Freund, R.

Hanik, Norbert

Jeppesen, Palle

Molle, L.

Peucheret, Christophe

We perform a systematic numerical optimization of pre- and post-compensation normalized sections using standard and dispersion compensating fibers for non-return to zero 10-Gbits/s single channel systems. By independently varying the power at the different types of fibers inputs and the compensation ratio, we find that the post-compensation performs better than precompensation at the expense of stricter parameter tolerance. Moreover, we show that both pre- and post-compensated systems can be significantly improved by using passive predistortion at the transmitter

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Optimization of pre- and post-dispersion compensation schemes for 10-Gbits/s NRZlinks using standard and dispersion compensating fibersPeucheret, Christophe; Hanik, Norbert; Freund, R.; Molle, L.; Jeppesen, PallePublished in:I E E E Photonics Technology LettersLink to article, DOI:10.1109/68.867985Publication date:2000Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Peucheret, C., Hanik, N., Freund, R., Molle, L., & Jeppesen, P. (2000). Optimization of pre- and post-dispersioncompensation schemes for 10-Gbits/s NRZ links using standard and dispersion compensating fibers. I E E EPhotonics Technology Letters, 12(4), 992-994. DOI: 10.1109/68.867985992 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 8, AUGUST 2000Optimization of Pre- and Post-DispersionCompensation Schemes for 10-Gbits/s NRZ LinksUsing Standard and Dispersion Compensating FibersChristophe Peucheret, Norbert Hanik, Ronald Freund, Lutz Molle, and Palle Jeppesen, Member, IEEEAbstract—We perform a systematic numerical optimizationof pre- and post-compensation normalized sections using stan-dard and dispersion compensating fibers for non-return to zero10-Gbits/s single channel systems. By independently varying thepower at the different types of fibers inputs and the compensationratio, we find that post-compensation performs better thanprecompensation at the expense of stricter parameter tolerance.Moreover, we show that both pre- and post-compensated systemscan be significantly improved by using passive predistortion at thetransmitter.Index Terms—Dispersion compensation, optical fiber communi-cation, optical fiber nonlinearity, optical networks.I. INTRODUCTIONTHE USE OF dispersion compensating fiber (DCF) is anefficient way to upgrade installed links made of standardsingle mode fiber (SMF) [1]. It has also been suggested that thedesign of future optical transparent networks could be facilitatedby the use of so-called “normalized sections” [2], [3]. Any paththrough the network would then consist of a cascade of identicalnormalized sections and, therefore, both network scalability andmanagement would rely on the knowledge of the accumulatedsignal degradation after transmission through a certain numberof these sections. Spans made of SMF and DCF are good candi-dates as their high local dispersion is known to reduce the phasematching giving rise to four-wave mixing in wavelength divi-sion multiplexing (WDM) systems. Signal degradation in suchsystems is due to the combined effects of group velocity dis-persion, Kerr nonlinearity, and accumulation of amplified spon-taneous emission noise due to periodic amplification. Becauseof the nonlinear nature of propagation, system performance de-pends on the power levels at the input of the different types offibers, on the position of the DCF [4], and on the amount ofresidual dispersion [5], [6]. Of particular interest are the pre- andpost-compensation schemes where each link is made of spanswhere the DCF is located before or after the SMF, respectively.In this letter, the cascadability of 80-km-long normalizedsections using the post- and precompensation schemes isinvestigated in a systematic fashion using computer simulationManuscript received February 22, 2000; revised April 17, 2000. This workwas supported by the European Commission under AC354 DEMON.C. Peucheret and P. Jeppesen are with Research Center COM, Technical Uni-versity of Denmark, DK-2800 Lyngby, Denmark.N. Hanik is with T-Nova Technologiezentrum, D-10589 Berlin, Germany.R. Freund and L. Molle are with Virtual Photonics Incorporated, D-10587Berlin, Germany.Publisher Item Identifier S 1041-1135(00)06246-7.Fig. 1. (top) Post- and (bottom) pre- compensation systems under investigationwith and without the optional predistortion fibers.for single channel non-return to zero (NRZ) modulation at 10Gbits/s. We add another degree of freedom compared to earlierwork [4], [5] by allowing for independent variation of the SMFand DCF input powers in order to define transmission optimaand enable a comparison of the different normalized sections.The influence of the compensation ratio defined as the absolutevalue of the ratio of the total dispersion accumulated in theDCF to the total dispersion accumulated in the SMF for onespan is also systematically investigated. We generalize theresults of [5] and show that better performance can be obtainedfor post-compensation provided correct power levels are usedat the SMF and DCF inputs. Moreover, we also investigatethe influence of predistortion consisting of an added piece ofDCF at the transmitter in the post-compensation scheme or apiece of SMF in the precompensation case and correspondingcompensation at the receiver. We demonstrate that predistortioncan significantly improve the performance of both pre- andpost-compensated systems and shifts the optimum power levelstoward higher values which, in turn, provides a higher opticalsignal-to-noise ratio (SNR) throughout the link.II. RESULTS AND DISCUSSIONThe systems under investigation are shown in Fig. 1. Con-tinuous wave (CW) light is externally modulated at 10 Gbits/swith an NRZ pseudorandom binary sequence in a chirplessMach–Zehnder modulator having a 13-dB extinction ratio and25-ps rise time, before being transmitted in a link consisting ofa cascade of pre- or post-compensation normalized sections.These sections consist of 80 km of SMF and a variable lengthof DCF, which is adjusted to define the compensation ratio.The fiber parameters are given in Table I. Two erbium-dopedoptical fiber amplifiers (EDFA) with 5-dB noise figure areused in each section in order to independently set the SMF andDCF input powers. After a number of cascades, the signal is1041–1135/00$10.00 © 2000 IEEEAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 9, 2010 at 02:56 from IEEE Xplore.  Restrictions apply. PEUCHERET et al.: OPTIMIZATION OF PRE- AND POST-DISPERSION COMPENSATION SCHEMES FOR 10-GBITS/S NRZ LINKS 993TABLE ISMF AND DCF PARAMETERSamplified to a level of 0 dBm, filtered by a first order opticalband-pass Gaussian filter with a 3-dB bandwidth of 25 GHz andfed to the receiver, which consists of a PIN photodiode with aresponsivity of 1 A/W and a single-sided thermal noise densityof 15 pA Hz, followed by a fifth order low-pass Besselfilter with a 3-dB bandwidth of 10 GHz. Optionally, a pieceof DCF (respectively SMF) can be added at the transmitter inthe post-compensation (respectively precompensation) case,the dispersion of which is compensated for at the receiverby an additional piece of SMF (respectively DCF). EDFAsare used to exactly compensate for the loss in these pre- andpost-distortion fibers.The calculation of the propagation in the optical fibers wasperformed using a standard split-step algorithm with adaptivestep-size [7]. The EDFAs were modeled by wavelength inde-pendent gain and noise addition. Signal and noise interactionwas included during propagation in the fibers. Noise was calcu-lated in a 320-GHz bandwidth centered around the transmitterfrequency, which is equivalent to the use of a 2.5-nm squareband-pass in-line filter. Sequence lengths of 1024 bits were usedin order to obtain realistic Q factor estimations in the receivermodel.For each compensation ratio, the powers at the SMF input andDCF input were varied systematically by setting the gains of theEDFAs, and the Q factor was calculated for each set of powerafter a defined number of cascaded spans. Fig. 2 shows Q factorcontour plots obtained after 10, 20, 30, and 40 cascades for 98%post-compensation without any predistortion. The powers dis-played on the axes are signal average power, meaning that thenoise power in a given bandwidth centered on the laser wave-length is not taken into account. The existence of a transmis-sion optimum for certain sets of powers is clearly seen in thesecontour plots. This optimum corresponds to input powers ofabout 2 and 7 dBm into the SMF and DCF, respectively. It canmoreover be observed that in this case, the position of the op-timum does not vary much when the number of cascaded spansis increased. This allows for flexibility in network design as thepower levels can be set independently of the path followed by asignal.The compensation ratio has been varied between 97%and 101% for both pre- and post-compensation schemes andoptimum Q values and fiber input power levels have beenextracted from contour plots similar to the ones in Fig. 2. Theresults are summarized in Fig. 3, where the maximum Q factorsand the corresponding powers are plotted as a function of thecompensation ratio for 30 cascaded sections correspondingto a 2400-km link. Post-compensation shows better perfor-mance than precompensation and exhibits a clear optimumFig. 2. Evolution of the Q factor as a function of SMF and DCF input powersfor 10, 20, 30, and 40 cascaded sections in the case of 98% post-compensationwithout predistortion.Fig. 3. Maximum Q factor and corresponding SMF and DCF input powersas a function of the compensation ratio for 30 cascades of the pre- andpost-compensation sections.corresponding to 98%. The Q factor and optimum power levelsvary significantly when the compensation ratio is changed,which is not observed for precompensation showing poorer butmore stable performance. It should be pointed out that in oursystem, a 1% change in the compensation ratio correspondsto only 12.8 ps/nm variation in dispersion, which is a highaccuracy requirement when ordering dispersion compensatingmodules from fiber manufacturers. Moreover, even if weassume that multichannel nonlinear effects have little influenceon systems making use of SMF and DCF, and unless DCFswith slope compensation are used, the compensation ratio willvary from one channel to another in WDM systems resulting inunequal channel performance. Therefore, running a system atits maximum performance using post-compensation does notallow for much parameter tolerance.It can also be seen in Fig. 3 that when the compensationratio is decreased, optimum powers at both SMF and DCF in-puts need to be increased in the post-compensation scheme. For100% compensation, fiber input power levels are set by SNRrequirements and not by the need to counteract dispersion im-balance by self phase modulation (SPM). On the other hand,Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 9, 2010 at 02:56 from IEEE Xplore.  Restrictions apply. 994 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 8, AUGUST 2000Fig. 4. Q-factor for 30 cascades as a function of SMF and DCF input powersfor 100% post- (left) and pre- (right) compensation with (bottom) or without(top) 50% predistortion.when the compensation ratio is decreased, some amount of SPMis required to make up for the undercompensation. SPM in theanomalous dispersion regime leads to pulse compression [7]and, therefore, a higher power into the SMF is desirable. Fig. 3shows that some amount of SPM in the DCF can also be ben-eficial for an undercompensated system, which is in agreementwith [6], which studied a single span system.Introducing a predistortion fiber reduces the maximumamount of total accumulated dispersion and realizes somepassive prechirping at the transmitter. In Fig. 4, we show therelative performance of pre- and post-compensation with andwithout predistortion for 100% compensation and 30 cascades.In this case, the absolute value of the dispersion of both pre- andpost-distortion fibers corresponds to half the total dispersionaccumulated in one span of SMF. With this value, the excursion(from 0 ps/nm) of the total accumulated dispersion is minimizedand is moreover the same for pre- and post-compensation.However, it might not correspond to an absolute optimum asnew design parameters (lengths and input powers of pre- andpost-distortion fibers) are introduced. Again, a higher max-imum Q factor is obtained for post-compensation compared toprecompensation. For a given compensation ratio (here 100%,but the same trend has been observed for other ratios), thetransmission performance can be significantly improved byusing predistortion. In the 100% case, results are comparablewhether pre- or post-compensation is used. This is due to thefact that in this case, if the pre- and post-distortion fibers areexcluded, these two dispersion maps are only translated by oneDCF length. predistortion has also the effect of shifting theoptimum powers toward higher values. This is beneficial as itprovides a higher SNR ratio throughout the system.III. CONCLUSIONWe have performed a systematic compensation ratio andfiber input power optimization of pre- and post–dispersioncompensation schemes using SMF and DCF. We have shownthat if no predistortion is used, post-compensation performsbetter at the expense of more stringent parameter tolerance.A compensation ratio of 98% has been found to constitutean optimum for post-compensation. On the other hand, whenprecompensation is used, little influence of the compensationratio on the optimum Q factor and power levels has beenobserved. We have also shown that introducing predistortioncan significantly improve the performance of both pre- andpost-compensation schemes.REFERENCES[1] N. Kikuchi, S. Sasaki, and K. Sekine, “10 Gbit/s dispersion-compen-sated transmission over 2245 km conventional fibers in a recirculatingloop,” Electron. Lett., vol. 31, no. 5, pp. 375–377, 1995.[2] E. J. Bachus, M. Eiselt, K. Habel, K. D. Langer, E. U. Scheuing, andF. C. Tischer, “Photonic network design based on reference circuits,” inOptical Network Design and Modeling ONDM’97, Vienna, Austria, pp.56–69.[3] N. Hanik, A. Gladisch, and G. Lehr, “An effective method to designtransparent optical WDM-networks,” in Technology and InfrastructureNOC’98, Manchester, U.K., pp. 190–197.[4] D. M. Rothnie and J. E. Midwinter, “Improved standard fiber perfor-mance by positioning the dispersion compensating fiber,” Electron.Lett., vol. 32, no. 20, pp. 1907–1908, 1996.[5] G. Bellotti, A. Bertaina, and S. Bigo, “Dependence of self-phase modu-lation impairments on residual dispersion in 10 Gbit/s-based terrestrialtransmissions using standard fiber,” Photon. Technol. Lett., vol. 11, pp.824–826, July 1999.[6] R. J. Nuyts, Y. K. Park, and P. Gallion, “Performance improvement of10 Gb/s standard fiber transmission systems by using the SPM effect inthe dispersion compensating fiber,” Photon. Technol. Lett., vol. 8, pp.1406–1408, Nov. 1996.[7] G. P. Agrawal, Nonlinear Fiber Optics. San Diego, CA: Academic,1995.Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 9, 2010 at 02:56 from IEEE Xplore.  Restrictions apply. 

Optimization of pre- and post-dispersion compensation schemes for 10-Gbits/s NRZ links using standard and dispersion compensating fibers

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Optimization of pre- and post-dispersion compensation schemes for 10-Gbits/s NRZ links using standard and dispersion compensating fibers

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