Growing participant numbers and the increase in use of multimedia content drive a permanent rise of data transmitted over the internet. Telecom providers are thereby forced to quickly expand their network infrastructure, especially the core networks. Today, the core networks transmit data optically over all-glass fiber. New transmission technologies increase the capacity of present fiber tracks, helping to save costs for new tracks. The main issue of new transmission technologies is polarization multiplex. Channel capacity is doubled by transmitting two data signals with orthogonal polarizations over a single fiber. The two channels have to be split (demultiplexed) in the receiver. If demultiplexing is done in the optical domain, simple and cost-saving direct detection receivers can be used. This requires a polarization controller, which compensates all polarization changes occurring during transmission. This work documents the realization of such a polarization control system. This system is able to track endless polarization changes with a speed of up to 140 krad/s, corresponding to about 8000 full rotations of the lightwave‘s oscillation plane per second. Several different error signal detectors have been constructed and analyzed for accurate measuring the polarization state deviations. Immunity against temperature and wavelength changes has been proven within practice-oriented ranges. At the end, suitability of the system for automatic polarization demultiplex is demonstrated in several transmission experiments. Thereby, the worldwide highest channel data rate of 200 Gbit/s in polarization-agile PDM-DQPSK transmission was achieved.