  This second paper on the Fabry-Perot cavity presents a semi-classical
approach, which means that we consider the transition from wave optics to
geometrical optics. The basic concepts are the periodic orbits and their
stability. For the plano-concave Fabry-Perot cavity in the paraxial
approximation, the derivation of the trace formula demonstrates that the
spectrum is based only on the axial periodic orbit and its repetitions.
Experiments with micro-lasers illustrate the relation to periodic orbits. The
methods presented in this paper are not limited to laser cavities and can be
applied to a large range of wave systems.
