Patent ID: 8477825

Claim:
Optical apparatus, comprising: a concave mirror having an optical axis; a thin-disk gain-medium having a reflective backing, and located on the optical axis of the concave mirror facing the concave mirror; a first retro-reflecting mirror arrangement located below the optical axis of the concave mirror and facing the concave mirror; a second retro-reflecting mirror arrangement located above the first retro-reflecting mirror arrangement, above the optical axis of the concave mirror, and facing the optical axis of the concave mirror; at least a first reflecting surface located between the first and second retro-reflecting mirror arrangements and inclined to the optical axis of the concave mirror; and wherein the positions and angular orientations of the surfaces of the concave mirror, the first and second retro-reflecting mirror arrangements, the at least first reflecting surface, and the thin-disk gain-medium are cooperatively arranged such that when a light-ray, that is plane-polarized with a predetermined polarization-orientation enters the apparatus parallel to and spaced apart from the optical axis of the concave mirror and is incident on the concave mirror, the light-ray is caused to be incident on the thin-disk gain-medium at least four times, with each of the four incidences thereon being from a different direction and wherein the polarization-orientation of the light-ray is about the same at each of the at least four incidences thereof on the thin-disk gain-medium.