IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 1, JANUARY 1, 2009 51 Experimental Assessment of Interactions Between Nonlinear Impairments and Polarization-Mode Dispersion in 100-Gb/s Coherent Systems Versus Receiver Complexity Oriol Bertran-Pardo, Jérémie Renaudier, Gabriel Charlet, Patrice Tran, Haïk Mardoyan, Massimiliano Salsi, and Sébastien Bigo Abstract—We investigate experimentally the tolerance to polarization-mode dispersion (PMD) of 100-Gb/s polarization- division-multiplexing quaternary phase-shift keying systems under nonlinear impairments. We show that nonlinear effects decrease the robustness of the system against PMD. Moreover, we study the dependence of this robustness on the complexity of the receiver. Index Terms—Coherent detection, digital signal processing, nonlinearities, optical fiber communication, phase-shift keying, polarization-mode dispersion (PMD). I. INTRODUCTION C ONTRARY to direct detection schemes , receivers based on coherent detection can be extremely resistant to polarization-mode dispersion (PMD). Since coherent receivers provide access to the amplitude, the phase, and the polarization of the optical field, they offer the possibility to compensate for several linear impairments, thanks to appropriated digital signal processing. High tolerance to some of these linear impairments have been pointed out with polarization-division-multiplexing (PDM) quaternary phase-shift keying (QPSK) modulation format, such as to optical noise, to chromatic dispersion and to differential group delay [1], and to optical filtering [2] at 100 Gb/s and to PMD [3] at 80 Gb/s. Nevertheless, this mod- ulation format can suffer from nonlinear impairments in some cases [4], [5]. Thus, it would be interesting to investigate how the tolerance to linear impairments is reduced due to nonlinear effects, as in [6]. In this letter, we give further insight on how linear and non- linear effects interact in 100-Gb/s PDM-QPSK coherent sys- tems. We optimize the complexity of the carrier phase estima- tion (CPE) algorithm implemented in the coherent receiver to Manuscript received June 05, 2008; revised October 03, 2008. First published October 31, 2008; current version published January 05, 2009. This work was supported by the French Government in the frame of the COHDEQ 40 and TCHATER projects. The authors are with Alcatel-Lucent Bell Laboratories, Centre de Vil- larceaux, 91620 Nozay, France (e-mail: oriol.bertran_pardo@alcatel-lucent.fr; jeremie.renaudier@alcatel-lucent.fr; gabriel.charlet@alcatel-lucent.fr; haik.mardoyan@alcatel-lucent.fr; patrice.tran@alcatel-lucent.fr; massimiliano. salsi@alcatel-lucent.fr; sebastien.bigo@alcatel-lucent.fr). 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.2008.2008206 Fig. 1. Experimental setup. cope with nonlinear limitations. In this scenario, we characterize how the robustness to PMD is affected when a realistic amount of nonlinear effects has accumulated along the line. Finally, to verify the possibility of reducing the receiver complexity, we study the dependence on the number of taps of the equalizer of the PMD tolerance when nonlinear effects are taken into ac- count. II. EXPERIMENTAL SETUP The test-bed, depicted in Fig. 1, involves 80 lasers spaced by 50 GHz and separated into two independently modulated, spec- trally interleaved combs. The light of each set is sent into a dif- ferent QPSK modulator operating at 28 Gbauds. Both modula- tors are fed with -bit long words at 28 Gb/s, including 7% forward-error correction and protocol overhead. Polariza- tion multiplexing is then performed by dividing the resulting signal by a 3-dB coupler, decorrelating both polarization trib- utaries by an arbitrary 100 symbol delay and finally recom- bining the QPSK data into a polarization beam combiner (PBC) [Fig. 1(inset a)]. Thus, a 112-Gb/s PDM-QPSK data is obtained. After passing through a polarization scrambler (PS), oper- ating at low speed ( 10 Hz), the multiplex is boosted and sent into a typical dispersion-managed link with four 100-km-long spans of standard single-mode fiber, separated by dual-stage erbium-doped fiber amplifiers (EDFAs), incorporating disper- sion-compensating fiber. The PMD of the line does not exceed 1 ps. When needed, a PMD emulator [Fig. 1(inset b)] can be in- serted at the beginning or at the end of the transmission link. It consists of ten sections of polarization-maintaining fiber (PMF) 1041-1135/$25.00 © 2009 IEEE