  A physical theory is presented for polarized light from an aspect of
polarization singularity.This is carried out by analyzing the evolution
equation of the Stokes parameters that is derived from the nonlinear
Schrodinger type equation. The problem is explored from two aspects:The first
is concerning the single mode Stokes parameter that is described by the
propagation distance $ z $. The trajectory of the polarization singularities is
simply given by the points satisfying a rule: $ S_3 (z) = +(-)S_0 $ and $ 0 $
($ S_3 $ represents the third component of the Stokes parameters), which form a
tube like surface centered with the C-line surrounded by L-surfaces in the
presence of the linear as well as the nonlinear birefringence. It is pointed
out that the optical activity (or chirality) plays an "obstacle" to reveal the
polarization singularity. As the other aspect, we consider the field theoretic
aspect of the Stokes parameters, for which a special solution, called an
optical skyrmion, is constructed to explicate the polarization singularity.The
skyrmion forms a {tube}, and the dynamical content of which is briefly
discussed. On the basis of the work presented here further analysis would be
expected to reveal hidden aspects of polarization optics.
