Lasers with injected signals: Fluctuations and linewidths

We consider a single mode laser coupled with an external periodic field. The statistical properties of the laser field (photon statistics) are studied under the combined action of fluctuations due to spontaneous emission processes and an external coherent signal. The main focus of our analytical and numerical investigations is on the eigenvalue spectrum, the field correlation function and the line shape function. We find that the linewidth of the single mode laser increases with increasing external field.