Nearly flat-top laser beams from unstable resonators with internal spatial filtering

We theoretically show that negative branch confocal unstable resonators with an aperture at the confocal plane can be used to generate nearly flat-top laser beams at large FresneI numbers. Such a behavior is due to the fact that these resonators can be viewed as image relay systems that provide magnified versions of the internal aperture. Modal discrimination increases with magnification and, for most conditions, should provide single transverse mode oscillation. Other unstable resonators with an internal aperture can be used to generate nearly flat-top laser beams provided they satisfy a simple analytical criterion. (C) 1997 Elsevier Science B.V.