This invention relates to optical devices such as a fiber-optic frequency shifter and an optical interferometer. This invention also relates to a method of generating optical interference signals to obtain reliable sensor signals independent of polarization fluctuations of the optical waves in the fiber. Fiber-optic interferometers have many advantages such as high sensitivity and convenient way of using even in poor surrounding environments. These interferometer, however, exhibit polarization dependent signal degradation since the polarization of a light wave passing through an optical fiber irregularly changes depending on the birefringence of the fiber. This invention is characterized in that the frequencies of two orthogonal eigen polarizations of a light wave are shifted to different frequency values at one optical path of the interferometer. They produce two beat interference signals after interfering with the light wave passed through the other optical paht of the interferometer. The two beat interference signals have amplitudes varying complementarily. Selecting the larger one from the two beat signals can prevent polarization dependent signal degradation. Moreover, the beat signals from the optical interferometer according to the invention requires simpler signal processing compared to the output of conventional fiber-optic interferometer.