  We study the optic control for birefringence of a polarized light by an
atomic ensemble with a tripod configuration, which is mediated by the
electromagnetically induced transparency with a spatially inhomogeneous laser.
The atom ensemble splits the linearly polarized light ray into two
orthogonally-polarized components, whose polarizations depend on quantum
superposition of the initial states of the atom ensemble. Accompanied with this
splitting, the atom ensemble behaves as a birefringent lens, which allows one
polarized light ray passing through straightly while focus another orthogonal
to this polarization with finite aberration of focus.
