Temporal behavior of an unstable optical cavity.

The relaxation of a geometrically unstable Fabry-Perot cavity is theoretically and experimentally investigated. It is observed that the usual sum of the exponential decays of the intensities in the different transverse modes of the cavity is replaced by a more complex behavior. In particular, light couplings into the fundamental mode of the cavity with initial wave excitation factors larger than unity, i.e., larger than in the case of mode-matched injection, are directly observed. The influence of the cavity Fresnel number and of transverse mode crossings and anticrossings on these cavity decays is isolated.