Abstract-Chromatic dispersion (CD) and polarization mode dispersion (PMD) severely limit the performance of optical transmission systems operating at data rates of 10 Gb/s and beyond. Electrical equalization techniques have been proposed to compensate dispersion in both coherent and intensity modulation/directdetection (IM/DD) systems. The former benefit from the fact that a complete compensation with zero penalty is possible, at least in principle, whereas in the latter the loss of phase information caused by the direct detection process results in a nonzero dispersion penalty even when optimal equalizers are used. In this paper, we investigate the combined adaptive digital equalization of all-order PMD, CD, and laser phase noise in highspeed coherent optical transmission systems. Although electrical equalization in coherent optical transmission systems has been addressed by previous literature [1]-[3], equalization of the combined effects of CD, PMD, and laser phase noise has not been reported so far. Simultaneous equalization of these impairments is particularly important in modulation systems that exploit polarization to increase the modulation efficiency, such as the joint polarization modulation and M-ary differential phase shift keying (JPMDPSK) system described in [4]. We propose a novel 4-dimensional equalizer structure for JPMDPSK systems. The specific example considered in this paper is 40 Gb/s transmission with a 10 GBaud symbol rate, using DQPSK modulation on each axis of polarization. Our results show that the new fourdimensional equalizer can compensate channel dispersion of up to 1000 km of standard single-mode fiber, with less than 3 dB penalty in signal to noise ratio (SNR). This is a dramatic improvement over 40 Gb/s IM/DD systems, even when they use electrical [5] or optical [6] equalization. The feasibility of the very large scale integration (VLSI) of coherent receivers in current technology is also discussed.
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