Theory of multiplicative noise caused by coupling loss and amplitude vector rotation in optical communication channels

From a general optical channel model, it is shown that the coupling loss intensity noise generated at connections of optical channels is caused by amplitude vector rotation and its dependency upon the coupling coefficient. The shape of the polarization is not important. This noise becomes multiplicative noise so that the noise disturbs signals directly even if fluctuations of rotation of the amplitude vector are of low frequency. Detailed spectral properties and SNR are given for this type of noise, and the existence of a lower bound of SNR is shown. The coupling loss intensity noise becomes serious in the coupling of fibers and optical circuits, while it will not be important for the splicing of fibers.