Adaptive digital equalization in the presence of chromatic dispersion, PMD, and phase noise in coherent fiber optic systems

Chromatic dispersion (CD) and polarization mode dispersion (PMD) severely limit the performance of optical transmission systems operating at and above 10 Gb/s. Electrical equalization techniques have been proposed to compensate dispersion in both coherent and intensity modulation/direct-detection (IM/DD) systems. We investigate the combined adaptive digital equalization of all-order PMD, CD, and laser phase noise in high-speed coherent optical transmission systems. Simultaneous equalization of these impairments has not been reported previously and is particularly important in modulation systems that exploit polarization to increase modulation efficiency. We propose a novel 4-dimensional equalizer structure for joint polarization modulation and M-ary differential phase shift keying (JPMDPSK) systems. The specific example considered 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 four-dimensional 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. The feasibility of the very large scale integration (VLSI) of coherent receivers in current technology is also discussed.