NATURAL LINEWIDTH OF A MICROMASER

The natural linewidth of a one-photon micromaser is studied through the two-time correlation function of the field. Both the exact and Fokker-Planck approaches are used and the results are compared. Near threshold, the exact linewidth can be nearly twice as large as the approximate linewidth given by the phase diffusion rate. Above threshold around integer multiples of 2pi of the micromaser pump parameter, the exact linewidth can be only about half of the approximate one given by the phase diffusion rate. Above threshold but before the collapse of the mean photon number happens, the phase diffusion rate is found to be a good approximation to the micromaser linewidth only when the mean photon number is large and the photon-number variance is much smaller than the square of the mean photon number. For very large pump parameter when the revival of the field intensity occurs, the phase diffusion rate fails to represent the micromaser linewidth in general.