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The light polarization fluctuations strongly affect the performance of coherent optical communication systems due to the fact that to recover the information contained in the phase of the signal beam, it is necessary to have interference with a reference beam. In the present work, a simple system based on the use of an optical fiber interferometer and two adaptive photodetectors, which is immune to polarization fluctuations provoked by the fiber in both signal and reference beams, is theoretically analyzed. In such study, two models of the optical fiber, the first with just one linear retarder and another more complex with an additional circular retarder, were used. The results for linearly polarized light in the Y axis direction showed that digital signal transmission received by the adaptive system is immune to polarization fluctuations (in both signal and reference beams) in a wide range of ellipticity and rotation variations. In particular, if TTL logic is used, a maximum fluctuation of 60% with respect to the maximum of the output signal would be allowed to transmit the information correctly, in this case, ellipticity and rotation variations could be up to ±30°.