Modes of empty off-axis unstable resonators with rectangular mirrors.

The effect of Fresnel edge diffraction on mode loss and far-field beam quality is investigated for empty offaxis unstable resonators with rectangular mirrors. For the cases investigated, mode-loss separation and beam quality of the lowest-loss mode are maximized nonconcurrently when the optical axis intersects the small convex mirror near one of its corners rather than its center. Although these properties indicate that the near-corner off-axis resonator can achieve superior beam quality to that achieved by an on-axis resonator, it is also shown that a large instability in beam quality can occur near the operating point which gives maximum mode-loss separation and superior beam quality.