MULTILEVEL COHERENT OPTICAL-SYSTEM BASED ON STOKES PARAMETERS MODULATION

This paper proposes a new multilevel transmission system that allows an efficient exploitation of a single-channel bandwidth in coherent optical communication systems. It is based on Stokes parameters modulation and a decision on the Poincaré sphere. A sensitivity improvement is obtained with respect to other conventional multilevel systems based on the transmission of a constant power signal. This may be due to the fact that in such conventional systems the points representing the transmitted symbols are located on a circle in the signal space. On the contrary, in the proposed system the symbols are represented by points belonging to a spherical surface on which the distances among the points decrease more slowly when increasing the number of points. Two fundamental problems of coherent optical systems are faced and solved. Polarization modulation assures wide insensitivity to laser phase-noise while an electronic feedforward algorithm allows the state of polarization fluctuations due to single-mode fiber birefringence to be compensated. © 1990 IEEE