Degenerate optical cavities.

An optical cavity is degenerate when an arbitrary ray retraces its own path after a single round trip. The condition for degeneracy is given for ring type cavities incorporating internal lenses, using geometrical optics methods. The simplest linear configurations require a spherical mirror or a corner cube, a thin lens, and a plane mirror. Planar rings with four plane mirrors require at least three thin focusing elements. A nonplanar ring is discussed which requires only two thin lenses. The alignment of degenerate cavities is, in general, as critical as the alignment of plane Fabry-Perot.