Correction of shape distortions in laser beams apodized with circular serrated apertures

Flat-top laser beams produced with apodizers comprising a circular serrated aperture and spatial filter are not optimal for propagation over long distances. Residual intensity fluctuations across the overall smooth profile at the apodizer exit significantly accelerate degradation of the beam at small Fresnel numbers. By solving the parabolic equation for uniform and Gaussian beams propagating through a serrated aperture apodizer, we show that a narrow opaque ring installed inside the serrated aperture can largely suppress unwanted diffraction effects and bring the output amplitude profile close to the flattened Gaussian function. With this correction, the usable propagation distance of the apodized beam can be extended to Fresnel numbers N F ≈ 2 … 5 .