Optical bistability and multistability in ladder-type four-level atomic coherence medium

Optical bistability（OB） and optical multistability（OM） behaviour of ladder-type four-level atom in a unidirectional ring cavity driven by three laser fields is studied. Three paths of photon transition are coupled with the laser fields. In this case, the laser field ΩB coupling with two middle levels is very important. When the ΩB is increased, optical bistability threshold significantly decreases and optical multistability appears. The effects of the laser field coupling with two upper levels on optical bistability are also discussed. Furthermore, we analyze the effects of the detuning of probe field and atomic cooperation parameter on OB and OM.