  We present a scheme to realize two-direction optical switch by a single-mode
optical cavity containing some four-level atoms. The high switching efficiency
can be obtained through low photon loss and large third-order nonlinear
susceptibility of this N-type atomic system in cavity. Without the microwave
source, it can be reduced to an atomic system where a coupling laser is used to
realize single electromagnetically induced transparency. Namely, the probe
field can be transmitted almost totally at resonance. Thus a two-direction
optical switch is operated and the state for forward (backward) direction is
set as open (closed). When microwave source is introduced, dressed splitting of
dark state happens. The probe field is reflected almost completely at resonance
and the state of the optical switch at forward and backward directions
(transmitted and reflected channels) is shifted as closed and open,
respectively. Moreover, this scheme is much advantageous to realize splitting
of dark state because weak microwave field induces the coupling between dark
state and one sub-level of ground state. While a strong pump laser which
couples dark state with an excited level is applied to realize this splitting
in Ref. [Phys. Rev. A 85 013814 (2012)].
