40 Gb/s and 4×40 Gb/s TDM/WDM standard fiber transmission

2276 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 17, NO. 11, NOVEMBER 1999

40 Gb/s and 4 40 Gb/s TDM/WDMStandard Fiber Transmission

C. M. Weinert, R. Ludwig, W. Pieper, H. G. Weber, D. Breuer, K. Petermann, and F. Küppers

Abstract—We investigate the possibilities of 40 and 4�� 40Gb/s time division multiplexing wavelength division multiplexing(TDM/WDM) return-to-zero (RZ) transmission over embeddedstandard single-mode fibers (SMF) at a transmission wavelengthof 1.55 �m both experimentally and theoretically. Dispersionof the SMF is compensated by a dispersion compensating fiber(DCF). Transmission over a span of 150 km of SMF in the single-channel case and of 100 km SMF in the multichannel case arereported. Numerical calculations are employed to investigate thepossibility of cascading the spans both for single-channel andmultichannel transmission. For single-channel transmission, itis shown that optimum performance is achieved with postcom-pensation of the DCF. The input power at the SMF and DCFinput have to be chosen carefully. For four channel transmission,the performance is mainly limited by residual dispersion in theoutermost wavelength channels. It is shown numerically thatimprovement is achieved by employing the newest type DCFwhich also compensates the dispersion slope of the SMF. Fora WDM channel separation of 2 nm no significant additionaldegradation due to cross-phase modulation (XPM) or four-wavemixing is observed.

Index Terms—Fiber transmission, optical communication, op-tical dispersion management, time division multiplexing (TDM)transmission, wavelength division multiplexing (WDM) transmis-sion.

I. INTRODUCTION

NEW challenges to modern telecommunications such as anexpanded internet and broadband distributive and inter-

active services demand for growing transmission capacities.Increasing the bandwidth can be either done by providingmore channels in a wavelength division mutliplexing (WDM)system or by enhancing the bit rate of already existing channelsusing time division multiplexing (TDM) or by a combinationof both. Capacity upgrading by TDM offers some advantagesfor network operators in view of economic efficiency. Thisis due to reduced network management efforts and becausealready installed single-band erbium-doped fiber amplifiers(EDFA’s) do not have to be replaced by broad-band amplifiersas used in the latest generation of WDM systems.

Manuscript received February 16, 1999; revised July 19, 1999.C. M. Weinert, R. Ludwig, W. Pieper, and H. G. Weber are with Heinrich-

Hertz-Institut fur Nachrichtentechnik Berlin GmbH, Berlin D-10587 Germany(e-mail: [email protected]).

D. Breuer and K. Petermann are with the Institut für Hochfrequen-ztechnik, Technische Universitat Berlin, Berlin D10587 Germany (e-mail:[email protected]).

F. Kuppers is with Deutsche Telekom, Technologiezentrum Darmstadt D-64307 Germany (e-mail: [email protected]).

Publisher Item Identifier S 0733-8724(99)08023-8.

Whereas 10 Gb/s TDM systems are already commerciallyavailable, even in WDM configurations with up to 32 channels,40 Gb/s TDM transmission is still subject to research anddevelopment. A lot of work has already been done includingimpressive laboratory demonstrations like 40 Gb/s solitontransmission over 70 000 km in a dispersion shifted fiber(DSF) loop [1], 8 40 Gb/s [2] and 4 40 Gb/s [3]TDM/WDM transmission over standard single-mode fiber(SMF), and 30 40 Gb/s WDM transmission over 85 kmof nonzero dispersion fiber (NZDF) [4]. In this work we willconcentrate on transmission over SMF which is still the basisof most fiber optic networks all over the world.

Fundamental investigations have demonstrated the useful-ness of SMF for single-channel 40 Gb/s transmission experi-mentally [5], also compared to DSF and NZDF [6]. Numerical[7] and theoretical [8] studies gave first ideas about the designof an appropriate passive dispersion management scheme forupgrading the existing SMF fiber basis. Also the choice of theappropriate modulation format [return-to-zero (RZ) instead ofnonreturn-to-zero (NRZ)] has been clarified [9], [10].

The increasing interest of network operators in 40 Gb/sTDM transmission is demonstrated by recent field trials whichhave taken place in Japan (NTT) [11], [12] and Europe (BritishTelecom [13] and Deutsche Telekom [14]). In particular,the field trials of Deutsche Telekom focussed on practicalproblems a network operator will face when high speed opticalsystems are operated on a fiber base which was not intendedfor carrying 40 Gb/s single-channel signals when installedmore than ten years ago. One problem is polarization modedispersion (PMD) but system manufacturers have realizedthis and first solutions like an automatic PMD compensationin 40 Gb/s optical transmission systems are proposed [15].The present state of 40 Gb/s SMF transmission (theory,numerical simulation, laboratory experiments and field trialsfor single-channel multispan transmission and first laboratoryexperiments for multichannel single-span transmission) let itappear advisable to do the next step forward toward mul-tichannel multispan transmission which will be investigatedhere based on our previous work. The focus will be onchromatic dispersion management schemes taking into accountthe newest types of dispersion compensating fibers.

II. THEORY

Neglecting effects of polarization and scattering effectslike stimulated Raman scattering and stimulated Brillouinscattering, propagation of optical pulses in fibers is described

0733–8724/99$10.00 1999 IEEE

WEINERT et al.: 40 Gb/s AND 4 40 Gb/s TDM/WDM STANDARD FIBER TRANSMISSION 2277

by the scalar nonlinear Schrödinger’s equation (NLSE) for thecomplex pulse envelope A [16], [17]

(1)

with

(2)

and denotes the transformation to a frame ofreference moving with the group velocity .

The first two terms on the right-hand side of (1) describechromatic dispersion. The dispersion parametersandresult from expansion of ( ) around the center frequency

and describe dispersion effects up to third order. Fiberdispersion is usually given by the dispersion and thedispersion slope . For high bitrate transmissionchromatic dispersion is one of the main limiting factorsbecause dispersion induced pulse broadening decreases thesignal to noise ratio. The increase in pulse width can beestimated from the analytical expression of broadening ofisolated Gaussian pulses which is also a good approximationfor sech2 pulses [16]. Since chromatic dispersion of the SMFis large at 1.55 m ( ps(nmkm) 1) it is necessary tocompensate dispersion. As discussed in Section I we will treatcompensation schemes with DCF’s. The DCF has a negative

and can therefore compensate the dispersion of the SMF.However, the particular DCF used in the transmission exper-iments (hereafter denoted DCF1) can only partly compensatethe slope of dispersion . This means that zero dispersioncan be achieved for one wavelength channel only whereas atother wavelength channels a residual dispersion remains. Thisresidual dispersion severely limits the bandwidth of WDMtransmission. Fiber loss is described by(given in dB/km) inthe third term on the right hand side of (1).

The last term on the right hand-side of (1) describes fibernonlinearity. It is proportional to the pulse intensity., asdefined in (2), is the nonlinear coefficient related to thenonlinear refractive index , the effective fiber core area ,and the velocity of light . The nonlinear effects includedin the NLSE are self-phase modulation (SPM), cross-phasemodulation (XPM), and four-wave mixing (FWM). Singlechannel transmission is affected by SPM only whereas inmultichannel transmission the combined effects of SPM, XPM,and FWM lead to signal degradation. Since SPM affects theoptical wave via its interaction with chromatic dispersionpartial cancellation of the nonlinear fiber degradation canbe achieved by using appropriate dispersion schemes [18].FWM, on the other hand, is expected to be negligible becausethe large dispersion value in each span prevents the phasematching condition which is necessary for FWM to becomeeffective [17].

The NLSE will be solved numerically using the well knownsplit-step fast Fourier transform (FFT) algorithm [16]. Simu-lations were done for a PRBS of length using sech2

pulses of 4 ps FWHM. The amplifier noise was modeled aswhite noise created by a Gaussian random generator and addedto the optical field amplitude at the output of each amplifier.

TABLE IPARAMETERS FOR THE SMF AND DCF USED FOR

NUMERICAL SIMULATION OF THE EXPERIMENTS

SMF DCF1 DCF2Fiber attenuation (dB/km) 0.22 0.5 0.5Dispersion D @ 1.551�m (ps/(km nm)) 16.4 –90.7 –100

Dispersion slope S @ 1.551�m (ps/(km nm2)) 0.06 –0.23 –0.34

Nonlinear refractive indexn2(10�20m2/W) 2.6 2.6 2.6

Effective core area (�m2) 80 30 30

The receiver was modeled by an optical filter with a measuredbandwidth of 125 GHz for single-channel transmission and of87.5 GHz for multichannel transmission. The photodiode ismodeled as a square law detector followed by an electricallow pass filter.

In Table I, we list the parameters for the fiber span includingtwo different DCF’s. DCF1 was used in the experiment. DCF2is a new fiber which became available very recently. Thereforeit was only used in the numerical simulation. DCF2 nearlyperfectly compensates both and .

The calculated quantities which we compare with experi-ment, are the pulse width of the RZ pulses and the eye closurepenalty. The pulse width is determined by averaging over theindividual pulse widths of the bit sequence. The eye closurepenalty is evaluated from the eye closure at the receiver forthe complete transmission path as compared to back-to-backeye closure.

III. EXPERIMENTAL

A. Single-Channel 40 Gb/s Transmission

The schematic of the experimental set-up is depicted inFig. 1. The data transmitter comprises a tunable mode-lockedlaser, operating at 10 GHz repetition rate [19]. The pulsetrain is intensity modulated with a pseudorandom bit sequence(PRBS) of length or using an external modulator.The 10 Gb/s optical data signal is sequentially bit interleavedby a fiber delay-line multiplexer. In the multiplexer, the bitsequences were shifted against each other bybit periods ( = 2, 4) in order to ensure a PRBS,40 Gb/s single-polarization (no polarization multiplexing) datasignal. The pulsewidth of the transform-limited pulses is4 ps with a sech2 pulse shape. The 40 Gb/s signal is thentransmitted over 150 km SMF. The overall link dispersionis compensated to zero for the signal wavelength of

nm by using 27 km of DCF1 (see Table I). The DCFis placed either at the transmitter (precompensation) or atthe receiver (postcompensation). The optical power launchedinto the DCF is always low (<5 dBm) to ensure operation inthe linear transmission regime. At the receiver a SLALOM-based configuration is used as an optical demultiplexer [20].Fig. 2 shows measured bit error rates for two word lengthsand for the two different compensation schemes. An error-free transmission (BER = 109) was achieved with receiversensitivities of 27 dBm for PRBS . For PRBSthere was an additional penalty of 1 dB for both compensationschemes which we attributed to the system electronics. Themeasurements of the four TDM channels showed identical


Fig. 1. Experimental setup for transmission of one WDM channel.

(a)

(b)

Fig. 2. Measured bit error rates for the transmission of one WDM channel.(a) Precompensation and (b) postcompensation.

results. Therefore, in Fig. 2 only one channel is depicted.The receiver sensitivity in Fig. 3 refers to 40 Gb/s. Thereceived power was measured at the end of the transmissionline before the power was coupled either into the opticaldemultiplexer in the precompensation scheme or into theinput of the postcompensation scheme. In the precompensationscheme the launched signal power into the SMF was 11 dBm.In the postcompensation scheme a signal power of 14 dBm

Fig. 3. Bit error measurements using the postcompensation scheme in theone channel experiment.

was launched into the fiber. In the following we describemeasurements with PRBS = only in order to comparethese results with calculations in the subsequent sections.Fig. 3 shows bit error ratio measurements with various opticalinput powers into the 150 km SMF in the postcompensationscheme. Similar measurements were also performed in theprecompensation scheme. From these results we evaluated thesystem penalty versus the input power, which is discussed andcompared with theoretical results in Section IV.

B. 4 40 Gb/s TDM/WDM Transmission

The experimental setup is shown in Fig. 4. The four WDMchannels to were generated by two modelocked semi-conductor lasers (FWHM 1.3 ps) and with the use ofspectral slicing technique based on an arrayed waveguidegrating (AWG). The AWG has a channel spacing of 2 nmand a FWHM of 0.9 nm. Using this technique, optical pulses(FWHM 4.0 ps) at four different wavelengths towith a wavelength spacing of 2 nm were obtained. The fourpulse trains were coupled together into one intensity modulator(10 Gb/s, PRBS and ). Each of the four10 Gb/s data signals was then multiplexed four times by thesame fiber delay-line multiplexer as described in Section III-A.


Fig. 4. Experimental setup for transmission of four WDM channels.

Fig. 5. Optical spectra of the four WDM channels each with 40 Gb/s. Also,the measured pulse widths at the input of the demultiplexer are shown.

Finally, we obtained four 40 Gb/s OTDM single-polarizationWDM channels at wavelengths to . All four WDMchannels carry the same data pattern. Without dispersion, theinterchannel interference would be maximum because the datathen travel synchronously. The large local dispersion causes awalk-off between the pulses of different wavelength channelsand thus the interchannel interaction is reduced and averagedout. We therefore expect no change when using nonidenticalmodulation in the WDM channels.

The 4 40 Gb/s data signal was then transmitted over 100km of SMF. The dispersion compensating fiber (17.5 km ofDCF1) was placed at the receiver. Behind the DCF a tunableoptical filter (FWHM 2 nm) was used to select one ofthe four 40 Gb/s WDM channels. Note, that no individualdispersion compensation of the WDM channels was applied.Fig. 5 shows the optical spectra of all four WDM channelsat the output of the transmission line. The width (FWHM) ofthe optical pulses at the input of the demultiplexer (SLALOM)varied between 4.5 and 8 ps depending on how close a channelwas with respect to the optimum wavelength for dispersioncompensation. This optimum wavelength was chosen to beat about 1551 nm. A comparison of the measured pulse

Fig. 6. Measured bit error rates for the four WDM channels.

width with the calculated pulse broadening over the fiberspan verified the total fiber dispersion and its slope. Theselected channel was then demultiplexed in the time domain asalready described in Fig. 1. The SLALOM demultiplexer hasa small polarization dependence which may lead to reducedcontrast ratio for separation of the different TDM channels.Therefore, for each WDM channel, polarization was adjustedfor minimum BER before the BER curve was measuredmeasurement. Fig. 6 shows the BER-measurements on each ofthe four WDM channels. The measurements of the four TDMchannels showed identical results. Therefore, in Fig. 6 onlyone channel is depicted. The received power was measuredat the output of the 100 km transmission line. The datapresented were taken for a PRBS of length to allowfor a comparison with simulations. However, the performanceexhibited small dependence (1 dB) for pattern lengths upto similar to the results in Fig. 2. From the errorratio performance of the system, both before and after thetransmission, we can see that the transmission penalty (BER

10 9) is about 3 dB. This penalty was attributed to thedependence of the demultiplexer on the pulse width. Thedemultiplexer had an optimum performance for pulse widthless than 4 ps. Similar to the investigations in Section III-A,the penalty was investigated versus the optical power at theinput of the SMF. As compared to single-channel transmissionno additional penalty due to nonlinear effects was obtainedfor four channel transmission with a total fiber coupled inputpower up to 20 dBm.

IV. NUMERICAL SIMULATION AND DISCUSSION

A. Single-Channel Single-Span Transmission

For the theoretical analysis the setup depicted in Fig. 1was assumed. According to Fig. 1 the 40 Gb/s signal wastransmitted over 150 km SMF. At the transmission wavelengthof 1548 nm the accumulated SMF dispersion was completelycompensated by the 27 km of DCF1 (see Table I) whichwas either placed at the receiver (postcompensation) or at thetransmitter (precompensation). The input pulse widthof thesech2 pulses was 4.3 ps. In the numerical calculations a PRBSpattern of length was chosen to allow a comparison be-


Fig. 7. System penalty for 150-km SMF transmission against average fiber(SMF) input power for pre- and postcompensation scheme.

tween the experimental and theoretical results. As shown in theexperimental part it was verified that a pattern length ofdid not cause a significant change. Both compensation schemespre- and postcompensation were investigated. Fig. 7 showsthe penalty against the average fiber input power for the twodifferent compensation schemes assuming complete dispersioncompensation. For the experimental data the penalty wasextracted from the BER curve for postcompensation as shownin Fig. 3 with respect to back-to-back for a BER of 109.A similar BER extraction was used for precompensation. Asshown in Fig. 7 a good agreement between experimental andtheoretical data is achieved. An EDFA noise figure of 6 dB wasassumed. At low fiber input powers, the system performanceis hampered due to a low signal-to-noise ratio (SNR) and athigh input powers, the system performance is degraded dueto the increasing impact of nonlinear self-phase modulation.In the postcompensation scheme the system penalty increasesstrongly for fiber input powers exceeding 16 dBm, whereasin the precompensation scheme the penalty increases alreadyat power levels exceeding 13 dBm. In the linear regime forlow fiber input powers there is no difference between pre- andpostcompensation.

Fig. 8 shows the eye-diagrams for both compensationschemes after 150-km SMF transmission at an average fiberinput power of 16 dBm. To show the principle difference ofthe two compensation schemes and to avoid burdening theinterpretation by amplifier noise, we used the eye-diagramsof the theoretical study neglecting the amplifier noise. Theeye-diagrams in Fig. 8 show that signal distortions in theprecompensation scheme arise mainly due to strong bit-pattern dependent variations of the pulse peak power. Inthe postcompensation scheme, however, the signal distortionsat 16 dBm are significantly lower and a penalty of about2 dB is achieved. A mixed compensation scheme with 1/3precompensation and 2/3 postcompensation and vice versawas also investigated. No improvement of a split compensationscheme was achieved in this case. Since for NZDF a significantdifference in the spectra for pre- and postcompensation hasalready been observed [21] we also monitored the spectrumafter 150 km SMF transmission for both transmission schemes.However, in contrast to NZDF the spectra were almost

(a)

(b)

Fig. 8. Theoretical eye-diagrams after 150 km SMF fiber at 16 dBm for pre-and postcompensation. (a) Precompensation and (b) postcompensation.

identical. We attribute this to the high local dispersion of theSMF. Due to the high chromatic dispersion of the SMF a largephase mismatch between the different frequency componentsoccurs. This leads to a reduced influence of SPM in the SMFcompared to the influence of SPM in the NZDF.

The system behavior in the precompensation scheme maybe explained as follows: in the precompensation scheme thedata signal is at first transmitted over the DCF. Due to thereduced power in this fiber the signal is mainly affected bychromatic dispersion. This causes severe pattern dependentdispersive waveform distortions. If this signal is now launchedinto the SMF the nonlinearity in this fiber causes strong signaldistortions of the already perturbed pattern. This leads to largevariations in the peak power at the end of the transmissionline. Simulation showed that these distortions are not causedby higher order dispersion ().

To investigate the optimum compensation ratio and the dis-persion tolerance for 40 Gb/s RZ single-channel transmissionFig. 9 shows the penalty versus the residual link dispersion forSMF transmission for the postcompensation scheme for twodifferent input powers. For both fiber input powers 10 and 16dBm the optimal system performance occurred for completedispersion compensation. The penalty shows a symmetricalbehavior around zero average dispersion like in a lineartransmission scheme indicating that at high power levels nooptimization due to under-compensation is feasible. A similar


Fig. 9. Penalty against residual dispersion for 150 km SMF transmission forthe postcompensation scheme for different fiber input powers.

Fig. 10. Investigated compensation schemes for cascaded span transmission.

behavior has been reported for RZ transmission at 10 Gb/s [9].The dispersion tolerance for 1 dB penalty at an input power of10 dBm is about 15 ps/nm corresponding to a SMF lengthof about 1 km.

B. Single-Channel Cascaded Span Transmission

To investigate the potential of cascading single-channel40 Gb/s transmission over multiple spans we performed nu-merical calculations using different dispersion compensationschemes. In this study we considered a postcompensation, asymmetrical compensation, and an alternating compensationscheme as depicted in Fig. 10. Particularly for 10 Gb/s RZtransmission the symmetrical and alternating schemes showedsuperior performance compared to pure postcompensation[22]. Precompensation was not considered, since already insingle-span transmission it was less effective than postcom-pensation. The amplifier spacing was reduced to 100 km. In allcompensation schemes DCF1 was considered to be operated inthe linear regime. Fig. 11 shows the calculated penalty against

Fig. 11. Penalty after 300-km SMF transmission against fiber input powerfor post-, symmetrical, and alternating compensation scheme.

the average fiber input power for the three compensationschemes for three spans corresponding to 300 km SMF. Forlow input power all schemes show almost identical behavior.The performance is limited by the amplified spontaneousemission noise. For higher input powers there is only a differ-ence of about 0.5 dB between the post- and the alternating-compensation scheme. The penalty is about 1.3 to 1.8 dB for afiber input power of 12 dBm. In the symmetrical compensationscheme, however, the penalty increases significantly at powerlevels exceeding 9 dBm. The eye closes due to variation of thepeak power like in the pure precompensation scheme in single-span transmission. We attribute the superior performance of thepost- and alternating compensation scheme to the fact that inboth compensation schemes the first fiber part is of SMF fiber(like in pure postcompensation), whereas in the symmetricalscheme the first fiber part is of DCF type (precompensation).

C. Multichannel Cascaded Span 40 Gb/s Transmission

We first show the calculated results for the 440 Gb/stransmission over 100 km of SMF with postcompensation by17.5 km SMF [3]. In order to compare the measured resultswith experiment, we first look at the pulse broadening in thefour channels at 1547, 1549, 1551, and 1553 nm which will belabeled channels 1, 2, 3, and 4, respectively. For the averageSMF input power of 10 mW and using the values for the fibernonlinearity and for the dispersion (DCF1) as given in Table Ithe calculated values of the pulse width (FWHM) are 8.5, 6.2,4.5, and 6.2 ps, respectively, which compare very well withthe measured pulse widths given in Fig. 6. For the calculationthe dispersion zero was placed at 1551 nm. The eye-diagramsof the four channels are shown in Fig. 12. Because of thedispersion zero at 1551 nm channels 2 and 4 are very similarwhereas channel 1 shows the largest eye closure penalty due tothe large pulse broadening. The pulse broadening of the fourchannels are mainly due to residual dispersion. This can beshown by a simple estimate of pulse broadening of channel 1which is 4nm away from the dispersion zero. Using the valuesgiven in Table I the sum of the residual dispersion in channel1 amounts to ps/nm. Using the well known pulsebroadening formula [16], the pulse broadens from


Fig. 12. Calculated eye-diagrams for channel 1 to 4 of the 4� 40 Gb/ssingle-span transmission. The FWHM of the pulses averaged over the2

7� 1

bits is given as parameter.

to 8.8 ps at the end of the fiber span which is close to themeasured pulse width.

The effects of FWM were tested numerically by launchingpower into three channels only and monitoring the effect ofFWM in the fourth channel. The simulations showed thatno noticable FWM products appear in the fourth channel.This was expected because of the phase mismatch due to thedispersion map and because of large channel spacing.

We also investigated possible signal degradation due toXPM numerically by comparing the eye-diagram of channel2 of the 4 40 Gb/s system with the eye-diagram of asingle channel at the same wavelength. We chose identicalbit patterns and parallel polarization for all channels. The largelocal dispersion leads to a walk-off between pulses in differentchannels. Consequently we see no difference in XPM crosstalkif the bit strings are delayed or changed between the channels.Comparison of the average pulse width and the eye closurepenalty showed no difference between the four channel andthe single-channel case.

In order to test the cascadability of 440 Gb/s transmissionwe perform numerical simulations of the repeated span with100 km SMF postcompensated by 17.5 km DCF1. However,we now minimize residual dispersion in the outer wavelengthchannels by choosing the dispersion zero at 1549 nm and thefour channels at 1546, 1548, 1550, and 1552 nm. In Fig. 13,we depict the eye-diagrams for the inner channel at 1548 nmand the outer channel at 1546 nm for one and three cascadedspans. The eye-diagrams of the channels at 1550 and 1552 nmare not shown since they are essentially the same as the onesat 1548 and 1546 nm, respectively. This reflects the symmetrywith respect to the dispersion zero.

From Fig. 13, it is seen that the outer channel 1 at 1546 nmshows the largest degradation. This degradation of the eyecomes from pulse broadening which limits transmission totwo or maximum three cascades. As seen from comparison

(a)

(b)

Fig. 13. Calculated eye-diagrams for the 4� 40 Gb/s transmission spancascaded one to four times: (a) for channel 1 at 1546 nm and (b) for channel2 at 1548 nm.

Fig. 14. Calculated eye closure penalty versus SMF input power for channel1 (1546 nm) after three cascaded spans solid: line—compensation with DCF1used in the single-span experiment, dashed line—compensation with DCF2,and dotted line—compensation with DCF1 plus individual channel dispersioncompensation at the receiver.

with the inner channel, this broadening is caused by theresidual dispersion due to the imperfect compensation of thedispersion slope. In order to verify this, we show in Fig. 14the calculated eye closure penalties for the outer channel 1after 3 cascades with the experimentally used DCF1 (solidline) and for compensation with DCF2 (dashed line) whichnearly completely compensates bothand . It is found thatboth curves are similar in shape with a minimum in the rangebetween 3–10 dBm per channel. However, the curve for theexperimental DCF1 is shifted by about 3dB to higher penaltyvalues compared to compensation by DCF2.

It is also interesting to compare perfect dispersion com-pensation of DCF2 with the combined effects of incomplete


dispersion compensation with DCF1 plus individual dispersioncompensation for each wavelength channel at the receiver.In Fig. 14 the dotted line depicts the calculated penalty ofchannel 1 after 3 cascaded spans of compensation with DCF1plus compensation of the residual dispersion of this channelafter the wavelength filter (e.g., by a suitable fiber grating).In the regime of low input power (linear behavior), bothcurves coincide whereas for an input power larger than 6dBm the receiver compensation exhibits a slightly largerpenalty. Note, however, that for three cascaded spans com-pensation of residual dispersion at the receiver is almost asgood as perfect dispersion compensation with DCF2 in eachspan.

As for the single-span transmission we investigated theeffects of XPM and FWM. There are found minor pulse broad-ening effects due to XPM whereas FWM remains negligible.

We also numerically investigated the effects of incompletedispersion compensation by slight reduction of the DCF length(undercompensation). The channels which have to be im-proved are the outermost wavelength channels. Shorteningthe DCF in general improves the low wavelength channelbut degrades the high wavelength channel. Therefore, nonet improvement is achieved for multiwavelength channeltransmission by undercompensation with the DCF.

V. CONCLUSION

In conclusion, we reported recent achievements in fiberoptic 40 Gb/s TDM/WDM transmission. Theory and resultsof numerical and experimental investigations were presentedand discussed starting with a single-channel single-span (150km SMF) configuration for which pre- and postcompensationschemes were compared with the result that postcompensationallows for higher input powers. The number of channels wasincreased to four with a channel spacing of 2 nm. For 100km SMF no additional penalty compared to single-channeltransmission could be observed. For single-channel multispan(3 100 km SMF) transmission the different compensa-tion schemes showed almost identical behavior at low signalpower levels. For higher power levels post- and alternatingcompensation schemes showed superior system performancewhereas pre- and symmetrical compensation suffers from thehigh nonlinear distortions in the DCF. Finally we investigatednumerically a 4 40 Gb/s WDM/TDM transmission over3 100 km SMF and found that system behavior is domi-nated by the residual chromatic dispersion of the individualWDM channels. Because of the residual chromatic disper-sion in the outer channels undercompensation schemes whichare advantageous for single-channel transmission fail for themultichannel transmission. Nonlinear channel interaction likeXPM and FWM were of minor importance which is due tothe high local dispersion of a dispersion compensated SMFtransmission line. Using the newest type of DCF which offersan appropriate dispersion slope to compensate for chromaticdispersion exactly over a broad wavelength range, every singlechannel of the WDM system behaves like a single-channelsystem with exact compensation. Similar good results forindividual channel compensation at the end of the cascaded

fiber span are predicted by numerical simulation . The resultscan be used to establish engineering rules and design toolsfor upgrading existing SMF based networks toward highercapacity.

ACKNOWLEDGMENT

The authors would like to thank Deutsche Telekom AGand the Bundesministerium fur Bildung und Forschung forsupport of the work. The authors would also like to thank Lu-cent Technologies Denmark A/S for providing the dispersioncompensating fiber (DCF2).

REFERENCES

[1] K. Suzuki, H. Kubota, A. Sahara, and M. Nakazawa, “40 Gbit/ssingle channel optical soliton transmission over 70 000 km using in-line modulation and optical filtering,”Electron. Lett.,vol. 34, no. 1, pp.98–99, Jan. 1998.

[2] D. Garthe, R. A. Saunders, W. S. Lee, and A. Hadjifotiou, “Simulta-neous transmission of eight 40 Gbit/s channels over standard singlemode fiber,” in Proc. Optic. Fiber Commun.’97,Dallas, TX, 1997,postdeadline paper PD 20.

[3] W. Pieper, R. Ludwig, C. M. Weinert, B. Kuhlow, G. Przyrembel,M. Ferstl, E. Pawlowski, and H. G. Weber, “4-channel� 40 Gb/sunrepeatered OTDM transmission over 100k-m standard fiber,”IEEEPhoton. Technol. Lett.,vol. 10, pp. 451–453, 1998.

[4] C. D. Chen, I. Kim, O. Mizuhara, T. V. Nguyen, K. Ogawa, R. E.Tench, L. D. Tzeng, and P. D. Yeates, “1.2Tbit/s (30� 40 Gbit/s)WDM transmission over 85km fiber,”Electron. Lett.,vol. 34, no. 10,pp. 1002–1004, May 1998.

[5] R. Ludwig, W. Pieper, H. G. Weber, D. Breuer, K. Petermann, F.Kuppers, and A. Mattheus, “Unrepeatered 40-Gbit/s RZ single channeltransmission over 150 km of standard fiber at 1.55�m,” in Proc. Optic.Fiber Commun.’97, Tech. Dig.,1997, pp. 245–246.

[6] D. Breuer, H. J. Ehrke, F. Kuppers, R. Ludwig, K. Petermann, H.G. Weber, and K. Weich, “Unrepeatered 40-Gbit/s RZ single channeltransmission 1.55�m using various fiber types,” inProc. Optic. FiberCommun.’98, Tech. Dig.,1998, pp. 115–116.

[7] D. Breuer, K. Jurgensen, F. Kuppers, A. Mattheus, I. Gabitov, and S.K. Turitsyn, “Optimal schemes for dispersion compensation of standardmonomode fiber based links,”Opt. Commun.,vol. 140, pp. 15–18, July1997.

[8] S. K. Turitsyn, V. K. Mezentsey, and E. G. Shapiro, “Dispersion-managed solitons and optimization of the dispersion management,”Optic. Fiber Technol.,vol. 4, pp. 384–452, 1998.

[9] C. Caspar, H.-M. Foisel, A. Gladisch, N. Hanik, F. Kuppers, R. Ludwig,A. Mattheus, W. Pieper, B. Strebel, and H. G. Weber, “10Gbit/s NRZ/RZtransmission over 2000 km standard fiber with more than 100 kmamplifier spacing,” inProc. ECOC’97,1997, vol. 1, pp. 87–89.

[10] D. Breuer and K. Petermann, “Comparison of NRZ- and RZ-modulationformat for 40-Gb/s TDM standard-fiber systems,”IEEE Photon. Technol.Lett., vol. 9, pp. 398–400, Mar. 1997.

[11] A. Sahara, K. Suzuki, H. Kubota, T. Komukai, E. Yamada, T. Imai,K. Tamura, and M. Nakazawa, “Single channel 40 Gbit/s solitontransmission field experiment over 1000 km in Tokyo metropolitonoptical loop network using dispersion compensation,”Electron. Lett.,vol. 34, no. 22, pp. 2154–2155, Oct. 1998.

[12] K. Suzuki, H. Kubota, T. Komukai, E. Yamada, T. Imai, K. Tamura,A. Sahara, and M. Nakazawa, “40 Gbit/s soliton transmission fieldexperiment over 1360 km using in-line soliton control,”Electron. Lett.,vol. 34, no. 22, pp. 2143–2145, Oct. 1998.

[13] D. Nessetet al., “40 Gbit/s transmission over 186.6 km of installedfiber using mid-span spectral inversion for dispersion compensation,” inProc. OFC’99, Tech. Dig.,1999, ThI3.

[14] W. Weiershausen, H. Schöll, F. Kuppers, R. Leppla, B. Hein, H.Burkhard, E. Lach, and G. Veith, “40 Gb/s field test on an installedfiber link with high PMD and investigation of differential group delayimpact on the transmission performance,” inProc. OFC’99, Tech. Dig.,1999, ThI5.

[15] D. Sandel, M. Yoshida-Dierolf, R. Noe, A. Schopflin, E. Gottwald, andG. Fischer, “Automatic polarization mode dispersion compensation in40 Gbit/s optical transmission system,”Electron. Lett.,vol. 34, no. 23,pp. 2258–2259, Nov. 1998.


[16] G. P. Agrawal, Nonlinear Fiber Optics,2nd ed. San Diego, CA:Academic, 1995.

[17] D. Marcuse, A. R. Chraplyvy, and R. W. Tach, “Effect of fibernonlinearity on long-distance transmission,”J. Lightwave Technol.,vol.9, pp. 121–128, 1991.

[18] N. Hanik, A. Gladisch, and G. Lehr, “An effective method to de-sign transparent optical WDM-networks,” inProc. NOC-98, Technol.Infrastructure,1998, pp. 190–197.

[19] R. Ludwig, S. Diez, A. Ehrhardt, L. Kueller, W. Pieper, and H. G.Weber, “A tunable femtosecond modelocked semiconductor laser forapplications in OTDM-systems,”IEICE Trans. Electron.,vol. E81-C,no. 2, pp. 140–145, 1998.

[20] M. Eiselt, W. Pieper, and H. G. Weber, “Semiconductor laser amplifierin a loop mirror,”J. Lightwave Technol.,vol. 13, pp. 2099–2112, 1995.

[21] D. Breuer, H. J. Ehrke, F. Kuppers, R. Ludwig, K. Petermann, H.G. Weber, and K. Weich, “Unrepeatered 40 Gbit/s RZ single-channeltransmission at 1.55�m using various fiber types,”IEEE Photon.Technol. Lett.,vol. 10, pp. 822–824, 1998.

[22] D. Breuer, F. Kuppers, A. mattheus, E. G. Shapiro, I. Gabitov, andS. K. Turitsyn, “Symmetrical dispersion compensation for standardmonomode fiber based communication systems with large amplifierspacing,”Opt. Lett.,pp. 982–984, 1997.

C. M. Weinert , photograph and biography not available at the time ofpublication.

R. Ludwig, photograph and biography not available at the time of publication.

W. Pieper, photograph and biography not available at the time of publication.

H. G. Weber, photograph and biography not available at the time ofpublication.

D. Breuer, photograph and biography not available at the time of publication.

K. Petermann, photograph and biography not available at the time ofpublication.

F. Kuppers, photograph and biography not available at the time of publi-cation.

Documents

40 Gb/s and 4×40 Gb/s TDM/WDM standard fiber transmission