  An effect of polarization-mode dispersion, nonlinearity and random variation
of dispersion along an optical fiber on a pulse propagation in a randomly
birefringent dispersion-shifted optical fiber with zero average dispersion is
studied. An averaged pulse width is shown analytically to diffuse with
propagation distance for arbitrary strong pulse amplitude. It is found that
optical fiber nonlinearity can not change qualitatively a diffusion of pulse
width but can only modify a diffusion law which means that a root mean square
pulse width grows at least as a linear function of the propagation distance.
