We study the optical bistability (OB) in an active Raman gain atomic medium by means of a unidirectional ring cavity. The system considered is a resonant n-type four-level atomic ensemble, which can be realized at room temperature and not only have a lot of tunable parameters, such as detuning, atomic concentration, pump and control field, but also possess gain-free (or little gain) transparent windows. We discuss the conditions and the range of these parameters for realizing OB, and also the condition for realizing optical multistability. Our results will provide the theoretical basis for experimental realization.

We examine the effects o f driving field linewidth on a one-atom dressed state laser. Unexpectedly, the linewidth leads to anomalous effects on the cavity field. The mean photon number of the cavity field is raised or the normalized variance is reduced to a certain degree as the linewidth increases for an appropriate range of parameters. The responsible mechanism is attributed to the fluctuation-induced modification o f the electromagnetic reservoir where the atom stays.