  By using the normal ordering method, we study the state evolution of an
optically driven excitons in a quantum well immersed in a leaky cavity, which
was introduced by Yu-xi Liu et.al. [Phys. Rev. A {\bf 63}, 033816 (2001)]. The
influence of the external laser field on the quantum decoherence of a
mesoscopically superposed states of the excitons is investigated. Our result
shows that, thought the characteristic time of decoherence does not depend on
the external field, the phase of the decoherence factor can be well controlled
by adjusting the external parameters.
