One of these mirrors, the output coupler, allows a small percentage of light transmission. The first is a plane parallel cavity consisting of two flat plane mirrors with the radius of curvature of infinity. Although this cavity is relatively easy to construct, it is extremely sensitive to alignment. Thus creating an unstable cavity. Refer to figure 1.
Stability is based on the cavity's ability to contain the beam.
To ensure that a plane parallel cavity is stable the beam entering the cavity must be parallel. The second cavity is a concave concave cavity comprised of two concave mirrors. Beam alignment in this cavity is much easier to expedite because this cavity has a focusing characteristic. The beam is focused during each pass. Refer to figure 2.