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.
© 1969 Optical Society of America
Original Manuscript: May 31, 1968
Published: January 1, 1969
J. A. Arnaud, "Degenerate Optical Cavities," Appl. Opt. 8, 189-196 (1969)