The steady-state behavior in a unidirectional cavity has been analyzed
by taking into account the effect of phase fluctuation in the control
field based on a Lambda-type atomic model. It is shown that even if b
oth fields are in resonance with the corresponding atomic transitions,
optical bistability can still be realized due to a phase fluctuation
in the control field. Furthermore, the bistable hysteresis loop become
s wider as the linewidth increases.