Collective modes in coupled unstable resonators

The results of analytic and numerical investigations of collective modes in two coupled unstable optical resonators are reported. The dependences of the Q factors of the modes and of their frequencies on the mismatch between the resonator lengths and on the phase shift (advance) in the coupling channel are derived. It is shown that an important role is played by the difference between the distributions of the fields in the resonators and by their inhomogeneity, as well as by the effects of diffraction at the edges of mirrors and coupling apertures.