IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 13, JULY 1, 2007 1017

Behavior of MLSE-EDC With Self-Phase ModulationLimitations and Various Dispersion Levels in

10.7-Gb/s NRZ and Duobinary SignalsJohn D. Downie, Jason Hurley, and Michael Sauer

Abstract—We investigate experimentally the effectiveness ofmaximum likelihood sequence estimation electronic dispersioncompensation (MLSE-EDC) for signals limited by self-phasemodulation (SPM) and with various dispersion levels. We studynonreturn-to-zero (NRZ) and duobinary signals modulatedat 10.7 Gb/s and find that the improvement obtained with anMLSE-EDC receiver depends strongly on the level of residualchromatic dispersion (after partial optical compensation) in bothcases. In general, greater SPM tolerance to high channel powersis derived from the MLSE in the presence of larger residualdispersion values. An increase in the allowable channel launchpower of more than 2 dB is observed for NRZ signals, whereas,the increase for duobinary may be almost 6 dB.

Index Terms—Duobinary, electronic dispersion compensation(EDC), optical communication, self-phase modulation (SPM).

I. INTRODUCTION

ELECTRONIC dispersion compensation (EDC) has at-tracted significant study and commercial interest in the

recent past as a means to mitigate optical signal distortion inthe electrical domain after detection in a photoreceiver. Typesof distortion amenable to at least some degree of correction arechromatic dispersion, polarization-mode dispersion, electricalfilter bandwidth limitations, and optical filtering effects [1]–[7].The ability to electronically compensate for some of theseimpairments may allow looser component tolerances and/orsystem design rules, thereby promoting system cost savingsand alternative network configurations. For example, EDC hasbeen demonstrated to be effective in extending uncompensatedreach length and in compensating for narrowband optical filterinduced distortion [6], [7]. Such properties may be particularlyvaluable in metro network designs, for promoting the use ofreconfigurable optical add–drop multiplexers, and/or allowingtighter channel spacings.

The version of EDC perhaps most studied to date is maximumlikelihood sequence estimation (MLSE) technology. MLSE is adigital signal processing approach that operates on a sequenceof bits rather than on a single bit at a time. It attempts to find thesequence of digital data that is statistically most likely to havegenerated the detected optical signal. As noted, this equaliza-tion approach has already been shown to be quite effective in

Manuscript received February 23, 2007; revised April 3, 2007.The authors are with Corning Incorporated, Corning, NY 14831 USA (e-mail:

downiejd@corning.com).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2007.898768

Fig. 1. Experimental setup used to measure MLSE effectiveness with SPMimpairments.

mitigating the effects of several linear impairments. Recently,MLSE performance was studied in the presence of nonlinear ef-fects where it was found that it can be effective in the presenceof self-phase modulation (SPM) alone, but is largely ineffectivewith both SPM and strong cross-phase modulation [8]. We ex-tend the investigation of MLSE-EDC here with a deeper lookat the performance of an MLSE receiver with clearly SPM-lim-ited signals, for both nonreturn-to-zero (NRZ) and duobinarymodulation formats. We experimentally measure and comparethe performances of an MLSE receiver and standard receiver inexperiments with different combinations of SPM nonlinearityand residual chromatic dispersion. We find that the advantagein terms of SPM tolerance offered by the MLSE-EDC dependsgreatly on the level of residual dispersion. In general, MLSEprovides more SPM tolerance to signals with larger uncompen-sated dispersion, for both modulation formats studied. Further-more, the maximum measured allowable launch power advan-tage conferred by the EDC is nearly 6 dB for duobinary signals,but 2 dB for NRZ signals.

II. EXPERIMENTAL CONFIGURATION

The experimental setup used for all measurements is shownin Fig. 1. An optical channel at 1550 nm was modulatedat 10.7 Gb/s in either the NRZ or duobinary format with aMach–Zehnder modulator. A pseudorandom bit sequence oflength was generated by a pulse pattern generatorand used to drive the modulator. The channel was amplifiedand launched into the first of two fiber spans with a controlledlaunch power. The channel was launched into the secondspan with the same power. The fiber spans were each 107 kmlong and were comprised of a G.652-compliant single-modefiber with an enhanced stimulated Brillouin scattering (SBS)

1041-1135/$25.00 © 2007 IEEE

1018 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 13, JULY 1, 2007

threshold [9]. Given the higher SBS threshold by 3 dB overstandard single-mode fiber, and the use of frequency ditheringin the transmitting laser to further increase the SBS thresholdof the NRZ signal, we ensured that signal impairments causedby high channel launch powers were due solely to SPM and notto SBS. Frequency dithering was unnecessary for the duobi-nary signal since it exhibits an intrinsically much higher SBSthreshold than NRZ due to its lack of a carrier in its spectrum.

The optical signal-to-noise ratio (OSNR) of the receivedsignal was controlled in the experiments by varying the channelpower into the fiber amplifier at the output of the secondfiber span with a variable optical attenuator (VOA). Anotheramplifier stage including a red-pass WDM filter to eliminateexcessive amplified spontaneous emission (ASE) buildup in theblue end of the -band and dispersion-compensation fiber insome cases, an optical filter of bandwidth 0.6 nm, and a VOA tomaintain fixed received signal power followed. Clock and datawere recovered in the receiver and passed to a bit-error-ratetester (BERT) for measurement of the signal bit-error rate(BER).

Two different commercially available receivers were com-pared in the system experiments. One was a standard receiver(Rx) with a PIN photodetector, transimpedance amplifier, andassociated clock and data recovery circuitry. The second Rx wasfrom the same manufacturer, but had MLSE-EDC circuitry inthe back-end electronics. The MLSE Rx digital equalizer com-prises a 3-bit analog-to-digital (A/D) converter operating at upto 25 Gsamples/s and a four-state (memory ) Viterbi de-coder. The general equivalence of the PIN photodetectors in thetwo receivers allowed comparative evaluation of the effects ofthe MLSE-EDC technology in the second receiver.

The basic experimental procedure involved varying thechannel launch power into the fiber spans (equal in both spans)and measuring the required signal OSNR value at the Rx toachieve a BER value of . The measurements weretaken for different lengths of dispersion-compensation fiberin the Rx, and with the two different receivers to understandthe role that MLSE-EDC can play in extending the allowablelaunch power when NRZ or duobinary signals are limited bySPM.

III. EXPERIMENTAL RESULTS

We first investigated signals modulated with the NRZ format.The total fiber link length was 214 km of the G.652 fiber witha nominal dispersion value at 1550 nm of 17 ps/(nm km), for atotal link dispersion of about 3638 ps/nm. We varied the amountof optical dispersion compensation in the receiver to evaluatethe performance of the MLSE-EDC against SPM as a func-tion of residual chromatic dispersion. For the NRZ signal, weperformed experiments with residual dispersion values of 238,1088, and 1938 ps/nm. The results from two of these experi-ments are shown in Fig. 2. In general, the MLSE-EDC providessome advantage to allowable launch power level, or increasedSPM tolerance. However, the degree of this advantage dependsstrongly on the level of residual dispersion in the signal when re-ceived. These results are in general agreement with some of thesingle-channel results in [8]. For moderate residual dispersion(1088 ps/nm), the SPM tolerance increase is small ( 0.5 dB),but for residual dispersion of 1938 ps/nm, the increase providedby the MLSE-EDC is significantly greater, and allows launch

Fig. 2. OSNR required to achieve a BER value of 1�10 for an NRZ signalas a function of channel launch power. Residual dispersion values are (a) 1088and (b) 1938 ps/nm.

Fig. 3. Optical eye diagrams for NRZ signal with 1938-ps/nm dispersion andlaunch powers of (a) 12 and (b) 16 dBm. Y-axis: intensity (a.u.).

powers up to 17 dBm. The MLSE afforded negligible or nega-tive advantage for very small dispersion levels due to the minorback-to-back penalty exhibited by the MLSE Rx, possibly dueto the limited resolution of the A/D converter [6].

Optical eye diagrams of the received NRZ signal with uncom-pensated dispersion of 1938 ps/nm are shown in Fig. 3 for twolaunch power levels. At 12 dBm, the eye is open due to the in-teraction between SPM and dispersion. At 16 dBm, the eye isessentially closed by the greater SPM nonlinear impairment butcan be still well-corrected by the MLSE-EDC Rx. The OSNRwas 30 dB for both eye diagram figures.

We found similar trends in the results for a duobinary signal.Measurements were taken for residual dispersion values of1088, 1938, 2788, and 3638 ps/nm. The range of residualdispersion studied is about twice that for NRZ because ofduobinary’s inherently greater dispersion tolerance. Results forthe three largest of these dispersion values are shown in Fig. 4.

DOWNIE et al.: BEHAVIOR OF MLSE-EDC WITH SPM LIMITATIONS AND VARIOUS DISPERSION LEVELS 1019

Fig. 4. OSNR required to achieve a BER value of 1 � 10 for a duobinarysignal as a function of channel launch power. Residual dispersion values are(a) 1938, (b) 2788, and (c) 3638 ps/nm.

As with NRZ, the SPM tolerance for the duobinary signal ob-tained from MLSE-EDC increases with residual dispersion, butis also generally larger for duobinary. However, the maximumlaunch power decreased with increasing dispersion for theduobinary signal for these larger residual dispersion values.

The results for both formats are summarized in Fig. 5, ex-pressed as the difference between maximum allowed channelpowers using the MLSE-EDC Rx and the standard Rx. The max-imum channel power is defined at the 2-dB penalty point com-pared to the required OSNR for BER for the stan-dard Rx in the linear regime ( 0-dBm launch power), for each

Fig. 5. Allowable launch power advantage using MLSE-EDC Rx.

residual dispersion value. This involved extrapolating our mea-sured data back to lower power levels in some cases.

IV. SUMMARY

We have performed a systematic study of the effectivenessof MLSE-EDC for increasing the SPM tolerance for NRZ andduobinary signals at 10.7 Gb/s. We found for both formatsthat the SPM advantage from MLSE is highly dependenton the amount of uncompensated dispersion at the end of a214-km fiber link. In general, greater SPM tolerance advan-tages were obtained from the MLSE technology for largervalues of residual dispersion. The biggest increase of nearly6 dB obtained with the MLSE Rx was found for a completelyuncompensated duobinary signal.

REFERENCES

[1] G. S. Kanter, A. K. Samal, and A. Gandhi, “Electronic dispersion com-pensation for extended reach,” in Proc. Optical Fiber CommunicationConf. (OFC 2004), Washington, DC, Paper TuG1.

[2] T. Nielsen and S. Chandrasekhar, “OFC 2004 workshop on optical andelectronic mitigation of impairments,” J. Lightw. Technol., vol. 23, no.1, pp. 131–142, Jan. 2005.

[3] H. Griesser, J.-P. Elbers, C. Fuerst, H. Wernz, and C. Glingener, “In-creasing the dispersion tolerance of 10 Gb/s duobinary modulation byelectrical distortion equalisation,” in Proc. Eur. Conf. Optical Commu-nications (ECOC 2004), Stockholm, Sweden, 2004, Paper We4.P.106.

[4] M. D. Feuer, S.-Y. Huang, S. L. Woodward, O. Coskun, and M.Boroditsky, “Electronic dispersion compensation for a 10-Gb/s linkusing a directly modulated laser,” IEEE Photon. Technol. Lett., vol.15, no. 12, pp. 1788–1790, Dec. 2003.

[5] H. Haunstein and R. Urbansky, “Application of electronic equalizationand error correction in lightwave systems,” in Proc. Eur. Conf. Op-tical Communications (ECOC 2004), Stockholm, Sweden, 2004, PaperTh.1.5.1.

[6] J. D. Downie, M. Sauer, and J. Hurley, “Experimental measurementsof uncompensated reach increase from MLSE-EDC with regard tomeasurement BER and modulation format,” Opt. Express, vol. 14, pp.11520–11527, 2006.

[7] M. Rubsamen, P. J. Winzer, and R.-J. Essiambre, “MLSE receivers fornarrowband optical filtering,” in Proc. Optical Fiber CommunicationConf. (OFC 2006), Washington, DC, Paper OWB6.

[8] S. Chandrasekhar and A. H. Gnauck, “Performance of MLSE receiverin a dispersion-managed multispan experiment at 10.7 Gb/s under non-linear transmission,” IEEE Photon. Technol. Lett., vol. 18, no. 23, pp.2448–2450, Dec. 1, 2006.

[9] A. Kobyakov, S. Kumar, D. Q. Chowdhury, A. B. Ruffin, M. Sauer,S. R. Bickham, and R. Mishra, “Design concept for optical fibers withenhanced SBS threshold,” Opt. Express, vol. 13, pp. 5338–5346, 2005.