PMD induced system impairments in long-haul optical communication system

Abstract

With increasing bit-rate polarization mode dispersion (PMD) is becoming a major system impairment. The fact that PMD is a statistical parameter of the system, depending sensitively on environmental condition and deployment, analysis of impairment caused by PMD is difficult. PMD is defined as the statistical ensemble average of differential group delay (DGD) between the two principal states of polarization (PSP). First order PMD impairments arises from distortions caused by the DGD that is assumed constant as a function of wavelength. We discuss how to budget PMD induced impairment for such systems where statistical distribution of DGD is taken into account. We also discuss the validity of this analysis in presence of fixed DGD elements, e.g., EDFAs and other discrete optical elements. In reality, because of random distributed mode coupling between the two eigenpolarization directions, DGD is wavelength dependent. In addition, the direction of PSP varies as a function of wavelength causing added penalties. Variation of DGD and PSP direction over the signal bandwidth causes the 2nd order PMD impairments. We discuss the amount of 2nd order PMD for various system configurations and the impact of this 2nd order PMD for high bit-rate systems. We conclude by reviewing the current state of PMD compensation and comment on the effectiveness of 1st order PMD compensation for high bit-rate systems.