Polarization-Mode-Dispersion Impairments in 112-Gb/s PDM-QPSK Coherent Systems

We analyze PMD impairments in 112-Gb/s PDM-QPSK coherent systems and quantify outage probabilities. We show that 1st-order PMD under-estimates PMD impairments, but modeling 2nd-order PMD as polarization-dependent chromatic dispersion significantly over-estimates PMD penalties. Introduction Polarization-mode dispersion (PMD) has long been considered as one of the limiting factors for optical communication systems [1]. Although various PMD compensation techniques have been proposed and demonstrated, PMD impairments can only be partly compensated in direct-detection systems due to the existence of higher-order PMD and its stochastic characteristics [2]. Recently optical digital coherent detection emerged as a promising technology for optical networks. One of the advantages of digital coherent detection is that the full optical field information is available after detection, and thus PMD impairments in principle can be completely compensated in the electrical domain by digital signal processing if electronic equalizers in coherent receivers are complex enough [3]-[6]. In reality, the complexity of electronic equalizers is limited. Therefore, it is important for a system designer to know the amount of PMD that a coherent receiver with given complexity can compensate. There have been some investigations on PMD impairments in digital coherent optical communication systems, st including both deterministic 1 -order PMD measurements and statistic studies, but the PMD tolerance of an optical coherent system has not yet been quantified in outage probabilities.