all-optical sampling orthogonal frequency-division multiplexing scheme for high-speed transmission...

4848 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 21, NOVEMBER 1, 2009

All-Optical Sampling OrthogonalFrequency-Division Multiplexing Scheme

for High-Speed Transmission SystemHongwei Chen, Member, IEEE, Minghua Chen, Member, IEEE, and Shizhong Xie, Senior Member, IEEE

Abstract—A novel high spectral efficiency all-optical samplingorthogonal frequency-division multiplexing (AOS-OFDM) schemewith optical cyclic postfixes (OCPs) inserted is proposed. Ultra-short pulses are used as optical samples and processed by all op-tical discrete Fourier transformers based on fiber Bragg gratings(FBGs) technique. This scheme includes five subcarrier channelswith 20 Gb/s differential quadrature phase-shift keying (DQPSK)modulation of each channel. So, the total bit rate is 100 Gb/s. Thespectral efficiency is higher than 1.4. Furthermore, with the helpof OCPs, the dispersion tolerance of this 100 Gb/s AOS-OFDMsystem increases to more than 300 ps/nm, which is much higherthan a traditional 100 Gb/s DQPSK system.

Index Terms—Cyclic postfix (CP), optical sampling, orthogonalfrequency multiplexing.

I. INTRODUCTION

O PTICAL orthogonal frequency-division multiplexing(OFDM) method recently arises as a potential tech-

nology for future high-speed communication system [1],[2]. It is considered to have large tolerance to different fibertransmission impairments, such as chromatic dispersion, polar-ization-mode dispersion, and optical fiber nonlinearity [3]–[5].Many existing optical OFDM systems use electrical circuit tomultiplex parallel data into multiple subcarriers due to OFDMprinciple and modulate these signals in optical domain bya modulator. Thus, electrical OFDM modulation is limitedby electronics process speed in forward and inverse discreteFourier transform (DFT/IDFT) module and also the bandwidthof digital-to-analog/analog-to-digital converter (DAC/ADC).Recently, high spectral efficiency and real-time detection havegreat improvements for coherent OFDM (CO-OFDM) system.Coherent pulse-duration modulation-OFDM (PDM-OFDM)scheme is proposed in [6] with net SE of 7 b/(s Hz). A field-pro-grammable gates array (FPGA) based CO-OFDM receiver isdemonstrated with real-time implementation [7]. These tech-niques push the optical OFDM system to practical applications.

Manuscript received April 10, 2009; revised June 16, 2009, July 09, 2009.First published August 04, 2009; current version published September 10, 2009.This work was supported in part by Tsinghua Basic Research Fund under GrantJC2007020, in part by the National Science Foundation of China under Con-tract 60736002, in part by the National 863 Program of China under Contract2007AA01Z264, and in part by the Research Fund for the Doctoral Program ofHigher Education of China under Contract 20070003015.

The authors are with the Department of Electronic Engineering, State KeyLaboratory on Integrated Optoelectronics, Tsinghua National Laboratory forInformation, Science and Technology (TNList), Tsinghua University, Beijing100084, China (e-mail: [email protected]; [email protected];[email protected]).

Digital Object Identifier 10.1109/JLT.2009.2029063

On the other hand, if the DFT process can be realized byoptical method, the OFDM signal process will be very fast andthe transmission data rate will also increase greatly. All-opticalDFT methods combining optical delays and phase shifters areintroduced recently. Continuous wave with data modulationis used for transmission and Mach–Zehnder interferometer(MZI) is used as IDFT module [8], [9]. Also, coherent wave-length-division multiplexing (WDM) signal utilizing OFDMprinciple is proposed with either coherent comb optical source[10] or coherent detection [11]. And a scheme using ultrashortoptical pulses as samples for optical DFT/IDFT process isproposed in [12], which has a complex structure of DFT/IDFTmodules. Considering the good performance of fiber Bragggratings (FBGs) in optical code-division multiple-access(OCDMA) systems [13], it is promising to use FBGs as alloptical DFT/IDFT modules.

In this paper, we report a novel optical DFT/IDFT scheme forall-optical OFDM system applications. This scheme uses ultra-short optical pulses as sample pulses and can operate differentsubcarrier channels in parallel. Each AOS-OFDM channel ismodulated and processed by FBGs. And for the first time, op-tical cyclic postfixes (OCPs) are inserted to improve fiber dis-persion tolerance.

II. PRINCIPLE

The basic principle of all-optical OFDM modulation is sim-ilar with that of electrical ones. With optical delay lines andphase shifters, one can use ultrashort optical pulses as samplesto do DFT/IDFT process all optically [12]. However, such op-tical DFT/IDFT process is based on linear convolution that isdifferent from electrical DFT/IDFT process. So, at the receiver,after optical IDFT process, only one sample pulse in one periodcan keep orthogonal. Thus, synchronous pulse carver modula-tion should be used to extract the correct sample [12], which isdifficult in a high-speed system. In order to solve this problem,the optical samples are partly cyclic extended, which can keepsubcarrier orthogonal in the cyclic prefix or postfix samples.

The principle of optical DFT for parallel processing is shownin Fig. 1(a). In fact, the sum of all subcarrier samples in onesymbol period (equals to bit period) can be expressed as

(1)

0733-8724/$26.00 © 2009 IEEE

CHEN et al.: ALL-OPTICAL SAMPLING ORTHOGONAL FREQUENCY-DIVISION MULTIPLEXING SCHEME FOR HIGH-SPEED TRANSMISSION SYSTEM 4849

Fig. 1. (a) Principle of optical DFT for parallel processing. (b) Single subcarrierchannel process.

where is the number of samples, is the number of sub-carriers, and is the sample value. So, in this case, every sub-carrier channel samples can be processed separately as long asthey are synchronous. Thus, optical DFT and IDFT modules fordifferent subcarrier channels are composed of time delays andphase shifters, as shown in Fig. 1(b). The pulses in Fig. 1(b) rep-resent optical intensities. The analyses can be shown as follows.

Considering one sample pulse of th subcarrier channel afterthe corresponding ODFT in AOS-OFDM system without CPcan be expressed by

(2)

where is the sample value, ( ) is the shape of an opticalsample pulse, is the number of samples in one bit period ,

is the time delay between each sample, is the phaseshift of th sample, which is equal to . Usually,the sample value keeps the identical in one symbol period,so it can be neglected in the following descriptions.

As shown in Fig. 1(b), the OIDFT has the similar structure(time delays and phase shifters) as ODFT. Thus, this signal after

th OIDFT module can be expressed as

(3)

This signal is the result of linear convolution, so the total numberof samples after OIDFT becomes . At the moment

Fig. 2. Illustration of demutiplexed samples (a) without and (b) with CPs.

, i.e., , there is a superposition ofsamples, which can be expressed as

(4)

The schematic of demultiplexed samples are shown in Fig. 2(a).If the signal of the other subcarrier channel passes through

noncorresponding OIDFT module, for example, th channelsignal passes through th OIDFT module, then it will be givenby

(5)

Thus, at , the signal superposition can be ex-pressed as


(6)

It is quite clear that at moment, orthogonality of different sub-carrier channel samples can be obtained, while it is not true atthe other moments, i.e., when the signal passes through corre-sponding OIDFT, in orthogonal zone, there is one strong pulse,while there is no pulse when the signal passes through non-corresponding OIDFT, as shown in Fig. 2(a). The problem isthat there is only one orthogonal sample, which will be difficultfor the signal detection and need synchronous pulse carving atthe receiver [12]. This shortcoming can be overcome by addingpostfixes (CP) samples in one period. If the number of CPs is ,and the number of samples in one DFT interval is , so the totalnumber of samples is in one bit period T. Thus,the th subcarrier channel signal with CP can be expressed as:

(7)

Here, the time delay between each sample is , whichis different from (1). There are pairs of time delay and phaseshifters in one OIDFT module that is different from the ODFTones ( pairs). So, after passing through th OIDFT module,the signal can be expressed as

(8)

Thus, when the time moments are from to, there are superpositions of samples. Based on the pre-

vious analysis (4) and (6), these samples can keep orthogonality.Therefore, the orthogonal samples increase to , i.e.,

, as shown in Fig. 2(b).This ensures that the replicas of the OFDM symbol always

have an integer number of cycles within the DFT interval. Thus,if samples are in one DFT period and in cyclic postfix,correct demodulated samples after the optical IDFT module in-crease to . This method can help to enlarge the decisionrange in eye diagrams, and also it may give some benefits for

Fig. 3. Dispersion impact on AOS-OFDM scheme (a) without and (b) withCPs.

dispersion walk-off as the electrical CP’s function in wirelessmultipath environment [14].

Considering chromatic dispersion (CD) in optical fiber link,the phase changes of all the samples are identical and have noimpact on orthogonality. Here, we only consider the impact in-duced by the pulse shape expanding. As shown in Fig. 3(a),when the adjacent pulses broaden into the area of orthogonalsample pulse, the orthogonality will be damaged. Only in zoneII, the pulse is still keep orthogonality. With optical CPs, theorthogonal zone will expand, as shown in Fig. 3(b). Thus, thismethod can help to enlarge dispersion tolerance that will bedemonstrated by simulation.

Fig. 4 shows basic structure of optical OFDM modulation(ODFT and OIDFT module) using FBG technique. The FBGis designed to have many sample subgratings along its length.With the scan-exposure technique, each subgrating can have thesame refractive index modulation amplitude but different modu-lation phase shifts, i.e., the refractive index’s spatial modulationfunction of the FBG has the following form

(9)

where is the period of the grating, is the chip period,is the profile of each chip’s amplitude, and is each chip’sphase shift. If is very small, the FBG’s impulse responsecan be approximately given by [15]: ,where is a constant coefficient, is the effective refractiveindex, and is speed of light. So, the input optical pulseand the reflective signal have the relation

(10)

where

If we let ,then has the same structure as in (2). So, it is reason-able to use FBGs as ODFT and OIDFT modules in AOS-OFDMsystems. In Fig. 4, the optical pulses at different positions arealso shown. After ODFT FBG, the input sample pulse is ex-panded to multisamples that have identical amplitude and dif-ferent phase value. For different subcarrier channels (SC1, SC2,SC3, SC4, and SC5), the phase relationships of sample pulsesare shown in Fig. 4(a); it is clear that all the SC channels have


Fig. 4. Structure of (a) ODFT and (b) OIDFT based on FBG technique withoptical CPs inserted.

integer cycles in one DFT interval that is in line with the OFDMprinciple. Then, after OIDFT FBG, these expanded samples canbe demultiplexed. That means with corresponding OIDFT, theoutput signals have massive pulses in the middle of symbol pe-riod, while with noncorresponding OIDFT, the output signalshave null samples in the middle. This is quite the same as thedescriptions of Fig. 2.

III. SIMULATION

Fig. 5 shows the schematic of proposed 100 Gb/sAOS-OFDM system. Ultrashort pulse train from a mode-lockedlaser diode (MLLD) is split into five parts. Each part is modu-lated with independent data by a 20-Gb/s differential quadraturephase-shift keying (DQPSK) modulator driven by two groupsof 10 Gb/s data [16], and passes through an optical discreteFourier transformer (ODFT) with the structure shown in Fig. 4and the structure parameters are shown in Table I. The symbolperiod is 100 ps, number of samples in one DFT interval( ) is 32 and cyclic postfix number ( ) is 8. Then, the fiveparts are synchronously combined together and filtered by anoptical bandpass filter (OBPF) for better spectral efficiency.At the receiver, the OFDM signal is split into five parts andpasses through corresponding OIDFT modules and DQPSKdemodulators.

In our simulation, for bit-error-rate (BER) calculations,noises at the receiver are categorized according to the Gaussiandistribution and consisted of shot noise, signal spontaneousbeat noise, and spontaneous–spontaneous beat noise. Thesimulation parameters are shown in Table II.

Fig. 5. Schematic of 100 Gb/s AOS-OFDM system. DQPSK MOD: differentialquadrature phase-shift keying modulator; EDFA: erbium-doped fiber amplifier;MLLD: mode-locked laser diode; OBPF: optical bandpass filter; ODFT: opticaldiscrete Fourier transformer; SMF: single mode fiber.

TABLE IPARAMETERS OF ODFTS AND OIDFTS

IV. RESULTS

Fig. 6 shows the spectra of different modulation formats, in-cluding 100 Gb/s DQPSK, AOS-OFDM without CP, and AOS-OFDM with CP. The dB bandwidth and spectral efficiencyof AOS-OFDM without CP are 57.76 GHz and 1.73, respec-tively, and the dB bandwidth and spectral efficiency ofAOS-OFDM with CP are 67.3 GHz and 1.48, respectively. Itis clear that both AOS-OFDM with and without CP schemeshave better spectral efficiency than traditional DQPSK format.The AOS-OFDM with CP scheme has larger bandwidth than theone without CP is only because the redundant cyclic postfixesinserted.


TABLE IIPARAMETERS VALUES USED FOR SIMULATIONS

Fig. 6. Optical spectra of different formats.

Fig. 7. Dispersion tolerance of different formats.

In order to test the system performance, we change the lengthsingle-mode fiber (SMF) to get different CD. Polarization-modedispersion (PMD) and fiber nonlinearity is not considered here.Fig. 7 shows dispersion tolerance of different formats. We mea-sured the BER of both I and Q part of DQPSK signal, and thepower penalty is calculated in case, which is cor-

Fig. 8. Constellations and eye diagrams of received signals at�� ps/nm.(a) Original DQPSK data. (b) Traditional DQPSK signal. (c) AOS-OFDMwithout CPs. (d) AOS-OFDM with CPs.

responding to forward error coding (FEC) gain. Also, in simula-tion, the required optical SNRs (OSNRs) (resolution bandwidthof 0.1 nm) for all the schemes are kept to 20 dB. One can seethat the AOS-OFDM with CP signal has much better CD tol-erance than the other two formats. When power penalty equalsto 1 dB, the CD tolerance can be above 300 ps/nm. Differentchannels have different tolerances and the differences are quitelarge ( ps/nm), which is mainly because this AOS-OFDMsignal is bandwidth limited by the OBPF and the orthogonalityof edge channels is deteriorated. We also compared the constel-lations and eye diagrams of received signals in three formats,


when the CD value is 85 ps/nm. The second subcarrier chan-nels I data of AOS-OFDM with and without CP are selected forcomparison. It is very clear that the signal in AOS-OFDM withCP format has the best performance.

V. CONCLUSION AND DISCUSSION

A novel high spectral efficiency all-optical OFDM with cyclicpostfixes scheme has been proposed for 100 Gb/s optical trans-mission system application. In this scheme, ultrashort pulses areused as optical samples and processed by all ODFTs based onFBGs technique. The ultrashort pulses are reflected with certaintime delays and phase shifts, which correspond to OFDM prin-ciple, and OCPs can also be added in this novel AOS-OFDMscheme.

The 100 Gb/s AOS-OFDM system includes five subcarrierchannels with 20 Gb/s DQPSK modulation of each channel, thetotal bit rate is 100 Gb/s. Simulation results show that the spec-tral efficiency is higher than 1.4. Furthermore, with the help ofOCPs, the dispersion tolerance of this 100 Gb/s AOS-OFDMsystem increases to more than 300 ps/nm that is much higherthan traditional 100 Gb/s DQPSK system.

This scheme has many advantages that include the following.1) By using ultrashort optical pulses as samples, the system

only needs one single optical source for multichannelOFDM modulation.

2) At the receiver, only passive optical IDFT modules and tra-ditional receivers are used without coherent optical detec-tion or synchronous pulse-carving modulation.

3) The spectral efficiency of this scheme can be further in-creased by using multilevel modulation formats or polar-ization multiplexing technique.

4) Compared to electrical OFDM system, our scheme doesnot meet the limitation of DSP circuits. Furthermore, thisscheme can add OCPs to enlarge dispersion tolerance. Al-though the tolerance is not as large as CO-OFDM system,yet it will be suitable for traditional 10 Gb/s fiber links withsimilar residential dispersion tolerance.

5) This scheme can work online, which is quite different fromelectrical OFDM offline system.

6) By using optical integration technique, such as planar light-wave circuit (PLC), the optical DFT/IDFT modules canbe more accurate and stable than FBG devices. So, AOS-OFDM with OCPs inserted can be one of the candidatesfor future high-speed optical transmission systems

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[8] H. Sanjoh, E. Yamada, and Y. Yoshikuni, “Optical orthogonal fre-quency division multiplexing using frequency/time domain filteringfor high spectral efficiency up to 1 bit/s/Hz,” in Proc. OFC 2002, pp.401–402, Paper ThD1.

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Hongwei Chen (M’07) was born in Inner Mongolia,China, in 1979. He received the B.E. and Ph.D.degrees in electronic engineering from TsinghuaUniversity, Beijing, China, in 2001 and 2006,respectively.

He is currently with the faculty of the Departmentof Electronic Engineering, Tsinghua University. Hiscurrent research interests include radio-over-fibertechniques, high-speed optical communications, andoptical packet switching networks.

Dr. Chen received the Best Student Paper Award ofAsia-Pacific Optical Communications (APOC) 2004 and was a subcommitteemember of APOC 2005, 2007, 2008, and Pacific Rim Conference on Lasers andElectro-Optics (CLEO-PR) 2007.

Minghua Chen (M’04) received the Ph.D. degree inelectronics engineering from Southeast University,Nanjing, China, in 1998.

From 1998 to 2000, he was a PostdoctoralResearcher at Tsinghua University, where he iscurrently the Vice Director of the Information Opto-electronics Research Center within the Departmentof Electronics Engineering. He has supervised andcollaborated on several projects supported by theChinese National Science Funds and High-Tech(863) Projects. His current research interests include

optical networking and its key technologies, including dynamical wavelengthrouting, optical label switching, optical packet switching, and their key opticalcomponents.


Shizhong Xie (M’94–SM’98) graduated in 1970 andreceived the M.S. degree in electronic engineering in1981 from Tsinghua University, Beijing, China.

From 1987 to 1988, he was a Visiting Scholar atthe University of Southern California, Los Angeles,CA. In 1989, he was a Senior Visitor with a RoyalSociety British Telecom Fellowship at the UniversityCollege London, London, U.K. From 1970 to 1978,he was with the faculty of the Department of Elec-tronic Engineering, Tsinghua University, where he iscurrently a Full Professor and the Director of Optical

Communication Research Institute since 1981. He has led or participated many

major government programs in the area of optical network, including ChinaAdvance INfo-Optical Network (CAINONET), the National Science Foun-dation of China Network (NSFCNET), and the National High-PerformanceBroadband Information Network (3T’NET), and was a member of the expertgroups steering those programs. His current research interests include densewavelength-division multiplexing (DWDM) optical fiber communications,broadband optical networks, optical packet switching, UV-induced fiber Bragggratings, holey fibers, and their application in optical fiber communications.

Prof. Xie is a Senior Member of the IEEE Photonics Society [formerly knownas Lasers and Electro-Optics Society (LEOS)], the Chinese Institute of Elec-tronics, and the Chinese Optical Society.

all-optical sampling orthogonal frequency-division multiplexing scheme for high-speed transmission...

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