The effect of the optical fiber inhomogeneities, giving rise to stochastization of the transmitted signal. is investigated in the canonical formulation. The nonlinear stage of the modulation instability of the pump wave in the three-wave approximation is considered. The analytical analysis of the equations for the wave amplitudes. in the Hamiltonian form shows that a «stochastic layer», i.e. the region in which the phase trajectories: behave chaotically, is formed in the phase space of a dynamic system even at rather weak in- homogeneity of the fiber. The estimates indicate that the phenomenon under study is very likely to be observed in experiments.