Laser processing of a non-transparent material using a Bessel-Gauss beam is accompanied by serious ablation by the sidelobes. To suppress sidelobe ablation, the electric field of a Bessel-Gauss beam generated with a femtosecond laser and an axicon lens was modified by spatially filtering the input Gaussian beam using an annular filter. When the annular filter was applied, the intensity of the first sidelobe peak decreased from 15 of the unfiltered beam to 3.2 and 5.4, depending on the depth of focus. Laser drilling of 100 mu m-thick stainless steel foil using the spatially filtered Bessel-Gauss beam showed significant improvements in the hole quality as compared to the conventional Bessel-Gauss beam. The normalized sidelobe diameter (=sidelobe diameter divided by hole diameter) was reduced from 2.1-2.5 to 1.5-1.7. The maximum depth of sidelobe ablation was reduced by up to 72.5. It was also shown that nearly burr-free laser drilling is possible by properly selecting process conditions with the spatially filtered Bessel-Gauss beam.
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