The basic optical set-up for demodulation of phase signal propagating in singlemode fiber - optics was described in this article. In this arrangement two fiber - optics (modulated signal in first and reference in second) was used. Both fibers in the final part of this set-up are parallel to each other at distance between them equals "d". After propagation by fiber beam goes through the lens. The spatial distribution of light amplitude in the focal plane of the lens is proportional to the Fourier transform on the output light in fibers and modulated with frequency proportional to "1/d". The power spectral density of the spatial distribution of light by bicell photodetector was analyzed. The sum and difference of output signals from channels on the detector is depending on the power spectral density, bicell gap and cosine and sine of phase respectively. On the ground of this results it is possible to identify demodulate of different types of modulation without any a priori information.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.