Improvement of hole quality during laser drilling of stainless steel foil by using a modified Bessel beam

Abstract

A Bessel beam has been widely applied for laser processing such as laser drilling due to their long DOF (depth of focus) and non-diffractive characteristics. However, a sidelobe peak reaching up to 15% of the central peak causes unnecessary ablation in processing, especially for opaque materials. In this work, we improved the hole quality of stainless steel during laser drilling by reducing the sidelobe of the Bessel beam. To reduce the sidelobe peak, annular filters were designed through numerical algorithm. Two types of filters (filter 1 : 3.2% of sidelobe, filter 2 : 5.4 % of sidelobe) were determined and fabricated. Drilling experiments were performed on stainless steel foil (100 μm of thickness) through a modified Bessel beam with a reduced sidelobe. Femtosecond laser (1030 nm of wavelength, 450 fs of pulse duration, 1 kHz of repetition rate) was used for light source and Axicon lens (AX-252B, thorlabs) was used for generating Bessel beam. Bessel beam type (original, filter 1, filter 2), pulse energy (100~500 μJ), and number of shots (1500~4500 shots) were considered as experimental conditions. To investigate detailed morphology of holes, a total of 5 hole parameters (hole diameter and circularity, sidelobe ablation width, sidelobe ablation depth, burr height, taper angle) were measured with a 3D laser confocal microscope. The hole quality drilled by using original Bessel beam and modified Bessel beam was compared. It was confirmed that ablated area by the sidelobe was significantly reduced by using the modified Bessel beam. The sidelobe ablation width and depth decreased by up to 63% and 73% compared with original Bessel beam. Also, the morphology of the hole according to the filter type was compared. The sidelobe ablation depth was 1.8 times deeper when using modified Bessel beam with filter 2 (5.4% of sidelobe) than using modified Bessel beam with filter 1 (3.2% of sidelobe), which corresponds to the simulation result. In addition, the modified Bessel beam was able to create a straight hole with less burr (below 1 μm) and less taper angle compared to conventional Gaussian beam.