Combined longitudinal and transverse noise enhancement in lasers

Using a semiclassical approach we derive a general expression for the quant um-limited linewidth of a single-mode laser. We include both nonuniform pro perties of the laser medium and localized losses at mirrors and apertures. For such systems the transverse modes are known to be nonorthogonal, giving rise to an enhancement of the laser noise. The transverse factor varies, i n general, along the propagation direction. The combination of transverse a nd longitudinal noise enhancement is far from trivial. In particular, we sh ow that for an aperture in the cavity, the transverse excess noise factor i s the geometric mean of the factors pertaining to the two regions in which the aperture divides the cavity. [S1050-2947(98)06112-5].