A polarization modulation (PLM)/optical coherent detection system and its phase noise cancelling effect

Abstract

In coherent optical communication systems, the phase noise of the optical source is the dominant factor in limiting the receiving characteristics and its reduction is required. In this paper, a polarization modulationl optical coherent detection system is proposed and theoretical and experimental studies are carried out for this system. First, it is demonstrated that the polarization modulation signal is a differential phase-modulated optical signal between the two orthogonal polarization components. The fundamental principle for the optical coherent detection system is presented. Next, by taking into account the characteristics of the modulator, a general representation of the polarization modulation optical signals is derived and the inherent characteristics of the present system is shown. Further, for a polarization modulation/optical heterodyne detection system for a long-distance transmission, the detection and demodulation methods are presented. The phase noise cancellation effect is found as well as the fact that the polarization axis matching between the transmitter and the receiver is not necessary if a circular polarization modulation is used. The bit error rate of the polarization modulation/optical heterodyne detection system is analyzed theoretically. By comparing this system with other coherent optical communication systems, the receiving characteristics are evaluated. Finally, fundamental experiments are carried out for the optical heterodyne detection of the polarization modulation signals and the phase noise cancellation effect of the present system is confirmed.