Multilevel coherent optical system based on Stokes parameters modulation

The authors present a polarization-modulated system called N-Stokes parameter-shift keying, which is suitable for local and metropolitan area network applications. The proposed multilevel system is based on demodulation of polarization-modulated signals by Stokes parameter detection, exploiting the property that each state of the transmitted signal can be represented by a point on a Poincare sphere. The transmitted states of polarization, can be represented by a point set on the Poincare sphere and must be chosen to minimize the error probability. The metric characteristics of the spherical surface allow the system performance for a given number of levels to be improved with respect to conventional multilevel systems, in which the representative points of the transmitted field states are bound to a line or part of a plane surface. A heuristic explanation is that on a spherical surface, when the number of points increases, the distance between them decreases more slowly with respect to multilevel systems working on plane metrics. Two fundamental problems of coherent optical systems are solved: polarization modulation ensures wide insensitivity to laser phase noise, while an electronic feedforward algorithm allows the state of polarization fluctuations due to single-model fiber birefringence to be compensated.<>