  The interaction of two orthogonally polarized beams and a four-level GaAs
quantum well (QW) waveguide is investigated. It is shown that, by applying a
static magnetic field normal to the propagation direction of the driving beams,
the birefringence can be induced in the QW waveguide. Moreover, it is
demonstrated that the dephasing rate between two ground states of the QW
waveguide makes it a dichromatic medium and can also diminish the induced
birefringence. Our results show how a large and complete magneto-optical
rotation in the QW waveguide can be obtained via adjusting the intensity of the
magnetic field and also the length of the QW waveguide.
