NEARLY FLAT-TOP LASER-BEAMS FROM UNSTABLE RESONATORS WITH INTERNAL SPATIAL-FILTERING

We theoretically show that negative branch confocal unstable resonator s with an aperture at the confocal plane can be used to generate nearl y flat-top laser beams at large FresneI numbers. Such a behavior is du e to the fact that these resonators can be viewed as image relay syste ms that provide magnified versions of the internal aperture. Modal dis crimination increases with magnification and, for most conditions, sho uld provide single transverse mode oscillation. Other unstable resonat ors with an internal aperture can be used to generate nearly flat-top laser beams provided they satisfy a simple analytical criterion. (C) 1 997 Elsevier Science B.V.