27MURAI Hitoshi

1 Introduction

As a result of the growth of broadbandcommunications through the spread of ADSL(Asymmetric Digital Subscriber Line) andFTTH (Fiber to the Home) technologies, wecan expect to see a rapid increase in the com-munication capacities of backbone optical net-works. Meanwhile, many researchers havebeen conducting studies on DWDM (DenseWavelength Division Multiplexing), whichexpands communication capacity by accom-modating more wavelength channels denselyin a single fiber. Today, 40-Gb/s-basedDWDM optical transmission technology hasentered the stage of practical application andis gaining attention as a next-generation ultra-high-capacity optical transmission system toreplace current commercial 10-Gb/s opticaltransmission systems. At the same time,research and development of 160-Gb/s opticaltransmission systems is gradually spreading,mainly in Japan and Europe, on the view thatthese systems will allow implementation ofhigh-capacity DWDM with a small amount of

wavelength multiplexing and easy wavelengthmanagement. In particular, we have seen therecent introduction of the DPSK (Differential-Phase-Shift-Keying) modulation format,which offers better receiver sensitivity thanthe conventional IM/DD (Intensity-Modula-tion/Direct-Detection) modulation format.Such techniques have significantly improvedthe transmission performance of ultra-high-speed transmission. Following upon thesedevelopments, many field experiments for160-Gb/s transmission have been conducted inlaboratory facilities using installed opticalfiber cables［1］-［4］. One widely used methodfor transmitting and receiving ultra-high-speedsignals at a transmission rate of 160 Gb/s(regardless of the modulation format) is seenin the OTDM (Optical Time Division Multi-plexing) technique. The OTDM technique per-forms time-domain multiplexing and demulti-plexing of two or more optical data sequencesthat have the same wavelength. In principle,this method does not depend on the processingspeeds (normally approximately 100 Gb/s) ofelectronic devices and can generate ultra-high-

3-3 EA Modulator Based OTDM Techniquefor 160 Gb/s Optical Signal Transmission

MURAI Hitoshi

Ultra high-speed signal transmission at a bit rate of 160 Gb/s is one of key technologies toconstruct next generation ultra high-capacity optical network. In the “Research and Developmenton Ultrahigh-speed Backbone Photonic Network Technologies” project, promoted by NICT, wehave developed 160 Gb/s optical multiplexing/demultiplexing techniques with a capability forpractical use. In this report, we describe the overview of the 160 Gb/s OTDM technologiesbased on EA modulators, and we also discuss the applicability of the OTDM techniques to realsystem, reviewing 160 Gb/s field transmission experiment on JGNⅡ optical testbed.

KeywordsOTDM, Ultra high-speed optical transmission, EA modulator, JGNⅡ optical testbed, Field transmission experiment

28 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006

speed signals at a single wavelength, the speeddepending on the extent of multiplexing.Important elemental techniques for OTDMsignal generation include ultra-short opticalpulse generation, which prevents interferencebetween multiplexed optical data; and opticaltime division multiplexing, which bit-inter-leaves individually data-modulated opticalsignals. For signal reception, OTDM alsorequires a clock extraction technique, whichextracts the system clock synchronized to theOTDM signal; and optical demultiplexing,which extracts multiplexed channels withoutcrosstalk. In the “Research and Developmenton Ultrahigh-speed Backbone Photonic Net-work Technologies” project underway atNICT (National Institute of Information andCommunications Technology), we have con-ducted studies to develop a high-performance,practical OTDM transmitter-receiver. In thecourse of this project we have been workingon the development of 40-GHz ultra-shortoptical pulse generation technique and 4×40-Gb/s optical multiplexing/demultiplexingtechnique using EA (electroabsorption) modu-lators offering superior operability and stabili-ty. As a result of these efforts we have suc-ceeded in producing a high-quality, highly sta-ble prototype 160-Gb/s OTDM transmitter-receiver［5］.

In this report, we present an overview of

160-Gb/s optical time division multiplexingand demultiplexing techniques based on theuse of EA (electroabsorption) modulators. Todemonstrate the practicability of the devel-oped 160-Gb/s OTDM transmitter-receiver,we have also conducted a field transmissionexperiments using the JGNⅡ (Japan GigabitNetworkⅡ) optical testbed. Based on theresults of the field experiment, we were ableto identify problems in implementation of the160-Gb/s transmission system.

2 160-Gb/s Optical Time DivisionMultiplexing technique［5］

Figure 1 shows the configuration of the160-Gb/s OTDM transmitter-receiver. Theequipment consists of an ultra-short opticalpulse source and a time division multiplexer,which encodes optical pulse-trains accordingto input electrical data signals and convertsthese into the OTDM signal. The optical pulsesource is required to stably generate uniformoptical pulses at a constant rate of repetition(i.e., with low jitter performance). In order toconduct time division multiplexing withoutcausing inter-symbol-interference, the opticalpulses must be ultra-short, at <3 ps, and a highpulse extinction ratio (>35 dB) is alsorequired. The mode-locked laser diode(MLLD) is a representative optical pulse

Fig.1 Configuration of 160-Gb/s OTDM transmitter

29MURAI Hitoshi

source known to satisfy these conditions［6］［7］.However, as the repetition rate depends on thelaser cavity length, the MLLD needs to bedesigned taking system clock frequency intoaccount. Meanwhile, we have been investigat-ing the external modulation optical pulsesource using EA (electroabsorption) modula-tors, with particular attention to flexibility inoperation. The optical pulse generation withthe external modulation supports arbitraryclock frequencies in principle and can gener-ate extremely stable optical pulses using low-jitter modulation signals. Here, the systemuses two EA modulators connected in series togenerate optical pulses that can be applied tothe 160-Gb/s OTDM signal. The two EAmodulators are driven by the 40-GHz clocksignals with adjusted timing and convert theinput CW (Continuous Wave) beam intoGaussian pulses with a FWHM (Full-Width-Half-Maximum) of approximately 3 ps. The40-GHz optical pulse train is sent to the opti-cal time division multiplexing circuit and isconverted into 160-Gb/s OTDM signal. TheOTDM circuit performs data modulation foreach multiplexed channel individually, thusrequiring multiple optical modulators. A160-Gb/s OTDM circuit with a basic rate of40 Gb/s requires four optical modulators. It isimportant that these modulators be efficientlyinstalled within a compact package. Accord-ingly, we have developed an OTDM circuitbased on a free-space integration technology,which we determined would accommodate theoptical modulators relatively easily. Figure 2shows a general view of the OTDM circuit.The optical modulators adopted are EA modu-lators, which are small and offer high-speedmodulation. The four EA modulators are indi-vidually encapsulated in airtight packages tosecure stability and reliability in modulationand are placed in four deferent spatial pathscomposed of half mirrors and prisms. Themodulators each convert the 40-GHz opticalpulse-trains into 40-Gb/s optical signals. Eachof the four 40-Gb/s optical signals is sequen-tially bit-interleaved to 80 Gb/s and then to160 Gb/s, and two sets of 80-Gb/s signals and

160-Gb/s signals are then output. The OTDMmodule offers delay time accuracy of 6.28 ps±0.01 ps and insertion loss of 18 dB. ThisOTDM module is unique in that the opticalphase difference between adjacent bits (i.e.,the carrier-phase difference) is variable. Theoptical phase difference is an important para-meter that influences the degree of fiber non-linear effects, which are among the factorsleading to degradation of transmitted signals.Generally, an OTDM method that multiplexesfour or more optical signals has difficulty sta-bilizing optical phase between multiplexedsignals propagating different paths. This mod-ule adopts a free-space integration technologyand has suppressed variation in the opticalphase difference between adjacent bits within5˚/˚C (environmental temperature). By chang-ing the driving temperature of the EA modula-tors, which exhibit the thermo-optic effect, itis also possible to set an arbitrary opticalphase difference between adjacent bits. In thisway, it is now possible to apply a superiormodulation format even to 160-Gb/s signals,including CS-RZ (Carrier Suppressed Returnto Zero)［8］, which is effective in reducing sig-nal degradation due to fiber non-linearity. Fig-ure 3 shows the optical spectrum (a) and opti-cal sampling (b) of 160-Gb/s CS-RZ signalsgenerated using the developed transmitter.Each of the four multiplexed 40-Gb/s signalsis a PRBS (Pseudo-Random-Bit-Sequence)signal with a data length of 215－1. The opticalphase difference between adjacent bits of theCS-RZ signals rotates 180˚ every bit, so that

Fig.2 160-Gb/s OTDM module

30 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006

the optical spectrum exhibits a characteristicshape, with suppression of the center carriercomponents. From the shape of the opticalsampling waveform, the uniformity and well-defined eye-opening of the generated 160-Gb/s signals are clear.

This transmitter features an automaticoptical phase control function to generate sta-ble phase control signals even in environmentsof wide shifts in temperature［9］. To detect thephase difference between adjacent bits of theOTDM signals, a 1-bit-delay interferometer isused. This system makes use of the character-istic change in the output level of the 1-bit-delay interferometer according to the phasedifference. Here the phase differences betweenadjacent bits of the 80-Gb/s and 160-Gb/sOTDM module output signals are quantifiedseparately and the driving temperature of theEA modulators in the OTDM module is con-trolled to obtain the desired phase difference.Figure 4 shows the optical signal spectrum ofthe 160-Gb/s CS-RZ signals measured contin-

uously for 12 hours. The environmental tem-perature (i.e., the laboratory temperature)changed between 22˚C and 30˚C during theperiod of the experiment. Nevertheless, thespectral form showed very little change,thanks to the automatic phase control. Here,the fluctuation of the optical phase is 180˚±5˚,which demonstrates the stability of the gener-ated 160-Gb/s CS-RZ signals.

Figure 5 (a) shows the configuration of the1:4-time division demultiplexer. After the160-Gb/s signals are demultiplexed by thedemultiplexing gate with the EA modulatorsdriven at 40 GHz, they are sent to the 40-Gb/sreceiver and detected. The time width of thedemultiplexing gate is approximately 4 ps.The suppression ratio of the adjacent channelsin the 40-Gb/s demultiplexing signal is 15 dBor greater. The 40-GHz clock synchronized tothe input 160-Gb/s signals is extracted usingthe opt-electronic hybrid phase-locked loopcircuit, as shown in Fig. 5 (b)［10］. The input160-Gb/s signals are sampled at a frequencyof 40－∆f GHz by the EA modulators installedin the front of the device. The phase differenceof the local oscillator signal (LO, ∆f =250 MHz) is detected using the beat signal atthe frequency of 4×∆f (= 1 GHz) contained inthe output signal as the reference signal. Theresult of the phase comparison is sent to the40-GHz VCO (Voltage Controlled Oscillator)

Fig.3 Optical signal spectrum (a) andoptical sampling waveform (b) of160-Gb/s OTDM CS-RZ signals

Fig.4 Results of 12-hour continuous mea-surement of CS-RZ signal spectrum

Fig.5 (a) Configuration of 1:4-opticaltime division demultiplexer and (b)configuration of 40-GHz clockextraction unit

31MURAI Hitoshi

as the control signal. By controlling the VCOoutput frequency such that the reference signaland the local oscillator signal have the samephase, the 40-GHz clock synchronized to theinput 160-Gb/s signal is extracted. The RMStime jitter of the recovered 40-GHz clock isapproximately 60 fs. The synchronization cap-ture range is 12 MHz, and the locking range is1.2 MHz. The EA modulators used in clockextraction and the demultiplexing gate aredesigned to respond to the input optical sig-nals in an arbitrary polarization state, so thatthey are polarization-insensitive as receivers.There is a concern that the clock extractionfunction may degenerate due to the way theunit is designed when the signal waveform isdistorted by chromatic dispersion or PMD(Polarization Mode Dispersion). However,laboratory experiments and the field transmis-sion experiment described later have demon-strated that the system operates stably within arange of chromatic dispersion between－5 Ps/nm and 5 Ps/nm and within a range ofDGD (differential group delay) by PMDbetween 0 Ps and 3 Ps. Thus in practice thesystem does not present a problem in terms ofthis concern.

The back-to-back performance of thedeveloped equipment was evaluated in termsof the Q-factor estimated based on the mea-surement of BER (Bit Error Rate) for each ofthe four 40-Gb/s demultiplexing signals. Weconfirmed that the equipment provides a Q-factor of 27 dB or more for all demultiplexingchannels. This value corresponds to a BERvalue of 10－100 or less, attesting to excellentback-to-back performance. Favorable resultswere also obtained in terms of system stabili-ty, an essential factor in practical applications.Figure 6 illustrates stability in back-to-backperformance evaluated based on over fourhours of continuous Q-factor measurement forthe 40-Gb/s demultiplexing signals. Althoughfluctuations in the Q-factor of approximately1.3 dB were noted, due mainly to measure-ment error, the figure reveals an extremely sta-ble Q-factor of 27 dB on average. A similarevaluation in the wavelength range of

1,540 nm to 1,560 nm also shows equivalentback-to-back performance. The device isdesigned to meet the requirements of WDMtransmission flexibly, with a basic rate of160 Gb/s.

3 160-Gb/s field trial on JGNⅡ(Japan Gigabit NetworkⅡ)optical testbed

Due to its wide signal band, (at or over1 nm in a 3-dB band), 160-Gb/s transmissionresponds sensitively even to slight residualdispersion in the optical transmission line,leading to degradation in transmission charac-teristics. Waveform degradation due to non-linear effects also become conspicuous, whichrestricts the input level into the optical fibersand makes it more difficult to maintain a suffi-cient SNR (Signal to Noise Ratio) over a longdistance. We have already experimentally con-firmed that applying the CS-RZ modulationformat to 160-Gb/s signals can reduce degra-dation of transmission performance caused byresidual dispersion and the non-linear opticaleffects of the optical fibers. We conducted are-circulating loop transmission experiment inthe laboratory demonstrating error-free

Fig.6 Results of evaluation of stability in160-Gb/s back-to-back perfor-mance

32 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006

(BER<10－9) transmission over 640 km (8×80 km) without the use of FEC (forward-error-correction)［5］. On the other hand, for ultra-high-speed transmission of up to 160 Gb/s, thedegradation in transmission characteristics dueto PMD of the optical fiber transmission linerepresents a serious problem. Among otherexperiments, a field transmission experimentusing installed optical fiber cables has demon-strated the necessity of applying the PMD com-pensation technique［1］-［4］. Unlike in relativelystable laboratory environments, for installedoptical fibers in actual use, PMD and chromat-ic dispersion varies according to external fac-tors such as weather conditions［1］. For practi-cal applications, the desired transmission char-acteristics must be maintained under circum-stances in which the optical fiber characteris-tics are subject to continuous change. As thefinal achievement of the “Research and Devel-opment on Ultrahigh-speed Backbone Photon-ic Network Technologies” project, whichended in 2005, we performed a field transmis-sion experiment using the JGNⅡoptical test-bed and verified the practicability of thedeveloped 160-Gb/s transmission technique.Figure 7 shows the network configuration ofthe optical testbed and the experimental trans-mission system. The optical testbed consists of10 standard SMFs (Single Mode Fibers)installed between the Keihanna Human Info-Communication Research Center (SeikaTown, Kyoto) and the Dojima Base Station

(Dojima, Osaka). Each transmission line is63.5 km long, offering a maximum total con-nected length of 635 km, with five loop-backpaths. Five sets of optical amplifiers (EDFAs,or Erbium Doped Fiber Amplifiers) areinstalled at Keihanna and Dojima to form a10-span link structure with a repeater spacingof 63.5 km. The transmission loss per span isapproximately 15 dB. The optical amplifierhas a two-stage configuration with a DCF(Dispersion Compensating Fiber) between thestages. Management of settings such as outputsignal level is integrated at the Keihanna facil-ity. Here, the signal input levels into the SMFand the DCF are set at 4.5 dBm and 1 dBm,respectively. The residual dispersion per spanis typically +10 Ps/nm, and compensation fordispersion slope is nearly 100%-compensated.Chromatic dispersion over the entire transmis-sion line is adjusted at 0 Ps/nm, with the addi-tional DCFs installed at input-end and output-end of the transmission line. Slight pre-com-pensation of the dispersion (performed byinstalling a DCF prior to transmission) alsohelps reduce signal degradation attributable tofiber non-linearity. A PMD compensator(PMDC) is placed directly in front of thereceiver. The PMDC features the first-orderPMD compensation scheme consisting of apolarization controller, a tunable DGD genera-tor, and a monitor to detect the degree ofpolarization (DOP). The PMDC compensatesfor PMD by adjusting the polarization of the

Fig.7 Configuration of JGNⅡ (Japan Gigabit Network Ⅱ) optical testbed and the experimentalsystem

33MURAI Hitoshi

input signals and the extent of DGD genera-tion, maximizing the DOP of the output sig-nals. In the experiment, the PMDC was con-trolled manually.

Figure 8 shows the results of Q-factormeasurement performed every 127 km from254 km to 635 km. The Q-factor was evaluat-ed for all tributary channels demultiplexed to40 Gb/s from 160 Gb/s. We obtained goodtransmission characteristics with a Q-factor of16.2 dB (BER<10－10) on average over a trans-mission distance of 508 km, which corre-sponds to the distance between Tokyo andOsaka. The Q-factor was estimated at 15.7 dB(BER approximately 10－9) on average evenafter transmitting over 635 km. Figure 9shows the optical sampling waveforms aftertransmitting the signals over 508 km and over635 km, as observed by an optical samplingoscilloscope following PMD compensation.

Both waveforms indicate clear eye-opening.On the other hand, the transmission character-istics varied with time even at the same trans-mission distance, causing fluctuations in theQ-factor. This is mainly due to the fact that theeffects of PMD on transmission characteristicschange according to the external environment.It has been confirmed that the amount of DGDgeneration due to PMD changes from 3 Ps to7 Ps at maximum at a transmission distance of635 km. Figure 10 shows the results of contin-uous Q-factor measurement at transmissiondistances of 381 km and 508 km. The arrowsindicate the time at which the PMD compen-sator was readjusted in response to Q-factordegradation. In both cases, the PMD compen-sator required readjustment after 10 to 20 min-utes in order to maintain satisfactory transmis-sion characteristics. Nevertheless, the resultsindicate that introducing the automatic PMDcompensation technique will secure the long-term stability required for practical applica-tions. In terms of the accuracy of PMD com-pensation, experiments and simulations haveindicated that the degradation of signal qualityis slight with a DGD of 1 Ps or less. For thesereasons, we have concluded that the adaptivePMD compensation technique, which is tech-nically mature as a 40-Gb/s transmission tech-nology, can meet the necessary requirements.In the same vein, although this experiment didnot lead to the detection of any distinct effects,it has been reported that the effects of higher-

Fig.8 Results of Q-factor measurement ateach transmission distance

Fig.9 Optical sampling waveforms of 160-Gb/s transmission signals

Fig.10 Stability of transmission character-istics

34 Journal of the National Institute of Information and Communications Technology Vol.53 No.2 2006

order PMD are also serious in 160-Gb/s long-distance transmission［3］. We believe thatdesign of the transmission system must takethese possible effects into consideration aswell.

At the same time, in addition to the evalu-ation of the transmission characteristics withPRBS signal, we believe that comprehensivesystem evaluation is required, including anassessment of signal processing for accommo-dating real data in 160-Gb/s optical signals.Accordingly, we have also performed a trans-mission experiment, as part of a compre-hensive field transmission experiment, for160-Gb/s optical signals incorporating high-definition image data, as part of a collabora-tive project with the University of Electro-Communications. This experiment demon-strated stable demodulation of images after254 km of transmission with quality equiva-lent to that of the transmitted images［11］. Thisrepresents the first field transmission experi-ment anywhere for 160-Gb/s optical signalscarrying actual data and is a significant firststep toward practical applications.

4 Conclusions

160-Gb/s ultra-high-speed optical trans-mission is expected to become an importantfundamental technology in the construction ofnext-generation high-capacity optical net-works. Accordingly, we have been developing160-Gb/s optical time division multiplexing/demultiplexing techniques based on EA (elec-troabsorption) modulators, focusing on practi-

cality and operability. In the course of ouractivities, we have developed a free-spaceintegrated OTDM module that supports indi-vidual data modulation for all multiplexed sig-nals and have established a technique for gen-erating stable and high-quality 160-Gb/sOTDM signals while maintaining practicabili-ty. These are regarded as core technologies forgenerating ultra-high-speed OTDM exceedingthe processing speeds of electronic devices.CS-RZ is an example of a superior modulationformat suitable for ultra-high-speed transmis-sion. We have also enabled the introduction ofsuch formats into 160-Gb/s signals, takingadvantage of the stable carrier phase, a charac-teristic of a space-integrated OTDM module.In a field transmission experiment using theJGNⅡoptical testbed connecting the KeihannaHuman Info-Communication Research Center(Seika Town, Kyoto) and the Dojima BaseStation, we have demonstrated satisfactory160-Gb/s ultra-high-speed long-distance trans-mission over 635 km and succeeded for thefirst time anywhere in a 254-km transmissionexperiment with 160-Gb/s signals containinghigh-definition image data.

Minor problems remain, such as the long-term stability of transmission performance.However, along with rapidly progressing opti-cal signal processing techniques—involvingmodulation format, adaptive chromatic disper-sion, PMD compensation, and 3R signalregeneration—we are optimistic that effortstoward practical application of our ultra-high-speed transmission system will soon yieldresults.

35MURAI Hitoshi

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MURAI Hitoshi, Dr. Eng.

Oki Electric Industry Co., Ltd. R&DCenter, NW device laboratory

Ultra High Speed Optical Transmis-sion, Optical Signal Processing