Abstract
The properties of unstable resonators in which localized amplitude or phase perturbations are induced at the center of the coupling mirror are investigated. Unstable resonators with a non-reflecting central zone have zero geometrical feedback; however, there exists a residual feedback due to diffraction at the mirror edges. Such a residual feedback can be significantly reduced using variable reflectivity mirrors. Negative branch unstable resonators with an opaque focal zone are even more efficient to quench this residual feedback. Enhancement of the feedback coefficient by factors up to 3 is predicted when the phase of the wave reflected at the coupling mirror changes over a restricted zone near its center.
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