Recently, significant progress was achieved in the creation of high bit-rate optic fiber communication systems based on single-mode technology. The reduction of semiconductor lasers emission band, the increase in the stability of their parameters, as well as the development of lightguides preserving polarization of the propagating optic emission have provided the possibility to apply efficient methods of signal transmission and processing allowing a great increase of the capacity and the range of communication. Heterodyning (homodyning), angular modulation of carriers, and signal division with a relatively small frequency spacing are only a few of them. Single-mode technology has also allowed implementation of discrete phase modulation with all the advantages inherent to it. However, together with the simplicity of technical implementation of the phase modulators based on the transversal electro-optic effect certain problems arise in the creation of demodulator functional units. They are caused due to a rather small wavelength (lambda) of the optic emission compared to the dimensions of optic elements and result in inaccurate signal phase relationships at various points on the optic circuit unless special measures are taken. Besides, it is extremely difficult to perform synchronization of the heterodyne oscillator required for coherent reception. Under these conditions the application of DPSK (or PDM) modulation, in particular, of higher orders becomes more preferable. In this case signal demodulation with PDM can be performed based on autocorrelation, correlation, or on coherent methods possessing their own peculiarities and restrictions in the optic frequency range.