Unstable laser resonators with helical mirrors

Summary form only given. Helical laser beams, often referred to as vortex beams or the beams with a phase singularity on the beam axis, have been constantly attracting researchers interest because of their "doughnut" intensity distribution and nonzero values of topological charge and orbital angular momentum. All these features can be useful in a number of applications. Usually laser beams with helical wavefronts are obtained via diffraction of conventional laser beams on computer-generated holograms or with the use of cylindrical lens converters. Such converters can transform the Hermite-Gaussian beams into Laguerre-Gaussian ones that have the nonzero topological charge. However, these techniques have limited usefulness in generating high-power helical beams that are of interest in connection with investigation of vortex solitons in nonlinear media and in some other applications. We present the results of the computer simulation of large-aperture lasers with unstable resonators comprising a helical mirror with the charge m=-2 instead of a conventional coupling mirror. It is shown that such a laser oscillates at the fundamental mode that has a helical wavefront dislocation with the charge n=l and axially symmetric intensity distribution provided the rear mirror of the resonator is replaced for by a roof-reflector. The intensity and phase distributions of radiation within a CO/sub 2/ laser with unstable resonator having magnification M=2, equivalent Fresnel number N/sub eq/=9.43 and the helical coupling mirror with diameter 2a=4 cm are presented.