104 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005 WDM Transmission at 6-Tbit/s Capacity Over Transatlantic Distance, Using 42.7-Gb/s Differential Phase-Shift Keying Without Pulse Carver G. Charlet, E. Corbel, J. Lazaro, Member, IEEE, A. Klekamp, R. Dischler, P. Tran, W. Idler, H. Mardoyan, A. Konczykowska, F. Jorge, and S. Bigo, Member, IEEE Abstract—We report the transmission of a record 6 Tbit/s capacity over 6120 km distance, involving channels modulated at 42.7-Gb/s bit-rate with differential phase-shift keying (DPSK). The performance is found similar to DPSK with subsequent pulse carving, namely RZ-DPSK. Index Terms—Differential phase-shift keying (DPSK), informa- tion spectral density, ultralong haul. I. INTRODUCTION J UST a few years ago, laboratory experiments emulating WDM submarine systems at 40-Gb/s channel rate were laying far behind those emulating systems at 10-Gb/s channel rate [1], [2] in terms of capacity and performance. The most recent achievements have highlighted the progress made at 40 Gb/s [3]–[6], as accounted for by the successive introductions of new technologies. One of the most helpful has been op- tical data modulation based on differential phase shift keying (DPSK) and balanced detection [7]. Up to now, DPSK modu- lation has almost exclusively been used in combination with so-called return-to-zero (RZ) pulse carving, yielding RZ-DPSK modulation. Here, we use 40-Gb/s data modulated with raw DPSK format without pulse carving, to reach 0.8 bit/s/Hz information spec- tral density when packing WDM channels as close as 50 GHz. We show that the performance is very similar to that obtained with RZ-DPSK, while alleviating the complexity and the cost of transmitters. A record 6-Tbit/s capacity is transmitted over a transatlantic distance. II. EXPERIMENTAL SETUP Our transmitter involves 149 channels with 50-GHz fre- quency spacing, ranging from 1535.04 to 1565.08 nm in the C band and from 1569.58 to 1601.94 nm in the L band. In each band, two sets of even and odd channels are multiplexed into array-waveguide gratings (AWGs) with 100-GHz spacing and modulated independently according to either DPSK or Manuscript received July 23, 2004; revised September 3, 2004. G. Charlet, E. Corbel, P. Tran, H. Mardoyan, A. Konczykowska, F. Jorge, and S. Bigo are with the Alcatel Research and Innovation, 91460 Marcoussis, France. J. Lazaro, A. Klekamp, R. Dischler, and W. Idler are with the Alcatel Research and Innovation, D-70499 Stuttgart, Germany. Digital Object Identifier 10.1109/JLT.2004.840348 Fig. 1. DPSK versus RZ-DPSK modulation schemes. RZ-DPSK modulation schemes. These schemes are depicted in Fig. 1. DPSK modulation is performed by passing light into a push- pull Mach-Zehnder modulator driven by the two complemen- tary outputs of a 42.7-Gb/s electrical precoder. This differential precoder working at the line rate is fed by a separate 42.7-Gbit/s pattern generator delivering long pseudorandom bit se- quences (PRBS) out of four sequences at 10 Gbit/s with 7% overhead, i.e., at 10.7 Gbit/s. The overhead emulates the pres- ence of forward error correction (FEC). Whenever necessary, RZ-DPSK data are obtained by over- modulating the DPSK data with a second push-pull modulator driven by a clock at 42.7 GHz, which carves RZ pulses with 50% duty cycle. The sets of odd and even modulated chan- nels are then passed into periodical optical filters (Gaussian second-order-like shape with 50-GHz bandwidth at 3 dB and with a chromatic dispersion of approximately ), as depicted in the experimental setup of Fig. 2. This narrows the spectrum of each individual channel, thereby helping to contain linear crosstalk. In both bands, odd and even channels are then interleaved with orthogonal polarizations via a polar- ization beam combiner (PBC). Hereafter, the signal is fed in a double-stage booster incorporating dispersion compensating fiber (DCF), before entering the recirculating loop through a switch and a 3-dB coupler. Our repeaters are based on Raman amplification, operated only in backward direction, for simplicity. To minimize noise accumulation, while containing nonlinear effects, we chose to operate with fiber spans consisting of three concatenated sec- tions of UltraWave™ Ocean fibers. The average loss, dispersion and effective area of the fiber (respectively, 0733-8724/$20.00 © 2005 IEEE