Multi-core fiber transmissions provide great capacity scalability for future optical backbone and access networks. Unfortunately, the polarization-mode dispersion has not been experimentally investigated so far in single-mode multi-core fibers. In this scenario, the differential group delay may present a time-varying nature in each core because of the temporal fluctuations of the optical medium. In order to investigate this phenomenon, we present a coupled-mode theory based on local modes to simulate numerically the polarization-mode dispersion in such optical media and we report extensive experimental measurements of the time-varying differential group delay using a homogeneous 4-core multi-core fiber considering three different months. Our results indicate that the differential group delay presents a similar, but not identical, temporal evolution in the four cores.
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