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MMM 2003 VOLUME 15 NUMBER 3 IPTLEL (ISSN 1041-1135) PAPER

Copyright © 200X IEEE.

Reprinted from IEEE Photonics Technology Letters, vol. 15, no. 3, pp. 467-469 25 x 40-Gb/s Copolarized DPSK Transmission Over 12 x 100-km NZDF With 50-GHz Channel Spacing A.H. Gnauck, G.Raybon, S. Chandrasekhar, J. Leuthold, C. Doerr, L. Stulz, and E. Burrows

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 3, MARCH 2003

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40-Gb/s Copolarized DPSK Transmission Over 12 100-km NZDF With 50-GHz Channel Spacing

A. H. Gnauck, Senior Member, IEEE, G. Raybon, Member, IEEE, S. Chandrasekhar, Fellow, IEEE, J. Leuthold, Member, IEEE, C. Doerr, Member, IEEE, L. Stulz, and E. Burrows

Abstract—We report the first transmission of copolarized 40-Gb/s channels with 0.8-b/s/Hz spectral efficiency. We transmit 25 channels over 1200 km of nonzero-dispersion fiber (1600 km with adjacent channels orthogonally polarized). This result was obtained using differential phase-shift keying, and without forward error correction. Index Terms—Differential phase-shift keying (DPSK), modulation formats, optical fiber communications, return-to-zero (RZ), wavelength-division-multiplexing (WDM) transmission.

I. INTRODUCTION

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ECENTLY, 40-Gb/s wavelength-division-multiplexed (WDM) transmission systems have reached record distances due to the use of forward error correction (FEC) and distributed Raman amplification. Impressive results have been obtained for systems operating with 100-GHz channel spacing (0.4-b/s/Hz spectral efficiency) and 100-km fiber spans. Using carrier-suppressed return-to-zero ON–OFF keying (CSRZ-OOK) modulation format and standard FEC (SFEC) 80-channel 2000-km transmission has been demonstrated in nonzero-dispersion fiber (NZDF) [1], and 40-channel 3600-km transmission has been reached in dispersion-managed (hybrid) fiber spans [2]. Applying enhanced FEC (EFEC) allowed the latter result to be increased to 5200 km [3]. Using return-to-zero differential phase-shift-keying (RZ-DPSK) format and SFEC, 64-channel transmission over 4000 km has been reached in NZDF [4], and 80-channel transmission over 5200 km has been accomplished in dispersion-managed spans [5]. The results with 50-GHz channel spacing (0.8-b/s/Hz spectral efficiency) have also been dramatic. Transmission of 128 channels over 16 80-km spans of standard single-mode fiber (SMMF) was accomplished using CSRZ and SFEC [6], while 159 channels have been transmitted over 21 100-km spans of fiber (dispersion of 8 ps/nm/km) using a variant of duobinary modulation and EFEC [7]. However, in all experiments reported thus far at 40 Gb/s with 0.8-b/s/Hz spectral efficiency, adjacent channels were orthogonally polarized to minimize linear crosstalk and nonlinear transmission penalties. This adds to system complexity, especially when using optical add–drops. Manuscript received October 18, 2002; revised December 3, 2002. The authors are with Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/LPT.2002.807933

Fig. 1. Experimental setup.

Here, we report the first transmission of copolarized 40-Gb/s channels with 0.8-b/s/Hz spectral efficiency, thus testing the worst case for linear and nonlinear penalties. We transmit 25 channels (1 Tb/s) in a 10-nm -band over 12 100-km spans of NZDF. We also report 1600-km transmission using orthogonally polarized adjacent channels. We use DPSK format, balanced detection, and distributed Raman amplification to 10 achieve these results. We reach bit-error rates of 1 without using FEC. II. EXPERIMENT AND DISCUSSION The experimental setup is shown in Fig. 1. The 25 distributed feedback laser sources operated on a 50-GHz grid, and ranged from 1547.31 to 1556.96 nm (193.75 to 192.55 THz). Two 40.0-Gb/s CSRZ-DPSK [8] transmitters were used. Each transmitter consisted of laser sources operating on a 100-GHz grid, combined in an arrayed-waveguide grating (AWG) router, followed by two external LiNbO modulators. The first modulator was used to generate a 40.0-GHz 16-ps CSRZ pulse train. It was biased at the null in its transmission curve, and driven with a half bit rate (20.0-GHz) sine wave at twice the switching voltage. The second modulator performed phase modulation [4], [9] and was driven by a 40.0-Gb/s nonreturn-to-zero data stream. The data was generated by electrically multiplexing four copies of a 10.0-Gb/s pseudorandom bit stream (PRBS) . For DPSK, the data would normally have to of length be differentially encoded to accommodate delay-interferometer demodulation at the receiver. However, due to the properties of pseudorandom sequences, this was not necessary. An interleaver with 43-GHz passbands combined the outputs of the

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 3, MARCH 2003

Fig. 3. Copolarized spectrum at 0 km (top) and at 1200 km (bottom). Insets: Eye patterns.

Fig. 2. Single-channel spectrum and eye pattern before (left) and after (right) the interleaver. The eye patterns were detected using a 32-GHz-bandwidth photodiode.

two 100-GHz-spaced sets of channels onto a 50-GHz grid with uncorrelated data on adjacent channels. It should be noted that the use of a CSRZ pulse train instead of an RZ pulse train would do nothing more than invert the differentially encoded data if the pulse widths were identical. However, the method described above creates 66%-duty-cycle RZ pulses, helping to minimize the spectral width of the RZ signals. In turn, the loss in the 43-GHz-passband interleaver is minimized. In back-to-back measurements of system performance after interleaving, we found similar performance using either 66% CSRZ-DPSK or 33% RZ-DPSK, because the final signal waveform is largely dictated by the narrow filtering. However, the loss through the interleaving filter was approximately 2 dB greater for 33% RZ-DPSK. Fig. 2 shows the optical spectrum and directly detected eye pattern of a single channel before and after the interleaver. Spectral dips occur at 55 GHz, reducing linear crosstalk in the interleaved channels. Small peaks at 75 GHz have a negligible effect on the channels at 100 GHz. The eye pattern before the interleaver is, as expected, a series of pulses (with information encoded in the optical phase). Narrow filtering by the interleaver produces distortion of the signal amplitude. Nevertheless, as will be shown, good system performance can still be obtained. Polarization controllers were used to control the relative polarization of adjacent channels. In the case of copolarized adjacent channels, the signals were passed through a polarizer after interleaving. When the adjacent channels were orthogonally polarized, the polarizer was replaced with a 10% optical coupler. The tapped light was sent to a polarizing beamsplitter that was used to monitor the polarization states of the two transmitters. Dispersion precompensation of 122 ps/nm (1200 km) or 174 ps/nm (1600 km) was applied. The transmission experiment was performed in a recirculating loop that consisted of four 100-km spans of TrueWave reduced-slope (TW-RS) fiber. Each span included an erbium-doped fiber amplifier (EDFA), the TW-RS fiber, a Raman pump, and dispersion-compensating fiber (DCF). The measured average dispersion of the TW-RS fibers used in the

experiment was 4.35 ps/nm/km at 1550 nm and the average dispersion slope was 0.0475 ps/nm /km. The DCF was capable of nearly 100% slope compensation for TW-RS fibers, and each span was undercompensated by 20 ps/nm. The loss of each 100-km span, including the Raman WDM coupler, was 21 dB. Distributed Raman amplification was achieved by backward pumping the transmission fibers. The Raman pump powers injected into the four transmission fibers were 0.5, 0.5, 0.35, and 0.35 W, respectively, and the corresponding Raman gain values were 20, 20, 15, and, 15 dB. The launched power per channel into each span was approximately 2 dBm. A programmable gain-equalizing filter (GEF) was inserted in the loop to compensate for the gain slope in the system across the signal channels. After transmission, a deinterleaver and an AWG demultiplexer having 100-GHz channel spacing and 51-GHz 3-dB bandwidth were used to extract a single channel. Dispersion postcompensation was adjusted for optimum performance using DCF and short lengths of conventional single-mode fiber. After amplification and filtering through a 1.3-nm-bandwidth tunable filter, the signal was reamplified and sent to a Mach–Zehnder delay-interferometer (MZDI) demodulator. The differential delay was 25.0 ps, the polarization-dependent wavelength shift was less than 500 MHz, and the extinction ratio was better than 25 dB. The two outputs from the demodulator were connected to a balanced-photodiode circuit. The output of the balanced-photodiode circuit drove a 40-Gb/s electronic decision circuit/demultiplexer. A 10% optical coupler following the tunable filter provided signal for a 40-GHz clock recovery circuit. Fig. 3 shows the signal spectrum for copolarized channels at the transmitter and after 1200-km transmission. The insets show the eye patterns at one port of the demodulator. The average received optical signal-to-noise ratio (OSNR) was 22.5 dB for 1200-km (copolarized) transmission, and 21.5 dB for 1600-km (orthogonally polarized) transmission. The measured values for all the channels in both cases are shown in Fig. 4. The minimum is 15.6, corresponding to a bit error rate of 1 10 . In back-to-back measurements, OSNR for values of 19.7 and 19.1 dB were required to reach copolarized and orthogonally polarized channels, respectively. Therefore, the transmission penalties are 2.8 dB and 2.4 dB, respectively.

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[2] F. Liu, J. Bennike, S. Dey, C. Rasmussen, B. Mikkelsen, P. Mamyshev, D. Gapontsev, and V. Ivshin, “1.6 Tbit/s (40 42.7 Gbit/s) transmission over 3600 km UltraWave™ fiber with all-Raman amplified 100 km terrestrial spans using ETDM transmitter and receiver,” in Proc. OFC 2002, 2002, Postdeadline Paper FC7. [3] C. Rasmussen, S. Dey, F. Liu, J. Bennike, B. Mikkelsen, P. Mamyshev, M. Kimmitt, K. Springer, D. Gapontsev, and V. Ivshin, “Transmission of 40 42.7 Gbit/s over 5200 km UltraWave® fiber with terrestrial 100 km spans using turn-key ETDM transmitter and receiver,” in Proc. ECOC 2002, 2002, Postdeadline Paper PD4.4. [4] A. H. Gnauck, G. Raybon, S. Chandrasekhar, J. Leuthold, C. Doerr, L. Stulz, A. Agarwal, S. Banerjee, D. Grosz, S. Hunsche, A. Kung, A. Marhelyuk, D. Maywar, M. Movassaghi, X. Liu, C. Xu, X. Wei, and D. M. Gill, “2.5 Tb/s (64 42.7 Gb/s) transmission over 40 100 km NZDSF using RZ-DPSK format and all-Raman-amplified spans,” in Proc. OFC 2002, 2002, Postdeadline Paper FC2. [5] B. Zhu, L. Leng, A. H. Gnauck, M. O. Pedersen, D. Peckham, L. E. Nelson, S. Stulz, S. Kado, L. Gruner-Nielsen, R. L. Lingle, Jr., S. Knudsen, J. Leuthold, C. Doerr, S. Chandrasekhar, G. Baynham, P. Gaarde, Y. Emori, and S. Namiki, “Transmission of 3.2 Tb/s (80 42.7 Gb/s) over 5200 km of UltraWave™ fiber with 100-km dispersion-managed spans using RZ-DPSK format,” in Proc. ECOC 2002, 2002, Postdeadline Paper PD4.2. [6] D. F. Grosz, A. Agarwal, S. Banerjee, A. P. Kung, D. N. Maywar, A. Gurevich, T. H. Wood, C. R. Lima, B. Faer, J. Black, and C. Hwu, “5.12 Tb/s (128 42.7 Gb/s) transmission with 0.8 bit/s/Hz spectral efficiency over 1280 km of standard single-mode fiber using all-Raman amplification and strong signal filtering,” in Proc. ECOC 2002, 2002, Postdeadline Paper PD4.3. [7] G. Charlet, J.-C. Antona, S. Lanne, P. Tran, W. Idler, M. Gorlier, S. Borne, A. Klekamp, C. Simmonneau, L. Pierre, Y. Frignac, M. Molina, F. Beaumont, J.-P. Hamaide, and S. Bigo, “6.4Tb/s (159 42.7Gb/s) capacity over 21 100 km using bandwidth-limited phase-shaped binary transmission,” in Proc. ECOC 2002, 2002, Postdeadline Paper PD4.1. [8] Y. Miyamoto, A. Hirano, S. Kuwahara, Y. Tada, K. Murata, and H. Miyazawa, “Carrier-suppressed differential phase shift keying format for ultra-high-speed channel transmission,” in Proc. OAA 2002, 2002, Paper OTuB2. [9] T. Chikama, S. Watanabe, T. Naito, H. Onaka, T. Kiyonaga, Y. Onoda, H. Miyata, M. Suyama, M. Seino, and H. Kuwahara, “Modulation and demodulation techniques in optical heterodyne PSK transmission systems,” IEEE J. Lightwave Technol., vol. 8, pp. 309–325, Mar. 1990.

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Q measurements at 1200 km (squares) and 1600 km (triangles). III. CONCLUSION

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We have reported transmission of 25 40-Gb/s WDM channels on a 50-GHz grid over 12 (copolarized adjacent channels) or 16 (orthogonally polarized adjacent channels) 100-km spans of NZDF. This is the first reported demonstration of copolarized channels in 40-Gb/s WDM transmission with 0.8-b/s/Hz spectral efficiency. It was achieved using DPSK and counterpropagating Raman pumping in NZDF, and without the use of FEC.

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REFERENCES [1] B. Zhu, L. Leng, L. E. Nelson, L. Gruner-Nielsen, Y. Qian, J. Bromage, S. Stulz, S. Kado, Y. Emori, S. Namiki, P. Gaarde, A. Judy, B. Palsdottir, and R. L. Lingle, Jr., “3.2 Tb/s (80 42.7 Gb/s) transmission over 20 100km of nonzero dispersion fiber with simultaneous C L-band dispersion compensation,” in Proc. OFC 2002, 2002, Postdeadline Paper FC8.

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