Optical quality of tilted spherical mirror unstable resonators

A new class of resonators has recently been proposed which has the property of having different magnifications Mx and My in orthogonal directions perpendicular to the propagation direction; the most interesting cases are those with collimated output analagous to conventional confocal unstable resonators. The designs utilize spherical mirrors which are tilted substantially from the mirror normal. The conventional orientation requires the mirror normal to be along (or very nearly parallel to) the propagation direction. Use of spherical mirrors by this new class of resonators eliminates the difficulties of fabricating mirrors with more complex figures, which would otherwise be required to achieve the desired asymmetric magnification. The critical problem of keeping beam-quality degradation (aberrations) due to mirror tilt within acceptable limits has been examined experimentally and analytically. The results confirm the possibility of achieving acceptable small aberrations for large tilt angles (convex mirror tilt as large as 70 deg, corresponding to 140 deg beam deflection) to operation at infrared wavelengths.