320 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 5, MARCH 1, 2010 Practical XPM Compensation Method for Coherent Optical OFDM Systems Liang Bangyuan Du, Student Member, IEEE, and Arthur J. Lowery, Fellow, IEEE Abstract—We show that cross-phase modulation in coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is mitigated with a simple intensity-dependent phase-modulation before the wavelength demultiplexer. A single low-bandwidth photodiode is used to estimate the optical power of all wave- length-division-multiplexing channels entering the receiver, and this determines a phase modulation applied to all channels. This method works well in conjunction with self-phase-modulation pre/postcompensation, allowing significant improvements in the performance of OFDM systems over optically dispersion-managed links. Using simulations, we show a 4-dB signal quality improve- ment in a 2375-km 60-Gb/s CO-OFDM system in the nonlinear limit. Index Terms—Chromatic dispersion (CD), cross-phase modula- tion (XPM), fiber nonlinearity compensation, optical Kerr effect, self-phase modulation (SPM), wavelength-division multiplexing (WDM). I. INTRODUCTION O PTICAL orthogonal frequency-division multiplexing (OFDM) systems are a possible solution for 100-Gb/s Ethernet (100 GE) long-haul transport [1]–[3]. Experimental demonstrations have shown that polarization-multiplexed (Pol-Mux) coherent optical OFDM (CO-OFDM) has sim- ilar back-to-back sensitivity and nonlinear performance to Pol-Mux coherent single-carrier quadrature phase-shift keying (CO-QPSK) [4], [5]. Recent studies have suggested that inline dispersion compensation increases the nonlinear penalty for CO-OFDM systems [5]. This is caused by increased non- linear mixing across several wavelength-division-multiplexing (WDM) channels, commonly referred to as cross-phase mod- ulation (XPM), because the walk-off between the WDM channels [6] is reduced by the dispersion compensation. Recent demonstrations have shown the XPM from the orthog- onal polarization on a single wavelength can be effectively com- pensated [7], [8]. However, it is commonly assumed that XPM across different wavelength channels cannot easily be mitigated by electronic processing, because mitigation would require full knowledge of all of the channels transmitted within the fiber. Manuscript received October 30, 2009; revised November 21, 2009; accepted December 04, 2009. First published January 12, 2010; current version published February 10, 2010. This work was supported in part under the Australian Re- search Council’s Discovery funding scheme (DP0772937). The authors are with the Department of Electrical and Computer Systems Engineering, Monash University, Clayton, Victoria, 3800, Australia (e-mail: liang.du@eng.monash.edu.au; arthur.lowery@eng.monash.edu.au). 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.2009.2039210 Thus, XPM between wavelengths is seen as the ultimate limit to the information capacity of optical fibers [9]. In this letter, we show that XPM in CO-OFDM can be par- tially compensated with a simple low-bandwidth system. When used together with self-phase-modulation (SPM) compensation, a 4-dB increase in signal quality can be obtained in 60 Gb/s per polarization CO-OFDM systems operating in the nonlinear regime. II. SPM COMPENSATION METHODS Fiber nonlinearity can be perfectly compensated with a single power-dependent phase modulation if the link is dispersionless, by applying a phase modulation opposite to that produced by the nonlinear Kerr effect [10]–[12]. For fibers with some dis- persion, SPM products can be partially mitigated, which im- proves performance in the nonlinear regime for both CO-QPSK [10] and CO-OFDM systems [12]–[14]. The phase modulation can be applied at the transmitter, receiver, or at both ends [15], using opto-electronics [11] or digital signal processing (DSP) [8], [10]. Near perfect nonlinearity compensation is possible in disper- sive links by using split-step methods to solve the inverse non- linear Schrödinger equation (NLSE) digitally, either at the trans- mitter [16] or receiver [17], [18]. The effectiveness of compen- sation is determined by the number of steps used and the op- tical bandwidth modeled. Practical implementation is difficult because compensation of a single-channel 25-span link requires 100 times the computational complexity of linear equalizers. Compensation for multiple WDM channels is theoretically pos- sible, but would require even greater simulation bandwidth and more steps [18]. III. PROPOSED XPM COMPENSATION METHOD XPM is the phase modulation imposed on one wavelength channel by the intensity fluctuations of other channels. In links with periodic inline dispersion compensation, XPM contributes significantly to nonlinear degradation. Residual dispersion (or uncompensated dispersion) in each span suppresses XPM; how- ever, in all legacy links using inline dispersion compensation, XPM effects are significant, so limit the benefit that can be achieved from SPM compensation techniques alone [13], [17]. Chromatic dispersion (CD) causes the efficiency of XPM products to decrease with both the modulating signal’s fre- quency (in this case, a component of the signal in a channel) and the frequency spacing of the channels. For example, for a 50-GHz WDM grid, only frequency components of the signal 1 GHz will impose a significant amount of XPM on adjacent channels. This bandwidth decreases for more distant channels 1041-1135/$26.00 © 2010 IEEE