The beam is focused during each pass. Refer to figure 2. However, concave mirrors do not necessarily guarantee a stable cavity. A stable cavity is also dependent on the dimensions of the cavity and the mirrors' radius of curvature. One condition that warrants a stable cavity is when the radius of curvature is greater than half the mirror separation.
Interferometers are optical instruments employed to study light characteristics.
The Fabry-Perot interferometer used in this experiment is identical to the concave concave optical cavity. The mirrors are separated by a distance of 50 meters. The radius of curvature of these mirrors is 34 meters - satisfying the above stability condition. Light inside the interferometer propagates similar to a standing wave. Refer figure 3.