CALCULATION OF THE MICROMASER SPECTRUM .1. GREEN-FUNCTION APPROACH AND APPROXIMATE ANALYTICAL TECHNIQUES

We first calculate the 'exact'' micromaser spectrum by solving numeric ally for the Green's function of the appropriate master equation for t he one-atom (micro-) maser. We then proceed to calculate approximate a nalytical expressions for the maser linewidth using (i) a phase-operat or approach, (ii) an ansatz similar to the standard quantum theory of the laser, and (iii) a linear expansion of the correlation function. N ovel features, quite distinct from the familiar Schawlow-Townes linewi dth, are found, e.g., the linewidth decreases as the number of thermal photons increases.