourier transform of the input object,and this concept forms the basis of the entire opticalinformation processor. As an example, we can obtain theDFT function Eq. (14), by equally spacing the input andoutput waveguides. In [37], a relevant configuration of theAWG design for the multiport encoder/decoder (or DFTdevice) has been realized: In this design, the array pitches dand do of Fig. 10a have to satisfy the following condition:

d = d0 =

RN

, l = R, (15)

where l is the slab length. In addition, the delay difference introduced by two adjacent waveguides should satisfythe condition = 1/BN, where B is the OFDM symbolrate. The same AWG architecture has been described in[26, 31, 33]. In some previous publications, the confocalslab couplers that perform the DFT have been replaced bya set of phase shifters [18, 20].

The same AWG device can also be used to generateOFDM symbols. If a single short laser pulse is fed back-ward to one of the output ports of an AWG of Fig. 9, thecorresponding Fourier code is simultaneously generated atthe device input port. Each optical Fourier code is equiva-lent to an AO-OFDM symbol that corresponds to a carrierand the orthogonality between the code detection is used todemultiplex OFDM carrier at the receiver.

3.2. Coupler-based DFT device design

A device that performs DFT in the optical domain canbe attained by different physical implementations. The firstarchitecture that performs the DFT was proposed by Marhic[39] and revisited in a work by Siegman [40]. The WCMcomposed of passive couplers and phase shifters shown inFig. 10a performs the DFT of a parallel optical field inputsignal am (m = 0,1, . . N1) to produce a parallel opticalfield output bn (n = 0,1, . . N1), according to Eq. (4). TheNth-order DFT of the parallel input signal is evaluated bya chain of log2N stages, wherein each stage is composed of2 2 3-dB couplers that are equivalent to the second-orderDFT as shown in Fig. 11b.

These schemes make the DFT have a parallel input. Tobe used in optical communications, where data symbols areserially transmitted on a fiber, a serial-to-parallel converter,composed of a splitter and a set of delay lines, is required,as shown in Fig. 12. To realize a complete AO-OFDM




Figure 11 Waveguide and coupler mesh schematic diagram (a)composed of couplers and phase shifters that perform the DFTof N = 8 parallel inputs. The phase shifters are represented bythe boxes with phase-shift values 2q/N, where q is indicatedinside the boxes; and (b) the mathematical model for the 2 2coupler.

demultiplexer design, this schematic should be incorporatedwith optical delay arrays as shown in Fig. 12, which wasfabricated for the first time by Takiguchi et al. [41, 42].The same principle has also been used for 2 2 DFTsignal processing in an all-optical OFDM [21]. In addition,a generalization of the architectures of Figs. 11 and 12,replacing the 2 2 couplers with M M hybrids has beenproposed [43].

The DFT schemes of Figs. 11 and 12 are highly sensi-tive to small phase errors on all waveguides since a WCM isa complex interferometer, wherein multiple MachZehnderinterferometers (MZI) are nested with one another, as stud-ied in [44]. According to this report, a WCM device ismore sensitive to device-fabrication errors of waveguidephase-shift control than an equivalent AWG device. In or-der to overcome this drawback, a new configuration of thewaveguide DFT approach was formulated for a discrete se-rial input by Cincotti [45], and reinvestigated in [46]. In thisnovel design as shown in Fig. 13, the time delay array isrepositioned into MZIs, and MZIs are separated from oneanother through a tree architecture, as shown in Fig. 13:

Figure 13 (online color at: www.lpr-journal.org) Cascaded op-tical FFT composed of MZIs with couplers and phase shifters,which provides AO-OFDM demultiplexing equivalent to combinedfunctions of serial-to-parallel conversion, delay arrays, and DFT.The time delays and phase shifters are represented by the boxeswith time-delay values (multiples of ) and phase shift values(fractions of ), respectively.

The Nth-order DFT is then obtained by a chain of log2Nstages, each of which consists of an MZI with an outputcoupler.

This architecture has been fabricated by Hillerkeusset al. [43] (Fig. 14) and used in the record-breaking 26Tbit/s transmission experiment [6]. In this design, N carri-ers can be demultiplexed in M (< N) carrier AO-OFDM,and hence each output of the AO-OFDM output port hasN/M carriers, showing a cyclic pattern in the spectrum. Themultiple carriers of a port are separated by a coarse WDMdemultiplexer. Overall, this hybrid design of a cascaded op-tical FFT and WDM demultiplexer has demonstrated deviceefficiency as well as strong performance.

4. System implementations

As discussed throughout this review paper, severaladvanced all-optical techniques exist for all-opticalDFT/IDFT processing, which can benefit from the ad-vantages of modulation format transparency, compact all-optical implementation, and high-speed all-optical signalgeneration and detection beyond the limit of the bandwidthof electronic components.

Figure 12 (online color at:www.lpr-journal.org) From [42]:Planar waveguide circuit for the8 8 DFT. The time delay in-troduced by L corresponds to , where and L are the effec-tive refractive index and the unitlength difference of the delay ar-ray waveguide, respectively.


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4.1. System demonstrations of IM-DDAO-OFDM transmission

In the early development of AO-OFDM, there have beena few reports of two-carrier IM-DD AO-OFDM demon-strations [18], followed by several reports of system per-formance studies [20]. Nonetheless, experimental demon-strations for practical applications have been achieved onlyrecently [25, 26]. More recently, a reference AO-OFDMdesign with IM/DD that is depicted in Fig. 5 [20] has beendemonstrated experimentally by Shimizu et al. [33].

Figure 15a shows the experimental schematic of 12.5-Gbps 8-carrier AWG-based AO-OFDM transmitter andreceiver [33]. An optical comb for eight carriers with12.5 GHz frequency spacing is generated from a cw light

source with two cascaded EAMs modulated by 25 GHzand 12.5 GHz clocks, respectively. The frequency of thecw light source is 193.4875 THz. The optical comb is thensplit into two replicas and modulated by 12.5 Gbit/s pseu-dorandom bit sequence (PRBS) in an NRZ-OOK formatwith pattern lengths of 2151 and 2311 for odd and evenchannels, respectively. Each modulated data signal is splitinto four branches and sent to the AWG-based OFDM mul-tiplexer. Each branch has a different delay for data patterndecorrelation. The polarizations and powers of all the car-riers are aligned by polarization controllers and variableoptical attenuators, respectively. The AWG used in this ex-periment has 16 ports with 12.5 GHz carrier spacing. At thereceiver side, the signal is sent to the AWG-based OFDMdemultiplexer. The demultiplexed signal is applied to the

Figure 14 From [6]: Cascadedoptical-FFT and coarse-WDMAO-OFDM receiver model fora 325-carrier 26-Tbps transmis-sion demonstration. Note thata conventional WDM demulti-plexer is used before electroab-sorption modulators (EAMs).EAMs are used to time samplethe optical symbol at the orthog-onal points.

Figure 15 (online color at: www.lpr-journal.org) Pulse-mode AWG AO-OFDM system; (a) system schematic for IM-DD AO-OFDMtransmission of 8 12.5 Gbps, (b) demultiplexed receiver output optical eye diagram of carrier 5, (c) ICI waveform in carrier 5 outputfrom carrier 6, and (d) BER performance. LD: laser diode, EAM: electroabsorption modulator, LN-IM: LiNbO3 intensity modulator, AWG:arrayed waveguide grating, VOA: variable optical attenuator, PC: polarization controller, Rx: receiver, PPG: pulse pattern generator,BERT: bit error rate tester.




EAM to extract the orthogonal point in the symbol timeslot. The corresponding demultiplexed output eye diagramof the target carrier is shown in Fig. 14b. In this design, awaveform reshaping modulator is employed at the receiverside, which provides time-domain equalization (TDE) tocompensate for the AWG demultiplexing error. As a result,TDE can achieve efficient ICI cancellation at the samplingpoint as shown in Fig. 15, restoring orthogonality. This ex-periment confirms that the BER is reduced to 106 from104 by the use of TDE.

4.2. System demonstrations of high-speedAO-OFDM superchannel transmission

In recent years, significant advances in high-speed AO-OFDM superchannel technology and transmission capa-bilities have occurred. Such optical superchannels usingAO-OFDM are thus quite promising for realizing aggre-gate transmission data rates beyond 100 Gb/s, by enablingflexible capacity scaling via parallel addition of transmit-ters and receivers. At the transmitter side, the AO-OFDMsuperchannel approach is implemented based on opticalmethods to generate phase-locked optical carriers, fol-lowed by individual modulation of each carrier as a par-allel channel. At the receiver side, the optical carriers caneither be demodulated optically via an optical DFT cir-cuit, or electrically by first downconverting a spectral sliceof the AO-OFDM to the electrical domain with a digi-tal coherent receiver, then applying a digital fast Fouriertransform (FFT). However, since CD destroys orthogonal-ity between carriers, precise all-optical CD compensationmust be performed prior to optical detection if an optical

Fourier transform circuit is used, which can be a challengeto fully accomplish in the optical domain. On the otherhand, the digital FFT can be implemented in receiver-sideDSP as a complement to functions already implementedin typical 100 Gbit/s systems, including such as electronicCD compensation. Consequently, existing optical and elec-trical hardware developed for 100 Gb/s systems can toa large extent be reused, such that software-defined up-grades to beyond 100 Gb/s transmission become possi-ble. In this section, we consider experimental system-levelconsiderations for high-speed AO-OFDM superchanneltransmission.

The system-level setup of the 21.7-Tbit/s AO-OFDMfield trial over 1503 km of installed standard single-modefiber (SSMF) achieved in [7] is shown in Fig. 16. In to-tal, 22 external cavity lasers (ECLs) were used as seedlasers to produce 330 AO-OFDM carriers, with eitherpolarization multiplexed QPSK or 8-QAM modulationused on each carrier, depending on the underlying chan-nel quality. The optical tone generator consisted of sepa-rate phase modulators overdriven with sine-wave inputs,such that phase-locked optical carriers at 25 GHz spac-ing were created for each seed laser. A flexible band WSSwas used to equalize the optical carrier powers, as wellas it combined the carriers from the odd and evengroups into a single output. On each carrier, the symbolrate Rs = 12.5 Gbaud was exploited, with the per-carriermodulation format decided based on spectral efficiencyrequirements and optical signal-to-noise ratio (OSNR)conditions.

Following field-installed SSMF transmission, as inFig. 16, a WSS was used to filter out the target super-channel, while an ECL LO was tuned to downconvert the

Figure 16 (online color at:www.lpr-journal.org) System-level experimental setup of21.7Tbit/s AO-OFDM field trialwith transmitter and receiverside spectra and experimentalbit error rate (BER) results [7].


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target AO-OFDM carriers for digital carrier demultiplex-ing. Specifically, the LO was tuned midway between twooptical carriers, which were jointly processed in offline DSPas described in Fig. 8 with 30 GHz input bandwidth and6 times (M = 6) oversampling. Additional details of thesystem demonstration may be found in [7]. As shown bythe BER results in the inset of Fig. 16, 18 PDM 8-QAM su-perchannels were successfully transmitted over 1503 km offield-deployed SSMF within the BER limit of 4.5 103for 7% overhead hard-decision forward error correction,while 4 PM QPSK superchannels achieved the same trans-mission distance with more margin. By exhibiting boththe highest field-trial capacity (21.7 Tbps) and the high-est capacitydistance product (32.6 Pbps km) to date, theexperimental demonstration confirms the promising spec-tral efficiency and aggregate transmission rate capabili-ties of AO-OFDM for future >Tbit/s optical transmissionsystems.

5. Conclusions and further study

All-optical OFDM techniques have rapidly gained interestin the optical data transmission community in recent years,even though research efforts on this topic were initiallystarted on academic grounds. Nowadays, the principleunderstandings of orthogonality in multicarrier opticaldata transmission form the basis for Tbps superchanneland other highly spectrally efficient transmission systemsfor next-generation applications. This paper has discussedrecent developments in AO-OFDM techniques, includinga theoretical investigation of optical system require-ments, a review of AO-OFDM multiplexer/demultiplexerdevice technologies, and recent state-of-the-art systemdemonstrations of AO-OFDM IM/DD and superchannelsystems.

An AO-OFDM transmission system is modulation-format transparent, meaning that different carriers can beconfigured in different ways, enabling a mixture of low-cost direct detection and more advanced coherent detectionfor short-haul and long-haul transmission scenarios, respec-tively. Such flexibility with high spectral efficiency can bescaled to enable flexible hybrid optical networks that canbe utilized for optical flow switching and optical wavebandswitching. In addition, AO-OFDM processing can signif-icantly reduce overall energy consumption, and overcomecurrent energy consumption limits to enable beyond-Tbpsoptical transmission systems.

Although AO-OFDM technology has rapidly advancedwith proof-of-concept demonstrations and field trials, thereare many unsolved problems to be addressed in practicalapplications. First, various optical DFT device and sys-tem technologies need to be investigated in order to max-imize AO-OFDM carrier orthogonality in nonideal condi-tions. A good example in this avenue was reported in [6],where a high-order optical DFT was achieved by a com-bination of a low-order optical DFT and a conventionalWDM demultiplexer. In addition, orthogonality can also

be improved with optical signal processing as discussedin [33], where optical time-domain equalization is used tocompensate for the AO-OFDM demultiplexer design errors.Moreover, there exist various device-design solutions thatcan improve the accuracy of filter functions of optical DFTdevices. Secondly, fiber transmission impairment mitiga-tion technologies have to be exploited to optimize systemperformance against fiber chromatic dispersion and non-linearity. For example, fiber chromatic dispersion penaltycan be mitigated by optical or electrical compensationmethods. Self-phase modulation and intercarrier four-wavemixing penalty can likewise be managed by phase con-trol among carriers, or mitigated by coherent detectionand digital signal processing (e.g. nonlinear backpropa-gation [47]). All of these anticipated developments willadd to critical enabling technologies for practical realiza-tions of AO-OFDM for beyond-Tbps optical transmissionsystems.

Acknowledgment. The authors are grateful to ProfessorGabriella Cincotti of University Roma TRE, Italy, for indepth guid-ance and technical discussions.

Received: 30 June 2012, Revised: 8 December 2012,Accepted: 10 December 2012

Published online: 15 February 2013

Key words: All-Optical OFDM (AO-OFDM), Optical DiscreteFourier Transform (ODFT), Orthogonal Frequency Division Multi-plexing (OFDM), Superchannel.

June-Koo K. Rhee is an associate pro-fessor at the Korea Advanced Instituteof Science and Technology and a grad-uate of Seoul National Univ. with a B.E.(1988) and M.Sc. (1990), and the Univ.of Michigan, Ann Arbor, with a Ph.D.(1995), all in electrical engineering. Pre-viously, he was with Princeton Univ.(1995), NEC Res. Inst. (1996), Corning

Inc. (1998), and Samsung Adv. Inst. of Techno. (2003). His re-search has made contributions in the areas of ROADM, DPSKDWDM transmission, and WDM optical protection, green op-tical networking, and optical and flow switching.

Neda Cvijetic received the Ph.D. de-gree in electrical engineering from theUniversity of Virginia, Charlottesville,VA, in 2008. Since 2008, she hasworked as a Research Staff Member inthe Broadband and Mobile NetworkingDepartment at NEC Laboratories Amer-ica, Princeton, NJ. She is also currentlya member of the adjunct faculty at theDepartment of Electrical Engineering at

Columbia University. Her research interests include advancedmodulation, detection and digital signal processing (DSP)for high-speed optical transmission, optical-wireless conver-gence, and next-generation optical access networks.




Naoya Wada received the B.E.,M.E., and Dr. Eng. degrees in elec-tronics from Hokkaido University,Sapporo, Japan, in 1991, 1993,and 1996, respectively. In 1996,he joined the Communications Re-search Lab, Japan. He is currentlythe Research Manager of the Pho-tonic Network Group, National Inst.of Info. and Comm. Technology(NICT), Japan. His research inter-ests include photonic networks and

optical communications, such as OPS network, optical pro-cessing, and OCDMA system.

Ting Wang received the M.S. degreein electrical engineering from the CityUniversity of New York, New York, andthe Ph.D. degree in electrical engineer-ing from Nanjing University of Scienceand Technology, Nanjing, China. Since1991, he has been with NEC Laborato-ries America, Princeton, NJ, where heis currently the Department Head of op-tical networking research.

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