1312 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 9, MAY 1, 2012 Generation and Detection of 28 Gbaud Polarization Switched-QPSK in WDM Long-Haul Transmission Systems Jérémie Renaudier, Member, IEEE, Paolo Serena, Alberto Bononi, Member, IEEE, Massimiliano Salsi, Member, IEEE, Oriol Bertran-Pardo, Member, IEEE, Haïk Mardoyan, Patrice Tran, Eric Dutisseuil, Gabriel Charlet, Member, IEEE, and Sébastien Bigo, Member, IEEE Abstract—We report on an experimental long-haul transmission of a 28 Gbaud PS-QPSK signal in WDM scenarios. This report presents, to the authors’ knowledge, a novel implementation of an optical coherent receiver for the polarization-switched quadrature phase shift keying (PS-QPSK) modulation format, with minor variations on the standard coherent PDM-QPSK receiver. We compare the performance of PS-QPSK with that of PDM-QPSK, over a WDM system based on 100 Gb/s PDM-QPSK and standard single mode fiber (SSMF), and we demonstrate its interest for software-defined optical transceivers operating over dispersion managed and non-managed transmission links. Index Terms—Coherent communications, digital commu- nications, four-dimensional (4-D) constellations, optical fiber communications, polarization-switched quadrature phase-shift keying (QPSK). I. INTRODUCTION F OUR-DIMENSIONAL signaling featuring the combined use of real and imaginary components of light and its two orthogonal polarization states has been recently proposed [1]–[3]. Among all the modulation formats proposed for op- tical transmission, the polarization switched (PS-) quadrature phase shift keying (QPSK) format has attracted much attention because it is viewed to be the most power-efficient [2]. Indeed some theoretical considerations predict that PS-QPSK should exhibit a greater sensitivity to optical noise than that of polar- ization division multiplexed (PDM)-QPSK. This improvement in sensitivity to optical noise has been recently confirmed in a single channel experiment realized at 10 Gbaud [4]. More- over, numerical investigations have also reported an improved tolerance to nonlinearities in wavelength division multiplexed (WDM) transmission scenarios at a bit rate of 100 Gbit/s [5], Manuscript received September 16, 2011; revised December 09, 2011; ac- cepted January 09, 2012. Date of publication January 23, 2012; date of current version April 04, 2012. J. Renaudier, M. Salsi, O. Bertran-Pardo, H. Mardoyan, P. Tran, E. Dutisseuil, G. Charlet, and S. Bigo are with Alcatel-Lucent Bell Labs, 91620 Nozay, France (e-mail: jeremie.renaudier@alcatel-lucent.com). P. Serena and A. Bononi are with the Dipartimento di Ingegneria dell’Infor- mazione, Università degli Studi di Parma, 43100 Parma, Italy. 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.2012.2185682 [6]. Despite these advantages, PS-QPSK is not as spectrally ef- ficient as PDM-QPSK, which makes it less adequate for opti- cally-routed 100 Gb/s networks. Nevertheless, PS-QPSK may be a very attractive complement in the list of conventional mod- ulation formats to be used in adaptive bit-rate networks. In such networks, the data rate of each light path is dynamically adapted to both the capacity demand and the amount of physical impair- ments to be overcome [7]. Operating with the same hardware and baud rate as PDM-QPSK, PS-QPSK offers an interesting fall-back alternative to PDM-QPSK. Typically, over links where propagation impairments are strong, PDM-QPSK data may not reach their destination without intermediate O/E/O regenera- tion, whereas PS-QPSK format could survive the same impair- ments at the expense of 25% data-rate reduction [6]. We report here on a series of experiments conducted at 28 Gbaud using single carrier coherent PS-QPSK, expanding upon our initial results [8]. We propose a novel method for generating and detecting PS-QPSK signals compatible with the widely used constant modulus algorithm (CMA) in co- herent PDM-QPSK systems. We compare the performance of PS-QPSK to that of PDM-QPSK operating at the same baud rate over a WDM system based on 100 Gb/s PDM-QPSK and standard single mode fiber (SSMF), using the same transmitter hardware configuration and only minor adjustments of the DSP architecture at the receiver side. We also analyze the impact of electrical and optical dispersion compensation, and demonstrate almost 3 dB performance improvement brought by PS-QPSK in both cases. II. GENERATION AND DETECTION OF PS-QPSK A. PS-QPSK: A Set-Partitioned PDM-QPSK Signal The PS-QPSK format is an 8-level, 3 bit per symbol linear modulation format. At each symbol time one information bit se- lects one of two orthogonal polarizations of the light over which a QPSK symbol, selected by the remaining 2 information bits, is encoded. Nothing is transmitted on the remaining polarization at that symbol time. Fig. 1(a) shows a PS-QPSK transmitted symbol on the complex-envelope X and Y planes: one of 4 available QPSK symbols is selected on X , while zero power is transmitted on Y . In the example, the black dots represent one four-dimensional (4D) symbol. An alternative representation of the PS-QPSK symbols shown in Fig. 1(b) can be obtained from 0733-8724/$31.00 © 2012 IEEE