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20 Tbit/s Transmission Over 6860 kmWith Sub-Nyquist Channel Spacing

Jin-Xing Cai, Fellow, IEEE, Fellow, OSA, Carl R. Davidson, Senior Member, IEEE, Alan Lucero,Hongbin Zhang, Member, IEEE, Dmitri G. Foursa, Senior Member, IEEE, Oleg V. Sinkin, Member, IEEE,

William W. Patterson, Alexei N. Pilipetskii, Georg Mohs, Senior Member, IEEE, andNeal S. Bergano, Fellow, IEEE, Fellow, OSA

Abstract—We demonstrate that channel spacing can be re-duced to values smaller than the Nyquist channel spacing overtransoceanic distance. Modulation memory induced by con-strained transmitter bandwidth together with multisymboldetection can reduce intersymbol interference for systems withsub-Nyquist channel spacing.We transmit 198 100G bandwidthconstrained polarization-division-multiplexed return-to-zero qua-ternary phase shift keying channels with 400% spectral efficiencyover 6860 km using 52 km spans of 150 m fiber and simplesingle-stage erbium-doped fiber amplifiers without any Ramanamplification. We also show that 100 G coherent nonlinear per-formance scales differently with distance on uncompensateddispersion maps compared with direct detection transmission.

Index Terms—Coherent communications, interchannel inter-ference, intersymbol interference (ISI), maximum likelihoodsequence estimation (MLSE), maximum a posteriori probability(MAP) detection, optical fiber communications, phase-shift keying(PSK), polarization-division multiplexing (PDM), wavelengthdivision multiplexing (WDM).

I. INTRODUCTION

A N IMPORTANT parameter of long-haul communicationsystems is the potential capacity at full loading. In gen-

eral, the higher the capacity per fiber pair, the lower the costper bit since the cost of the cable and its installation can beamortized over a larger capacity. It is therefore not surprisingthat considerable research effort has been devoted to achievethe largest possible capacity. Currently, the record for totalcapacity per fiber core is 100 Tbit/s [1] demonstrated over165 km. Achieving large capacity over transoceanic distancepresents further challenges. Before our new demonstration, theprevious record capacity–distance product of 116 Pbit/s km(15.5 Tbit/s over 7200 km) was set in 2009 [2]. Both of theaforementioned records were achieved using either Raman-as-sisted erbium-doped fiber amplifiers (EDFAs) or pure Ramanamplification to boost the received optical signal to noise ratio(OSNR) for 100 G signals, and using both C- and L-band am-

Manuscript received August 12, 2011; revised November 14, 2011, De-cember 06, 2011; accepted December 07, 2011. Date of publication December15, 2011; date of current version February 01, 2012.The authors are with the TE Subsea Communications LLC, Eaton-

town, NJ 07724 USA (email: jcai@subcom.com; cdavidson@subcom.com;alucero@subcom.com; hzhang@subcom.com; dfoursa@subcom.com;osinkin@subcom.com; wpatterson@subcom.com; apilipetskii@subcom.com;gmohs@subcom.com; nbergano@subcom.com).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JLT.2011.2179975

Fig. 1. Experimentally demonstrated single-fiber transoceanic capacityrecords.

plification technology. These technologies have not yet madetheir way into commercial undersea communication systems.The previous record for the largest capacity demonstrated overtransoceanic distance in the conventional C-band wasTbit/s [3].Fig. 1 summarizes experimentally demonstrated trans-

mission capacities over a transoceanic length path since theinvention of optical fiber amplifiers. The maximum aggregatecapacity almost doubled per year in the first ten years thanksto advancements in wavelength division multiplexing (WDM)technology, dispersion management, amplifier technology,modulation format, and forward error correction (FEC) algo-rithms. One important note is that all the capacity records priorto 2002 were achieved with simple ON–OFF keying. Since theintroduction of phase modulation in 2002 [4], the total capacityimprovement has mainly been driven by advanced modulationformats. The progression of modulation formats transitionedfrom simple differential phase shift keying including dif-ferential binary phase shift keying (BPSK) and differentialquaternary phase shift keying (QPSK) with differential de-coding by delay interferometer to more advanced BPSK andQPSK with coherent detection. With the advent of digitalcoherent receivers [5], [6], several impressive transoceanic 100G experiments with large capacity and high spectral efficiency(SE) have been demonstrated [1]–[3], [7]–[9]. In particular,15.5 Tbit/s were transmitted over 7200 km in both C- andL-band with a capacity–distance product of 116 Pbit/s km [2].In this paper, we review our new capacity-distance milestone

of 141 Pbit/s km [10] using only the C-band by transmitting

0733-8724/$26.00 © 2011 IEEE

652 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 4, FEBRUARY 15, 2012

Fig. 2. 112 Gbit/s BW-constrained PDM RZ-QPSK transmitter. DFB:distributed feedback laser, ECL: external cavity laser, PM: polarization main-taining, and RZ: return to zero.

198 100 G channels with 400% SE (i.e., 4 bits/s/Hz). Our am-plifier chain consists of single-stage EDFAs with 40 nm band-width (BW) in theC-band and 150 m large effective area fiberwith 52 km amplifier spacing. We use a transmission format ofBW constrained polarization-division-multiplexed (PDM) re-turn-to-zero (RZ)-QPSK and achieve a transmission distance of6860 km. The BW constraint is created by aggressive filtering ofthe signal which results in significant intersymbol interference(ISI) and a complex signal constellation that can be interpretedas symbol correlation or memory in the modulation format [7].We developed a suite of algorithms including maximum like-lihood sequence estimation (MLSE) to take advantage of thesymbol correlation and mitigate the linear ISI penalty associ-ated with the tight prefiltering. More than 5 dB multisymbol de-tection gain is demonstrated at 400% SE. Furthermore, we alsoinvestigate the optimum preemphasis as a function of systemlength and find that beyond a certain distance, the nonlinearphase shift is randomized by the uncompensated dispersion mapwhich leads to a weak dependence on distance, unlike in directdetection systems that use dispersion management.

II. EXPERIMENTAL SETUP

A schematic of the transmitter is shown in Fig. 2. We com-bine 198 lasers into two groups of 99 lasers each. We add fouradditional external cavity lasers (ECL) per set of lasers that aretuned to eight contiguous channels and replace the coincidinglasers during the bit error ratio (BER) measurements. Each setof 99 lasers is modulated independently with pseudo-randombinary sequence tributaries of length at 28 GBaud usingthe RZ-QPSK format. Our baud rate accounts for 7% overheadfor FEC1. The optical RZ-QPSK signal was generated with acascade of an RZ modulator and a nested Mach–Zehnder QPSKmodulator which is driven by a pair of independent data streamsas I and Q channels. The RZ modulator is biased at maximumtransmission and driven by a 14 G sinusoidal clock signal. Wethen split each set of 99 channels into two parts and delay onepart with respect to the other before recombining both partsusing a polarization beam combiner. The delay serves to decor-relate the symbol streams in each part to emulate independentpolarization multiplexing. In addition, to emulate four indepen-dent rails, we also decorrelate the nearest neighbors on both

1Note that our symbol rate of 28 GBaud gives a transmitted bit rate ofGbit/s. The 7% FEC overhead reduces the user data rate to

Gbit/s, which we generically label as 100 Gbit/s.

Fig. 3. Circulating loop test bed and 112 Gbit/s PDMRZ-QPSK receiver. BPF:tunable optical bandpass filter, GEF: gain equalization filter, LO: local oscillator,LSPC: loop synchronized polarization controller, OIF: optical interleaving filter,and SLAF: super large effective area fiber.

the even and odd channels using back-to-back 50 GHz opticalinterleaving filters with a fiber delay. Even and odd channels arethen combined with a 25 GHz interleaver filter that also con-strains the BW of each channel. A second 25 GHz interleaverfilter is added to each odd/even rail for additional pre-filtering.At all times during our transmission experiments, we transmit198 PDM RZ QPSK channels.Our transmission path consists of twelve 52 km spans of

large effective area fiber ( m ) and simple single-stageC-band EDFAs with 4.4 dB noise figure (see Fig. 3). TheEDFAs are equalized to 40 nm BW and operate at 19.5 dB moutput power which corresponds to an average power perchannel of dBm launched into the transmission fiber.The 12 spans are configured into a transmission loop of 624km length including a loop synchronous polarization controller(LSPC) to properly account for polarization-dependent loss(PDL) and polarization mode dispersion (PMD) accumulationin the loop and a gain equalization filter to correct residualgain error. The average fiber loss is 0.183 dB/km, average fiberdispersion is 20.6 ps/nm/km, and the average differential groupdelay of the loop is 1.7 ps. We achieve 6860 km transmissiondistance by looping the signal 11 times. No pre-, post- or in-line-optical dispersion compensation is used in this experiment.Our digital coherent receiver (see Fig. 3) first demulti-

plexes the received spectrum using a combination of opticalinterleaving filters (OIF) and tunable bandpass filters. Opticaldemultiplexing or digital filtering in the receiver is not nec-essary if the receiver BW (RBW) is smaller than half of thechannel spacing. However, the BW of our receiver is 20 GHzwith digital enhancement which is much wider than half thechannel spacing (12.5 GHz) and, therefore, a 25 GHz OIF isneeded to suppress crosstalk from the two neighboring chan-nels in this experiment. After wavelength demultiplexing, thereceived channel is combined with a tunable local oscillator(LO) in a polarization diversity 90 optical hybrid which isconnected to four balanced photo detectors. The signals fromthe photo detectors are sampled at 50 GHz using a digitaloscilloscope with 16 GHz analog electrical BW. The recordedelectrical signals are digitally processed offline.We use offline processing for all data decoding. Fig. 4 depicts

the structure of our coherent receiver and the function blocksin our offline digital signal processing (DSP). After waveformrecovery and alignment, dispersion compensation is performeddigitally in the Fourier domain and the clock is extracted from

CAI et al.: 20 TBIT/S TRANSMISSION OVER 6860 KM WITH SUB-NYQUIST CHANNEL SPACING 653

Fig. 4. Offline digital signal processing flowchart.

the dispersion compensated data. With the recovered clock,the dispersion compensated waveform is then synchronouslyresampled. A constant modulus algorithm (CMA) is used forpolarization tracking followed by LO frequency offset compen-sation and carrier phase estimation using the Viterbi–Viterbialgorithm [11]. To enhance the performance relative to ourpreviously reported results on strongly filtered PDM RZ QPSKsignals at 400% SE [7], [12] we use MLSE [13], [14] instead ofmaximum a posteriori probability (MAP) detection [12], [15],which allows for more efficient utilization of the correlationbetween more distant symbols. We also employ a double-detec-tion technique that uses a nine-tap CMA for high-performancecarrier frequency offset and phase estimation. The estimatedfrequency offset and phase are used in a second branch that em-ploys only a single-tap CMA for polarization demultiplexing.The single-tap CMA preserves the symbol correlations inducedby the narrow filtering that are used by the MLSE to boost theperformance. We obtain on average about 1 dB improvementrelative to the DSP algorithms used in [7].

III. DETECTION OF BW-CONSTRAINED SIGNALS

From the Nyquist theorem, the maximum SE (with zero ISI)is achieved when the channel spacing equals the baud rate andthe pulses have a sinc shape [16]. For PDM-QPSK signal, there-fore, the maximum SE is 360% assuming 7% FEC overhead.Transpacific distance transmission has been demonstrated withthe Nyquist rate [3] for 100 G PDM-QPSK signals.To further improve SE and keep minimal penalty from

ISI, it is necessary to generate BW-constrained signals withcontrolled ISI or partial-response signals [17]. Examples ofpartial-response signals include the duobinary signal pulseand the modified duobinary signal pulse. Cascaded opticalinterleaving filters were used in our experiment to constrain theBW of the signal. The combined filtering BWs are 15.6, 19.8,and 31.9 GHz at 1, 3, and 20 dB, respectively. Fig. 5 comparesthe optical spectra for 28 GBaud RZ-QPSK under different BWconstraint conditions measured with 0.1 nm resolution BW.The aggressive prefiltering effectively removed crosstalk from

Fig. 5. Optical spectra for 28 GBaud RZ QPSK under different BW-constraintconditions (single channel without BW constraint, odd/even channels with BWconstraint, 25 GHz spaced WDM channels with BW constraint).

neighboring channels as shown for the odd/even channels withBW constraint.To achieve the optimum performance, a multisymbol detec-

tion scheme is necessary that takes advantage of the memorycontained in the received partial response signal. There are twomethods for detecting the information symbols at the receiverwhen the received signals contain controlled ISI. One way isbased on the computation of a MAP for each detected symbol.Although the MAP algorithm makes decisions on a symbol-by-symbol basis, each symbol decision is based on an observationof a sequence of received signals. The major problem with theMAP detection scheme is its computational complexity, i.e., thesize of the lookup table. For n-tap MAP detection of a QPSKsymbol, entries in the lookup table are needed. Hence, theMAP detection algorithm is practically limited to compensatingISI among three to five neighboring symbols. The MAP detec-tion scheme was used in our previous 300% SE [7] and 360%[3] experiments.When the channel spacing is significantly smaller than the

Nyquist rate, the ISI extends beyond five neighboring symbolsand the aggressively prefiltered waveforms produce signals withlong memory. This memory can be conveniently represented bya trellis. Therefore, an MLSE algorithm is a good detector forsignals when the channel spacing is smaller than the Nyquistrate. TheMLSE algorithm searches theminimumEuclidean dis-tance path through the trellis that characterizes memory in theBW-constrained signal. One important parameter for MLSE de-tection is the memory length L. We use in our algorithmthroughout this paper. A more detailed discussion of BW-con-strained signals and their detection can be found in [16].Fig. 6 shows the back-to-back performance of a single

channel (without BW constraint) and a WDM signal with400% SE for different detection schemes. Comparing theperformance at 400% SE without MAP or MLSE (single-tapCMA) with the single-channel performance shows that the ISIpenalty is large and the decoded Q-factor saturates at less than6 dB even for dB OSNR (in 1 nm RBW). With MAPdetection, the performance improved to 11.5 and 12.6 dB forfive taps and seven taps, respectively. However, the decodingtime increased significantly when going from five tap to seventap MAP. Further increasing the number of taps for the MAPdetection algorithm is therefore unattractive. With MLSE, we

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Fig. 6. Back-to-back performance comparison for the MAP and MLSEreceivers with 400% SE. The single-channel performance (without pre-filtering) and 400% SE performance without MAP or MLSE is also plotted forcomparison.

Fig. 7. MLSE benefit for 25 GHz spaced 28 Gbaud PDM RZ QPSK signals.

can efficiently implement 10 taps and obtain a further perfor-mance enhancement to 13.4 dB at 10 dB OSNR.Fig. 7 shows that MLSE benefit versus received OSNR. Sim-

ilar to the MAP detection, the MLSE benefit is OSNR depen-dent. At lower OSNR, the signal is noise dominated and theMLSE benefit is relatively small. At 7.7 dB OSNR (the averagereceived OSNR in our 198 100 Gbit/s experiment), we ob-serve 7.3 dB MLSE benefit and the Q-factor reaches 12.1 dBwith an MLSE length of 10.We also compare the transmission performance for MLSE

and MAP detection at different channel power levels in trans-mission using transmitter preemphasis on a group of eight con-tiguous channels. The results are shown in Fig. 8 where we plotthe performance of channel 100 (Ch100, 1547.116 nm) versusthe common preemphasis of all eight channels (Ch100 beingthe fourth in the group of 8). A preemphasis of 0 dB corre-sponds to the nominal operating point of the loop without trans-mitter preemphasis. We show two curves in Fig. 8 for Ch100,one with MLSE detection (magenta squares) and one with MAPprocessing (blue diamonds). We obtain about 0.6 dB improve-ment from MLSE relative to MAP which was used in [3] and[7]. Furthermore, we observed that the MLSE benefit becomessmaller relative to the MAP benefit at high power.Fig. 9 shows that the MLSE benefit gradually increases as

the channel power is increased. At 0 dB preemphasis (7.7 dB

Fig. 8. Eight channel preemphasis curve after 6860 km at 400% SE. Dashedlines correspond to second-order polynomial fits.

Fig. 9. MLSE benefit versus transmitter preemphasis after 6860 km (400%SE).

received OSNR in 1 nm RBW), MLSE detection providesdB performance improvement after 6860 km with 400% SE(note that the back-to-backMLSE benefit was 7.3 dB at the sameOSNR). As the channel becomes more nonlinear, the MLSEbenefit starts to degrade due to the fact that MLSE is not able tocompensate the self-phase modulation-induced ISI for the dis-persion uncompensated link.

IV. 20 TBIT/S TRANSMISSION OVER 6860 KM WITHSUB-NYQUIST CHANNEL SPACING AND 400% SE USING

BW-CONSTRAINED PDM-RZ-QPSK

The 20 Tbit/s (198 112 Gbit/s) WDM signals (25 GHzchannel spacing) are launched into the test bed without anyindividual channel power preemphasis at the transmitter.Fig. 10 shows the received spectrum and OSNR after 11 loops(6860 km) with flat channel launch at the transmitter. The insertshows details of the received optical spectrum in the center ofthe transmission band. The channel-to-channel power variationis controlled to within 1 dB. The average received OSNR (in1 nm RBW) is 7.7 dB with individual OSNR values rangingfrom 7.0 to 8.9 dB.The performance of the full loading experiment is shown in

Fig. 11. The BER of each channel is obtained by decoding fivesets of data with 2M samples each ( bits/ch). For eachchannel, we report the polarization-averaged BER converted to

CAI et al.: 20 TBIT/S TRANSMISSION OVER 6860 KM WITH SUB-NYQUIST CHANNEL SPACING 655

Fig. 10. Received OSNR (in 1 nm RBW) and optical spectrum after 6860 km.

Fig. 11. Performancemeasurement results after 6860 km transmission distanceat 400% SE.

Q-factor obtained from the five datasets for each channel. Theaverage Q-factor for all 198 channels (total bits ofdata decoded) is 10.2 dB with the individual channel perfor-mance ranging from 9.5 to 10.9 dB. Besides the Q-factor vari-ation from channel to channel, we also observe a positive tiltin the measured performance which is similar as the OSNR tiltshown in Fig. 10. Fig. 11 also shows the FEC threshold (8.2 dB)for a 7% continuously interleaved BCH code [18]. Comparedto this FEC, all channels had more than dB average FECmargin.In these experiments, we do not observe any cycle slips even

after decoding more than bits at 6860 km transmis-sion distance. We estimate that the probability of cycle slips inour experiments was less than with more than 99% con-fidence, which is sufficiently low to be mitigated using someDSP algorithm such as the one described in [19]. An LSPC isused to ensure that the five datasets for each channel are mea-sured with different polarization evolutions along the transmis-sion line. Hence, time-varying PDL and PMD were included inall measurements.

V. OPTIMUM PREEMPHASIS AS A FUNCTION OF SYSTEMLENGTH AND TRANSMISSION DISTANCE LIMIT

We also investigate the optimum preemphasis as a func-tion of system length. We vary the number of loops from 1(corresponding to 624 km) to 15 (9360 km) and determinethe optimum preemphasis with a second-order polynomial fitsimilar to the dashed line in Fig. 6. The results are plotted in

Fig. 12. Optimum preemphasis as a function of system length.

Fig. 13. Performance measurement results for Ch100 (1547.116 nm) beforeand after MLSE at 400% SE.

Fig. 14. MLSE benefit versus transmission distance at 400% SE.

Fig. 12 with MLSE processing. The optimum preemphasisdrops rapidly when increasing the system length from 624 to2000 km. Beyond 2000 km, the optimum preemphasis is nearlyconstant for increasing system length. During propagation inthe highly dispersive regime, the accumulated nonlinear effectsscale with the square root of distance, which is different thanthe linear dependence that is typically observed in dispersionmanaged systems. For dispersion uncompensated systems, theintensity of the optical signal is quickly dispersed and highlyrandomized; thus, nonlinear perturbations of the fiber refractiveindex also become randomized. The error bars in Fig. 12 indi-cate the uncertainty in the measurement and fitting procedure.The details of this study can be found in [20].

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Finally, we show the measured performance versus systemlength in Fig. 13 before (single tap CMA) and after MLSEfor Ch100 with optimum transmitter preemphasis. At 400% SEwithMLSE detection, the transmission distance can be extendedto km with 10 dB Q-factor, which is 3000 km more thanthe previous published results at the same SE but with MAP de-tection scheme. Fig. 14 shows the MLSE benefit versus trans-mission distance. The MLSE benefit decreases as transmissiondistance increases due to decreasing received OSNR. Neverthe-less, we obtained a performance benefit of more than 5 dB fromthe MLSE processing at 400% SE after 8000 km transmission.

VI. CONCLUSION

We successfully transmitted 198 100 G BW-constrainedPDMRZ-QPSK channels with 400% SE over 6860 km distanceusing 52 km spans of 150 m fiber and single-stage EDFAs.This corresponds to the new capacity distance milestone of 141Pbit/s km and includes 1.3 dB of margin over the threshold of anFEC code with 7% overhead. This result was enabled by usinga high-performance MLSE algorithm in our digital processing.Our MLSE detection algorithm enabled sub-Nyquist channelspacing by reducing ISI-induced penalty and improving the per-formance by dB after transoceanic distance taking advan-tage of the symbol correlations. We also demonstrate a weak de-pendence of the optimum channel preemphasis on system lengthdue to the randomization of the nonlinear phase shift in uncom-pensated dispersion maps.

ACKNOWLEDGMENT

The authors would like to thank Dr. Y. Cai for his contribu-tions to the study of MAP and MLSE detection algorithms.

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Jin-Xing Cai (S’97–M’00–SM’05–F’11) received the B.S. andM.S. degrees inelectronic engineering from Tsinghua University, Beijing, China, in 1988 and1994, respectively, and the Ph.D. degree in electrical engineering from the Uni-versity of Southern California, Los Angeles, in 1999.In 1999, he joined Tyco Submarine Systems Ltd. (now TE Subsea Commu-

nications LLC), Eatontown, NJ. His current research interests include ultra-long-haul transmission of high-speed dense wavelength division multiplexingchannels with massive system aggregate capacity.Dr. Cai is a Fellow of the Optical Society of America.

Carl R. Davidson, biography not available at the time of publication.

CAI et al.: 20 TBIT/S TRANSMISSION OVER 6860 KM WITH SUB-NYQUIST CHANNEL SPACING 657

Alan J. Lucero was born on December 18, 1949. He received the M.S. andPh.D. degrees in physics from the University of Connecticut, Storrs, in 1989and 1993, respectively.In 1995, he completed a two-year Postdoctoral Fellowship at Bell Laborato-

ries, CrawfordHill, NJ, and subsequently became amember of Technical Staff atAT&T Advanced Technologies Systems,Whippany, NJ, where he was involvedin research on the development of low power consumption erbium-doped fiberlasers and system design. In 1997, he was involved in the establishment of thePhotonics Research and Test Center, Corning, Inc., Somerset, NJ. In 2000, hejoined TE Subsea Communications LLC, Eatontown, NJ, where he is involvedin research on linear and nonlinear properties of 10- and 40-Gbit/s transport overnovel dispersion maps.Dr. Lucero is a member of the Phi Beta Kappa.

Hongbin Zhang (M’01) received the B.S. degree in electrical engineering fromZhejiang University, Hangzhou, China, in 1995, and the M.S. and Ph.D. degreesfrom the University of Arizona, Tucson, in 1996 and 2001, respectively.In 2001, he joined Tyco Submarine Systems, Eatontown, NJ. He is currently a

Distinguished Member of Technical Staff at TE Subsea Communications LLC,Eatontown. He holds 11 U.S. patents in the area of optical fiber transmission sys-tems. He is also the Technical Leader for the next-generation long-haul trans-mission equipment and line monitoring system. His current research interestsinclude ultralong-haul coherent transmission.

Dmitri G. Foursa (SM’10) was born in Moscow, Russia, in 1960. He receivedthe Diploma degree in physics from Moscow Physical and Technical Institute,Moscow, Russia, in 1983, and the Ph.D. degree from General Physics Institute,Moscow, in 1992.He was a Research Fellow with the General Physics Institute, Moscow,

Russia. During 1990–1991, he was an Academic Visitor with Imperial Col-lege London, London, U.K., concentrating on erbium-doped fiber lasers andhigh-repetition-rate trains of solitons. During 1994–1995, he held a Post-doctoral Fellowship from SSTC at Université Libre de Bruxelles, Belgium,where, from 1995 to 1999, he was a Senior Research Scientist investigatingexperimentally and numerically dark and gray solitons and their Raman am-plification in optical fibers, erbium-fiber lasers, and high-bit-rate pulse shapingtechniques. In 1999, he joined TE Subsea Communications LLC, Eatontown,NJ, where he is currently a Distinguished Member of Technical Staff. Hiscurrent research interests include advanced optical amplifiers and transmissionissues in submarine telecommunications.Dr. Foursa is a senior member of the Photonics Society.

Oleg V. Sinkin (M’01) was born in Protvino, Russia, in 1976. He received theM.S. degree (cum laude) in applied physics and mathematics from the MoscowInstitute of Physics and Technology, Moscow, Russia, and the Ph.D. degree inelectrical engineering from the University of Maryland Baltimore—BaltimoreCounty, Baltimore, in 1999 and 2006, respectively.During 1996–1999, he was a Research Engineer at IRE-POLUS, Moscow

(IPG Photonics), where he was involved in designing fiber lasers and amplifiers.During his Ph.D. studies, he was involved in research on developing analyt-ical and numerical techniques to model optical fiber communications, studyingnonlinear propagation effects in optical fibers, and modeling experiments. Heis currently a Senior Member of the System Modeling and Signal ProcessingResearch, TE Subsea Communications LLC, Eatontown, NJ.Dr. Sinkin has been a member of the Photonics Society since 2001.

WilliamW. Patterson was born in Maryville, TN. He received the B.S. degreein electrical engineering from the University of Tennessee, Knoxville, and theM.S. degree in computer science from the University of Illinois, Urbana, in 1980and 1988, respectively.In 1980, he joined Western Electric, where he was involved in designing

computer-automated test equipment, and then SLC Digital Loop Carrier circuitpacks. In 1988, he joined the Submarine Systems Division, AT&T Bell Labo-ratories, which was acquired by TE Subsea Communications LLC, Eatontown,NJ, in 1997, where he has since been involved in the testing of undersea sys-tems, and in the design of equipment used to test undersea systems.Mr. Patterson is a member of the Tau Beta Pi.

Alexei N. Pilipetskii received the M.S. degree in physics from Moscow StateUniversity, Moscow, Russia, in 1985, and the Ph.D. degree from the GeneralPhysics Institute Academy of Sciences, Moscow, in 1990, for his research innonlinear fiber optics.From 1985 to 1994, he was with the General Physics Institute, Russia. From

1994 to 1997, he was with the UMBC, where his interest shifted to the fiberoptic data transmission. Since 1997, he has been with TE Subsea Communica-tions LLC, Eatontown, NJ, where he has been involved in a number of researchand development projects. He is currently the Director of a research group withthe focus on the next-generation technologies for the undersea transmissionsystems.

Georg Mohs (M’05–SM’05) received the Diploma degree in physics from theUniversity of Dortmund, Dortmund, Germany, in 1993, and the M.S. and Ph.D.degrees in optical sciences from the University of Arizona, Tucson, in 1995 and1996, respectively.From 1996 to 1998, he was a Research Associate at the University of Tokyo,

Japan, where he was involved in semiconductor optics and ultrafast carrier dy-namics in semiconductors. In 1998, he joined the Optical Networks Researchand Development Group of Siemens, Munich, Germany, where he was involvedin transmission research on terrestrial optical communication systems. In 2001,he joined Tyco Telecommunications (now TE Subsea Communications, LLC)Laboratories, Eatontown, NJ, as a DistinguishedMember of the Technical Staff.He is currently Director of Transmission Research, engaging in forward-lookingexperimental work in high-capacity undersea optical communication systems.

Neal S. Bergano (S’80–M’88–SM’90–F’99) received the B.S. degree in elec-trical engineering from the Polytechnic Institute of New York, New York, andthe M.S. degree in electrical engineering and computer science from the Mass-achusetts Institute of Technology, Cambridge, in 1981 and 1983, respectively.In 1981, he joined the technical staff of Bell Labs’ undersea systems divi-

sion. In 1992, he was named a Distinguished Member of the Technical Staff ofAT&T Bell Labs, where he became an AT&T Technology Consultant in 1996and AT&T Technology Leader in 1997. He is currently the Managing Directorof the System Research and Network Development, TE Subsea Communica-tions LLC, Eatontown, NJ. He holds 31 U.S. patents in the area of optical fibertransmission systems. His main research has been devoted to the understandingof how to improve the performance and transmission capacity of long-haul op-tical fiber systems, including the use of wavelength division multiplexing inoptical-amplifier-based systems.Mr. Bergano is a Fellow of the Optical Society of America (OSA), AT&T,

and TE Connectivity. He is on the Board of Directors for the OSA, and hasserved on the Board of Governors for the IEEE Lasers and Electro-Optics So-ciety from 1999 to 2001. He is a long-time volunteer and supporter of the OpticalFiber Communication Conference and Exposition and the National Fiber OpticEngineers Conference (OFC/NFOEC) meeting, which includes General Chairand Technical Chair in 1999 and 1997, Chair of the steering committee from2000 to 2002, and is currently the Chair of OFC/NFOEC’s long-range planningcommittee. He is the recipient of the 2002 John Tyndall Award, for outstandingtechnical contributions to and technical leadership in the advancement of globalundersea fiber-optic communication systems.