1560 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 14, JULY 15, 2006 Chirp-Managed Laser and MLSE-RX Enables Transmission Over 1200 km at 1550 nm in a DWDM Environment in NZDSF at 10 Gb/s Without Any Optical Dispersion Compensation S. Chandrasekhar, Fellow, IEEE, A. H. Gnauck, Senior Member, IEEE, G. Raybon, L. L. Buhl, D. Mahgerefteh, X. Zheng, Y. Matsui, K. McCallion, Z. Fan, and P. Tayebati Abstract—The small form factor chirp-managed laser (CML) source at 1550 nm was used in conjunction with postdetection electronic signal processing to successfully achieve transmission over 1200 km of nonzero dispersion-shifted fiber in a dense wavelength-division-multiplexing environment, without using any optical dispersion compensation. In a comparative study, the dispersion tolerance and the transmission performance of the CML were found to be better than the duobinary format. Index Terms—Dispersion compensation, optical communica- tion, signal processing. I. INTRODUCTION R ECENTLY, there has been increased interest and ac- tivity in achieving 10-Gb/s transmission over standard single-mode fiber (SSMF) without using any in-line optical dispersion-compensating fiber (DCF). A variety of approaches have been demonstrated, including electronic predistortion techniques at the transmitter to achieve long uncompensated reaches [1], new transmitter techniques such as the chirp-man- aged laser (CML) that have wide dispersion tolerance [2], [3], and electronic signal processing at the receiver such as maximum-likelihood sequence estimation (MLSE) [4] and microwave domain dispersion compensation [5] to mitigate dispersion-related intersymbol interference (ISI). In these reported publications, the fiber type studied was SSMF with a nominal chromatic dispersion of 17 ps/nm-km at a wave- length of 1550 nm. However, there are many unlit installed links of nonzero dispersion-shifted fiber (NZDSF), such as TrueWave RS (TWRS) and LEAF, where fiber dispersion is in the range of 3 to 6 ps/nm-km. In such links, a dispersion tolerance ranging from 4000 to 5000 ps/nm clearly implies that transmission distance over 1000 km is possible without the need for any DCF. The implications from an overall system cost point of view are very important, in that there is a large Manuscript received March 23, 2006; revised April 27, 2006. S. Chandrasekhar, A. H. Gnauck, G. Raybon, and L. L. Buhl are with Bell Laboratories, Lucent Technologies, Holmdel, NJ 07733 USA (e-mail: sc@lucent.com). D. Mahgerefteh, X. Zheng, Y. Matsui, K. McCallion, Z. Fan, and P. Tayebati are with Azna LLC, Wilmington, MA 01887 USA (e-mail: daniel@aznacorp. com). Digital Object Identifier 10.1109/LPT.2006.879810 savings both from avoiding the use of DCFs as well as from using simpler amplifiers (without the need for midstage access). Electrical low-pass filtered (LPF) duobinary format was used in [6] to transmit 50-GHz-spaced channels at 10.7 Gb/s over 1200 km of LEAF using lumped optical precompensation at the transmitter and MLSE at the receiver. In this letter, we use the small form factor CML source, which has been demonstrated to have large dispersion tolerance and be amenable to receive-side signal processing, in conjunction with a commercially available MLSE receiver to bridge distances up to 1200 km of TWRS, without any optical dispersion compensation, in a dense wavelength-division-multiplexing (DWDM) environment. We also compare quantitatively the dispersion tolerance and the transmission performance of the CML with LPF duobinary. II. EXPERIMENTAL SETUP The experimental setup is shown schematically in Fig. 1. Seven distributed feedback (DFB) lasers on a 50-GHz grid ranging from 193.15 to 193.45 THz (1552.12 to 1549.71 nm) were used as sources. Two transmitters were used, corre- sponding to odd and even channels. Each had DFB lasers operating on a 100-GHz grid, combined in an arrayed-wave- guide grating (AWG) router, and followed by an external X-cut nominally zero-chirp LiNbO Mach–Zehnder modulator. To obtain the duobinary format, the modulator was biased at a null, a 3.0-GHz electrical LPF was added in the drive-signal path, and a peak-to-peak drive voltage corresponding to twice the switching voltage was applied. The data rate was 10.664 Gb/s with a pseudorandom bit stream of length . (The clock recovery used in this investigation failed under large chromatic dispersion for longer word lengths.) The outputs of the trans- mitters were combined with a 100- to 50-GHz interleaver. A monitor containing a polarizing-beam splitter filter was used to ensure that the odd and even channels were copolarized. Using a second pattern generator, a CML source was set up, similar to that reported in [2]. The frequency of the CML was set to 193.3 THz (1550.92 nm), which is the center frequency among the seven DWDM channels. For transmission measurements with the CML, the corresponding DFB laser in the duobinary transmitter was turned off and the CML signal was combined with the remaining channels using an optical coupler. Thus, the CML format was surrounded by LPF duobinary formats, which are spectrally quite similar [2]. 1041-1135/$20.00 © 2006 IEEE