Abstract: Using the linearized nonlinear Schrödinger equation obtained with the aid of the small signal approximation,analytical expressions for the amplitude and phase of a perturbation propagating in anoptical fiber have been deduced.The evolution characteristics of the amplitude and phase of the perturbation has been calculated and discussed.The moving trajectories portrayed by the perturbation phasors have been obtained under the polar coordinates.The characteristics and the asymptotic behaviors of the trajectories have been analyzed.The results show that the evolution of the perturbation is intensively influenced by the initial perturbation phase.