  We report the experimental observation of a novel transmission phenomenon in
optical long-haul communication systems. Un-polarized ASE depolarizes via
nonlinear fiber interactions a cw laser light during their co-propagation which
leads to small but measurable ultra-fast polarization state fluctuations at the
fiber output. We provide a phenomenological approach and a theory that
qualitatively corroborates our experimental results. One of our major findings
suggests that the applicability of the often used Manakov equation needs to be
scrutinized for highly accurate studies of nonlinear polarization state
evolutions in noisy environments. The described phenomenon leads to a
qualitatively different microscopic understanding of nonlinear light
propagation in fiber and can contribute towards an explanation for todays
commonly perceived gap between simulated and experimentally obtained system
performance in optical data transmission.
