Planar and non-planar multi-reentrant 2-mirror ring cavities

Summary form only given. Planar and non-planar multi-reentrant ring cavities are demonstrated. We have successfully derived the reentrant condition and obtained good agreement with the experimental results. The stability criteria have also been analyzed. Long round-trip length can be achieved with short cavity length by using the multi-reentrant ring laser cavities. They have the advantage for a mode-locked laser to have a ten-fold reduction in cavity length compared to a conventional linear cavity. They can also be used in trace gas detection for increasing the sensitivity compared to the multi-pass cell technique.