Transverse excess noise factor in geometrically stable laser resonators

The excess noise factor due to the nonorthogonality of the transverse modes of a geometrically stable cavity subject to large diffraction losses is calculated. This calculation is based on an exact determination of the transverse field distribution of the cavity fundamental eigenmode. It is shown that when the modes become essentially determined by diffraction, the transverse modes are far from being orthogonal, leading to the appearance of a large excess noise factor that must multiply the standard Schawlow-Townes linewidth. Moreover, in the presence of two diffracting apertures, the excess noise factor is shown to exhibit a resonant behavior reminiscent of the one observed in unstable cavities. We estimate that, in the case of circular apertures, excess noise factors as large as 100 can be experimentally measured using a high-gain gas microlaser.