Modeling Compensation for Optical Fiber Communication Systems

Today the vast majority of telecommunication and Internet messages are sent along fiber optic cables buried underground. Binary data (encoded as a sequence of pulses of light) may travel thousands of kilometers to reach its final destination. The fibers that are used for this data transfer necessarily contain manufacturing impurities that lead to fast and slow polarization states for the propagating signal. This imperfection in the fiber results in a random distortion effect known as polarization-mode dispersion (PMD). As binary data travels along these fibers, the pulses spread, causing the ones to decrease in value and the zeros to increase. Thus, the received message may contain errors. To decrease the likelihood of errors in the received signal, a device known as a compensator can be placed at the receiver. Determining an optimal setting for the compensator involves rotating the fiber in the compensator to best align its slow axis with the fast axis of the transmission fiber. Such a rotation should cancel out some of the effects of PMD. Modeling this system numerically requires that one generate fiber realizations with large amounts of PMD. To measure rotation angle goodness of fit between compensation and transmission fiber requires that one choose a feedback signal for the compensator. We compare the eye opening, spectral line, and degree of polarization ellipsoid feedback signals. While the eye opening feedback mechanism is the most accurate measure, it is difficult to optimize numerically. The degree of polarization and spectral line feedback signals act as smooth surrogates for the eye.