Intracavity mode competition between classes of flat–top beams

Abstract

There are many applications in which a laser beam with a flat–top intensity profile would be ideal, as compared to a laser beam with a non–uniform energy distribution. Standard stable optical resonators will unfortunately not generate such a laser beam as the oscillating mode. Single–mode oscillation would typically be Gaussian in profile, while multimode oscillation might deliver a beam with an averaged flat–like profile in the near field, but would diverge very quickly due to the higher order modes. In addition, if the modes are coherently coupled, then large intensity oscillations could be expected across the beam. Techniques exist to generate flat–top beams external to the cavity, but this is usually at the expense of energy, and almost always requires very precise input beam parameters. In this paper we present the design of an optical resonator that produces as the stable transverse mode a flat–top laser beam, by making use of an intra-cavity diffractive mirror. We consider the modal build–up in such a resonator and compare the mode competition between flat–top like beams, including Flattened Gaussian beams, Fermi–Dirac beams, and super–Gaussian beams. Finally, we remark on the use of an intra–cavity piezoelectric unimorph mirror for selecting a particular class of flat–top beam as the fundamental mode of the resonator.