Linewidth of four-level microcavity lasers

We use standard quantum laser theory to examine the Linewidth of lasers wit h a four-level atomic gain medium as a function of the spontaneous emission rate into the lasing mode and the decay rates from the upper and lower las ing levels. We find that the laser linewidth ceases to exhibit the customar y decrease with increased photon number when a critical photon number, prop ortional to the ratio of the decay rate from the lower lasing level to the rate of spontaneous emission into the lasing mode, is reached. This critica l photon number would decrease as the size of the optical cavity is decreas ed. In addition we show that, in general, the laser linewidth depends criti cally on the ratio of the overall decay rates from the upper and lower lasi ng levels. [S1050-2947(99)00903-8].