31MIYAZAKI Tetsuya

1 Introduction

Internet traffic continues to increasedespite the current difficult economic condi-tions; these circumstances demand opticaltransmission technology that can efficientlyaccommodate this growing traffic. When con-structing photonic networks for the near futureand beyond, technology to increase data-trans-mission capacity per wavelength will beessential, in addition to conventional technolo-gy relating to Wavelength Division Multiplex-ing (WDM). The 160-Gb/s ultrafast opticalcommunication system［1］［2］, with a bit rateper wavelength exceeding the maximum pro-cessing speeds of current electronic circuits(approximately 100 Gb/s), will be able tocarry the same traffic in far fewer wavelengthsthan the 10-Gb/s systems now commerciallyavailable (and still fewer than in the 40-Gb/sWDM systems soon to enter the market).Ultrafast communication is thus expected to

simplify future network operations and man-agement to a significant extent, such as estab-lishing wavelength path and signal qualitymonitoring for switching in the event of fail-ure. Recent progress in optical device technol-ogy has enabled indoor and field transmissionexperiments with 160-Gb/s optical transmis-sion systems using installed fiber optic equip-ment. These experiments, conducted mainlyby research institutions involved in nationalprojects within Europe, have been the subjectof a steady stream of recent reports［3］［4］.

Installed fiber optic equipment differsfrom corresponding laboratory equipmentdesigned with ideal characteristics for 160-Gb/s transmission, in that the former isinstalled in severe outdoor environmentsinvolving significant temperature variationand environmental vibration. In a particularchallenge to actual implementation, 160-Gb/soptical pulse signals feature small repetitionintervals; as a result, Polarization Mode Dis-

3 Photonic Network Technology

3-1 Ultrafast 160 Gb/s-based TransmissionExperiment on JGNⅡ

MIYAZAKI Tetsuya

We have been investigating basic technology to establish a Peta-bit/s class photonic net-work, such as ultrafast photonic processing, novel modulation/demodulation format and ultrafasttransmission technology. We would like to report our recent research works including collabora-tive work with private company using JGNⅡ optical test bed to curve the expansion of the footprint and electrical consumption power in network nodes and to release network managementby lowering required the wavelength number against endlessly growing demand of internet traf-fic.

KeywordsUltrafast optical communication, Highly efficient modulation/demodulation formats, Peta-bit/s-class photonic networks

32 Journal of the National Institute of Information and Communications Technology Vol.52 Nos.3/4 2005

persion (PMD) is the main cause of degrada-tion in transmitted signal quality［1］-［4］. Nev-ertheless, we succeeded in stable 160-Gb/sfield transmission for the first time in Japan inJuly, 2004, using the fiber optic testbed estab-lished for the JGNⅡproject［5］, by implement-ing a simple system based on an automaticPMD compensator. This experiment was per-formed in collaboration with KDDI R&DLaboratories［6］. We also evaluated the sup-pression of PMD signal degradation, adoptinga multi-level format for a data capacity of 160Gb/s per wavelength. The obtained data indi-cated satisfactory results［7］. Further, in March2005, we performed the world’s first stableinter-city WDM optical transmission, span-ning 200 km at a total capacity of 1.28 terabitsper second, applying 160-Gb/s 8-wavelengthWDM; success in this case relied heavily onthe strength of the simple automatic PMDcompensator［8］.

2 Characteristics of fiber opticline［6］

Figure 1 shows the network configurationof the optical testbed. Optical repeaters,including Dispersion Compensating Fibers(DCFs), are installed at the Ootemachi and

Tsukuba JGNⅡResearch Center stations andat the relay station (Kashiwa); these stationsare connected with Single-Mode Fibers(SMFs; compatible with ITU-T G.652)approximately 50 km in length. In the currentconfiguration, the optical signals fromOotemachi are rerouted at Tsukuba, in a 4-span transmission-line structure featuring atotal length of 200 km (50 km×4 spans). Asaerial cable routes were already present inSpans 2 and 3, a practical optical testbed isthus formed. Each station is equipped withoptical amplifiers featuring gain deviation of ±0.5 dB or less for wavelengths from 1,535 nmto 1,565 nm. Dispersion Compensating Fibers(DCFs) are installed between the stages ofeach optical amplifier. In the optical amplifiersat Kashiwa, Dynamic Gain Equalizers (DGEs)are installed to adjust link gain flatness. Theoptical amplifiers and DGEs within the systemcan be remotely controlled from other stations.For the 200-km transmission link, the accumu-lated dispersion is plus or minus 5 ps/nm orless over a bandwidth of 30-nm (1,535 nm to1,565 nm) in the C band.

As shown in Fig.2, signal quality degradesif PMD causes Differential Group Delay(DGD)— i.e., optical pulses splitting to causedifferences in speed of the optical pulses inthe direction of polarization. Figure 3 showsthe wavelength dependence of DGD measuredevery 30 minutes over 8 hours from day intoevening, with the 30-minute segments over-laid. Within the optical signal wavelengthindicated with the dotted line (1,558 nm), amaximum of approximately 3 ps of DGD isobserved. This indicates an extremely severetransmission environment for 160-Gb/s opticalsignals at 6.25-ps pulse intervals.

Fig.1 JGNⅡ optical testbed networkTx: transmitter system, Rx: receiver system,DCF: Dispersion Compensating Fiber, SMF:Single-Mode Fiber, Rep: optical repeater Fig.2 Polarization Mode Dispersion (PMD)

33MIYAZAKI Tetsuya

3 Field transmission experiments

3.1 160-Gb/s single-wavelength DPSKtransmission［6］

We applied DPSK modulation with highreceiver sensitivity to achieve stable 160-Gb/stransmission, introduced a polarization stabi-lizer in the receiver as a simple PMD compen-sator, and evaluated the suppression of PMDvariation.

Figure 4 shows the 160-Gb/s DPSK trans-mitter and receiver systems.

The transmitter (Tx) generates 40-GHzoptical pulse trains using a set of 2-stage Elec-tro-Absorption Modulators (EAMs), appliesDifferential Phase Shift Keying (DPSK) at 40Gb/s, compresses the pulses using highly non-linear fiber (2.7 ps), and transmits single-polarization 160-Gb/s-DPSK signals (PN 7stages) using polarization-maintaining Optical

Time Domain Multiplexing (OTDM) equip-ment. The receiver (Rx) contains an automaticPMD compensator consisting of an automatictracking polarization stabilizer and a polarizer.The Demultiplexer (DEMUX) uses a 2-stageEAM operating at a clock rate of 40 GHz gen-erated by the hybrid Phase-Locked Loop(PLL) clock extraction circuit. After theDEMUX, the 160-Gb/s DPSK signals weredemultiplexed into 40-Gb/s signals, and then,the 40-Gb/s signals into 10-Gb/s signals byelectrical time division, and the Bit Error Rate(BER) is measured.

Figure 5 shows the correlation characteris-tics between the Q-factor (higher values indi-cating fewer errors) calculated from the aver-age error rate of the received sixteen 10-Gb/ssignals demultiplexed from the 160-Gb/s sig-nals and the DGD near the signal wavelength.When the polarization is adjusted on the trans-mitter side, (the black circle indicating thebest value and the white circle indicating theworst value), stable transmission characteris-tics are obtained following 200-km transmis-sion (18 dB<Q) even with DGD of over 2 ps(which occurred during over 32 percent of thetransmission time-slot). The results indicatethat signal quality is also maintained for PMD(DGD) larger than that obtained without theuse of the stabilizer (indicated by the black tri-angle in the figure).

3.2 160 Gb/s APSK transmission［7］Simultaneous Amplitude and Phase-Shift

Keying modulation (APSK, or ASK-DPSK)

Fig.4 160-Gb/s DPSK transmitter andreceiver systems

Fig.3 Time variation in wavelengthdependence of DGD characteris-tics

Fig.5 DGD dependence of signal quality(Q-factor)

34 Journal of the National Institute of Information and Communications Technology Vol.52 Nos.3/4 2005

allows us to obtain 2-bit/symbol multilevelmodulation by the superposition of Differen-tial Phase-Shift keying (DPSK) modulationonto Amplitude Shift Keying (ASK) modula-tion. As shown in Fig.6, the ASK and DPSKcomponents, each featuring a bit rate of 80Gb/s, produce 160-Gb/s APSK modulationsignals. In this case, the pulse interval is 12.5ps, which is twice the value of normal 160-Gb/s optical pulses and equivalent to that ofthe 80-Gb/s pulses; resistance to PMD fluctua-tion is therefore expected to be higher. Thus,using the JGNⅡ optical testbed (betweenOotemachi and Tsukuba), which is subject tosevere PMD conditions, we evaluated the sup-pression of degradation in receiver sensitivitydue to PMD fluctuation, adopting 160-Gb/sAPSK modulation without adaptive PMDcompensators.

The transmitter applies 10-Gb/s ASK(with the extinction ratio degraded to approx.6 dB) to the 10-GHz pulse train from theMode-Locked Laser Diode (MLLD; wave-length: 1,558 nm, pulse width: 2.2 ps), super-poses the 10-Gb/s ASK on the 10-Gb/s DPSKto generate 20-Gb/s APSK optical signals, and

transmits the 160-Gb/s APSK optical signals(consisting of eight tributary channels) usingan Optical-Time-Domain-Multiplexing Multi-plexer (OTDM-MUX). The receiver receivesboth the 10-Gb/s ASK and PSK componentsusing two-stage EAM, and the Bit Error Rate(BER) is evaluated. Figure 8 shows the BERcharacteristics of the ASK (white and blackcircles) and DPSK (white and black squares)components of the 160-Gb/s APSK signalsbefore and after transmission with the 160-Gb/s RZ-ASK (white and black triangles).Here, the white symbols represent back toback conditions and the black symbols after200 km transmission. At a BER of 10-7, thereceiver sensitivity penalty before and aftertransmission is 8 dB or more for the ordinaryRZ-ASK 160-Gb/s optical signals, while thepenalty for both components of the 160-Gb/sAPSK optical signals is 4 – 5 dB or less. Theseresults indicate suppression of the PMD penal-ty by using the 2-bit/symbol multilevel modu-lation format.

Fig.6 Avoiding pulse overlap due to PMDby multilevel modulation

Fig.8 Bit Error Rate characteristics of 160-Gb/s APSK

Fig.7 160-Gb/s APSK transmitter andreceiver system

35MIYAZAKI Tetsuya

3.3 160-Gb/s×8-wavelength divisionmultiplexing-1.28-terabit DPSKtransmission［8］

Figure 9 shows the transmitter system for160-Gb/s×8-WDM transmission. First, out-puts of the eight CW light sources, which arespaced at intervals in the 300-GHz wavelengthband, are wavelength-division multiplexed byan Arrayed-Waveguide-Grating (AWG); then,40-GHz, 3.9-ps optical pulses are generatedusing a two-stage cascade LiNbO3 modulatorunder push-pull operation, with 40-GHz sinu-soidal signals for the first stage and 20-GHzsinusoidal signals for the second stage. Thetransmitter then applies DPSK modulation toall wavelengths together, in the same manneras the transmitter in Fig.4, and transmits the160-Gb/s×8-WDM DPSK-RZ optical signals.The receiver system is a DPSK receiver sys-tem featuring the same configuration as thatshown in Fig.4, after selecting the receivingwavelength channel for the receiver evaluationusing an optical tunable filter with a transmis-sion bandwidth of 2 nm. The output of eachchannel of the repeater in the transmissionroute is set to 6.5 dBm for each span. Figure10 shows the DGD dependence of the Q-fac-tor calculated from the measured BER foreach channel. Here, the input polarization sta-tus is manually adjusted on the transmitter toprovide the worst BER in measurement. Addi-tionally, for 8-WDM measurement, the auto-matic PMD compensator used in the single-wavelength transmission experimentsdescribed in 3.1 is introduced in the system.By using the compensator, Q-factors for theworst channel was 14.4 dB or more even forlarge DGD values near 2 ps. These results

demonstrate that stable transmission at 10-13 orless is possible using standard in-band For-ward Error Correction (FEC).

4 Summary

To test the fundamental technologyrequired for efficient photonic networks,NICT conducted a number of experiments incollaboration with KDDI R&D Laboratories,using the JGNⅡoptical testbed. These experi-ments included ultrafast single-wavelengthoptical transmission at a bit rate of 160 Gb/sper wavelength, 2 bits/pulse multi-level 160-Gb/s optical transmission, and WDM trans-mission at a total capacity of 1.28 terabits persecond using 8-wave WDM at a transmissionspeed of 160 Gb/s. In the course of theseexperiments NICT succeeded in the world’sfirst stable inter-city optical transmission, cov-ering a total 200 km. These results demon-strate that the JGNⅡ optical testbed can beused for research and development on terabit-class optical network construction technology,as has been anticipated since initial testbedoperations. JGNⅡwill thus see continuous usein collaborative joint projects with privatecompanies, leading the way in cutting-edgeresearch and development in the field.

Fig.9 160-Gb/s x 8-WDM transmitter sys-tem

Fig.10 DGD dependence of Q-factor foreach channel after 200-km trans-mission

36 Journal of the National Institute of Information and Communications Technology Vol.52 Nos.3/4 2005

Acknowledgments

We would like to express our sincere grati-tude to Researcher Masahiro Daikoku, SeniorResearcher Itsuro Morita, Group LeaderHideaki Tanaka, and Executive OfficerMasatoshi Suzuki of KDDI R&D Laboratoriesfor their help and collaboration in the field

demonstration. We would also like to thankSenior Researcher Yukiyoshi Kamio andSenior Researcher Yoshinari Awaji (now aCabinet Office officer) of the NICT UltrafastPhotonic Network Group for their coopera-tion, as well as Director Fumito Kubota andExecutive Director Yuichi Matsushima fortheir valuable advice.

References01 R.Ludwig, U.Feiste, C.Schmidt, C.Schubert, J.Berger, E.Hillinger, M.Kroh, T.Yamamoto, C.M.Weinert,

and H.G.Weber, “Enabling Transmission at 160 Gbit/s”, Optical Fiber Communication Conference (OFC

'02), TuA1, 1-2, Anaheim, USA, 2002.

02 H.Murai, M.Kagawa, H.Tsuji, and Kozo Fujii, “Single Channel 160 Gbit/s Carrier-Suppressed RZ Trans-

mission over 640 km with EA Modulator based OTDM Module”, European Conference on Optical Com-

munications (ECOC '03), Mo3.6.4, Rimini, Itally, 2003.

03 S.Kieckbusch, S.Ferber, H.Rosenfedt, R.Ludwig, C.Boerner, A.Ehrhardt, E.Brinkmeyer, and H.G.Weber,

“Adaptive PMD compensation in 160Gb/s DPSK transmission over installed fiber”, Optical Fiber Com-

munication Conference (OFC '04), Post-Deadline paper, PDP31, Los Angels, USA, 2004.

04 M.Schneiders, S.Vorbeck, R.Leppla, E.Lach, M.Schmidt, S.B.Papernyi, and K.Sanapai, “Field transmis-

sion of 8×170 Gbit/s over high loss SSMF link using third order distributed Raman amplification”, Opti-

cal Fiber Communication Conference 2005, PDP39, 2005.

05 http://www.jgn.nict.go.jp/e/index.html

06 T.Miyazaki, M.Daikoku, I.Morita, T.Otani, Y.Nagao, M.Suzuki, and F.Kubota, “Stable 160-Gb/s DPSK

transmission using a simple PMD compensator on the field photonic network test bed of JGNⅡ”, Opto-

Electronics and Communications Conference (OECC '04), Post-Deadline paper, PD1-3, Yokohama,

Japan, 2004.

07 T.Miyazaki, Y.Awaji, Y.kamio, and F.Kubota, “Field Demonstration of 160-Gb/s OTDM Signals Using

Eight 20-Gb/s 2-bit/symbol Channels over 200Km”, Optical Fiber Communication Conference (OFC

2005), OFF1, Anaheim, USA, 2005.

MIYAZAKI Tetsuya, Dr. Eng.

Group Leader, Ultrafast Photonic Net-work Group, Information and NetworkSystems Department

Ultra-fast Photonic Network