Interpretation of polarization dispersion in a single-mode fiber

In a weak birefringent fiber such as a typical single-mode fiber, the group delay difference between two eigen modes decays due to the mode conversion effect. It tends to saturate with distance, and its second moments, contributing to the pulse broadening, increases at a constant rate. However, strictly speaking, no polarization mode is defined for a fiber having birefringence perturbation along a fiber axis. Thus, polarization dispersion experimentally evaluated for a real fiber is not yet clear. This paper shows a new interpretation of polarization dispersion of a long single-mode fiber. The group delay difference is evaluated through the evolution of SOP on the Poincare sphere, and its length dependence is briefly estimated. The group delay difference observed at a fiber end contains information of the power distribution ratios at local parts of a fiber, and it grows with distance, like a pulse width. As a result, the evaluated group delay difference is expected to increase with distance at an almost constant ratio over a 1000 km-length. Evaluation of polarization dispersion is noted as a potential limitation in long-haul transmission systems.