Multi-pass optical imaging apparatus includes a concave mirror in combination with two retro-reflecting mirror pairs and at least one reflective surface. The mirror, the retro-reflecting mirror pairs and the reflecting surface are arranged such that a light-ray input into the apparatus parallel to and spaced apart from the optical axis of the concave mirror and incident on the concave mirror is caused to be incident on the thin-disk gain-medium at least four times, with each of the four incidences on the gain-medium being from a different direction. If the input ray is plane-polarized, the arrangement provides that the polarization orientation of the ray on each incidence on the gain-medium is in the same orientation.