  We develop a general method allowing one to construct the consistent theory
of light pulse propagation through an atomic medium in arbitrary nonlinear
regime with respect to the field strength, taking into account the light
polarization, temporal (frequency) and spatial dispersions. The method is based
on the reduced Maxwell equation, atomic density matrix formalism and adiabatic
approximation. In order to demonstrate the efficiency of our method we
investigate in detail the case of propagation of the polarization pulse under
conditions of coherent population trapping in a medium of two-level atoms with
degenerate energy levels. Equations describing the evolution of various field
parameters are derived. It is shown that pulses of ellipticity and spatial
orientation of polarization ellipse propagate with slowing. Analytical
expressions for the slowing factor are obtained. A previously unknown effects
of the stimulated phase modulation and the generation of a phase pilot pulse by
the variation of spatial orientation of the polarization ellipse are predicted.
In addition, we show that the spatial dispersion can be interpreted as "wind
effects".
